The possibility to use a fork detector for partial defect verification of spent LWR fuelassemblies has been investigated in Task JNT A 1071 “Partial Defect Test on Spent FuelLWRs”. The task was arranged as a joint task between the Finnish, Swedish and Belgiansupport programmes to IAEA safeguards.
This task studied the prospects of both a conventional fork detector and an enhancementwhere the gross gamma and neutron signals of a conventional fork are combined withsimultaneous gamma spectrometry using a CdZnTe detector.
The fork method was investigated by measuring BWR and VVER-440 spent fuel assem-blies and a fresh MOX mock-up assembly. Correction methods were developed to improvethe analysis of measurement results. Also model calculations were performed to clarify theeffect of the geometrical configuration of the defect.
The investigations have shown that a general partial defect test based on the fork methodis not possible without making use of operator’s declared data. There exist configurationseven with 50% of pins removed, which cannot be detected, either with the conventionalfork or with the enhanced fork detector. Using the operator declared data cannot beavoided due to the influence of both the fuel design and the irradiation history to themeasured signals. If operator’s data are available and considered reliable, the detectionlimit of a partial defect is at about 20% of pins missing for BWR assemblies with theburnup 18 MWd/kg or higher. For developing a reliable, operator data independent partialdefect verification device a totally different approach must be applied.
2002. , 27 p.