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

## TL;DR

This paper introduces a boron coated straw-based neutron multiplicity counter designed for accurate neutron assay of TRISO fuel pebbles, enhancing nuclear fuel characterization and safety in pebble bed reactors.

## Contribution

The paper presents a novel neutron counter with optimized boron coated straw detectors for improved efficiency and accuracy in measuring uranium-235 in TRISO fuel pebbles.

## Key findings

- Achieved 4.71% singles detection efficiency.
- Achieved 0.174% doubles detection efficiency.
- Estimated uranium-235 content with less than 2% error in 100 seconds.

## Abstract

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.