Reference level of the occupational radiation exposure in a deep geological disposal facility for high-level nuclear waste: A Monte Carlo study

Highlights: •Calculation of effective dose in a geological repository for SNF with MCNP6. •ICRP ISO geometry underestimates effective dose in the geological repository. •ICRP AP or ROT geometry provide give adequate estimation of effective dose. -- Abstract: In deep geological repositories for high-level nuclear waste, radiation field around the disposed nuclear waste package is characterized by highly scattered radiations due to the surrounding host rock layers or cement liner. Calculation of the reference level of the occupational radiation exposure in such a facility is hence of interest, since geometrical conditions of the occupational exposure in the facility cannot be readily represented by the standard irradiation geometries considered by ICRP. In this study, a horizontal emplacement drift inside rock salt was modeled to represent a deep geological disposal facility. A nuclear waste package, simulated with a shielding cask loaded with spent nuclear fuel, was placed on the ground of the rock salt drift. A "reference worker" inside the drift was represented by the ICRP/ICRU reference adult voxel phantom. The reference level of the occupational radiation exposure was then calculated with a Monte Carlo code in terms of the effective dose based on the ICRP 2007 recommendation. In order to investigate the occupational exposure of a worker during different working scenarios in the drift, the effective dose was calculated with the voxel phantom placed at various distances and different body orientations with respect to the shielding cask. Furthermore, the effective dose obtained with voxel phantom was compared with that obtained with the fluence-to-effective-dose conversion coefficients for the standard irradiation geometries provided by ICRP. It was found out that (1) usage of the dose conversion coefficients for the isotropic (ISO) geometry, which is recommended by ICRP for highly scattered radiation fields, generally underestimates the effective dose in the rock salt emplacement drift; (2) depending on the orientation of the worker in the drift, the dose conversion coefficients for the anterior-to-posterior (AP) or the rotational (ROT) geometry should be used, in order to obtain an adequate estimation of the effective dose in the rock salt drift.