EVALUATING PEAK AREA UNCERTAINTIES IN CONNECTION TO PASSIVE GAMMA MEASUREMENTS OF SPENT NUCLEAR FUEL

In many countries, spent nuclear fuel is planned to be stored in a geological repos­itory. Before the final encapsulation, safety parameters such as decay heat, criti­cality, and dose rate need to be ensured. A gamma scan of the spent nuclear fuel assemblies in the pool can extract valuable information needed to verify or update the declared values before the encapsulation. Gamma scans can be used to esti­mate values such as burnup, cooling time, or initial enrichment [1], but also decay heat [2]. This paper presents results from a full energy peak area evaluation study of experimental gamma­-ray spectra acquired from measurements using a high­purity germanium detector on 47 spent nuclear fuel assemblies from Sweden in 2016 and 2019. The assemblies chosen are UO2 fuel and represent a large span in cooling­ time, burnup, and initial enrichment [3]. The gamma spectra were acquired in the spent fuel pool of the Clab facility. As part of the measurement analysis, one wishes to determine the full­-energy net peak areas associated with selected fission prod­ucts. This work presents results obtained using different methods to evaluate the full­-energy peak areas, including the use of different background estimations. In the determination of important safety parameters using gamma spectroscopy, it is crucial to consider uncertainties originating in the peak area analysis. The un­certainty from the full­ energy peak area without the background has been evaluated and compared between the different models