Investigating the Cherenkov light production due to cross-talk in closely stored nuclear fuel assemblies in wet storage

Volume: 57, December 2018, pages 66-74

Authors: Erik Branger, Sophie Grape, Peter Jansson, Erik Andersson Sundén, Staffan Jacobsson Svärd

_{Uppsala University}

Abstract: The Digital Cherenkov Viewing Device (DCVD) is one of the tools available to a safeguards inspector performing verifications of irradiated nuclear fuel assemblies in wet storage. One of the main advantages of safeguards verification using Cherenkov light is that it can be performed without moving the fuel assemblies to an isolated measurement position, allowing for quick measurements. One disadvantage of this procedure is that irradiated nuclear fuel assemblies are often stored close to each other, and consequently gamma radiation from one assembly can enter a neighbouring assembly, and produce Cherenkov light in the neighbour. As a result, the measured Cherenkov light intensity of one assembly will include contributions from its neighbours, which may affect the safeguards conclusions drawn.

In this paper, this so-called near-neighbour effect, is investigated and quantified through simulation. The simulations show that for two fuel assemblies with similar properties stored closely, the near-neighbour effect can cause a Cherenkov light intensity increase of up to 3% in a measurement. For one fuel assembly surrounded by identical neighbour assemblies, a total of up to 14% of the measured intensity may emanate from the neighbours. The relative contribution from the near-neighbour effect also depends on the fuel properties; for a long-cooled, low-burnup assembly, with low gamma and Cherenkov light emission, surrounded by short-cooled, high-burnup assemblies with high emission, the measured Cherenkov light intensity may be dominated by the contributions from its neighbours.

When the DCVD is used for partial-defect verification, a 50% defect must be confidently detected. Previous studies have shown that a 50% defect will reduce the measured Cherenkov light intensity by 30% or more, and thus a threshold has been defined, where a ≥30% decrease in Cherenkov light indicates a partial defect. However, this work shows that the near-neighbour effect may also influence the measured intensity, calling either for a lowering of this threshold or for the intensity contributions from neighbouring assemblies to be corrected for. In this work, a method is proposed for assessing the nearneighbour effect based on declared fuel parameters, enabling the latter type of corrections.

Keywords: DCVD; partial defect verification; Cherenkov light; Geant4; Cross-talk

Reference guideline:

Branger, E., Grape, S., Jansson, P., Andersson Sundén, E., & Jacobsson Svärd, S. (2018). Investigating the Cherenkov light production due to cross-talk in closely stored nuclear fuel assemblies in wet storage. ESARDA Bulletin - The International Journal of Nuclear Safeguards and Non-proliferation, 57, 66-74. https://doi.org/10.3011/ ESARDA.IJNSNP.2018.12