A MONTE CARLO SIMULATIONS APPROACH FOR IRIS INTERNAL SHIELDS OPTIMIZATION

IRIS (International Reactor Innovative and Secure) is a medium-power (~1,000 MWt) advanced light water reactor that is being developed by an international consortium led by Westinghouse. IRIS features an integral primary system configuration to enhance its safety performance. An annular region surrounding the core accommodates steam generators in its upper portion (above the core) and forms a thick downcomer (~1.7 m) next to the core. Compared to loop PWRs where the downcomer is only ~20 cm thick, IRIS configuration provides a neutron fluence reduction at the pressure vessel by several orders of magnitude. Additional internal shields consisting of steel plates may be placed in the downcomer region and in the lower plenum to provide further shielding and dose reduction at the pressure vessel outside surface. Transport theory Monte Carlo numerical simulations were performed to evaluate several alternatives of the internal shield design. The fast neutron fluence at the pressure vessel is sufficiently low that the pressure vessel surveillance program will not be required. The neutron and gamma dose (while the reactor is operating) are cut down to levels that may allow elimination of the external biological shield, whereas the reduced vessel activation lowers the cost and minimizes the personnel dose during the maintenance and D&D activities. This paper presents results of the analyses performed so far and describes studies currently under way.