Will $^{37}$Ar emissions from light water power reactors become an obstacle to its use for nuclear explosion monitoring?

TL;DR

This paper assesses whether emissions of $^{37}$Ar from light water reactors could interfere with its use as an indicator for underground nuclear explosions, finding that routine emissions are generally below background levels, with exceptions during specific events.

Contribution

It identifies and quantifies various production pathways of $^{37}$Ar from reactors and evaluates their impact on nuclear explosion monitoring.

Findings

Routine reactor emissions are below cosmogenic background levels.

Inadvertent reactor restart can cause high $^{37}$Ar emissions.

High $^{41}$Ar emissions can help discriminate reactor signals from nuclear explosions.

Abstract

$^{37}$Ar is a promising candidate for complementing radioxenon isotopes as indicators of underground nuclear explosions. This study evaluates its potential anthropogenic background caused by emissions from commercial pressurised water reactors. Various $^{37}$Ar production pathways, which result from activation of $^{36}$Ar and of $^{40}$Ca, respectively, are identified and their emissions quantified. In-core processes include (1) the restart of operation and degassing of the primary cooling water after maintenance and refueling shutdown, (2) the replacement of primary coolant water for limiting its tritium concentrations, and (3) the leakage of $^{37}$Ar produced from calcium impurities in UO$_{2}$ after fuel rod failures. Activation of air and of calcium in concrete within the biological shield are major out-of-core production pathways. Whereas emissions from in-core processes are transient, a rather constant $^{37}$Ar source term results from its out-of-core production. Generic atmospheric dispersion simulations indicate that already at moderate distances from the emitter, concentrations of $^{37}$Ar caused by routine reactor operations are far below its cosmogenic background in air. The only exception results from an inadvertent reactor re-start without operation of the primary cooling water degassing system for prolonged time. Such an event also causes high emissions of $^{41}$Ar which can be used for discriminating its $^{37}$Ar signal from an underground nuclear explosion.