Monte Carlo simulations for shielding and activation of the 17 MeV MYRRHA injector beam line

The objective of the MYRRHA project is to couple a sub-critical reactor with a 600 MeV, 4 mA proton LINAC, as a very first prototype of Accelerator Driven System (ADS). In this configuration, the neutron population is sustained by the LINAC, which accelerates and focuses a proton beam to a spallation target inside the reactor. The construction road map is divided into three phases. In the first phase, MINERVA, a research facility that will also function as the first section of the LINAC (up to 100 MeV), will be designed and constructed. In the second phase, the accelerator construction will be completed (600 MeV). In the third phase, the reactor will be built. According to current estimates, the construction will be completed in 2036, with MINERVA already available in 2026. For the complete design of MINERVA, many aspects still need to be considered and, most importantly, its safe operation must be fully demonstrated. This thesis focuses on the shielding and activation of the first component of MINERVA, the injector, in which protons are accelerated from 30 keV (at which they are generated in the ion source) to 17 MeV, and its beam dump, which will absorb a maximum of 68 kW of proton beam energy (17 MeV, 4 mA). This last component will be used during the injector commissioning and maintenance tests. The objective of this thesis is to demonstrate that the shielding provisions suggested so far at SCK CEN, and the ones that are suggested in this work, are enough to guarantee that the safety standards for people and environment are respected. Two types of radiation sources are responsible for radiation along the beam line, and around the beam dump. The large amount of prompt doses and radioactivity in the beam dump and in its shield are produced due to interactions of the primary protons with the beam dump materials. As far as the beam line is concerned, continuous proton beam losses (1 W/m) along the beam line are assumed. Radiation will be created due to the interaction of the lost protons with the beam line components and the surrounding materials. The shielding design of the injector and of the beam dump is done by means of the Monte Carlo transport code MCNP6.2, developed at the Los Alamos National Laboratory. Activation calculations are performed with the SCK CEN depletion code ALEPH2. In order to speed up the calculations and to obtain better statistics, a massive use of variance reduction techniques is made. In particular, mesh-based weight windows and DXTRAN spheres are studied and applied. The results obtained with these tools show that it is possible to satisfy the safety requirements concerning the dose rate limits imposed by the Belgian legislation and by SCK CEN, by implementing simple shielding designs, based on concrete and other shielding materials. In particular, dose rates inside and outside the injector tunnel are calculated. Thanks to these results, it is possible to demonstrate that the dose rate outside the injector tunnel is below the dose rate limit for workers. Moreover, the dose absorbed by some internal devices, such as the lighting fixtures inside the tunnel, are computed, in order to suggest a maintenance and replacement strategy. An entrance is foreseen to access the injector tunnel, in case of maintenance. During normal operation, a shielded door must be used in order to shield workers outside of the tunnel. In this work, a simple three-layers layout is proposed for the shielded door, featuring the use of lead and borated polyethylene, which have been proven to be effective in photon and neutron shielding, respectively. For the beam dump, a shielding design was already proposed at SCK CEN. Its effectiveness is demonstrated by this work, and the activation of the concrete is studied.