Neutronics analysis around the spallation target for the MYRRHA ADS design

The present study has been done in the framework of the Central Design Team european project (CDT), which has the goal to design the FAst Spectrum Transmutation Experimental Facility (FASTEF), able to demonstrate efficient transmutation of high level waste and associated ADS technology. On the FASTEF design will be based the MYRRHA facility at SCK•CEN in Mol (Belgium), which should start the construction phase in 2015. The heart of the system is a 100 MW lead-bismuth eutectic (LBE) cooled reactor, working both in critical and sub-critical modes. 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 is provided by a linear accelerator and hits a LBE spallation target located inside the reactor core.
Starting from the initial need to assess the shielding of the reactor building and to characterize the irradiation of the materials in the last part of the proton beam-line, an extensive simulation study has been done to define the radiation fields around the spallation target, with special attention to the neutron component. Using a description that includes the last part of the proton beamline and the LBE spallation target, neutron yields and spectra have been computed with both Monte Carlo codes FLUKA (version 2011.2) and MCNPX (version 2.6.0), where in the second case different fragmentation/ evaporation models have been compared. As second step the neutron fluence behavior has been estimated in the whole structure around the reactor core, including fission neutrons. In this case a full MCNPX model has been used, including the vertical part of the proton beamline, the spallation target, the reactor core and the structure around, from the coolant until the external vessel, the reactor cover and the shielding walls. With the aim to compare the results, an additional simulation has been performed with the FLUKA code, using neutron source terms evaluated in the previous MCNPX calculations on suitable surfaces close to the reactor core. The results of the neutronics analysis are presented, together with the main implications on the design solutions.