Experimental verification of simulated predictions from the DDSI instrument

The Differential Die-away Self Interrogation (DDSI) instrument was researched for many years under the Next Generation Safeguards Initiative Spent Fuel effort. Later a prototype instrument was manufactured and used to make non-destructive measurements of spent nuclear fuel in the Swedish Central Interim Storage Facility for Spent Nuclear Fuel (Clab) in Sweden in 2018. Results of DDSI research, based on either simulations or measurement time, have indicated that the instrument could successfully be used to draw safeguards-relevant conclusions about spent nuclear fuel.In this work we investigate how well the modelled response of the DDSI instrument, based on Serpent and MCNP simulations, corresponds to measured data of 17x17 pressurised reactor fuel. We also studied repeatability, i.e. to what extent repeated measurements on the same fuel assembly gave consistent results. We also investigated the dependence of tau on the selected time window. The results show that tau values determined from measurement data are consistently higher than tau values determined from simulations, and that the magnitude of tau is dependent on the choice of time window. We also note that tau is relatively insensitive to positioning in the DDSI instrument.