Sensitivity study to optimize the thickness of a moderator for use in the AP1000 source, intermediate, and power range ex-core detectors

The purpose of this paper is to discuss the coupled deterministic- stochastic analysis techniques and codes used to optimize the thickness of a polyethylene moderator for use in the AP1000 ex-core source, intermediate, and power range detectors. The analysis incorporated a 'bootstrapping' approach whereby an outward directed cylindrical boundary source was generated by DORT, source particle cumulative density functions (CDFs) in space, direction, and energy were formulated, these CDFs were processed by another code to create discrete source particles, and these discrete source particles were used by MCNP to calculate response functions which were meant to approximate detector signal responses. Furthermore, a simplistic stochastic study was performed to determine the maximum moderator thickness that should be considered for this application. For the AP1000, the optimum moderator thickness was the maximum allowed due to the mechanical design (1.38 cm) whereas for a general case the optimum moderator thickness is approximately 7.5 cm. (authors)