Nuclear Fission
Abstract
Nuclear fusion, the energy source that fuels the Sun and stars, can potentially be exploited to generate electricity. The process in the Sun involves the fusion of hydrogen atoms at extremely high temperature and pressure. Those conditions cannot be recreated on the earth but another fusion reaction, between deuterium and tritium, is feasible and forms the basis for fusion research. Fusion takes place as such a high temperature that atoms dissociate into a plasma electrons and nuclei. This must be contained if fusion is to be exploited. Two approaches are being developed. In one a magnetic field is used to control the plasma, while in the second a fusion reaction is triggered in a small capsule of the reactants so quickly that they are prevented from escaping by their own inertia. A massive international project called ITER is exploring the commercial development of the first of these.
Keywords
Fusion; deuterium; tritium; magnetic confinement; inertial confinement; tokamak; ITER; National Ignition Facility
Nuclear fusion, the reaction that fuels the Sun and the stars, has excited scientists and technologists ever since the process was identified during the 1930s. Similar to fission, the key to unraveling the mechanism of fusion depends on recognizing that some elements in the periodic table are more stable than others. In particular, elements toward the middle of the periodic table become relatively more stable than those at the extremes. Thus, helium with two protons and two neutrons is more stable than hydrogen with one proton. Identifying this during the early part of the last century culminated in the realization by a British astrophysicist, Sir Arthur Eddington, that fusion of hydrogen atoms to produce helium would release around 0.7% of their mass as energy. This led to the first theory of fusion in the stars, elaborated by German physicist Hans Bethe in 1939.
Unsuccessful attempts at fusion took place during the 1930s at the Cavendish Laboratory in Cambridge, UK, but work on fusion halted during World War II. Experimental work restarted during the late 1940s, again in the United Kingdom, at Harwell and it was here that the Zero Energy Toroidal Assembly or ZETA operated between 1954 and 1958 and provided a body of experimental observation that proved vital for later fusion research. Since then a series of fusion reactors have been built around the world. Around 20 are in operation today.
In 1958, at an Atoms for Peace conference in Geneva, fusion research was established as an international collaborative venture and at least one strand of fusion development, that based on magnetic confinement, has remained international in flavor ever since. The necessity for this was reinforced during the 1970s when it became clear that the cost of developing fusion was likely to be beyond the resources of any one nation. Before that a key magnetic confinement discovery was made by Russian scientists Igor Tamm and Addrei Sakharov who, in 1968, unveiled a magnetic confinement device called a tokamak that was capable of sustaining much higher temperatures than any previous device. Although other systems are still being explored, this is now the dominant method of using a magnetic field to contain a fusion plasma.
The latest large reactor exploiting this technology is the International Thermonuclear Experimental Reactor (ITER) under construction in the south of France. If successful when it reaches full power, by 2030 or earlier, it will be the first fusion reactor to produce more energy that is needed to start the reaction in the first place. However a commercial fusion reactor is not likely before the middle of the century, at the earliest.
While large fusion reactors based on magnetic confinement were being built in the United States, Europe, and Japan, other fusion developments remained hidden behind the security of nuclear armaments research. Fusion is the basis for the hydrogen bomb and so much of the research into its development and control remained secret until very recently. It is this research that has led to as second approach to generating electrical power from fusion, the idea of inertial confinement. During the last 5 years, the veil of secrecy has at least partly dropped from this research and a major program in the United States aims to develop a demonstration power plant based on these fusion techniques during the 2020s. If successful, it may reach fruition at around the same time as ITER.
Fusion Basics
Nuclear fusion, like nuclear fission, can provide energy from mass. In the case of fusion, this energy is released when very light atoms are turned into slightly heavier, but more stable atoms. Since the primary precursor is hydrogen, one of the most abundant elements on the earth, fusion—in principle at least—offers a limitless supply of energy. Moreover the fusion reaction, while producing a large amount of energy, generates less toxic waste than nuclear fission. The reaction is not entirely clean because very high-energy particles are generated and these will cause nuclear reactions in plant components, leaving some radioactive remnants. One of the key hydrogen isotopes involved in fusion is radioactive too. Nevertheless, it is generally judged much more benign, environmentally than fusion.
The fusion reaction that powers the Sun and stars is a reaction in which hydrogen atoms combine to produce deuterium and then deuterium and hydrogen atoms fuse to make helium with the release of energy. This reaction takes place in the center of the Sun at a temperature of 10 million to 15 million degrees celsius and under extreme pressure. Under these conditions, the hydrogen atoms disintegrate to form a sea of electrons and nuclei, which are held close together by the massive gravitational force within the Sun (gravitational confinement). The conditions required to allow this reaction to take place are considered almost impossible to recreate on the necessary scale on Earth. However, there is another fusion reaction, between deuterium and a third isotope of hydrogen called tritium, that requires less extreme conditions and these can be recreated, albeit with extreme difficulty, on our planet. It is this reaction that forms the basis for fusion research.
As with fission, this fusion reaction releases its energy primarily as kinetic energy that is carried away by a neutron that is generated during the reaction. In a fusion reactor this energy must be captured and used to generate steam for power production. The amount of energy available is enormous. In theory 1 tonne of deuterium could provide the equivalent of 3×1010 tonnes of coal.
There is one problem with the deuterium–tritium reaction; tritium does not occur naturally and must itself be made during a nuclear reaction. It can be produced from lithium using the high-energy neutrons in the fusion reactor so like a breeder reactor, a fusion reactor will have to be able to produce its own fuel as well as energy. This significantly complicates the design of such reactors.
Magnetic Confinement
The fusion reaction between deuterium and tritium (the DT reaction) discussed above is the easiest to achieve in a reactor but even so it requires extremes of both temperature and pressure. The temperature required to achieve fusion with DT is over 40 million degrees celsius. This temperature is necessary to provide the nuclei with enough thermal energy to overcome the electrostatic repulsion between the two positively charged particles and allow them to approach close enough to one another to react. However at temperatures anywhere near this, atoms disintegrate to create a sea of electrons and nuclei, a fourth state of matter called a plasma.
If enough energy can be provided to allow the DT to enter the plasma state and for fusion to begin, then in principle the reaction itself will be self-sustaining because the enormous amount of energy released by each reaction will maintain the high temperature. However the problem of achieving this is complicated by the fact that the plasma must be maintained for a sufficiently long time for the reaction to take place, and the density of the plasma1 must be high enough so that it can produce more energy than it absorbs.
There are no materials in existence that can be used to build a container to hold a plasma so an alternative way has to be found to contain and control the hot sea of particles. The most promising solution is by means of a magnetic field. If a strong magnetic field is applied to the plasma, then the motion of the charged particles in the plasma will be affected by this field. By judicious application of magnetic fields, the charged particles can be made to orbit inside the reactor without touching its walls. This is the basis of magnetic confinement.
Magnetic confinement was recognized early in fusion research as the only way to maintain a fusion plasma but it was not until the 1950s that the best form of magnetic field, the toroidal field, was identified by scientists in Russia. It was there that a device called a tokamak was developed and in the early 1960s experimental results showed that the high temperatures required for fusion could be achieved with this device.
Since then, a series of ever larger tokamak reactors have been constructed. The most important of these were the Joint European Torus (JET) at Culham in the United Kingdom, the Tokomak Fusion Test Reactor (TFTR) at Princeton in the United States, JT-60U in Naka, Japan, and T-15 in Moscow, Russia. Both TFTR and JET experimented with DT fuel from the beginning of the 1990s, and in 1997 JET established the record for the greatest amount of energy generated by a fusion reactor, 16 MW. A schematic of a magnetic confinement fusion plant is shown in Fig. 8.1.
Even with this output, the reactor consumed more energy than it generated. JET achieved a power-in-to-power-out ratio (the gain of the reactor) around 0.7. A gain of 1 represents the break-even point. Even more crucially, JET could only maintain the plasma burst for 5 seconds. If it continued for longer, its systems would begin to overheat. During the experiment, the temperature at the center of the JET plasma reached 170 million degrees celsius. At this temperature the plasma behaves like a boiling liquid, creating eddy currents that make is unstable. Controlling turbulence within the plasma is one of the keys to building an efficient fusion reactor that can operate with a gain of greater than one.
The next stage in magnetic confinement fusion development is the International Thermonuclear Experimental Reactor (ITER–the word is also Latin for “the way”). This project was first conceived in 1988 and an agreement to build the plant was finally signed in 2007 by the EU, Russia, Japan, the United States, China, India, and South Korea.
ITER will have a plasma volume of 800 m3 and a power output of 500 MWth, 30× that of JET. Fusion energy generation is a matter of size and at this size it is hoped that ITER will have a gain of 10, producing 500 MWth from an input of 50 MWth. This will be sufficient to prove fusion as a net source of energy but ITER has not been designed to generate power so it will not have all the features needed for a demonstration plant. Its main goal is to generate 500 MW of thermal fusion power for 400 seconds to prove that commercial fusion is possible. The first demonstration power plant will have to wait until a successor to ITER is built.
Even so, ITER is a massive project, and possibly the most difficult engineering project on the earth today. It involves the 7 collaborating nation members and the components will be built in 34 countries. If all proceeds according to plan, then the first tests are expected around 2020.
Inertial Confinement
As the research into magnetic confinement edged forward during the past 60 years, there was, behind the facades of defense research establishments across the world, a completely different method of achieving fusion being explored. This had more interest in how to create an instantaneous fusion reaction to release an explosive amount of energy, in other worlds a bomb. Secrecy meant that this research remained hidden until very recently. Today, however, the commercial potential of this research is being explored and much more knowledge about the technique called inertial confinement is available publicly.
While magnetic confinement seeks to create a stable continuous plasma in which fusion can take place the alternative, inertial confinement, seeks instead to generate energy from a series of discrete fusion reactions, each producing a burst of energy. In an inertial confinement reactor, small capsules containing around 150 mg of a mixture of deuterium and hydrogen are exposed to a massive pulse of energy from multiple lasers. When the laser beams strike the capsule, they create an explosion of X-rays from its surface and these in turn (by the mechanical principle of action and reaction) create a pressure pulse—a shock wave—that heats and compresses the DT mixture within the capsule with such vigor that the conditions for fusion are generated at its core. One fusion starts, the reaction radiates outward through the DT mixture faster than the actual molecules can expand and escape (they are “confined” by their inertia) and so the whole charge undergoes fusion and releases a pulse of energy. The inertial confinement fusion process is shown schematically in Fig. 8.2.
To make this into a means of generating power, these small exploding Suns must be created at a relatively rapid rate of perhaps 15 each second. This sounds both exacting and ambitious but it is exactly what a US program proposes. To achieve it, the US government has built the National Ignition Facility (NIF), a $5 billion project that is intended to serve both military and civilian research.2
NIF is provided with 192 lasers capable of providing up to 5 MJ of energy in a 20-nanosecond pulse and it has so far produced 1.8 GJ in a single short pulse, equivalent to a delivery rate of 500 TW of energy. The lasers produce infrared light which is converted first into visible light and then into ultraviolet before striking its target. All the energy contained in the laser beams is focused onto 150 mg of DT inside a tiny capsule, about 2 mm in diameter, called a hohlraum.
NIF is only capable of single shot experiments rather than the continuous operation required for a power plant. However the amount of laser power it has at its disposal in similar to that which would be required for a 1000 MW power station. Since it started in 2009, it has carried out a series of experiments trying to achieve ignition, the point at which the DT produces more fusion energy than the lasers pump into it. By 2013 it was a factor or two or three short of ignition.
Running alongside NIF is another project, the Laser Inertial Fusion Energy (LIFE) project. LIFE is a collaboration of scientists, technologists, utilities, and regulators that are seeking to design a power plant capable of exploiting inertial confinement. The target for LIFE is to be in a position to build a demonstration power plant within 10 years of ignition being achieved at NIF. This demonstration plant would have an initial generating capacity of 400 MW but would be designed to be capable of scaling up to 1000 MW. Current plans envisage this demonstration project will be constructed between 2020 and 2030 with commercial plants available by 2030 or soon afterward. However, it is a saying in fusion research circles that a commercial plant is always 30 years away. While the future for fusion looks more promising than it has done before, there is still an enormous amount of work to be done.
Tritium Production
For either magnetic confinement or inertial confinement to become a successful source of electrical power, each needs a source of tritium. Tritium is radioactive with a half-life of 10 years so any naturally formed tritium quickly decays. However a supply can be generated in a nuclear reaction from lithium. When lithium is exposed to neutrons it splits for form an atom of tritium and an atom of helium. One isotope of lithium, lithium-6 will react with slow neutrons while another isotope, lithium-7 reacts with fast neutrons.
In order to provide self-sustaining power plants, both a magnetic confinement and an inertial confinement plant would have to produce, or breed, its own tritium. Under current designs, it is conceived that this will be manufactured in a “blanket” that surrounds the core of the fusion reactor, or be placed around the ignition chamber in an inertial confinement plant. The design of the blanket for either type of reactor is still a matter of speculation. However, it will have to serve two functions. The first is to absorb the high-energy neutrons that are generated during the fusion reaction, capturing their heat in the process. The second is to generate tritium for the reactor.
How lithium will be utilized within a plant is uncertain although several options exist. One is to use lithium as the plant coolant as well as the tritium breeder. If used in this way, liquid lithium would circulate in primary coolant circuits, capturing heat and carrying it to a heat exchanger and steam generator where steam is produced to drive a steam turbine generator. Tritium that is generated in the blanket will dissolve in the hot lithium and would then have to be harvested. Various schemes for this have been proposed but so far most are only experimental. An alternative approach is to have the lithium present in ceramic blocks through which a coolant such as helium passes. This would separate the cooling system from the tritium breeding system but would require the ceramic blocks to be removed to harvest the tritium from them.
Safety and waste disposal are also issues that have to be addressed in both types of fusion plant. As already noted, the fusion reaction produces high-energy neutrons and these will cause nuclear reactions within the components of the power plant, creating a radioactive waste problem. The main source of this waste is likely to be the steel that is used to construct the reactor, of either type. There are low activation steels available that will produce short-lived radioactive isotopes when exposed to neutrons. These then decay quickly so that the level of radioactivity falls rapidly once the reactor is decommissioned and the steel can be consigned to low-level radioactive waste repositories.
Another consideration is the tritium. This hydrogen isotope that plays a key role in the fusion reaction is radioactive, though again with a short half-life so that it will decay rapidly. A fusion reactor is expected to contain less than 1 kg of tritium while it is operating. Further, a plant failure of any type is likely to lead to the fusion reaction shutting down since the reaction is not self-sustaining, reducing the overall risk level. Even so it presents an issue for regulators if, and when fusion plants become commercial.