Chapter 2

Monitoring of Radioactive Waste Disposal
Cells in Deep Geological Formation ¹

2.1. Context

The French agency for radioactive waste management (Andra) is in charge of the long-term management of radioactive waste generated in France. Within the framework of this mission, it uses its expertise and its know-how to enable the state to implement safe management solutions for all French radioactive waste in order to protect the current and future generations from the risk this waste represents. Andra manages two disposal facilities in operation in the Aube district accepting very low-level radioactive waste to low- and intermediate-level short-lived radioactive waste, as well as a closed center in the monitoring phase in the Manche district.

In 2005 Andra concluded that deep geological disposal for high-level long-lived waste and intermediate-level long-lived waste was feasible. It was put in charge of designing and implanting a disposal center for these wastes (called Cigéo, the Industrial Geological repository Center) by the program law of June 28, 2006. An underground research laboratory was built in the early 2000s in the town of Bure in Meuse district. This laboratory implements experiments of a scientific and technological nature. It is envisaged that by 2015 all the elements necessary for an authorization request for the creation of Cigéo will be ready. Subject to the necessary authorizations, it is envisaged that Cigéo will be operational in 2025, with the necessary construction work starting in 2017. The underground facilities of the repository center will be progressively built upon and will run for a century. The authorization will determine the minimal duration during which, for the sake of caution, the disposal reversibility must be ensured; the law from June 28, 2006 states that this duration of reversibility cannot be fewer than 100 years.

Within the framework of this geological repository project, the monitoring (called by Andra “observation and surveillance”) of the environment and structures must incorporate the knowledge required to run the disposal and its reversible management. It must also contribute to the safety analyses while Cigéo is in operation and after closure.

The complete monitoring system must fulfill a set of regulations and societal expectations; expressed:

– in the law of June 26, 2006 on the management of radioactive waste and the law of June 13, 2006 on transparency and security;

– in the “safety guide relative to the permanence of nuclear waste in deep geological formation”, coming from a revision process initiated by the authority on radioactive safety of the fundamental safety rule n° III.2.f;

– in the environment code, requesting an environmental reference state in agreement with the extent of the industrial project; and

– within the framework of public debate, exchanges on reversibility, etc.

To meet these requirements, the monitoring system considered by Andra is based on a global strategy including the monitoring of structures and the environment.

2.2. Monitoring of the environment

Andra has set up a perennial observatory of the environment. This observatory, recently labeled SOERE (systems of long term observation and experimentation for the environmental research) by AllEnvi (French National Alliance of Environmental Research) allows Andra to collect the data necessary to carry out an impact assessment before construction. Cigéo is going to be run over a century, beside the evolutions generated by industrial activities in the area. Several changes will take place during this period, be it at the local scale with socioeconomic evolutions (agriculture and forestry) or at a more global scale with climate change. It will be important for Andra to be able to discriminate against the relative contributions of these different evolutions. On top of a fine mesh of sampling and analysis, Andra is putting a network of instrumented stations in place to monitor water, the air and biogeochemical cycles in the soils of the region of interest. An environmental specimen bank is also under construction.

2.3. Monitoring of geological repository structures

The second key element in the monitoring strategy of the geological repository is structural monitoring. Besides the equipment relatively classically implemented to ensure the operational safety of the centre's exploitation, a complementary and parallel monitoring system will be developed. The time range will be increased: the system will be implemented from construction onwards and maintained beyond the first partial closures of the disposal cells.

In contrast, the specifications on the reactivity time or the availability of the information from monitoring are less strict. The goal is to check the expected behavior and to bring additional elements of knowledge to those coming from the modeling and studies currently being carried out in surface laboratories and in the underground laboratory at Meuse Haute-Marne, to supply the decision-making along the repository process. The focus here is placed on the verification of phenomenological evolutions by aiming in particular to:

– Confirm the knowledge involved in the evaluation of the long-term safety and accuracy of the models, on the basis of data obtained in situ (scale, construction, operation), within the framework of periodical re-evaluations of the structures. It is about the confirmation of phenomenological models and parameters involved in the evaluation of the long-term safety of Cigéo. By comparing the measurements carried out in situ on the first decades of the repository's life with the results of the calculations, the confidence in the evaluation of the long-term safety is strengthened.

– Observe the evolutions of the structures and environmental conditions necessary for reversible management, or even the possible withdrawal of waste packages (re-evaluation of the life spans, etc.). The dimensioning of the disposal cells are otherwise often based on envelope hypotheses and fault scenarios. The monitoring must enable the margins thus involved to be defined.

– Monitor certain evolutions that are likely to influence the safety of the operation, complementing and linked to the operational safety regulations foreseen by “command and control”.

The monitoring system considered must supply a thermo-hydro-mechanical-chemical-radiological characterization of phenomena around the disposal of radioactive waste cells. The constraints to fulfill are rigorous:

– The duration of operation needs to be in multiples of 10 years. Certain important evolutions for long-term safety, however, have a characteristic time that is almost inaccessible at the century-scale, so that the variation in the time range can be decreased and the accuracy required is very high;

– The monitoring equipment placed in the waste disposal cells, as soon as the first radioactive waste package is placed inside, is inaccessible. This restricts the maintenance of the instruments (apart from a few automated devices).

– The instruments must not be invasive, since they should not affect the conditions favorable to long-term safety.

– The locally aggressive environmental conditions (temperature, chemistry, irradiation, etc.) must be taken into account.

In order to suggest solutions adapted to these specificities and constraints, many studies and reflections have been carried out. The developments deal with: (i) the monitoring strategy, in particular on the distribution of instrumented structures within the repository; (ii) the design of monitoring units according to a thorough qualification procedure; and (iii) research and development to adapt, complete and qualify the sensing devices.

The response to monitoring requirements lies in the classical and proven approaches, which have been adapted to the context of the geological repository by taking into account the available feedback in the monitoring of some tunnels and confinement areas of nuclear power plants and dams. Andra is also leading an ambitious research and development program into instrumentation and aims to develop specific monitoring equipment. At each significant step in technological progress in a relevant monitoring method for the geological repository, a “qualification” process is started. It involves testing and qualifying the complete measurement chain in progressive steps, knowing these steps and being able to anticipate them, determining the failure rates and mastering the possible long-term drifts.

To design a perennial monitoring system (to last multiple 10-year durations), the strategy focuses first on the implementation of complementary monitoring approaches: embedded sensors completed by the classical approaches of visual inspection and non-destructive evaluation methods. This redundant and complementary procedure is also applied to the embedded instrumentation. Excess sensors would have to be placed among the monitoring units and be linked up according to their complementarities: proven technologies next to innovative sensors, and localized measurements associated with devices providing distributed measurements. This combination of approaches and means enables us to strengthen our confidence in the endurance of the global system but it creates difficulty during the merging of data from these multiple measurement systems that are disparate from the point of view of technologies used and the parameters measured (direct and indirect measurements, localized or distributed, etc.).

It is one of the first expectations Andra has of GIS: the implementation of decision aid systems. The merging of data coming from the monitoring system of the repository aims to supply indicators that are of direct use to the operator. This could be progressive in order to benefit from what is learned from the spatial and temporal redundancy of the global monitoring system. The focus of Andra towards more classical industrial applications means that the acceptable reactivity times for monitoring long-term safety are very slow — at the day-scale which lifts technological constraints that are often strong (calculation times). The duration of monitoring and the risks of associated drifts, the inaccessibility and the associated impossibility of maintenance, however, are relatively original optimization constraints.

To find the equilibrium between the monitoring requirements already expressed, the implementation constraints and the associated costs of monitoring equipment, Andra looks to maintain a reasonable density of embedded sensors. For this purpose, Andra will take advantage of the complementarities of different technical approaches to monitoring that are available by putting progressively more emphasis on visual inspections and on non-destructive tests, equipment that can be mutualized, and decreasing the number of embedded sensors. For this purpose, it optimizes the arrangement of sensors to spread the instrumentation in a largely inhomogeneous way by taking advantage of the similarity between structures, particularly the kilometers of access tunnels and the thousands of disposal cells for long-life high-activity waste (5,000 cells are currently foreseen). By taking advantage of the similarity of some expected phenomenological evolutions (linked to the large homogeneity of the host rock and reserving the selection of disposal cells for similar radioactive package waste), the monitoring strategy suggests following a sequence of structures, referred to as dummy, current and non-instrumented levels, whose density of embedded instrumentation is progressively decreased. Each structure thus labeled has a precise function in monitoring. The “dummy” structure is chosen from among the first structures built. It must exhaustively fulfill the technical monitoring goals. Beside the first constructions, dummy structures will be chosen in specific locations enabling a relevant monitoring. The “current” structure is less well instrumented, monitored by comparison with a “dummy” structure. The non-instrumented structures will only contain essential equipment for operating safety and will be the target of occasional inspection and control. Andra has integrated GIS in order to benefit from optimization methods enabling the optimal number of dummy structures to be chosen by taking into account the variability of phenomena and limits of reproducibility of measurements.

It is for this reason that Andra got involved in the PARDI project, whose results are detailed in this book. This project will enable it to apply the optimization of the instrumentation at the level of disposal. One of the specificities involves spreading the dummy structures both in space and time:

– Almost 5,000 foreseen high-level long-lived cells are spread in zones or modules, each containing a few hundred cells. Each module will be built in one step and then put into operation. The question considered by the PARDI project is how we can statistically define how many cells to use, knowing that the first qualitative evaluations led by Andra are in the range of a few percent, at least for the first modules built.

– The construction and operation of the disposal facility will be very progressive and will last almost a century. The need for information can be progressively decreased as soon as the operation has shown that the evolutions agree with the initial expectations and are sufficiently similar between structures of the same type. In other words, the monitoring strategy must be integrated into the temporal phasing of the exploitation: progressive construction of the typical disposal cells (mineshafts, wells, tunnels, cells, plugs, sealing and embankments) and parallel operation of new phases of the repository with possible evolutions in design. The PARDI project and the concept of learning that is developed therein must enable us to deepen this reasoning by supplying quantitative elements to take advantage of feedback from the first structures monitored.

2.4. Conclusion and perspectives

The inclusion of Andra in GIS is relatively recent and happened after the launch of the organization. Andra is integrated in this community and has an interest in operating safety. It wants to transpose the knowledge and models available in this area to the system of monitoring of the geological repository. In order to optimize the instrumentation plan and the development of measurement interpretation methods, it will establish the diagnostics involved in operating the disposal. Afterwards, the area surrounding the security and safety operation of facilities that Andra manages, and the transport of radioactive materials, will also be able to benefit from the results of GIS.

1 Chapter written by Stéphane BUSCHAERT and Sylvie LESOILLE.