Strategies for numerical verification of nuclear waste packages of compliance with the permissible activity release during a postulated damaging fire

The waste acceptance criteria for the final repository Konrad are defined in the Konrad waste acceptance requirements (status: December 2014, SE-IB-29/08-REV-2). The requirements are derived on the basis of the results of a site-specific safety analysis. They include general requirements for waste containers, waste packages as well as specific requirements for the waste product and the activity limitations for individual radionuclides and mass limitations for non-radioactive hazardous substances.

Depending on the activity inventory of the waste packages, higher-quality packaging may be required. In this case, the owners of radioactive waste must provide evidence that a possible release of radionuclides during the postulated accidents is limited down to permissible values.

As part of the "plan approval decision for the construction and operation of the Konrad mine in Salzgitter", the underground fire of a transport vehicle was defined as design basis for the thermal accident to determine the release, which is modelled by a corresponding time-flame temperature model curve.

According to the Konrad waste acceptance requirements and the applicable implementing provisions (product control of radioactive waste, radiological aspects, SE-IB-30/08-REV-1), both experimental and computational considerations are permitted for verification procedures, provided that the equivalence of the thermal test with the postulated accident is demonstrated regarding heat input and spatio-temporal temperature profile.

In addition to the caloric properties of packaged waste products, the material composition and structure of the waste container and its installations have an influence on the heat transfer and thus on the temperature reached in the waste product. The release behaviour at a given temperature depends on the chemical/physical composition of the waste product. Covering heat penetration calculations lead to high calculated temperatures in the waste product, therefore restrictions are necessary for the properties of the waste product to be packaged. Nevertheless, there remain degrees of freedom in the packaging strategy. On the other hand, if the specific packaging variant is used as basis, more accurate heat penetration calculations can be carried out, which lead to lower temperatures. Thus a sufficiently low release of radionuclides can be demonstrated for a large number of waste products. This presentation compares different approaches of the numerical verification to suggest the waste owners to choose the most efficient packaging strategy for their waste products.