Searching a site for a repository: scientific methodology for a site comparison

02.12.2020

Samples of the potential host rocks crystalline, claystone and salt in GRS's geoscientific laboratory (© GRS)

In the interim report on subareas, regions in Germany were identified for the first time which are candidates for a repository for high-level radioactive waste. In a joint research project, experts from Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, BGE Technology GmbH and the Federal Institute for Geosciences and Natural Resources have developed the methodological bases and recommendations for the site selection procedure. The synthesis report and ten supplementary technical reports are now available on the GRS website.

On 28 September, the federal company for radioactive waste disposal, Bundesgesellschaft für Endlagerung mbH (BGE), published its interim report on subareas “Zwischenbericht Teilgebiete” and thus completed the first of two steps of the first phase of the search for a site for a repository for high-level radioactive waste. Subareas are those regions in Germany which, measured against the requirements of the Site Selection Act (StandAG), give reason to expect “favourable geological conditions for the safe disposal of high-level radioactive waste”.

In the second step of the first phase, so-called representative preliminary safety analyses are to be conducted for all subareas. On the basis of the results of these analyses and the application of the statutory geoscientific weighing criteria, the siting regions are then identified that are to be explored from the surface in the second phase of the selection procedure. From the siting regions that prove to be suitable in this process, potential sites are then selected for underground exploration in the third and final phase.

On behalf of BGE, GRS scientists, together with experts from BGE Technology GmbH and the Federal Institute for Geosciences and Natural Resources, have developed essential methodological bases for preliminary safety analyses and formulated recommendations for the application of the geoscientific weighing criteria in the assessment of siting regions or sites within the RESUS research project.

The objective: providing safe disposal for one million years

Preliminary safety analyses are to be conducted in all three phases of the selection procedure to assess whether safe containment of radioactive waste can be expected in the respective siting region or at the respective site. In doing so, the respective geological conditions and, based on these, those properties of a repository that result from these conditions must be taken into account. In simplified terms, experts must first plan fictitious repositories adapted to the respective geology in order to be able to carry out the preliminary safety analyses. Such so-called disposal concepts describe the concrete design of a repository – starting with the drifts and shafts, the waste containers and the geotechnical facilities such as the sealing structures.

In the RESUS project, the researchers have focused on the key elements of the preliminary safety analyses. The question in this regard is whether and, if so, what quantities of radioactive substances could be released from the repository over a period of up to one million years. The related analyses are based on complex calculations in which, among other things, the transport of radioactive substances from the waste containers through the surrounding host rock is calculated using simulation models. These analyses are also used to examine whether the integrity of the geological barrier – i. e. the ability of the host rock to contain the radioactive substances – is maintained.

The researchers have developed methodological recommendations for such calculations. One of the greatest challenges here was the fact that the disposal concepts differ considerably depending on the host rock, as Dr. Jörg Mönig, RESUS project manager, explained: “While, for example, in a repository in salt or claystone, the containment of the radioactive material is primarily to be achieved by the host rock itself, in a repository in crystalline rock, the waste containers usually have to perform this task”.

For the host rocks clay and salt, the experts were able to build to a greater extent on the results of earlier research and development projects, in which the methodological bases for such analyses had already been laid. For crystalline rock – often referred to as “granite” as one of these rocks – they were able to draw on the international state of the art in science and technology in addition to their own development work. Here, they benefited, among other things, from the long-standing cooperation of GRS with scientific institutions from countries such as Finland or Sweden where repositories in crystalline rock are being constructed.

Uniform criteria for different host rocks

The different properties of the three potential host rocks and the resulting differences between the respective disposal concepts also represent one of the fundamental challenges in the application of the geoscientific weighing criteria. The StandAG specifies a total of eleven of these criteria for assessing in all three phases of the selection procedure whether a “favourable overall geological situation” can be expected in a region or at a site. These criteria include, for example, the permeability of the geological formation for groundwater or the size of the rock zone intended for the containment of radioactive waste. A total of 40 indicators are assigned to these criteria in the Act with the aim of assessing whether the overall geological situation is “favourable”.

Although the geoscientific weighing criteria according to the StandAG are to be applied to all host rocks, their respective significance for the safety of a repository may differ, sometimes considerably, depending on the host rock or disposal concept. “In order to be able to make an appropriate, quasi 'fair' comparison between regions or sites with different host rocks, we need a method that is able to adequately take into account these differences when assessing the overall geological situation”, Dr. Mönig said.

In order to determine the relevance of the individual geoscientific weighing criteria for the different host rocks or disposal concepts and thus also to make the bases of the methodology, the researchers at first developed ten different application cases corresponding to typical geological situations in Germany. Each of these cases was then based on a disposal concept as it would be conceivable in Germany according to the current state of the art in science and technology. On this basis, the researchers made extensive calculations for each geoscientific weighing criterion on the integrity of the barrier rock and on the containment of the radionuclides, as they also have to be performed in a preliminary safety analysis.

The results of these calculations have been incorporated into the development of a total of three schemes. With these schemes, the results from the application of the geoscientific weighing criteria can be summarised in such a way that they can be used as a basis for the safety assessment of the overall geological situation as required by law.

Project results include feedback from experts

The first results of the two-year research project were presented for discussion in the spring of this year in the form of draft reports. Via the GRS website, interested parties were given the opportunity to comment on the drafts and to propose amendments. “On the one hand, of course, we wanted to get technical input from our scientific environment but, on the other hand, we also wanted to show that we as scientists try to make our work as comprehensible as possible,” explained Dr. Mönig.

The feedback received has been taken into account and is included in the results published. The summary synthesis report and the ten related individual reports are now available for download here and in the GRS database of publications.