Aspects of the core shielding assessment for the design of FASTEF-MYRRHA

To support the construction of the MYRRHA accelerator driven system at SCKCEN in Mol (Belgium), in the years 2009-2012 the FP7 European project Central Design Team (CDT) worked at the design of the Fast Spectrum Transmutation Experimental Facility (FASTEF), to demonstrate efficient transmutation of high level waste and associated technology. The heart of the system is a lead-bismuth eutectic (LBE) cooled reactor, working both in critical and subcritical mode. The neutrons needed to sustain fission in the sub-critical mode are produced via spallation processes by a 600 MeV,  4 mA proton beam, which hits a LBE spallation target located inside the reactor core. Between the many challenges of the design, radiation shielding and minimization of induced activation are key points. To assess the shielding of the reactor core, both critical and sub-critical operation modes have been studied. Since in FASTEF the reactor is foreseen to operate at 100 MW core power in the critical mode and at 94 MW in the subcritical one, the critical mode exhibits the highest lateral neutron fluence at the fuel level, and can be reasonably considered the conservative case for the lateral radiation containment. At the contrary, because of the backscattered radiation from the spallation target and due to the presence of the beam pipe channel, the subcritical operation drives the vertical design. Starting from the MCNPX Monte Carlo models of the core defined in CDT in both the operation modes, neutron spectra have been fully characterized on suitable surfaces and used as input of a second row of FLUKA simulations, where complex source terms have been used. FLUKA has the unique possibility to compute, in the same simulation, the transport of the radiation due to the system in operation and the coupled residual fields, due to the activated materials. The FLUKA/MCNPX comparison of the neutron fluence rates inside the external vessel at different radial and vertical distances from the core barrel shows a very good agreement - at the percent level - and has been used to validate the FLUKA analysis. Dose distributions have been then evaluated from the core barrel to the external containment and the shielding walls in the horizontal direction, up to the last magnet of the proton beam-line and the final roof in the vertical one. Moreover, the activation of key materials has been characterized for typical irradiation patterns. This simulation addressed the optimization of key elements of the design, from the cover plate to the local shielding structure above the last magnet.