Shielding and activation calculations for the MYRRHA ADS design in the subcritical operation mode

Accelerator-driven systems (ADS) are one of the options studied for the transmutation of nuclear waste in the European Community. The design of sub-critical ADS requires high energy and high power proton accelerators, of the order of hundreds MeV and some MW for the proposed demonstration experiments. The use of high energy Mega-Watt proton beams, in combination with a nuclear reactor core operating in sub-critical or critical mode, presents many challenges for various aspects of the design, being radiation shielding and minimization of the induced activation key points.
The present study has been done in the framework of the Central Design Team european project (CDT), which worked with the goal to design the FAst Spectrum Transmutation Experimental Facility (FASTEF) to demonstrate efficient transmutation of high level waste and associated ADS technology. On this 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.
With the goal to assess the shielding of the reactor building and to study the activation of the materials in key points around the reactor and in the vertical part of the proton beam-line, an extensive simulation study has been done. Both the Monte Carlo codes MCNPX (version 2.6.0) and FLUKA (version 2011.2) have been used, also with the aim to do a code-to-code comparison and to cross check the results. Starting from the MCNPX model of the reactor core in the sub-critical operation mode, which includes the last part of the vertical proton beamline with the spallation target and the LBE coolant material around the core, until the external vessel, the radiation fields have been fully characterized on suitable surfaces around the core and used as input in a second row of simulations. These calculations have been done with the FLUKA code, which has the unique possibility to compute, in the same simulation, the transport of both the prompt radiation (due to the ADS in operation) and the residual one (due to the activated materials). The neutron fluence behaviour, together with the dose distributions due to the prompt and to the residual radiation, has been then studied. Dose profiles have been evaluated from the core vessel 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.
The results of the shielding and activation analysis are presented and discussed, together with the main implications on the design solutions.