Demonstration of Gamma Ray Insensitivity of Boron Coated Straw-based Neutron
Multiplicity Counter

Pebble bed reactors (PBRs) are advanced reactor designs that can potentially improve the safety, efficiency, and economics of the nuclear energy production. PBRs rely on tristructural-isotropic (TRISO) fuel for enhanced fission product retention and improved spent fuel management. Unique identification of individual fuel pebbles would allow determining the fuel transit time for validation of computational models, preventing excessive burnup accumulation or premature fuel discharge, and improving fuel accountability. One of the unique signatures for fuel identification is the 235U mass and burnup level, which can be extracted through neutron coincidence counting. However, neutron coincidence counting of partially spent fuel can be challenging due to the intense gamma-ray background emitted by fission products. Boron coated straw (BCS) detectors, featuring high neutron detection efficiency and low sensitivity to gamma rays, are suitable for this application. In this work, we have designed and tested a high-neutron-efficiency BCS-based neutron multiplicity counter (NMC, external diameter 17.5 cm, inner diameter 5 cm, length 40 cm) optimized for TRISO fuel assay. We used the NMC to measure a 8.1 Ci 192Ir gamma-ray source and obtained an intrinsic gamma-ray efficiency of 10-12 under an average gamma-ray exposure rate of 340.87 R/h. We simulated the active interrogation of used TRISO-fueled pebble with burnup ranging from 9 GWD/t to 90 GWD/t and achieved a relative uncertainty and error below 2.5% in 235U mass assay in 100 s