Spent VVER fuel characterisation combining a fork detector with gamma spectrometry: Interim report on Task JNT A 1071 FIN of the Finnish Support Programme to IAEA Safeguards

According to the IAEA's criteria a partial defect verification of spent fuel assemblies has to be performed before they become difficult to access. A partial defect test for spent fuel should be able to detect if half or more of the fuel pins have been removed from an assembly or possibly replaced by dummies. Therefore a partial defect test procedure needs to be developed by evolving the measurement systems and the analysis methods of the measurement data. 18 VVER assemblies were measured with an enhanced fork detector at the Loviisa KPA Store in May 2000. This measurement campaign is a follow-on to the campaigns conducted in 1999 at the Olkiluoto KPA Store, where BWR assemblies were measured using the same instrument. The validity of correction methods developed in data analysis of Olkiluoto measurements was investigated in the analysis of Loviisa measurements. The share of 244Cm neutron source out of the total neutron counts is derived from the results calculated with the PYVO code. The enrichment correction method to the neutron data corresponding to that used for BWR assemblies was applied for VVER assemblies. The contribution of other gamma emitting nuclides than 137Cs was eliminated from the gross gamma signal with the help of gamma spectroscopy using the method developed for the BWR data. All these corrections were found to improve the essential correlations. An assembly may have off-reactor cycles between irradiation cycles. The measured 137Cs gamma signal can be corrected for off cycles using the recipes introduced in this report. Also the effect of off cycles to the neutron signal is contemplated. The correction for off cycles may be very important for a correct burnup verification of those assemblies, which have not been irradiated in sequential cycles.