Activation calculations for the MYRRHA accelerator-driven system design

The MYRRHA facility at SCK-CEN in Mol (Belgium), which is at present in an advanced phase of the design, aims to demonstrate efficient transmutation of high level waste and associated Accelerator-Driven System (ADS) technology. The system is based on a lead-bismuth eutectic (LBE) cooled reactor, working both in critical and in sub-critical operation modes. 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. The use of a high energy/high current proton beam, coupled with a nuclear reactor operating in subcritical mode, presents many challenges for various aspects of the design, being minimization of the induced activation a key point. In order to assess the main activation and shielding problems, a method based on the combined use of the two Monte Carlo codes MCNPX and FLUKA has been developed, with the goal to perform detailed analyses of both the radiation fields due to the system in operation, and the coupled residual radiation due to the activated materials. Activation has been then evaluated for typical irradiation patterns and key structural materials, from the spallation target to the structure above the core, the reactor cover and critical points along the proton beamline.
The results of this simulation work are presented, with some implications on the design solutions.