Disposal with a temperature limit?
GRS studies on the temperature on the outer surface of disposal casks
In Germany, high-level radioactive waste is to be disposed of in a repository deep underground. Radioactive decay will continue to take place in the waste even after it has been emplaced. The heat released during this process will be transferred to the surrounding host rock via the surface of the disposal casks. In a study commissioned by the German Federal Office for the Safety of Nuclear Waste Management (BASE), GRS experts have investigated the influences of heat release and the permissible temperatures with regard to the safety of a repository concept.
How heat can affect rock
If rock is exposed to a heat input, this affects the temperature and thermal properties, such as thermal conductivity. The increase in temperature can lead to changes in the mineralogical, mechanical and chemical properties of the rock. For example, a higher rock temperature can increase susceptibility to decomposition or cracking or may affect the pH of the groundwater in the rock. These processes also have a reciprocal effect and can have a positive or negative impact on repository safety.
How warm does a disposal cask get?
The temperature on the outer surface of a cask after its emplacement depends on several factors. Issues that play a role here are the amount of heat generated in the cask and the thermal conductivity of the materials surrounding the cask. With regard to the repository concept as a whole, it must also be considered how many casks are stored in one area and, accordingly, jointly give off heat to the environment. However, the properties of the geotechnical barrier, e.g. the so-called "buffer" located between the cask and the geological environment, are also influencing factors that affect the temperature on the cask surface.
Investigations by GRS and state of research in Germany
For the development and planning of a repository concept, it is important to know how the rock behaves at different temperatures and whether it is therefore reasonable e.g. to specify a maximum permissible temperature for the casks to be emplaced, the so-called "limit temperature" referred to in the Repository Site Selection Act (StandAG). These questions were looked into by GRS researchers in the study "Investigations relating to the "maximum physically possible temperatures" according to § 27 StandAG with regard to the limit temperature on the outer surface of waste disposal casks" for the three host rock types in question in Germany: rock salt, claystone, and crystalline rock. For this purpose, temperature-dependent processes and properties were specified and their relevance for the determination of a permissible "limit temperature" was assessed. In addition, the international state of research was ascertained with regard to the determination of a permissible "limit temperature" for the emplacement of radioactive waste.
Since Germany is currently still at the beginning of the search procedure for a repository site, a provisional limit temperature of 100 degrees Celsius was adopted in the StandAG, which says: "As long as the maximum physically possible temperatures in the respective host rocks have not yet been specified on the basis of pending research, a limit temperature of 100 degrees Celsius on the outer surface of the casks is assumed for precautionary reasons".
Within the scope of their study, the GRS researchers do not arrive at a definite result in the form of a concrete indication of a blanket limit temperature for the host rock types investigated in Germany. It rather becomes clear that, due to the numerous interactions and dependencies, a temperature specification only makes sense if a concrete safety and repository concept is known. One reason for this is the individual properties of the host rocks that need to be considered in safety concepts. While, for example, higher temperatures in rock salt can support its containment-providing property - the so-called "creep", (too much) heat in clay and crystalline rock can worsen safety-relevant properties such as the sorption capacity and swelling capacity and, in the worst case, can create new pathways for water. This alone can result in different limit temperatures.
In addition, the scientists believe that if necessary, it should be possible to redefine the "limit temperature" for a specific site if a safety and repository concept is gradually adapted to the specific geological conditions. The study assumes that reliable data for this purpose can only be acquired in the course of underground exploration of a specific site.
Regarding the aspect of long-term safety and the retrieval or salvaging of the waste, the researchers also recommend that the advantages and disadvantages resulting from the specification of a "limit temperature" should be examined in safety analyses. Especially with respect to long-term safety, it should be avoided that, as a consequence of such a specification, there may be avoidable restrictions to possible safety-related optimisations even before a decision on safety and repository concepts is taken.
A review of the regulations in some other European countries and the USA also showed that no legal regulations have yet been specified for the temperature on the outside of the casks in terms of a limit temperature spanning across host rock types or concepts. Rather, temperatures are only specified on the basis of safety studies after a site or host rock has been identified.
Find out more
Report „Investigations relating to the "maximum physically possible temperatures" according to § 27 StandAG with regard to the limit temperature on the outer surface of waste disposal casks“ (GRS-A-3948, in German) on the BASE website