Verification of spent BWR fuel burnup in a long-term wet storage

High resolution gammaspectrometric measurements (HRGS) for the verification of burnup of spent BWR-type fuel assemblies, have been performed at the TVO spent fuel intermediate storage (TVO KPA-STORE) in Finland. Verification is based on the measurement of the relative Cs-137 gamma ray activity of individual assemblies. Uses made of the mechanical hardware of the facility operator and the measurement instrumentation and transportable calibration source are provided by STUK. As a further method of verification, the gross weight of the assemblies was measured using the installed equipment on the fuel handling machine. The paper describes the verification method and the results obtained by the State Authority, as well as the authentication procedure used by the IAEA