Determining Reactor Flux from Xenon-136 and Cesium-135 in Spent Fuel

TL;DR

This paper demonstrates how ratios of xenon and cesium isotopes in spent nuclear fuel can be used to infer reactor flux, aiding nuclear forensics and nonproliferation efforts.

Contribution

It introduces analytic expressions linking isotope ratios to reactor flux, validated against experimental data and simulations, with implications for reactor characterization.

Findings

Isotope ratios correlate with reactor flux and shutdowns.

Enrichment affects isotope ratios by up to 3% at high flux.

Ratios can distinguish reactor loading configurations.

Abstract

The ability to infer the reactor flux from spent fuel or seized fissile material would enhance the tools of nuclear forensics and nuclear nonproliferation significantly. We show that reactor flux can be inferred from the ratios of xenon-136 to xenon-134 and cesium-135 to cesium-137. If the average flux of a reactor is known, the flux inferred from measurements of spent fuel could help determine whether that spent fuel was loaded as a blanket or close to the mid-plane of the reactor. The cesium ratio also provides information on reactor shutdowns during the irradiation of fuel, which could prove valuable for identifying the reactor in question through comparisons with satellite reactor heat monitoring data. We derive analytic expressions for these correlations and compare them to experimental data and to detailed reactor burn simulations. The enrichment of the original uranium fuel affects the correlations by up to 3 percent, but only at high flux.