Individual dosimetry in disposal facilities for high-level nuclear waste

Monte-Carlo methods offer an alternative to the traditional methods to estimate doses for persons occupationally exposed to radiation. In particular, they can be applied in the pre-job planning phase to optimize work-flows In this work a methodology to assess personal working in nuclear waste disposal facilities has been developed. First, a validation of the selected tools has been performed. In the laboratory approach, it has been verified if the MCNP6 is able to reproduce experimental results and if simplifications of the model affect respective results. A neutron generator producing neutrons at 2.5 MeV is employed to simulate neutrons produced in spent nuclear fuel and neutron and gamma radiation are measured using a NE-213 detector. Layers of steel and polyethylene are positioned between the target of the neutron generator and the detector to mimic a shielding cask. Results of the measurements and corresponding simulations exhibit a good correlation confirming that MCNP6 can properly describe such scenarios. In the numerical approach, the impact of uncertainties in the cross section libraries on the simulated results has been studied, using the SAMPLER module of the SCALE package. Although results show that these perturbations impacts the obtained dose however, the magnitude is marginal and the confidence interval of calculated data to consider that the confidence interval of the results is practically not affected. Second, the influence of different parameters of a nuclear waste disposal facility on the dose received by workers has been studied by numerical means. Therefore, POLLUX ^registered casks with different spent nuclear fuel loadings are placed in different disposal galleries. The radiation field is calculated in the emplacement drifts as well as in a drift without surrounding walls to reveal the role of the backscattered radiation. Moreover, to study the effect of backscattered radiation on personal exposure, a mathematical phantom is used representing a worker inside a drift. Parametric simulations are performed where the orientations of the phantom to the cask is varied. Two conclusions are obtained. First, for the studied waste inventory neutrons dominate the radiation field. Second, the backscattered radiation plays an important role in such facilities. Therefore, it has been proposed that workers wear two dosimeters, one at the front and one at the back. The personal dose can then be derived by combining the dose rate obtained with both. Finally, a comparison of the personal dose during the emplacement of casks in a rock salt drift and in a drift with a reinforcement of concrete has been performed. Since the load capacity per cask for disposal in clay is lower than that for disposal in a rock salt, the disposal of the same amount of spent nuclear fuel leads to a higher personal dose in a clay formation compared to emplacement in rock salt due to the significantly enhanced time required for disposal.