Development of Differential Die-Away Instrument for Characterization of Swedish Spent Nuclear Fuel

Abstract: The Next Generation Safeguards Initiative Spent Fuel (NGSI-SF) project was established in 2009 by the U.S. Departmentof Energy with main objective to investigate, and potentially develop and test new technologies for spent nuclearfuel (SNF) characterization. In Sweden the SNF is currently being considered to be verified and encapsulated in canistersand deposited into a geological repository. The need for an independent instrument for SNF verification by theSwedish operator turned into the collaborative effort with NGSI-SF to develop an instrument for future deployment inSweden.One of the techniques investigated within this project is the differential die-away (DDA) technique, which followingthe theoretical investigation by means of high fidelity Monte Carlo simulations indicated the potential to be applied fordetermining of various spent fuel assembly (SFA) parameters.This work introduces the first deployable DDA instrument which was designed to be used for characterizing ofSwedish SFAs currently stored in the Central Interim Storage Facility for Spent Nuclear Fuel (Clab). All the instrumentcomponents relevant for DDA design functionality were evaluated to ensure reliable operation in Clab. Although mostof the components were tuned with special consideration given to concerns from the operator (The Spent Nuclear FuelandWaste Management Company) , several post-simulation modification of the design were made. These modificationsare described in this work.A complementary study of the detector responses to asymmetrically burned SFAs indicated a different detector responses,depending on which of the four different orientations was used to assay individual SFAs. This study illustratedthe sensitivity of detectors with respect to the SFA orientation if there is a strong burn-up gradient across the SFA andhence a strong asymmetry in isotopic distribution in the SFA. In addition, the study of asymmetry provided the informationon different operational scenarios of the DDA instrument. The DDA instrument may provide general informationabout the complete SFA as well as give local information about certain parts of the SFA.