Estimating gamma and neutron radiation fluxes around BWR quivers for nuclear safeguards verification purposes

Abstract

Non-destructive assay (NDA) methods are at the core of nuclear safeguards verification of spent nuclear fuel (SNF). In Sweden, the spent nuclear fuel from all the reactor sites is moved to the Swedish central interim storage facility for spent nuclear fuel (for which the Swedish acronym is Clab). A new facility, Clink, is planned at the site where the SNF will undergo a safeguards verification prior to encapsulation for long-term storage. The fuel to be encapsulated includes both regular fuel assemblies as well as "non-regular" fuel assemblies including fuel objects called quivers, which are specially designed containers to house damaged or failed and leaking spent fuel rods in a way to isolate the rods from the environment and prevent contamination. The quiver concept was recently introduced in the Swedish nuclear market by Westinghouse Electric Sweden AB and it has led to some unique challenges from a safeguards verification standpoint which stem from their construction. Their overall stainless steel build, while providing robustness to the structure, also greatly diminishes the possibility of detecting gamma or neutron radiation using traditional safeguards measurement devices. The current investigation looks into the practicalities of safeguards verification of boiling water reactor (BWR) quiver objects in the spent fuel pool from above, and also assesses the possibility of their verification from the side using the widely used Fork detector. The Fork instrument has been routinely employed by both operators and inspectors around the world to verify spent fuel for routine safeguards inspections. In the present work, we model the BWR quiver and the Fork instrument in the Monte Carlo particle transport code, Serpent2 to estimate the radiation flux around the quiver objects. We have shown that the gamma and neutron radiation from the BWR quiver were heavily attenuated by the stainless steel lid and could not be relied on to make a safeguards verification from above. Furthermore, it was established that while gamma radiation from the quiver remains measurable on the sides of the quiver by the Fork instrument, the neutron counts were low compared to a typical BWR fuel assembly of similar fuel content albeit within the limits of detectability of the Fork.