Calculation of radiation exposure of the environment of interim storage facilities for the dry storage of spent fuel in dual-purpose casks

Acceptance problems in the public concerning the transport of spent nuclear fuel elements and a new political objective of the Federal Government have forced the German utilities to embark on on-site interim storage projects for the temporary storage of spent nuclear fuel elements. STEAG encotec GmbH, Essen, Germany, was awarded contracts for the conceptual planning including necessary shielding calculations for the majority of the 13 nuclear sites which opted for the dry storage concept. The capacity of the storage facilities ranges from 80 to 100 casks, according to the storage needs of the plants. The average dose rate at the surface of each cask was limited to 0.5 mSv/h, independent of the type of radiation. These new buildings should not significantly increase the exposure of the public to radiation already originating from the existing nuclear power plant. The layout of the storage building therefore has to ensure that additional target values of 10-20 µSv/y are not exceeded. These very low target values as well as the requirement to avoid high mechanical impacts to the casks in case of external events led to a thickness of walls and ceilings of between 1.2 m and 1.3 m. The layout of the storage building is shown in Fig. 1.