Investigating deep disposal of radwaste - Part 2

Part 2

CONSTRUCTION

In 1995-9 a second access shaft to the underground facilities had to be sunk as mining constraints prevented the implementation of the Praclay project from the Hades facility.

A connecting gallery about 80 m long, which was completed in 2002, connects the base of this shaft to the facility. This has demonstrated the feasibility of a method that could be used on a large scale for excavating galleries while minimising the disturbance to the clay. The precautionary principle requires the disturbance to the clay caused by excavation to be minimised, in so far as this is technically and economically realistic. Since one method for minimising disturbance is to excavate the galleries at high rate, it will be important to ensure that logistical problems do not involve the redesign of the repository.

The open discontinuities at the base of the second shaft raise a series of important questions, especially with regard to their impact on long-term safety. A characterisation programme intended to answer these questions is currently in progress.

The general design requirements for the sealing of the access shafts are being identified by the RESEAL study. This study has first tested and then used a range of materials for the sealing of an underground shaft in the HADES-URF, the aim being to evaluate the technical feasibility of an actual seal of this type. This study is still in progress.

The sealing materials to be used remain to be specified in greater detail, as does the precise positioning of the seals in the main galleries and access shafts and the exact timing of the different stages of the closure phase. Bentonite materials are currently being considered and tested in RESEAL.

The scenario of the inadequate sealing of the repository will be further investigated in future long-term safety assessments.

Monitoring the different phases of a repository has not been dealt with in any great depth so far, but this aspect will grow in importance at international level. In particular, the link between monitoring and the demands of retrievability is an important subject that will require further study in the following phase of the disposal programme.

UNRESOLVED ISSUES

Without challenging the basic choice of the Boom Clay, enough issues remain unresolved at the present time, however, for it to be possible to produce a final statement on the technical feasibility of a repository in this host formation, on the long-term and operational safety of such a repository, or on its compliance with environmental

standards. The third phase of the ONDRAF/NIRAS programme of methodological R&D should thus consider the following priorities:

• Demonstrating the feasibility of implementing the disposal facilities.

• Improving the understanding of the processes of radionuclide retention at work in the Boom Clay (including the effect of lithological and structural discontinuities) and of the long-term evolution of the geochemical, hydrogeological and retention properties in this formation.

• Studying more in detail the aspects of chemical, biological, and physical compatibility of all of the repository materials with the host formation, and of the different disturbances induced by the various waste classes.

• Building confidence in the containment approach (reviewing the choice of material for the overpack and developing an integrated approach to designing the engineered barrier system based on preventing overpack corrosion).

• Defining and developing a long-term management and

transfer system for knowledge acquired to ensure trace-

  ability of decisions and technical choices and the transmission, integration, and synthesis of multi-disciplinary information.

ONDRAF/NIRAS has identified a number of aspects for each component of the disposal system that need further study or for which insufficient information is available.

These unresolved questions will guide the direction of future scientific research work.

• Further study is needed into the behaviour of the disposal tube in the short, medium and long term under the influence of a thermal load and an uneven swelling pressure. The practical feasibility of emplacing this tube over long distances must also be investigated.

• Other issues associated with the disposal tube include guaranteeing its watertightness, the design of the radiological seal and allowing for expansion.

• In developing the design for sealing the disposal gallery, consideration must be given to factors such as thermal effects, swelling pressure, possible expansion of the disposal tube, the pattern of forces at the intersection of a disposal gallery and a main gallery, the radiological seal and the risk of gas generation.

• The development of the design for the overpack is an iterative process. The dimensions, choice of material, method of fabrication (welding) and the addition of materials between the canister and the overpack may change.

• The design for the backfill material in the disposal galleries must provide answers to the following questions: What is the maximum permissible temperature in the backfill material? What swelling pressure must be considered? How will it be placed in the galleries? Is it economically viable to

  place tthe material in large quantities and over long

  distances? What is its optimal composition? Can homogeneous swelling be assumed? Are there hydration problems?

• The design for the gallery lining must lead to the choice of an appropriate material, for example the composition of the concrete, which satisfies requirements as regards chemical compatibility (eg enhanced alkalinity) and life during the operational phase and period of retrievability. Conceptual requirements and a range of design loads will determine the ultimate form and dimensions of the lining.

• The layout of the disposal galleries must take account of safety requirements and the demands of practical feasibility. A number of parameters of the design will be critically examined as part of future studies (the diameter and length of disposal galleries, the separation of disposal galleries, their dip). The layout will also have to take account of regulations governing tunnelling operations, working conditions during the operational phase, safety aspects and the ventilation of the underground facilities.

• In the meantime, new technical developments must be continuously monitored and evaluated, and this could give rise to alternative construction options, such as microtunnelling.

SOCIAL ISSUES

The work performed so far can only be really validated if it is presented in and supported by a decision-making context that meets the expectations of a modern society. Establishing such a dialogue is therefore a prerequisiteto searching for a solution without the risk of not being able to put it into practice.

In order to get off to a good start, the dialogue must be preceded by an exploratory phase during which a first definition of the problem will be made, the stakeholders will be identified, and the co-definition of the decision-making process and its main stages will be attempted.

In future ONDRAF/NIRAS will focus on:

• Identifying the stakeholders.

• Defining alternative or complementary options.

• Working out, in agreement with stakeholders, a decision-making process.

• Defining and implementing the future methodological technical programme.

The different stages of the technical methodological research programme after 2003 will be determined on the basis of the SAFIR 2 report and the recommendations of the scientific reading committee, as well as on international evaluation.

Once the methodological programme has been carried out, the government will have all the necessary information to make the final decisions about the option(s) chosen and the site(s) where this option should be implemented.

Overall conclusions

At present, none of the research findings indicates any prohibitive issue surrounding HLW disposal into the Boom Clay. This reinforces confidence in the studied solution and confirms that for the waste considered in the SAFIR 2 report, disposal within poorly-indurated clay remains a viable option.

By establishing an inter-disciplinary programme of R&D incorporating aspects that are fundamental, applied, or related to human sciences, it will be possible to further enhance the confidence acquired in the studied solution. In particular, confidence will be increased by considering management alternatives, developing concrete repository designs, allowing for non-radiological environmental effects, and considering societal aspects.