Retrievability in the BAMBUS-II Project. J.B. Grupa. NRG, the Netherlands

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1 Retrievability in the BAMBUS-II Project J.B. Grupa NRG, the Netherlands Summary In the context of radioactive waste disposal retrievability can be defined as the capability of waste package retrieval afforded by the repository system. The main reasons for incorporating retrievability are: Retrievability allows a staged and reversible decision process. Such a process, in which decisions are not irrevocable, meets concerns often expressed by stakeholder groups. Introducing retrievability provides greater control over the disposal system (e.g. possibilities to intervene in the disposal process if the repository system shows undesired behaviour), and increases the safety in this sense. In general, safety may incorporate, apart from a low inherent risk of harm, the level of oversight of the system (e.g. monitoring to check that the system is behaving as expected) and the level of control over the system. From the viewpoint of repository-design optimisation, the option of retrievability is provided by the design of the waste package and the disposal cell, the layout of the repository, and the behaviour of the backfill and the host rock. Further, the repository operational plan is relevant: the accessibility of the waste becomes progressively more complicated with each phase of the waste emplacement and each step in the closure plan of the repository. In the BAMBUS-II project, the consideration of retrievability and repository design optimisation has led to the following objectives for the Work Package retrievability : Assess the mechanical and thermal boundary conditions for retrieval of heat generating waste from emplacement drifts and boreholes Evaluate the time window for retrieval operations Evaluate and adapt previously developed mine structural designs Assess potential radiological consequences of the accessibility of the waste during the retrievability period. The work in this Work Package has been jointly carried out by NRG, The Netherlands, and DBE, Germany. A number of potential design modifications are presented that will enhance retrieval of the waste under the mechanical and thermal conditions that can be expected after disposal of heat generating waste. These can be combined with changes in the repository implementation project as a whole, to accommodate a staged decision process. Given that, with appropriate technical modifications, retrieval of waste canisters can be implemented in the design, the time window for retrieval operations is in practice determined by conventional mining issues. Experience shows that mines in rock salt can be in operation for a century. Beyond this period a new mining operation may be necessary.

2 An illustrative Performance Analysis shows that the accessibility of the waste does not have a significant impact on potential radiological consequences for a proper design. 1. Introduction At present, most disposal facilities are designed in such a way that they provide options to retrieve waste after emplacement. These options vary from the ability to reverse given steps in the disposal process to a requirement to be able to retrieve the waste for a very long time. Within the BAMBUS-II project, three tasks have been identified that relate to retrievability. For each task a task-report has been written: 1) Drift Emplacement (Ziegenhagen et al., 2003) 2) Thermal and Mechanical 3-D Analyses of Deep Lined Boreholes in Rock Salt used for the Retrievable Storage of Heat Producing Vitrified Radioactive Waste (Fokkens 2003) 3) The Safety Implications of Retrievability (Hart et al., 2003) These three reports are the basis for the text in this paper. Not all issues dealt with in these taskreports could be incorporated. Examples are: costs of retrievability, operational safety issues, details of design modifications, specific parts of Performance Assessment such as scenario development. 2. Drift Emplacement Drift Emplacement is foreseen in e.g. the Gorleben disposal concept and in the WIPP facility. The Gorleben concept, see Figure 1, has been evaluated with respect to retrievability, see (Ziegenhagen et al., 2003). This concept does not include any retrieval intentions. Because of this, the planned repository design and disposal technology offer only very restricted possibilities to retrieve disposed waste packages during the operational phase. Figure 1 Layout of disposal drifts of the concept Gorleben repository - Effect of heat-output reducing measures

3 The evaluation performed in BAMBUS-II shows that the temperature is the main factor that limits the access to the containers. Other factors, such as the repository design, especially (1) the emplacement scheme starting with filling the distant disposal fields and working towards the shafts, and (2) emplacement of canisters in a long row in drifts, and (3) backfilling and sealing, aggravate access to the casks, but do not make retrieval impossible. The following possibilities could be considered to decrease the thermal output of the containers in order to limit the temperature at the contact cask-salt to a level of about 100 C: Prolongation of the interim storage duration after unloading the fuel from the reactor and use of the same Pollux-8 container; Decreasing the fuel mass per disposal container. Calculations have been performed to estimate the effect of these measures (Ziegenhagen et al., 2003). The result is also shown in Figure Borehole Emplacement Borehole Emplacement is considered for disposal of vitrified waste in amongst others Germany and the Netherlands. In the German Gorleben concept, drift emplacement (for disposal of spent fuel) is combined with borehole emplacement (for disposal of vitrified HLW). Originally, both designs were not suited for retrieval of the waste. Therefore, the Dutch design was modified to allow retrieval of the canisters with vitrified waste. The main modifications are: 1) the addition of a liner (casing) to the boreholes. This liner must resist the rock pressure during the retrievability period and prevents the encapsulation of the canisters by rocksalt (creep/convergence), which would complicate future retrieval. 2) the addition of overpacks for the thin-walled canisters, both to support the pressure exerted by a stack of overpacked canisters and to withstand a collision of a falling canister, the design accident for this storage concept. After emplacement of the waste, the temperature of the rock salt will increase due to the decay heat of the vitrified waste. The largest temperature increase will occur at the depth of the middle of the borehole (> 100 m below the access drift level). The temperature increase will be in the order of 100 K. As a result of this increase of temperature, unconfined rock salt would expand 0.42% (volumetric expansion). To confine the heated rock salt to a constant volume, a stress of more than 200 MPa would be needed (for comparison, the rock pressure at 800 m depth is about 20 MPa). These potentially large stresses and associated stress gradients will drive creep of the rock salt. This process will allow the heated rock salt to expand, relaxing the thermally induced stresses. The actual time-development of the stresses will depend on the rate at which the temperature of the rock salt changes on one hand, and on the creep rate on the other hand. An illustrative result is given in Figure 2.

4 Figure 2 Geometry of the Generic Borehole Emplacement Concept - temperature induced stresses versus time The calculations show that the liner of the borehole will be able to endure these stresses, even if corrosion would reduce the strength of the liner. 4. Assessment of Safety and Radiological Consequences of the Accessibility of the Waste during the Retrievability Period During the retrievability period the radioactive material is isolated from the environment by the waste matrix, the canisters and overpack (including the lining of a disposal cell/borehole) and the seals of the disposal cell. To study the isolation performance of these barriers, it is assumed that the facility floods with groundwater (which converts to brine) during the operational phase. The large hydrostatic pressure (8 MPa) and the long time period (100 years) would enable the brine to penetrate the barriers. A small part of the radioactive material may migrate to the aquifers in the overburden, and eventually reach the biosphere. Figure 3 shows the development of the porosity of the borehole plugs, with and without the inflow of brine. The figure shows that, as soon as the brine enters the disposal cells at 29 years, the porosity decreases faster in the first few years as compared with the no-brine case. This is caused by the wetting of the salt grit by inflowing brine. After about 45 years, the rate of the porosity decrease, or the compaction rate, drops as a result of the pressure of the brine itself. At some point in time, the porosity will have decreased to a value comparable to the porosity of normal rock salt (about 1%). It is assumed that at this point the compacted salt will behave similar to rock salt, in particular it will have become impermeable just like rock salt. In the calculation the end porosity of 1% is reached at about 152 years; the compaction process stops and the porosity of the borehole plugs remains at that value.

5 No brine Plugs become impermeable y 0.50 Glass + waste Porosity (-) Released Mass from Borehole (g) E E E E E+04 Time (years) Figure 3 Porosity of the plugs, with and without brine inflow Time (years) Figure 4 Released mass from one borehole if the liner fails The estimated dose level in the biosphere as a result of this release is well below the in ICRP-60 recommended dose limit for accepted practises. This result suggests, that a complete safety study for a specific repository will show that, even in this operational period, the waste is sufficiently isolated. 5. Conclusion Evaluation and modification of previously developed mine structural designs For borehole emplacement, a desktop design has been developed that adds to existing designs (1) a steel casing, that counteracts the convergence of the borehole, and (2) an overpack that can carry the weight of the stack of (overpacked) containers in the borehole. A thermo-mechanical calculation has been performed to show that the steel casing can resist the thermal induced stresses that will occur after emplacement of the heat producing vitrified waste. For drift emplacement a number of measures that limit the temperature increase in the dis-posal fields have been considered. Most feasible measures are extension of the interim storage period (to ca. 80 years), and reduction of the number of spent fuel elements per disposed container. Evaluation of the time window for retrieval operations Given that modifications, such as mentioned above, technically allow retrieval of the waste canisters, the time window for retrieval operations is merely determined by conventional mining issues. Experience shows that mines in rock salt can be in operation for a century. Assessment of potential radiological consequences of the accessibility of the waste during the retrievability period The illustrative analyses presented earlier suggests, that a complete safety study for a specific repository will show that, even in this operational period, the waste is sufficiently isolated. However, the question: What are the long term safety implications of the introduction of retrievability in a deep underground repository in rock salt? is only a very limited aspect of long term safety : the question only covers the isolation performance in case of a brine intrusion scenario during operation. Safety is in concept much broader than isolation of the radioactive material from the environment. Safe does not mean the risk is small, but it means the (small) risk is accepted. Acceptance of a (small) risk is increased by increasing the level of oversight over the system (e.g. monitoring to check that the system is behaving as expected) and by increasing the level of control

6 over the system (e.g. possibilities to intervene in the disposal process if the system is behaving unexpected). Introducing retrievability has a positive effect on the level of control and the level of oversight of the disposal system, and increases the safety in this broad sense. The main difference between a retrievable and a non-retrievable disposal is that the latter facility may be longer in operation. As some aspects of the operational risk are proportional to the duration of the operation, one is inclined to think that the risk increases by extending the operational phase. However, it goes without saying that also during the operational period the waste must be safely isolated from our living and working environment, irrespective of the issue of retrievability. A safety consideration can therefore not be a reason to start the closure operation in a normal operating facility. Moreover, giving consideration to the legal, organisational and technical issues that may delay the start of the closure operation, any repository should be designed to allow a safe extension of the operational period, within reasonable margins. Nevertheless, some design modifications (technical efforts) have been considered to further limit the operational risk (e.g. a better control of the sealing of the disposal cells). In addition, operational risks can be reduced by developing a firm and sustainable control over the facility and the operator (organisational effort). Eventually, introducing retrievability may result in a safer disposal process due to design and organisational improvements. References ZIEGENHAGEN, J., LERCH, C.H., Drift Emplacement, Part 6.1 of Deliverable D6 of the BAMBUS-II project, Peine (2003). FOKKENS, J., Thermal and Mechanical 3-D Analyses of Deep Lined Boreholes in Rock Salt used for the Retrievable Storage of Heat Producing Vitrified Radioactive Waste, Part 6.2 of Deliverable D6 of the Bambus-II project, NRG 20533/ /I, Petten (2003). HART, J., GRUPA, J.B., The Safety Implications of Retrievability, Part 6.3 of Deliverable D6 of the Bambus-II project, NRG 20533/ /P, Petten (2003).