Boron Coated Straw-based Neutron Multiplicity Counter for Neutron Interrogation of TRISO Fueled Pebbles

Ming Fang - University of Illinois at Urbana-Champaign
Angela Di Fulvio - University of Illinois at Urbana-Champaign
Pebble bed reactors (PBRs) are advanced reactor designs that can potentially improve the safety, efficiency, and economics of 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 U-235 mass and burnup level, which can be extracted through neutron coincidence counting. Boron coated straw (BCS) detectors featuring high neutron detection efficiency and low sensitivity to gamma rays are a viable alternative to He-3 proportional counters. In this work, we have designed and tested a new BCS-based neutron multiplicity counter (NMC, external diameter 17.5 cm, inner diameter 5 cm, length 40 cm) optimized for TRISO fuel assay. Compared to existing BCS-based NMCs, we replaced round straws with newly developed pie-shape straws that feature a higher surface-area-to-volume ratio. The new design yielded a high singles and doubles detection efficiency (4.87% for singles, 0.14% for doubles) and a short die-away time of 16.8 ?s when measuring a Cf-252 source. We will test the device when interrogating a small TRISO fuel compact and uranium samples with a U-235 mass comparable to fresh TRISO fuels.