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

The LWR spent fuel assemblies have to be verified at the partial defect level before they become difficult to access. According to the IAEA's criteria the 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 and possibly replaced by dummies. Euratom applies similar criteria. Therefore a standard verification procedure needs to be developed using an appropriate combination of measurements and theoretical calculations. Two experiments with an "upgraded" fork detector were performed at the TVO KPA Store in September and in December 1999. On the whole, 26 assemblies were measured. In the "upgraded" fork detector the total neutron count and the gross gamma measurements are complemented with gamma spectroscopic measurement using an integrated measurement head. A cadmium-zinc-telluride (CZT) detector is placed on the same vertical level as the fission and ionisation chambers. This enables simultaneous gamma and neutron measurements at one location. In the upgraded fork model the fork prongs can also be removed and gamma spectrometric measurements can be done using only the CZT detector. This allows more versatile placement of the target fuel assembly allowing various kind of gamma spectroscopic scanning measurements. In this report a gamma spectroscopy based correction to the gross gamma data is introduced. This corrected gross gamma signal seems to describe more consistently the burnup of the assembly than the 137Cs intensity obtained by direct gamma spectrometry. Concerning the measured neutron data of assemblies with different enrichments, an enrichment correction method based on calculations made with the ORIGEN-S program is introduced in this report. In addition, the share of 244Cm neutrons of the total neutron source is derived from the results calculated with the PYVO program. These corrections to the neutron signal seem to improve the correlation of the neutron signal to the burnup and to the gross gamma signal. The PYVO program can be considered as an essential tool in the analysis. With help of the PYVO the 244Cm share of total neutron counts, the 244Cm neutron source term and the 137Cs activity of measured assemblies can be calculated. In addition, the axial activity profiles of one assembly are compared with the calculations made by ORIGEN-S. These comparisons show a remarkable agreement between the measured and calculated results.