  Pebble bed reactors (PBRs) can improve the safety and economics of the
nuclear energy production. PBRs rely on TRIstructural-ISOtropic (TRISO) fuel
pebbles for enhanced fission product retention. Accurate characterization of
individual fuel pebbles would enable the validation of computational models,
efficient use of TRISO fuel, and improve fuel accountability. We have developed
and tested a new neutron multiplicity counter (NMC) based on 192 boron coated
straw (BCS) detectors optimized for ${}^{235}$U assay in TRISO fuel. The new
design yielded a singles and doubles neutron detection efficiency of 4.71% and
0.174%, respectively, and a die-away time of 16.7 $\mathrm{\mu}$s. The NMC has
a low intrinsic gamma-ray detection efficiency of $8.71\times10^{-8}$ at an
exposure rate of 80.3 mR/h. In simulation, a high-efficiency version of the NMC
encompassing 396 straws was able to estimate the ${}^{235}$U in a pebble with a
relative uncertainty and error both below 2% in 100 s.
