VERIFICATION OF SPENT BWR FUEL BURNUP IN A LONG-TERM WET STORAGE; COMBINING THE STATE AND THE IAEA REQUIREMENTS

igh resolution gammaspectrometric measurements (HRGS) for the verification of burnup of spent BWR-type fuel assemb- lies, have been performed at the TVO spent fuel intermediate storage (TVO KPA-STORE) in Finland. Verification is based on the measure- ment of the relative Cs-137 gamma ray activity of individual assemblies. Uses made of the mechanical hardware of the facility operator and the measure- ment instrumentation and transportable calibration source are provided by STUK. As a further method of verifica- tion, the gross weihgt of the assemblies was measured using the installed equip- ment on the fuel handling machine. The results obtained show a good linear dependence between the Cs-137 ac- tivity and the declared burnup. The data points collected fall between limits with relative standard deviation (RSD) less than 1 % on the burnup (BU) range from 15 to 41 MWd/kgU. For authentication purposes, the measurement instrumentation was indepen- dently checked by the IAEA and a referen- ce assembly was measured and separately sealed in the storage pond. The results were used for inspection purposes. The paper describes the verification method and the results obtained by the State Authority, as well as the authenti- cation procedure used by the IAEA.