Chapter 22. Radioactive Waste Disposal

Materials that contain radioactive atoms and that are deemed to be of no value are classed as radioactive wastes. They may be natural substances, such as uranium ore residues with isotopes of radium and radon, products of neutron capture, with isotopes such as those of cobalt and plutonium, or fission products, with a great variety of radionuclides. Wastes may be generated as byproducts of national defense efforts, of the operation of commercial electric power plants and their supporting fuel cycle, or of research and medical application at various institutions. The radioactive components of the waste may emit alpha particles, beta particles, gamma rays, and in some cases neutrons, with half-lives of concern from the standpoint of storage and disposal ranging from several days to thousands of years.

Because it is very difficult to render the radioactive atoms inert, we face the fact that the use of nuclear processes must be accompanied by continuing safe management of materials that are potentially hazardous to workers and the public. The means by which this essential task is accomplished is the subject of this chapter.

22.1. The Nuclear Fuel Cycle

Radioactive wastes are produced throughout the nuclear fuel cycle sketched in Figure 22.1. This diagram is a flowchart of the processes that start with mining and end with disposal of wastes. Two alternative modes are shown—once-through and recycle.

Figure 22.1. Nuclear fuel cycles. The once-through shown on the left is used in the United States; the recycle shown on the right is used in other countries.

Uranium ore contains very little of the element uranium, approximately 0.1% by weight. The ore is treated at processing plants known as mills, where mechanical and chemical treatment gives “yellowcake,” which is mainly U³O⁸, and large residues called mill tailings. These still have the daughter products of the uranium decay chain, especially radium-226 (1599 y), radon-222 (3.82 d), and some polonium isotopes. Tailings are disposed of in large piles near the mills, with an earth cover to reduce the rate of release of the noble radon gas and thus prevent excessive air contamination. Strictly speaking the tailings are waste, but they are treated separately.

Conversion of U³O⁸ into uranium hexafluoride, UF⁶, for use in isotope enrichment plants produces relatively small amounts of slightly radioactive material. The separation process, which brings the uranium-235 concentration from 0.7 wt% to 3 to 5%, also has little waste. It does generate large amounts of depleted uranium (“tails”) at approximately 0.3% U-235. Depleted uranium is stored and could be used as fertile material for future breeder reactors. The fuel fabrication operation, involving the conversion of UF⁶ to UO² and the manufacture of fuel assemblies, yields considerable waste despite recycling practices. Because U-235 has a shorter half-life than U-238, the slightly enriched fuel is more radioactive than natural uranium.

The operation of reactors gives rise to liquids and solids that contain radioactive materials from two sources. One is activation of metals by neutrons to produce isotopes of iron, cobalt, and nickel. The other is fission products that escape from the fuel tubes or are produced from uranium residue on their surfaces.

Spent fuel, resulting from neutron irradiation in the reactor, contains the highly radioactive fission products and various plutonium isotopes, along with the sizeable residue of uranium that is near natural concentration. As shown on the left side of Figure 22.1, the fuel will be stored, packaged, and disposed of by burial according to current United States practice.

In some other countries the spent fuel is being reprocessed. As sketched in the right side of Figure 22.1, uranium is returned to the isotope separation facility for re-enrichment, and the plutonium is added to the slightly enriched fuel to produce “mixed-oxide” fuel. Only the fission products are subject to disposal.

22.2. Waste Classification

For purposes of management and regulation, classification schemes for radioactive wastes have evolved. The first contrasts defense and nondefense wastes. The original wastes were from the Hanford reactors used in World War II to produce weapons material. The wastes were stored in moist form in large underground tanks. Over subsequent years part of these defense wastes have been processed for two reasons: (a) to fix the wastes in stable form; and (b) to separate out the two intermediate half-life isotopes, strontium-90 (29.1 y) and cesium-137 (30.2 y), leaving a relatively inert residue. Additional defense wastes were generated by reactor operation over the years for the stockpile of plutonium and tritium for nuclear weapons, and the spent fuel from submarine reactors was reprocessed.

Nondefense wastes include those produced in the commercial nuclear fuel cycle as described previously by industry and by institutions. Industrial wastes come from manufacturers who use isotopes and from pharmaceutical companies. Institutions include universities, hospitals, and research laboratories.

Another way to classify wastes is according to the type of material and the level of radioactivity. The first class is high-level waste (HLW) from reactor operations. These are the fission products that have been separated from other materials in spent fuel by reprocessing. They are characterized by their very high radioactivity; hence the name.

A second category is spent fuel, which really should not be called a waste, because of its residual fissile isotopes. However, in common use, because spent fuel in the United States is to be disposed of in a high-level waste repository, it is often thought of as HLW.

A third category is transuranic wastes, abbreviated TRU, which are wastes that contain plutonium and heavier artificial isotopes. Any material that has an activity caused by transuranic materials of as much as 100 nanocuries per gram is classed as TRU. The main source is nuclear weapons fabrication plants.

Mill tailings are the residue from processing uranium ore. The main radioactive elements other than residual uranium are radium (1599 y) and thorium (7.54 × 10⁴ y). Nuclear Regulatory Commission (NRC) regulations call for covers of tailings piles to prevent the release of radon (3.82 d).

Another important category is low-level waste (LLW), which officially is defined as material that does not fall into any other class. LLW has a small amount of radioactivity in a large volume of inert material and generally is subject to placement in a near-surface disposal site. The name “low-level waste” is misleading in that some LLW can have a curie content comparable to that of some old high-level waste.

Two other categories are naturally occurring radioactive materials (NORM) such as byproducts of phosphate mining and accelerator-produced materials (NARM). Both have slight radioactivity.

Still other categories are used for certain purposes e.g., remedial action wastes, coming from the cleanup of formerly used facilities of the Department of Energy (DOE). A category called mixed LLW has the characteristics of LLW but also contains hazardous organic chemicals or heavy metals such as lead or mercury. An EPA Web site gives a full description of this type of waste (see References).

For a number of years the category “below regulatory concern” (BRC) referred to wastes having trivial amounts of activity and subject to unrestricted release. The NRC was unsuccessful in obtaining consensus on the subject and abandoned the category around 1990.

Some perspective of the fuel cycle and nuclear wastes can be gained from Table 22.1, adapted from an Australian Web site (see References).

Table 22.1. Typical Annual Weights (Tonnes) 1000 MWe Reactor, 3.5% Enriched Fuel

Fuel supplied and discharged	25.0
Enriched UF6	32.5
Depleted UF6	217.5
U3O8 from uranium mill	200
Uranium ore and tailings	50,000

The 25 tonnes of spent fuel is to be compared with the burning of 3.2 million tonnes of coal with a release of 7 million tons of CO².

22.3. Spent Fuel Storage

The management of spent fuel at a reactor involves a great deal of care in mechanical handling to avoid physical damage to the assemblies and to minimize exposure of personnel to radiation. At the end of a typical operating period of 1 y for a Pressurized Water Reactor (PWR), the head of the reactor vessel is removed and set aside. The whole space above the vessel is filled with borated water to allow fuel assemblies to be removed while immersed. The radiation levels at the surface of an unshielded assembly are millions of rems per hour. By use of movable hoists, the individual assemblies weighing approximately 600 kg (1320 lb) are extracted from the core and transferred to a water-filled storage pool in an adjacent building. Computer Exercise 22.A shows the arrangement of fuel assemblies in racks of a water storage pool. Approximately a third of the core is removed; fuel remaining in the core is rearranged to achieve the desired power distribution in the next cycle; and fresh fuel assemblies are inserted in the vacant spaces. The water in the 40-ft-deep storage pool serves as a shielding and cooling medium to remove the fission product residual heat. We may apply the decay heat formula from Section 19.3 to estimate the energy release and source strength of the fuel. At a time after shutdown of 3 months (7.9 × 10⁶ s) the decay power from all the fuel of a 3000 MWt reactor is If we assume that the typical particles released have an energy of 1 MeV, this corresponds to 1.4 billion curies (5.2 × 10¹⁹ Bq). To ensure integrity of the fuel, the purity of the water in the pool is controlled by filters and demineralizers, and the temperature of the water is maintained by use of coolers.

The storage facilities consist of vertical stainless steel racks that support and separate fuel assemblies to prevent criticality, because the multiplication factor k of one assembly is rather close to 1. When most reactors were designed, it was expected that fuel would be held for radioactive “cooling” for only a few months, after which time the assemblies would be shipped to a reprocessing plant. Capacity was provided for only about two full cores, with the possibility of having to unload all fuel from the reactor for repairs. The abandonment of reprocessing by the United States required utilities to store all spent fuel on site, awaiting acceptance of fuel for disposal by the federal government in accordance with the Nuclear Waste Policy Act of 1982 (NWPA). Re-racking of the storage pool was the first action taken. Spacing between assemblies was reduced, and neutron-absorbing materials were added to inhibit neutron multiplication. For some reactors this was not an adequate solution of the problem of fuel accumulation, and thus alternate storage methods were investigated. There were several choices. The first was to ship spent fuel to a pool of a newer plant in the utility's system. The second was for the plant to add more water basins or for a commercial organization to build basins at another central location. The third was to use storage at government facilities, a limited amount of which was promised in NWPA. The fourth was rod consolidation, in which the bundle of fuel rods is collapsed and put in a container, again to go in a pool. A volume reduction of about two can be achieved. A fifth was to store a number of dry assemblies in large casks sealed to prevent access by water. A variant is the storage of intact assemblies in dry form in a large vault. Dry storage is the favored alternative. An ideal solution would be to use the same container for storage, shipment, and disposal. A combination of methods may instead be adopted as DOE accepts spent fuel.

The amount of material in spent fuel to be disposed of annually can be shown to be surprisingly small. Dimensions in meters of a typical PWR fuel assembly are 0.214 × 0.214 × 4.06, giving a volume of 0.186 m³. If 60 assemblies are discharged from a typical reactor, the annual volume of spent fuel is 11.2 m³ or 394 cubic feet. For 100 United States reactors this would be 39,400 ft³, which would fill a standard football field (300 ft × 160 ft) to a depth of less than 10 inches, assuming that the fuel assemblies could be packed closely.

The amount of fission products can be estimated by letting their weight be equal to the weight of fuel fissioned, which is 1.1 g per MWd of thermal energy. For a reactor operating at 3000 MW this implies 3.3 kg/d or approximately 1200 kg/y. If the specific gravity is taken to be 10 (i.e., 10⁴ kg/m³), the annual volume is 0.12 m³, corresponding to a cube 50 cm on a side. This figure is the origin of the claim that the wastes from a year's operation of a reactor would fit under an office desk. Even with reprocessing the actual volume would be considerably larger than this.

The detailed composition of a spent fuel assembly is determined by the number of Mwd/tonne of exposure it has received. A burnup of 33,000 MWd/tonne corresponds to a 3-year operation in an average thermal neutron flux of 3 × 10¹³/cm² − s. Figure 22.2 shows the composition of fuel before and after. The fissile material content has only been changed from 3.3% to 1.43%, and the U-238 content is reduced only slightly.

Figure 22.2. Composition of nuclear fuel before and after irradiation with neutrons in a reactor.

(From Raymond L. Murray, Understanding Radioactive Waste, 2003, courtesy of Battelle Press, Columbus, OH.)

22.4. Transportation

Regulations on radioactive material transportation are provided by the federal Department of Transportation and the NRC. Container construction, records, and radiation limits are among the specifications. Three principles used are: (a) packaging is to provide protection; (b) the greater the hazard, the stronger the package must be; and (c) design analysis and performance tests assure safety. A classification scheme for containers has been developed to span levels of radioactivity from exempt amounts to that of spent nuclear fuel. For LLW coming from processing reactor water, the cask consists of an outer steel cylinder, a lead lining, and an inner sealed container. For spent fuel, protection is required against (a) direct radiation exposure of workers and the public, (b) release of radioactive fluids, (c) excessive heating of internals, and (d) criticality. The shipping cask shown in Figure 22.3(A) consists of a steel tank of length 5 m (16.5 ft) and diameter 1.5 m (5 ft). When fully loaded with 7 PWR assemblies, the cask weighs up to 64,000 kg (70 tons). The casks contain boron tubes to prevent criticality, heavy metal to shield against gamma rays, and water as needed to keep the fuel cool and to provide additional shielding. A portable air-cooling system is attached when the cask is loaded on a railroad car as in Figure 22.3(B). The cask is designed to withstand normal conditions related to temperature, wetting, vibration, and shocks. In addition, the cask is designed to meet four performance specifications that simulate real conditions in road accidents. The cask must withstand a 30 ft (~10 m) free fall onto an unyielding surface, a 40 in. (~1 m) fall to strike a 6-in. (~15 cm) diameter pin, a 30-min exposure to a fire at temperature 1475 °F (~800 °C), and complete immersion in water for a period of 8 h. Some extreme tests have been conducted to supplement the design specifications. In one test a trailer rig carrying a cask was made to collide with a solid concrete wall at 84 mph. Only the cooling fins were damaged; the cask would not have leaked if radioactivity had been present.

Figure 22.3. Spent fuel shipping cask.

(Courtesy of General Electric Company.)

Public concern has been expressed about the possibility of accident, severe damage, and a lack of response capability. The agencies responsible for regulation do not assume that accidents can be prevented but expect all containers to withstand an incident. In addition, efforts have been made to make sure that police and fire departments are familiar with the practice of shipping radioactive materials and with resources available in the form of state radiological offices and emergency response programs with backup by national laboratories.

22.5. Reprocessing

The physical and chemical treatment of spent nuclear fuel to separate the components—uranium, fission products, and plutonium—is given the name reprocessing. The fuel from the Hanford and Savannah River Plant weapons production reactors and the naval reactors has been reprocessed in the defense program at the federal government national laboratories. Commercial experience with reprocessing in the United States has been limited. In the period 1966–1972, Nuclear Fuel Services (NFS) operated a facility at West Valley, NY. Another was built by Allied General Nuclear Service (AGNS) at Barnwell, SC, but it never operated on radioactive material as a matter of national policy. To understand that political decision it is necessary to review the technical aspects of reprocessing.

On receipt of a shipping cask of the type shown in Figure 22.3, the spent fuel is unloaded and stored for further decay in a water pool. The assemblies are then fed into a mechanical shear that cuts them into pieces approximately 3 cm long to expose the fuel pellets. The pieces fall into baskets that are immersed in nitric acid to dissolve the uranium dioxide and leave zircaloy “hulls.” The aqueous solution from this chop-leach operation then proceeds to a solvent extraction (Purex) process. Visualize an analogous experiment. Add oil to a vessel containing salt water. Shake to mix. When the mixture settles and the liquids separate, some salt has gone with the oil (i.e., it has been extracted from the water). In the Purex process the solvent is the organic compound tributyl phosphate (TBP) diluted with kerosene. Countercurrent flow of the aqueous and organic materials is maintained in a packed column as sketched in Figure 22.4. Mechanical vibration assists contact.

Figure 22.4. Solvent extraction by the Purex method.

A flow diagram of the separation of components of spent fuel is shown in Figure 22.5. The amount of neptunium-239, half-life 2.355 d, depends on how fresh the spent fuel is. After a month of holding, the isotope will be practically gone. The three nitrate solution streams contain uranium, plutonium, and an array of fission product chemical elements. The uranium has a U-235 content slightly higher than natural uranium. It can either be set aside or re-enriched in an isotope separation process. The plutonium is converted into an oxide that is suitable for combining with uranium oxide to form a mixed oxide (MOX) that can form part or all of the fuel of a reactor. Precautions are taken in the fuel fabrication plant to protect workers from exposure to plutonium.

Figure 22.5. Simplified flowchart of nuclear fuel reprocessing.

In the reprocessing operations, special attention is given to certain radioactive gases. Among them are 8.04-d iodine-131, 10.73-y krypton-85, and 12.32-y tritium, which is the product of the occasional fission into three particles. The iodine concentration is greatly reduced by reasonable holding periods. The long-lived krypton poses a problem because it is a noble gas that resists chemical combination for storage. It may be disposed of in two ways: (a) release to the atmosphere from tall stacks with subsequent dilution, or (b) absorption on porous media such as charcoal maintained at very low temperatures. The hazard of tritium is relatively small, but water containing it behaves as ordinary water.

Reprocessing has merit in several ways other than making uranium and plutonium available for recycling:

• The isolation of some of the long-lived transuranic materials (other than plutonium) would permit them to be irradiated with neutrons, achieving additional energy and transmuting them into useful species or innocuous forms for purposes of waste disposal.
• Numerous valuable fission products such as krypton-85, strontium-90, and cesium-137 have industrial applications or may be used as sources for food irradiation.
• The removal of radionuclides with intermediate half-lives allows canisters of wastes to be placed closer together in the ground because the heat load is lower.
• Several rare elements of economic and strategic national value can be reclaimed from fission products. Availability from reprocessing could avoid interruption of supply from abroad for political reasons. Examples are rhodium, palladium, and ruthenium.
• The volume of wastes to be disposed of would be lower because the uranium has been extracted.
• Even if it were not recycled, the recovered uranium could be saved for future use in breeder reactor blankets.

Several countries abroad—France, the United Kingdom, Germany, Japan, and the former U.S.S.R.—have working reprocessing facilities and benefit from some of the preceding virtues.

An important aspect of reprocessing is that the plutonium made available for recycling can be visualized as a nuclear weapons material. Concern about international proliferation of nuclear weapons prompted President Carter in 1977 to issue a ban on reprocessing. It was believed that if the United States refrained from reprocessing, it would set an example to other countries. The action had no effect, because the United States had made no real sacrifice, having abundant uranium and coal reserves, and countries lacking resources saw full utilization of uranium in their best interests. It was recognized that plutonium from nuclear reactor operation was unsuitable for weapons because of the high content of Pu-240, which emits neutrons in spontaneous fission. Finally, it is possible to achieve weapons capability through the completely different route of isotope separation yielding highly enriched uranium. The ban prevented the AGNS plant from operating. President Reagan lifted the ban in 1981, but industry was wary of attempting to adopt reprocessing because of uncertainty in government policy and lack of evidence that there was a significant immediate economic benefit. However, the DOE is anticipating a revival of commercial reprocessing to ensure a sustainable fuel supply for the expected increased number of nuclear plants and to facilitate waste disposal, as discussed in Chapter 27.

22.6. High-Level Waste Disposal

The treatment given wastes containing large amounts of fission products depends on the cycle chosen. If the fuel is reprocessed, as described in the previous section, the first step is to immobilize the radioactive residue. One popular method is to mix the moist waste chemicals with pulverized glass similar to Pyrex, heat the mixture in a furnace to molten form, and pour the liquid into metal containers called canisters. The solidified waste form can be stored conveniently, shipped, and disposed of. The glass-waste is expected to resist leaching by water for hundreds of years.

If the fuel is not reprocessed, there are several choices. One is to place intact fuel assemblies in a canister. Another is to consolidate the rods (i.e., bundle them closely together in a container). A molten metal such as lead could be used as a filler if needed. What would be done subsequently with waste canisters has been the subject of a great deal of investigation concerning feasibility, economics, and social-environmental effects. Some of the concepts that have been proposed and studied are the following:

1. Send nuclear waste packages into space by shuttle and spacecraft. The weight of protection against vaporization in accidental re-entry to the Earth's atmosphere would make costs prohibitive.
2. Place canisters on the Antarctic ice cap, either held in place or allowed to melt their way down to the base rock. Costs and environmental uncertainty rule out this method.
3. Deposit canisters in mile-deep holes in the Earth. The method is impractical with available drilling technology.
4. Drop canisters from a ship, to penetrate the layer of sediment at the bottom of the ocean. Although considered as a backstop, there are evident environmental concerns.
5. Sink vertical shafts a few thousand feet deep, and excavate horizontal corridors radiating out. In the floors of these tunnels, drill holes in which to place the canisters, as sketched in Figure 22.6, or place waste packages on the floor of the corridor itself. The latter is the currently preferred technology in the United States high-level waste disposal program.

   Figure 22.6. Nuclear waste isolation by geologic emplacement.

   

The design of a repository for high-level radioactive waste or spent fuel uses a multibarrier approach. The first level of protection is the waste form, which may be glass-waste or an artificial substance, or uranium oxide fuel, which itself inhibits diffusion of fission products and is resistant to chemical attack. The second level is the container, which can be chosen to be compatible with the surrounding materials. Choices of metal for the canister include steel, stainless steel, copper, and nickel alloys. The third level is a layer of clay or other packing that tends to prevent access of water to the canister. The fourth is a backfill of concrete or rock. The fifth and final level is the geological medium. It is chosen for its stability under heat as generated by the decaying fission products. The medium will have a pore structure and chemical properties that produce a small water flow rate and a strong filtering action.

The system must remain secure for thousands of years. It must be designed to prevent contamination of water supplies that would give significant doses of radiation to members of the public. The radionuclides found in fission products can be divided into several classes as follows:

1. Nuclides of short half-life, up to about a month. Examples are xenon-133 (5.24 d) and iodine-131 (8.04 d). These would pose a problem in case of accident and give rise to heat and radiation that affect handling of fuel but are not important to waste disposal. The storage time for fuel is long enough that they decay to negligible levels.
2. Materials of intermediate half-life, up to 50 y, which determine the heating in the disposal medium. Examples are: cerium-144 (284.6 d), ruthenium-106 (1.020 y), cesium-134 (2.065 y), promethium-147 (2.62 y), krypton-85 (10.73 y), tritium (12.32 y), plutonium-241 (14.4 y), strontium-90 (29.1 y), and cesium-137 (30.2 y).
3. Isotopes that are still present after many thousands of years and that ultimately determine the performance of the waste repository. Important examples are radium-226 (1599 y), carbon-14 (5715 y), selenium-79 (2.9 × 10⁵ y), technetium-99 (2.13 × 10⁵ y), neptunium-237 (2.14 × 10⁶ y), cesium-135 (2.3 × 10⁶ y), and iodine-129 (1.7 × 10⁷ y). Radiological hazard is contributed by some of the daughter products of these isotopes; for example, lead-210 (22.6 y) comes from radium-226, which in turn came from almost-stable uranium-238.

Several candidate types of geologic media are found in various parts of the United States. One is rock salt, identified many years ago as a suitable medium because its very existence implies stability against water intrusion. It has the ability to self-seal through heat and pressure. Another is the dense volcanic rock basalt. Third is tuff, a compressed and fused volcanic dust. Extensive deposits of these three rocks as candidates for repositories are found in the states of Texas, Washington, and Nevada, respectively. Still another is crystalline rock, an example of which is granite as found in the eastern United States.

A simplified model of the effect of a repository is as follows. It is known that there is a small but continued flow of water past the emplaced waste. The container will be leached away in a few hundred years and the waste form released slowly over perhaps 1,000 y. The chemicals migrate much more slowly than the water flows, making the effective time of transfer tens of thousands of years. All of the short and intermediate half-life substances will have decayed by this time. The concentration of the long half-life radionuclides is greatly reduced by the filtering action of the geological medium. For additional details on the process of performance assessment, see References.

A pair of Computer Exercises provide an introduction to the mathematical modeling of the behavior of radioactive waste in a repository or disposal facility. A simple moving pulse with decay is studied in 22.B, and the spreading of a pulse by dispersion is shown in 22.C.

A plan and a timetable for establishment of an HLW repository in the United States was set by Congress. The NWPA called for a search of the country for possible sites, the selection of a small number for further investigation, and characterization of one or more sites, taking account of geology, hydrology, chemistry, meteorology, earthquake potential, and accessibility.

In 1987, Congress decreed that site studies in Texas and Washington State should cease and mandated that Nevada would be the host state. The location would be Yucca Mountain, near the Nevada Test Site for nuclear weapons. The project was delayed for several years by legal challenges from the State of Nevada, but characterization was begun in 1991, with cognizance by DOE's Office of Civilian Radioactive Waste Management. To test suitability of the site, an Exploratory Studies Facility was dug consisting of a corridor 10 m in diameter and 5 miles long. Among features investigated were the effect of heating to 300 °C and the flow of water down through the rock. As reported in the Viability Assessment document, the Yucca Mountain site is favorable because of the desert climate (only approximately 7 inches of water per year), the unsaturated zone with deep water table (2,000 ft), the stability of the geological formation, and a very low population density nearby. A Reference Design Document (RDD) was issued in January 1999. Some of the features cited are the following:

• 100 mi (160 km) northeast of Las Vegas, NV
• 70,000 tonnes of spent fuel and other wastes in 10,200 packages
• Underground horizontal tunnels (drifts)
• Diameter of drifts 18 ft (5.5 m); spacing 92 ft (28 m)
• Emplacement level approximately 1,000 ft (305 m) below the surface
• Waste packages hold 21 PWR or 44 BWR fuel assemblies

Multiple engineered barriers include the solid waste form (UO²), the metal fuel rod cladding, a container of special corrosion-resistant nickel alloy C-22,^[†] a “drip shield” to deflect water, a V-shaped trough for support, and underneath, the invert composed of stainless steel and volcanic rock to slow water flow. Figure 22.7 shows the proposed design.

^† Nominal percentages in Hastelloy C-22: 56 Ni, 22 Cr, 13 Mo, 3 Fe, 2.5 Co, 3 W, 0.5 other.

Figure 22.7. Spent fuel at Yucca Mountain.

Dedicated trains to carry spent fuel and high-level waste to Yucca Mountain are proposed by DOE. The choice as an alternative to trucks leads to fewer shipments, with 3,500 estimated.

The project was brought under question by the revelation in March 2005 of some e-mail messages in 1998 suggesting falsification of quality assurance data related to water infiltration. Excerpts of the messages are found in References. Investigations were made by Congress, DOE, United States Geological Survey (USGS), and the Federal Bureau of Investigation (FBI), and certain measurements were repeated as a corrective action needed to verify repository safety. The investigations were completed in 2005 as described in References.

Safety standards developed by the Environmental Protection Agency (EPA) (40CFR191) are to be used in licensing and regulation by the NRC (10CFR60). The EPA placed limits on the maximum additional radiation dosage to members of the public because of the release of radioactive material. Two time frames were established: (a) up to 10,000 y, with 15 mrem per year; and (b) to 1 million y, with 350 mrem per year, slightly under the United States average (see Section 16.2). The selection by EPA of a time span of 10,000 y for protection against hazard from waste deposits was based on a logical analysis. A comparison was made between two radioactivities. The first was that of natural uranium as found in the ground, a figure that remains constant. The second was the declining activity of spent fuel, as the fission products and activation products decay. It was assumed that when the two figures are equal, the radiation dose caused by the waste is no greater than that caused by the original uranium. Calculations led to a time of approximately 1,000 y, and a safety factor of 10 was applied. For further details, see an article in Nuclear News, February 2006. The longer time frame was set on the basis that the highest radiation from waste may occur beyond the 10,000-y period.

The EPA was reluctant to establish regulations that applied to a million years, stating in 2001, “It is not possible to make reliable estimates over such a long time frame.” (see References). The American Nuclear Society in a 2006 position statement with background information (see References) concurred, stating, “… extrapolating beyond 10,000 years is not scientifically sensible…”

The Yucca Mountain license application was delivered in 2008 by the Department of Energy to the NRC. DOE will certify that the repository will meet standards set by the EPA. A plan will be prepared for transportation of spent fuel by rail from reactors to the final destination in Nevada. The cap in loading of the repository of 70,000 tonnes might be raised by Congress. Money from the Nuclear Waste Fund will have to be authorized and approval for use of federal land obtained from the Department of the Interior.

In October 2005 DOE announced a change in the plan for handling spent fuel for disposal at Yucca Mountain. Instead of requiring several stages involving storage, packaging, shipping, and disposal, fuel is to be loaded into containers that can go directly into disposal. This is said to provide a “clean” (noncontaminated) repository. Benefits of the new method include elimination of expensive handling facilities and the avoidance of damage of fuel. An undesired consequence, however, is the necessary delay in submission of a license application by DOE to NRC. The new plan is ridiculed by representatives of Nevada.

The projected date for the start of burial is 2017. According to the Nuclear Waste Policy Act, DOE was required to accept spent fuel by 1998, but has not complied, to the concern of the nuclear industry.

The law called for a study of a monitored retrieval storage (MRS) system to serve as a staging center before disposal in a repository. Efforts to find a host were unsuccessful. Use of the Nevada Weapons Testing Grounds as a storage area for spent fuel has been promoted as a stopgap.

Financing for the waste disposal program being carried out by the federal government is provided by a Nuclear Waste Fund. The consumers of electricity generated by nuclear reactors pay a fee of 1/10 cent per kilowatt hour collected by the power companies. This adds only approximately 2% to the cost of nuclear electric power. Concern has been expressed about the fact that Congress has used some of the Fund for other purposes.

Progress in establishing the repository at Yucca Mountain has been slow, and the completion date has been repeatedly extended. The difficulties and uncertainties of the project have prompted consideration of alternatives. One is to irradiate certain radioisotopes in the spent fuel to destroy problem isotopes such as cesium-137 and strontium-90 that contribute to heating in the early period and neptunium-237, technetium-99, and iodine-129 that dominate the hazard at long times. These constitute only about a percent of the waste stream. If they are removed, the remaining waste needs to be secure for only approximately 100 y rather than the 10,000 y for spent fuel.

An R&D program titled Accelerator Transmutation of Wastes (ATW) was conducted over a number of years at Los Alamos National Laboratory. The concept first involves reprocessing spent fuel by pyroprocessing, which uses the IFR technology (see Section 13.3). The key fission products, actinides, and possibly plutonium would then be irradiated in a subcritical system with an accelerator that causes spallation (see Section 8.6). A beam of protons of 100 MW was to be directed to a molten lead target. A surrounding liquid would contain the isotopes to be burned, with heat removed by liquid lead. Electricity would be produced. Some 15 of such burners were estimated to be able to handle United States spent fuel. A roadmap for further development of the concept was prepared in 1999 (see References). However, spallation research on ATW at LASL was suspended and the ATW program was merged by DOE with the program Accelerator Production of Tritium (APT, Section 26.6) and essentially abandoned. There is a current lack of interest in ATW, but it remains a possibility for the more distant future.

22.7. Low-Level Waste Generation, Treatment, and Disposal

The nuclear fuel cycle, including nuclear power stations and fuel fabrication plants, produces approximately two thirds of the annual volume of LLW. The rest comes from companies that use or supply isotopes and from institutions such as hospitals and research centers.

In this section we look at the method by which low-level radioactive materials are produced, the physical and chemical processes that yield wastes, the amounts to be handled, the treatments that are given, and the methods of disposal.

In the primary circuit of the nuclear reactor the flowing high-temperature coolant erodes and corrodes internal metal surfaces. The resultant suspended or dissolved materials are bombarded by neutrons in the core. Similarly, core metal structures absorb neutrons, and some of the surface is washed away. Activation products as listed in Table 22.2 are created, usually through an (n,γ) reaction. Computer Exercise 22.D displays a series of radionuclides involved in the activation process and decay with release of radiation. In addition, small amounts of fission products and transuranic elements appear in the water as the result of small leaks in cladding and the irradiation of uranium deposits left on fuel rods during fabrication. The isotopes involved are similar to those of concern for HLW.

Table 22.2. Activation Products in Reactor Coolant

Isotope	Half-life (years)	Radiation Emitted	Parent Isotope
C-14	5715	β	N-14*
Fe-55	2.73	x	Fe-54
Co-60	5.27	β, γ	Co-59
Ni-59	7.6 × 104	x	Ni-58
Ni-63	100	β	Ni-62
Nb-94	2.4 × 104	β, γ	Nb-93
Tc-99	2.13 × 105	β	Mo-98, Mo-99[†]

^* (n,p) reaction.

^† Beta decay.

Leaks of radioactive water from the primary coolant are inevitable and result in contamination of work areas. Also, radioactive equipment must be removed for repair. For such reasons, workers are required to wear elaborate protective clothing and use a variety of materials to prevent spread of contamination. Much of it cannot be cleaned and reused. Contaminated dry trash includes paper, rags, plastics, rubber, wood, glass, and metal. These may be combustible or noncombustible, compactible or noncompactible. Avoidance of contamination of inert materials by radioactive materials is an important technique in waste reduction. The modern trend in nuclear plants is to try to reduce the volume of waste by whatever method is appropriate. Over the period 1980–1998, by a combination of methods, the nuclear industry reduced the LLW volume by a factor of more than 15. Costs of disposal have not decreased proportionately, however, because capital costs tend to be independent of waste volume.

One popular technique is incineration, in which the escaping gases are filtered, and the ash contains most of the radioactivity in a greatly reduced volume. Another method is compaction, with a large press to give a reduced volume and also to make the waste more stable against further disturbance after disposal. “Supercompactors” that reduce the volume greatly are popular. A third approach is grinding or shredding, then mixing the waste with a binder such as concrete or asphalt to form a stable solid.

Purification of the water in the plant, required for re-use or safe release to the environment, gives rise to a variety of wet wastes. They are in the form of solutions, emulsions, slurries, and sludges of both inorganic and organic materials. Two important physical processes that are used are filtration and evaporation. Filters are porous media that take out particles suspended in a liquid. The solid residue collects in the filter that may be a disposable cartridge or may be reusable if backwashed. Figure 22.8 shows the schematic arrangement of a filter in a nuclear plant. The evaporator is simply a vessel with a heated surface over which liquid flows. The vapor is drawn off leaving a sludge in the bottom. Figure 22.9 shows a typical arrangement.

Figure 22.8. Disposal-cartridge filter unit used to purify water and collect LLW.

(Courtesy ORNL.)

Figure 22.9. Natural-circulation evaporator used to concentrate LLW.

(Courtesy ORNL.)

The principal chemical treatment of wet LLW is ion exchange. A solution containing ions of waste products contacts a solid such as zeolite (aluminosilicate) or synthetic organic polymer. In the mixed-bed system, the liquid flows down through mixed anion and cation resins. As discussed by Benedict, Pigford, and Levi (see References), ions collected at the top move down until the whole resin bed is saturated, and some ions appear in the effluent, a situation called “breakthrough.” Decontamination factors may be as large as 10⁵. The resin may be re-used by application of an elution process, in which a solution of Na²SO⁴ is passed through the bed to extract the ions from the resin. The resulting waste solution will be smaller than before but will probably be larger than the exchanger. Whether to discard or elute depends on the cost of the ion-exchanger material.

The variety of types of LLW from institutions and industry is indicated in Table 22.3. The institutions include hospitals, medical schools, universities, and research centers. As discussed in Chapter 17, labeled pharmaceuticals and biochemicals are used in medicine for diagnosis and therapy and in biological research to study the physiology of humans, other animals, and plants. Radioactive materials are used in schools for studies in physics, chemistry, biology, and engineering and are produced by research reactors and particle accelerators. The industries make various products: (a) radiography sources; (b) irradiation sources; (c) radioisotope thermoelectric generators; (d) radioactive gages; (e) self-illuminating dials, clocks, and signs; (f) static eliminators; (g) smoke detectors; and (h) lightning rods. Radionuclides that often appear in LLW from manufacturing include carbon-14, tritium, radium-226, americium-241, polonium-210, californium-252, and cobalt-60. LLW disposal from the decommissioning of nuclear power reactors is of considerable future importance and is discussed separately in Section 22.8.

Table 22.3. Institutional and Industrial Low-Level Waste Streams

Fuel fabrication plant	Industrial
Trash	Trash
Process wastes	Source and special nuclear materials[†]
Institutions	Special
Liquid scintillation vials	Isotope production facilities
Liquid wastes	Tritium manufacturing
Biowastes	Accelerator targets
	Sealed sources, e.g., radium

^† SNM = Pu, U-233, etc.

(Adapted from the Environmental Impact Statement for NRC 10CFR61)

Although defined by exclusion, as noted in Section 22.2, low-level radioactive waste generally has low enough activity to be given near-surface disposal. There are a few examples of very small contaminations that can be disregarded for disposal purposes and some highly radioactive materials that cannot be given shallow-land burial.

The method of disposal of low-level radioactive wastes for many years was similar to a landfill practice. Wastes were transported to the disposal site in various containers such as cardboard or wooden boxes and 55-gallon drums and were placed in trenches and covered with earth without much attention to long-term stability.

A total of six commercial and 14 government sites around the United States operated for a number of years until leaks were discovered, and three sites at West Valley, NY, Sheffield, IL, and Maxey Flats, KY, were closed. One problem was subsidence, in which deterioration of the package and contents by entrance of water would cause local holes in the surface of the disposal site. These would fill with water and aggravate the situation. Another difficulty was the “bathtub effect,” in which water would enter a trench and not be able to escape rapidly, causing the contents to float and be exposed.

Three remaining sites at Richland, WA, Beatty, NV, and Barnwell, SC, handled all of the LLWs of the country. These sites were more successful, in part because trenches had been designed to allow ample drainage. Managers of the sites, however, became concerned with the waste generators' practices and attempted to reduce the amount of waste accepted. This situation prompted Congress to pass in 1980 the Low-Level Radioactive Waste Policy Act (LLRWPA), followed by the Low-Level Radioactive Waste Policy Amendments Act of 1985. These laws placed responsibility on states for wastes generated within their boundaries but recommended regional disposal. Accordingly, a number of interstate compacts were formed, with several states remaining independent. Figure 22.10 shows the division of the United States into states and compacts. The alignment of states has tended to change over the years.

Figure 22.10. United States interstate compacts for disposal of low-level radioactive wastes. States shaded are unaffiliated.

(Courtesy of Afton Associates and LLW Forum.)

At the same time, the NRC developed a new rule governing LLW management. Title 10 of the Code of Federal Regulations Part 61 (10CFR61) calls for packaging of wastes by the generator according to isotope type and specific activity (Ci/m³). Waste classes A, B, and C are defined in 10CFR61 and increasing levels of security prescribed. Greater-than-Class-C wastes are unsuitable for near-surface disposal and are managed by DOE as equivalent to high-level waste.

Computer Exercise 22.E describes an elementary “expert system” that determines the proper class of a given waste on the basis of half-life and specific activity.

The required degree of waste stability increases with the radioactive content. Limits are placed on the amount of liquid present with the waste, and the use of stronger and more resistant containers is recommended in the interest of protecting the public during the operating period and after closure of the facility.

Regulation 10CFR61 calls for a careful choice of the characteristics of the geology, hydrology, and meteorology of the site to reduce the potential radiation hazard to workers, the public, and the environment. Special efforts are to be made to prevent water from contacting the waste. Performance specifications include a limit of 25 mrems per year whole-body dose of radiation to any member of the public. Monitoring is to be carried out over an institutional surveillance period of 100 y after closure. Measures are to be taken to protect the inadvertent intruder for an additional 500 y. This is a person who might build a house or dig a well on the land. One method is to bury the more highly radioactive material deep in the trench; another is to put a layer of concrete over the wastes.

The use of an alternate technology designed to improve confinement stems from one or more public viewpoints. First is the belief that the limiting dose should be nearer zero or even should be actually zero. Second is the concern that some unexpected event might change the system from the one analyzed. Third is the idea that the knowledge of underground flow is inadequate and not capable of being modeled to the accuracy needed. Fourth is the expectation that there may be human error in the analysis, design, construction, and operation of the facility. It is difficult to refute such opinions, and in some states and interstate compact regions, legislation on additional protection has been passed to make a waste disposal facility acceptable to the public. Some of the concepts being considered as substitutes for shallow land burial are listed.

Belowground vault disposal involves a barrier to migration in the form of a wall such as concrete. It has a drainage channel, a clay top layer and a concrete roof to keep water out, a porous backfill, and a drainage pad for the concrete structure. Aboveground vault disposal makes use of slopes on the roof and surrounding earth to assist runoff. The roof substitutes for an earthen cover. Shaft disposal uses concrete for a cap and walls and is a variant on the belowground vault that conceivably could be easier to build. Modular concrete canister disposal involves a double container, the outer one of concrete, with disposal in a shallow-land site. Mined-cavity disposal consists of a vertical shaft going deep in the ground, with radiating corridors at the bottom, similar to the planned disposal system for spent fuel and high-level wastes from reprocessing. It is only applicable to the most active LLWs. Intermediate-depth disposal is similar to shallow-land disposal except for the greater trench depth and thickness of cover. Earth-mounded concrete bunker disposal, used in France, combines several favorable features. Wastes of higher activity are encased in concrete below grade and those of lower activity are placed in a mound with concrete and clay cap, covered with rock or vegetation to prevent erosion by rainfall.

Each of the interstate compacts embarked on investigations in accord with LLRWPA and 10CFR61. These involved site selection processes, geological assessments, and designs of facilities. The nature of the facilities proposed depended on the location, with shallow land burial deemed adequate for the California desert at Ward Valley, but additional barriers and containers planned for North Carolina in the humid Southeast. However, as the result of concerted opposition taking the form of protests, lawsuits, political action and inaction, and occasional violence, progress was very slow in establishing LLW disposal capability. Thus despite excellent planning and vigorous efforts, and the expenditure of millions of dollars in preparation, political and regulatory factors prevented most of the programs in the United States from coming to fruition. The only sites receiving low-level wastes as of the year 2008 were the Northwestern at Hanford and Barnwell in South Carolina, with certain materials accepted by Envirocare in Utah. Some 36 states have no outlet for LLW and must store it until new disposal facilities are available. A comprehensive review of the situation appears in a 2004 report by the General Accounting Office (GAO). Although noting the scarcity of disposal facilities, GAO was not very concerned. That view was strongly disputed by Alan Pasternak of the Cal Rad Forum (see References). He predicts higher costs for services and curtailment of biomedical research and medical use.

An elementary analysis by use of a spreadsheet of the behavior of a selected set of radionuclides in low-level radioactive waste is described in Computer Exercise 22.F. The effects of storage, decay, and retardation are displayed.

22.8. Environmental Restoration of Defense Sites

The legacy of World War II and the Cold War includes large amounts of radioactive waste and contamination of many defense sites. Priority was given to weapons production rather than environmental protection, leaving a cleanup task that will take several decades to carry out and cost many billions of dollars.

One of the most pressing problems to solve is the degraded condition of underground tanks at Hanford used to store the waste residue from reprocessing to extract plutonium. The single-wall tanks have leaked, and there is concern for the contamination of the nearby Columbia River. Some of the wastes have been processed to extract the valuable Cs-137 and Sr-90, and the contents of some tanks have been successfully stabilized to prevent hydrogen explosion. Ideally, all of the waste would be transferred to double-layered tanks or immobilized in glass. Similar tanks are located at the Savannah River Plant in South Carolina, where plutonium and tritium were produced.

Transuranic wastes (TRU) consist of materials and equipment contaminated by small amounts of plutonium. They have been stored or temporarily buried over the years, especially at Hanford, Idaho Falls, Los Alamos, Oak Ridge, and Savannah River. These wastes are scheduled to be buried in the Waste Isolation Pilot Plant (WIPP), a repository near Carlsbad, NM, that opened in 1999. The geological medium is salt, which has several advantages—its presence demonstrates the absence of water and it is plastic, self-sealing under pressure. The TRU is packaged in 55-gallon drums and shipped to WIPP in a cylindrical cask called TRUPAC II, which contains seven drums in each of two layers. The waste is buried approximately 2,160 ft (658 m) below the surface. Construction of WIPP was under the supervision of DOE, with advice by the National Research Council, and regulation by the EPA. Performance assessment was done by Sandia. For details of the roles of the various organizations, see References.

The monumental challenge of environmental restoration of sites used in the United States defense program is being addressed by the DOE. It has been recognized that it is not feasible to completely decontaminate the sites. Instead, cleanup to an extent practical is followed by “stewardship,” involving isolation, monitoring, and maintenance of certain locations for a very long period. To achieve the goal of protection of the public and the environment, the DOE Environmental Management (EM) program has initiated research on new efficient technologies to handle radioactive materials. As described in the DOE document Five Year Plan (see References), the mission of EM is the safe and successful cleanup of facilities, including stabilization of tank waste for treatment, disposal of all types of waste, and remediation of sites.

22.9. Nuclear Power Plant Decommissioning

“Decommissioning,” a naval term meaning to remove from service e.g., a ship, is applied to actions taken at the end of the useful life of a nuclear power plant (30 to 40 y). The process begins at shutdown of the reactor and ends with disposal of radioactive components in a way that protects the public. LLW disposal from dismantled reactors will be a major problem in decades to come.

The first action is to remove and dispose of the spent nuclear fuel. Several choices of what to do with the remainder of the plant are available. The options as identified formally by the NRC are (a) SAFSTOR or mothballing, in which some decontamination is effected, the plant is closed up, and then monitored and guarded for a very long period, perhaps indefinitely; (b) ENTOMB or entombment, in which concrete and steel protective barriers are placed around the most radioactive equipment, sealing it to prevent release of radioactivity, again with some surveillance; (c) DECON or immediate dismantlement, in which decontamination is followed by destruction, with all material sent to a LLW disposal site; (d) delayed dismantlement, the same as the previous case, but with a time lapse of a number of years to reduce personnel exposure. The distinction among these various options is blurred if it is assumed that the facility must eventually be disassembled. It becomes more a question of “when.” Aside from the aesthetic impact of an essentially abandoned facility, there is a potential environment problem related to the finite life of structural materials.

Operation of the reactor over a long period of time will have resulted in neutron activation, particularly of the reactor vessel and its stainless steel internal parts. Contamination of other equipment in the system will include the same isotopes that are of concern in LLW disposal. Various techniques are used to decontaminate—washing with chemicals, brushing, sand blasting, and ultrasonic vibration. To cut components down to manageable size, acetylene torches and plasma arcs are used. Because such operations involve radiation exposure to workers, a great deal of preplanning, special protective devices, and extra manpower are required. A very large volume of waste is generated. Some of it may be too active to put into a LLW disposal site but will not qualify for disposal in a high-level waste repository. Cobalt-60 dominates for the first 50 y, after which the isotopes of concern are 76,000-y nickel-59 and 24,000-y niobium-94.

The NRC requires nuclear plant owners to provide a License Termination Plan and to set aside funds for decommissioning. A standardized cost-estimation procedure has been developed. Costs vary with units but are $300 million or more. This cost, a small fraction of the value of electricity generated over the reactor life, is borne by the consumers of electrical power. Data on reactors that have been decommissioned are provided by NRC (see References).

An option that has not yet been fully explored is “intact” decommissioning, in which the highly radioactive region of the system would be sealed off, making surveillance unnecessary. The virtues claimed are low cost and low exposure. Ultimately, renewal of the license after replacing all of the worn-out components may be the best solution.

A number of reactors will need to be decommissioned in the first quarter of the 21st century. Factors that will determine action include the degree of success in reactor life extension, license renewal, and the general attitude of the public about the disposal of material from nuclear stations as low-level radioactive waste.

22.10. Summary

Radioactive wastes arise from a great variety of sources, including the nuclear fuel cycle, and from beneficial uses of isotopes and radiation by institutions.

Spent fuel contains uranium, plutonium, and highly radioactive fission products. In the United States spent fuel is accumulating, awaiting the development of a high-level waste repository. A multibarrier system involving packaging and geological media will provide protection of the public over the centuries the waste must be isolated. The favored method of disposal is in a mined cavity deep underground. In other countries, reprocessing the fuel assemblies permits recycling of materials and disposal of smaller volumes of solidified waste. Transportation of wastes is by casks and containers designed to withstand severe accidents.

LLWs come from research and medical procedures and from a variety of activation and fission sources at a reactor site. They generally can be given near-surface burial. Isotopes of special interest are cobalt-60 and cesium-137. Transuranic wastes are being disposed of in the Waste Isolation Pilot Plant. Establishment of regional disposal sites by interstate compacts has generally been unsuccessful in the United States. Decontamination of defense sites will be long and costly. Decommissioning of reactors in the future will contribute a great deal of low-level radioactive waste.

22.11. Exercises

1. Compare the specific activities (dps/g) of natural uranium and slightly enriched fuel, including the effect of uranium-234. Note the natural uranium density of 18.9 g/cm³ and the half-lives and atom abundances in percent for the three isotopes:

    

   Isotope	Half-life (y)	Natural	Enriched
   U-235	7.04 × 108	0.720	3.0
   U-238	4.47 × 109	99.2745	96.964
   U.234	2.45 × 105	0.0055	0.036

   What fraction of the activity is due to uranium-234 in each case?
2. With the data below (a) calculate the power capacity of all United States PWRs, Boiling Water Reactors (BWRs), and the Light Water Reactor (LWR) total, and (b) estimate the total annual amount of solid radioactive waste produced by United States power reactors.

    

   PWR	BWR
   No.	Average Power (MWe)	Waste (m3/GWe-y)	No.	Average Power (MWe)	Waste (m3/GWe-y)
   69	949.33	23.2	35	929.31	91.5

3. A batch of radioactive waste from a processing plant contains the following isotopes:

    

   Isotope	Half-life	Fission Yield, %
   I-131	8.04 d	2.9
   Ce-141	32.50 d	6
   Ce-144	284.6 d	6.1
   Cs-137	30.2 y	5.9
   I-129	1.7 × 107 y	1

   Form the products of the decay constants (in s⁻¹) and fission yields (in %) to serve as relative initial activities of the isotopes. Find the times where successive semilog graphs of activity would intersect by use of equality of activities (e.g., Aⁿ = Aⁿ⁺¹).
4. Traces of plutonium remain in certain waste solutions. If the initial concentration of Pu-239 in water were 100 parts per million (μg/g), find how much of the water would have to be evaporated to make the solution critical, neglecting neutron leakage as if the container were very large. Note: for H, σ^a = 0.332; for Pu, σ^f = 752, σ^a = 1022, ν = 2.88.
5. If the maximum permissible concentration of Kr-85 in air is 1.5 × 10⁻⁹ μCi/cm³, and the yearly reactor production rate is 5 × 10⁵ Ci, what is a safe diluent air volume flow rate (in cm³/s and ft³/min) at the exit of the stack? Discuss the implications of these numbers in terms of protection of the public.
6. Calculate the decay heat from a single fuel assembly of the total of 180 in a 3,000-MWt reactor at 1 day after shutdown of the reactor. How much longer is required for the heat generation rate to go down an additional factor of 2?
7. Data on fission products (in %) to accompany numbers in Figure 22.2 are as follows: U-238, 0.16; U-235, 1.98, Pu-239, 1.21; and Pu-241, 0.15.
   • Calculate the percentages of total power caused by each fissionable isotope.
   • Assuming that one third of the 180 fuel assemblies in the reactor are removed each year and that each contains 470 kg of U, find what weight of fission products the 60 assemblies contain.
   • What mass of fission products would be produced annually in the whole reactor if operated at its full rating of 3,000 MWt, knowing that 1.1 grams of fuel fissions per MWd?
   • Deduce a capacity factor (actual energy divided by rated energy) from the results of (b) and (c) above.
8. Assume that high-level wastes should be secured for a time sufficient for decay to reduce the concentrations by a factor of 10¹⁰. How many half-lives does this require? How long is this in years for strontium-90? For cesium-137? For plutonium-239?
9. A 55-gallon drum contains an isotope with 1 MeV gamma ray, distributed uniformly with activity 100 μCi/cm³. For purposes of radiation protection planning, estimate the radiation flux at the surface, treating the container as a sphere of equal volume of water, and neglecting buildup. (a) Show that the gamma flux at the surface, radius R, is given by SRP^e/3, where P^e is the escape probability and S is the source strength in dps/cm³. (b) With attenuation coefficient Σ and x = ΣRcalculate the gamma flux at R.

Computer Exercises

1. For a computer display of a stylized water pool for the storage of spent fuel at a nuclear plant, load and run the program FUELPOOL.
2. If buried radioactive waste is dissolved at a constant rate by water infiltration, it will be released as a square pulse. As the pulse migrates in an aquifer with some effective speed, the number of nuclei decreases because of decay. Program WASTPULS displays the motion in time. Load and run the program, trying a variety of combinations of distances, speeds, and half-lives.
3. The transport of a waste radionuclide by groundwater involves the flow with retardation because of holdup in pores. A process called dispersion causes an initial square pulse to be rounded as it moves along. Computer program WTT gives numerical values of the contaminant concentration observed at a point in space for various times. Run the program with the default values, then change individual parameters such as dispersivity to observe effects.
4. The sequence of products resulting from neutron capture in a nonradioactive nucleus is displayed in the program ACTIVE. Included are the activation product and the residual nucleus after decay. Load and run the program to observe the sequence. Suggest a set of specific nuclear species for which the diagram is appropriate, giving cross sections and half-lives wherever possible.
5. The NRC specifies in the Code of Federal Regulations 10 Energy Part 61 Section .55 (10CFR61.55) a classification scheme for low-level radioactive waste. The radionuclides present and their concentrations determine whether a shipment is Class A, B, or C. Computer program LLWES (LLW expert system) provides an easy way to classify a given waste. The program also illustrates an expert system, which yields answers by a specialist to questions by a worker. Load and run the program, then use the menus to learn about the NRC's rule and to test the expert's knowledge. Select some isotope or combination of isotopes and assign specific activity values to find out the classification. Note the effect of increasing or decreasing the concentration significantly.

22.12 References

Robert, 1999 G. Robert, Cochran and Nicholas Tsoulfanidis The Nuclear Fuel Cycle: Analysis and Management 2nd Ed. 1999 American Nuclear Society La Grange Park, IL

The Nuclear Waste Primer, 1993 The Nuclear Waste Primer, by The League of Women Voters Education Fund 1993 Lyons & Burford New York Brief and elementary information

Murray, April 1986 Raymond L. Murray, Radioactive Waste Storage and Disposal Proceedings of the IEEE Vol. 74 No. 4 April 1986552- A survey article that covers all aspects

Murray, 2003 Raymond L. Murray, Understanding Radioactive Waste 5th Ed. 2003 Battelle Press Columbus, OH An elementary survey intended to answer typical questions by the student or the public

Platt et al., 1985 A.M. Platt, J.V. Robinson, O.F. Hill, The Nuclear Fact Book 1985 Harwood Academic Publishers New York A large amount of useful data on wastes is included

Stewart, 1985 Donald C. Stewart, Data for Radioactive Waste Management and Nuclear Applications 1985 John Wiley & Sons New York

Roy G. Post, Ed Roy G. Post, Ed., Proceedings of the Symposium on Waste Management, Tucson, AZ, WM Symposia, Inc. An annual event with a collection of papers on all aspects of radioactive waste around the world.

Moghissi et al., 1986 Alan Moghissi, Herschel W. Godbee, Sue A. Hobart, Radioactive Waste Technology 1986 The American Society of Mechanical Engineers New York Sponsored by ASME and the American Nuclear Society

Mill tailings (Nuclear Regulatory Commission) Mill tailings (Nuclear Regulatory Commission)

http://www.nrc.gov/reading-rm/doc-collections/nuregs/brochures/br0216/#mill_tailings http://www.nrc.gov/reading-rm/doc-collections/nuregs/brochures/br0216/#mill_tailings

Mixed Low-Level Waste (Environmental Protection Agency) Mixed Low-Level Waste (Environmental Protection Agency)

http://www.epa.gov/radiation/mixed-waste http://www.epa.gov/radiation/mixed-waste

Status of “Below Regulatory Concern” (DOE) Status of “Below Regulatory Concern” (DOE)

http://homer.ornl.gov/nuclearsafety/nsea/oepa/guidance/radwaste/brc.pdf http://homer.ornl.gov/nuclearsafety/nsea/oepa/guidance/radwaste/brc.pdf

Radioactive Waste Management Radioactive Waste Management

http://www.uic.com.au/wast.htm http://www.uic.com.au/wast.htm

From Australian Uranium Association From Australian Uranium Association. Source of data for Table 22.1.

Nuclear Waste Policy Act As Amended Nuclear Waste Policy Act As Amended

http://ymp.gov/documents/nwpa/css/nwpa_2004.pdf http://ymp.gov/documents/nwpa/css/nwpa_2004.pdf

High-level waste and spent fuel High-level waste and spent fuel.

Nuclear Waste Policy Act of 1982 Nuclear Waste Policy Act of 1982

http://ocrwm.doe.gov/documents/nwpa/css/nwpa.htm http://ocrwm.doe.gov/documents/nwpa/css/nwpa.htm

Text of law on HLW and spent fuel. Includes update to 1987 policy Text of law on HLW and spent fuel. Includes update to 1987 policy.

Office of Civilian Radioactive Waste Management Office of Civilian Radioactive Waste Management

http://www.ocrwm.doe.gov http://www.ocrwm.doe.gov

Comprehensive discussion of repository Comprehensive discussion of repository. Select Yucca Mountain Repository, Transportation of Nuclear Wastes, or License Application, with links.

Investigation closes on project's questionable QA records, 2006Investigation closes on project's questionable QA records Nuclear News June 200652-53

Moghissi, 2006 A. Alan Moghissi, The origin of the EPA's 10,000-year Time Frame for the High-level Waste Repository Nuclear News, February 200641-

About Yucca Mountain and the Standards About Yucca Mountain and the Standards

http://www.epa.gov/radiation/yucca/about.html http://www.epa.gov/radiation/yucca/about.html

Answers to questions and discussion of legal aspects Answers to questions and discussion of legal aspects.

The EPA Radiation Standard for Spent-Fuel Storage in a Geological Repository The EPA Radiation Standard for Spent-Fuel Storage in a Geological Repository

ANS position statement and background information, November 2006 ANS position statement and background information, November 2006

http://www.ans.org/pi/ps/docs/ps81.pdf http://www.ans.org/pi/ps/docs/ps81.pdf

http://www.ans.org/pi/ps/docs/ps81-bi.pdf http://www.ans.org/pi/ps/docs/ps81-bi.pdf

Yucca Mountain Site Suitability Evaluation Yucca Mountain Site Suitability Evaluation

http://www.ocrwm.doe.gov/documents/sse_a/index.htm http://www.ocrwm.doe.gov/documents/sse_a/index.htm

DOE report of 2002 DOE report of 2002.

Berlin and Stanton, 1989 Robert E. Berlin, Catherine C. Stanton, Radioactive Waste Management 1989 John Wiley & Sons New York Many tables, charts, and diagrams on all types of nuclear wastes

Bebbington, December 1976 William Bebbington, The Reprocessing of Nuclear Fuels Scientific American December 197630-

Benedict, et al., 1981 Manson Benedict, Thomas H. Pigford, Hans Wolfgang Levi, Nuclear Chemical Engineering 2nd Ed. 1981 McGraw-Hill New York

Nuclear Wastes: Technologies for Separation and Transmutation, 1996 Nuclear Wastes: Technologies for Separation and Transmutation 1996 National Academy Press Washington, DC Study by a National Research Council committee on the feasibility of reducing long-lived isotopes by reprocessing followed by neutron irradiation

A Roadmap for Development of ATW Technology A Roadmap for Development of ATW Technology

http://www.osti.gov/bridge/product.biblio.jsp?osti_id=750787 http://www.osti.gov/bridge/product.biblio.jsp?osti_id=750787

Outdated plans Outdated plans.

DOE's Advanced Accelerator Applications Program DOE's Advanced Accelerator Applications Program

http://www.ne.doe.gov/pdfFiles/AAARptConMarch2001.pdf http://www.ne.doe.gov/pdfFiles/AAARptConMarch2001.pdf

The combination of ATW and APT into AAA The combination of ATW and APT into AAA.

United States Nuclear Regulatory Commission, November 1982 United States Nuclear Regulatory CommissionFinal Environmental Impact Statement on 10 CFR Part 61 Licensing Requirements for Land Disposal of Radioactive Waste Vols. 1–3 November 1982 NUREG-0945

Kittel, 1989 J. Howard Kittel, Near-Surface Land Disposal, Radioactive Waste Management Handbook Vol. 1 1989 Harwood, Chur Switzerland

Performance Assessment for LLW Disposal Facilities Performance Assessment for LLW Disposal Facilities

http://www.osti.gov/bridge http://www.osti.gov/bridge

Search on 563204 for article by S. M. Birk of INEEL Search on 563204 for article by S. M. Birk of INEEL.

Status of Low-Level Radioactive Waste Disposal Status of Low-Level Radioactive Waste Disposal

http://www.gao.gov http://www.gao.gov

Search on RCED-99-238 for 1999 status Search on RCED-99-238 for 1999 status.

Low-Level Radioactive Waste Disposal Low-Level Radioactive Waste Disposal

http://www.gao.gov/new.items/d04604.pdf http://www.gao.gov/new.items/d04604.pdf

Report GAO-04-604 on the status of disposal facilities (2004) Report GAO-04-604 on the status of disposal facilities (2004).

Cal Rod Forum's Alan Pasternak: Time is Running Out for a Permanent LLW solution December 2004Cal Rod Forum's Alan Pasternak: Time is Running Out for a Permanent LLW solution Nuclear News December 200422- Interview on disposal facilities

Improving the Regulation Improving the Regulation and Management of Low-Activity Radioactive Wastes

http://books.nap.edu/openbook.php?record_id=11595&page=R1 http://books.nap.edu/openbook.php?record_id=11595&page=R1

Nancy J. Zacha Nancy J. Zacha, “Low-level Radioactive Waste Disposal: Are we Having a Crisis Yet?”

Nuclear News Nuclear News, August 2007, p. 29.

Carleson et al., 1995 Thomas E. Carleson, Nathan A. Chipman, Chien M. Wai, Separation Techniques in Nuclear Waste Management 1995 CRC Press Boca Raton, FL Describes many processes in various areas of the world

Noyes, 1995 Robert Noyes, Nuclear Waste Cleanup Technology and Opportunities 1995 Noyes Publications Park Ridge, NJ Survey of the national problem and available technologies based on government reports

Gephart and Lundgren, 1998 Roy E. Gephart, Regina E. Lundgren, Hanford Tank Cleanup: A Guide to Understanding the Technical Issues 1998 Battelle Press Columbus, OH

The Waste Isolation Pilot Plant, 1996 The Waste Isolation Pilot Plant 1996 National Academy Press Washington, DC A review by a committee of the National Research Council

Improving Operations Improving Operations and Long-Term Safety of the Waste Isolation Pilot Plant

http://books.nap.edu/openbook.php?record_id=10143&page=R1 http://books.nap.edu/openbook.php?record_id=10143&page=R1

Online book Online book of National Academies Press, 2001.

EPA's WIPP EPA's WIPP Program

http://www.epa.gov/radiation/wipp http://www.epa.gov/radiation/wipp

Select Frequent Select Frequent Questions.

Science Science @ WIPP

http://www.wipp.energy.gov/science/index.htm http://www.wipp.energy.gov/science/index.htm

Physics and biology Physics and biology research underground.

DOE Environmental DOE Environmental Management (EM)

http://www.em.doe.gov/Pages/EMHome.aspx http://www.em.doe.gov/Pages/EMHome.aspx

Select Mission Select Mission, History, or EM Five-Year Plan (2008-2012).

Methodology and Technology of Decommissioning Nuclear Facilities, 1986 Methodology and Technology of Decommissioning Nuclear Facilities 1986 IAEA Technical Reports Series, No. 267 Vienna

Decommissioning Decommissioning of Nuclear Reactors

http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/decommissioning.html http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/decommissioning.html

Regulations and history Regulations and history from NRC.

Standard Review Standard Review Plan for License Termination

http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1700 http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1700

NRC requirements NRC requirements for decommissioning, 2003.

Decommissioning Nuclear Decommissioning Nuclear Facilities

http://www.uic.com.au/nip13.htm http://www.uic.com.au/nip13.htm

Information from Europe Information from Europe, Japan, and the United States.

Cohen, 1983 Bernard L. Cohen, Before It's Too Late: A Scientist's Case for Nuclear Energy 1983 Plenum Press New York Includes data and discussion of radiation, risk, and radioactive waste. A powerful statement by a strong advocate of nuclear power

Resnikoff, 1983 Marvin Resnikoff, The Next Nuclear Gamble: Transportation and Storage of Nuclear Waste 1983 Council on Economic Priorities New York Written by an opponent and critic of nuclear power

Frankena and Frankena, 1991 Frederick Frankena, Joann Koelln Frankena, Radioactive Waste as a Social and Political Issue: A Bibliography 1991 AMS Press New York Inclusive of pro- and anti-nuclear references on several technical aspects