  Tristructural-isotropic (TRISO) fuel is currently one of the most mature fuel
types for candidate advanced reactor types, namely pebble bed reactors (PBRs).
In PBRs, TRISO-fueled pebbles can be re-introduced into the core several times
before reaching their target burnup. Non-destructive techniques capable of
assaying ${}^{235}$U mass in the pebble are therefore needed for nuclear
material control and accountability during fuel recirculation. In this work, we
have developed a new boron-coated straw (BCS) based neutron multiplicity
counter (NMC) to estimate ${}^{235}$U mass in each pebble. BCS detectors are
chosen for their inherent high insensitivity to gamma rays that will enable
their use to assay also irradiated pebbles and high neutron detection
efficiency, comparable to ${}^{3}$He detectors. The BCS-based NMC that we have
designed was built by Proportional Technologies, Inc. (PTI) Houston, TX. In
this work, we report the system-level simulation of the BCS-based NMC and the
straw-level charge collection simulation coupled with a custom software to
tally the detected pulse integral from the list mode energy deposited. We have
developed a high-fidelity model of the NMC to simulate the response of a single
straw detector to a ${}^{252}$Cf source. The simulated die-away time, single
neutron count rate, and double neutron count rate agree well with measured
values, with a relative difference within $\pm$0.4\%. The simulated charge
spectrum agrees well with the measured one in the case of a round straw. We
plan to use the NMC to perform active and passive interrogation of fresh and
spent fuel pebbles.
