A Gamma-Compensated Helium-4 Ion Chamber for Spent Nuclear Fuel Monitoring.

Abstract

Management of the spent nuclear fuel (SNF) is an intriguing problem from a safeguards instrumentation perspective: for safety reasons, it must be packed with a large amount of shielding to prevent radiation exposure to facility workers and the public, and with neutron-absorbing materials to prevent criticality. SNF is typically also at high temperature in dry storage and during transportation, precluding the use of many types of radiation sensors internal to a fuel cask. The radiation environment of SNF casks includes intense gamma radiation originating mostly from fission fragments, and a smaller but still sizable amount of fast neutron radiation, both from spontaneous fission of transuranic isotopes and (alpha,n) reactions with oxygen in the fuel matrix. Fast neutrons emitted by SNF are of particular interest for safeguards, as plutonium is the most significant contributor to the neutron source term. A gamma-“compensated” 4 He ion chamber would in some ways be an ideal instrument for use in SNF safeguards: 4 He has a high cross-section for fast neutron elastic scattering, it does not contain any materials that are consumed or transmuted, and it is a simple gas-based detector that is resilient to high temperature and radiation damage. The detector must be primarily sensitive to fast neutrons, as the thermal neutron population in a cask is depressed by neutron-absorbing materials. The gamma-ray contribution to the chamber current can be compensated by sheathing two identical tubes in different thicknesses of tungsten, a strong attenuator of gamma rays. In this way, the fractional difference in current between the two tubes is directly proportional to the fractional contribution of gamma rays to the signal in the thinner tube, which can then be subtracted to yield the fast neutron-only signal. In this work, Monte Carlo simulations are used to predict the performance of a 4 He-based detector system deployed at a realistic SNF cask. The sensitivity to a fuel diversion scenario is evaluated, and the gamma-compensation relationship is validated across the lifetime of a typical SNF cask.