Feasibility of Neutron Coincidence Counting for Spent Fuel

TL;DR

This paper demonstrates a neutron coincidence counting method using boron coated straw detectors to accurately estimate uranium-235 in used TRISO fuel with high efficiency and low uncertainty, aiding fuel monitoring.

Contribution

A new high-efficiency neutron multiplicity counter based on boron coated straw detectors for precise 235U measurement in TRISO fuel is introduced.

Findings

Achieved <2.5% uncertainty in 100 s for 235U assay.

Demonstrated low gamma-ray sensitivity and high neutron detection efficiency.

Validated measurement approach with neutronics and fuel depletion calculations.

Abstract

High-temperature gas reactors rely on TRIstructural-ISOtropic (TRISO) fuel for enhanced fission product retention. Accurate fuel characterization would improve monitoring of efficient fuel usage and accountability. We developed a new neutron multiplicity counter (NMC) based on boron coated straw (BCS) detectors and used it in coincidence mode for 235U assay in TRISO fuel. In this work, we demonstrate that a high-efficiency version of the NMC encompassing 396 straws is able to estimate the 235U in used TRISO-fueled pebbles or compacts with a relative uncertainty below 2.5% in 100 s. We performed neutronics and fuel depletion calculation of the HTR-10 pebble bed reactor to estimate the neutron and gamma-ray source strengths of used TRISO-fueled pebbles with burnup between 9 and 90 GWd/t. Then, we measured a gamma-ray intrinsic efficiency of 10^-12 at an exposure rate of 340.87 R/h. The low gamma-ray sensitivity and high neutron detection efficiency enable the inspection of used fuel.