Evaluation of radiation field in a high level waste repository in view of gallery characterization and different scattering kernels

Abstract

Three main types of host rocks are now being considered to store highly radioactive wasteclay, granite and salt rocks. As a consequence of handling the waste during emplacement and possible retrieval in the future, the issue of occupational exposure to radioactive waste within the gallery has to be carefully taken into account. Specifically, the scattering of neutrons from the gallery wall materials, mentioned above, is of a great importance. This study evaluates the radiation field in salt and concrete covered waste repositories, with a particular focus on neutron scattering. The research covers several relevant aspects such as materials, geometries and scattering models which influence the neutron dose rates, using numerical simulations in the Monte Carlo neutron transport code MCNP6. Analysis of the ambient dose equivalent and flux spectrum reveals that neutron back-scattering from gallery walls significantly impacts the overall radiation dose. A comparative study of concrete and salt, shows that for concrete, lower scattering cross section and the ability to thermalize neutrons lead to reduced dose rates. Impact of geometry of the gallery was also investigated. In the planned L-shape galleries, neutron back-scattering is more pronounced near the corner, leading to higher dose rates for workers compared to straight galleries. Further evaluations involved the use of polyethylene rods to enhance neutron thermalization which strongly reduces the emitted neutron flux. The impact of different scattering models was further investigated. Comparing standard MCNP model with the newly implemented in MCNP6 Doppler Broadening Rejection Correction (DBRC) method, showed some influence on overall dose rate. However, other theoretical scattering evaluations using the Blatt-Biedenharn formula and optical model potential which are not yet fully implemented in Monte Carlo codes, showed differences in angular scattering distributions, raising uncertainties as far as the angular distribution is concerned, and in particular the back-scattering of neutrons from the gallery wall to the hosting cavity. Further dedicated investigation is planned on this back scattering effect in the RPI laboratory in Troy, United States.