Radiation environment in the I2S-LWR concept: Part I – Radial distribution and its impact on vessel fluence, neutron detectors placement, and radial reflector gamma heating

Abstract

The neutron and gamma environment surrounding the Integral Inherently-Safe Light Water Reactor (I2S-LWR) core was characterized to ensure safe operation over the 100-year lifetime of the plant with respect to important design parameters. To demonstrate that all requirements were met, a detailed shielding model was developed in the Scale6.1 code package and used with the MAVRIC shielding sequence. The neutron fluence to the reactor vessel at the end of the plant lifetime was shown to be almost two orders of magnitude below levels that would lead to neutron embrittlement (due to a wide downcomer region despite the increased plant lifetime and higher power density). Additionally, the feasibility of placing radiation-hardened high-temperature silicon carbide semiconductor detectors within the pressure vessel was demonstrated showing that flux levels could be monitored covering all ranges from startup (“source range”) through full-power operations. Furthermore, the gamma heating distribution within the radial neutron reflector directly outside of the core was determined and used to guide the design of cooling channels in order to remove the 6.6 MW of energy deposited in the reflector.