Development of methodologies for ex-core detector modeling for SMRs

This paper presents accurate and efficient methodologies to model the ex-core detection system for SMRs. This detection system is intended to be used for monitoring the reactor during startup, operation and shutdown. Additionally, the detection system will be useful during abnormal conditions. Two approaches have been employed to model the ex-core detection system, including i) 'forward' statistical Monte Carlo and ii) 'adjoint' deterministic discrete ordinates (Sn). For the Monte Carlo approach, the MCNPX code system with an implementation of the CADIS (Consistent Adjoint Driven Importance Sampling) variance reduction technique is utilized. For the discrete ordinates, the TITAN 3-D parallel hybrid transport code system is utilized. Using MCNPX with CADIS, the effect of a moderation sleeve on the response of a BF-3 ion chamber has been examined, and detector placement for achieving maximum response has been studied.