VERIFYING 235U ENRICHMENT AND 232U CONTENT OF FRESH VVR-440 FUEL ASSEMBLIES

Full text: Uranuim enrichment of homogenous and profiled fuel assemblies was verified by gamma spectrometry. The fuel rods in the assemblies contained uranium enriched to 1.6 to 4.4% among which those of 1.6, 2.4 and 3.6% were of homogenous composition. In one of the types of heterogenous (profiled) assemblies the enrichment in the central region is the highest (4%) and it is lower at the peripheral region (3.6 and 3.3%). In the other type it is 4.4% in the central part and 3.6 and 4% at the periphery. Three gamma detectors were used in the study two HPGe detectors of different resolution and efficiency and a CdZnTe (CZT) detector, which fits into the central tube of the assemblies In this way possible to obtain information from both the inner and outer parts of the assemblies. It proved to be possible to distinguish different types of assemblies within a reasonable measurement time(15-20 min). For the HPGe measurements the assemblies had to be lifted out from their storage rack, while when using the CZT detector, the assemblies could be left at their storage position. A routine project is going on for enrichment verification of -10% of the freshly arrived assemblies at a random basis in the fresh fuel storage under normal operational conditions, upon inspection, having minimum impact on the NPP work, it is also tested that 232U content of the examined assemblies is much smaller than what is expected for pellets made of reprocessed material.