Monte Carlo based study of radiation field in a deep geological repository for high-level nuclear waste with different host rock types

Disposal in deep geological formations with arrangements for monitoring and retrieval has been considered as one of the most promising management concepts of heat-generating, high-level nuclear waste. Typical design of a deep geological repository requires a shielding cask containing the waste, which is in turn located underground in an emplacement drift of a host rock formation. Various rock types, such as rock salt, clay stone and granite, are possible candidates as host rock for a deep geological repository. The massive host rock formation around the emplacement drift is the most important barrier against radionuclide release into the biosphere. However, the host rock layers have also influence on the radiation field in the emplacement drift, which should be taken into account when assessing occupational exposure in the repository. In the current study, impact of two different materials, i.e. rock salt and concrete (building material of the supporting liner for a drift in clay stone or granite), on radiation field around a nuclear waste package disposed in an emplacement drift was investigated with Monte Carlo method. The high-level nuclear waste was simulated with a 252Cf neutron source. Both neutron and gamma dose rates and spectra around the waste package were calculated. It was found out that neutrons dominate the radiation field in the drift and the dose rates in the drift are enhanced due to backscattered radiation by the surrounding material layers. Furthermore, due to different material compositions of rock salt and concrete, the resulted neutron spectra have also different characteristics. In general, concrete moderates neutrons better than rock salt, which leads to a lower dose rates in the emplacement drift.