Non-Destructive Determination of the Nuclear Material Content of Spent Fuel Pieces in Canisters

Passive high resolution gamma spectrometry was applied to determine the mass of the nuclear material in the inhomogeneous mixture of broken spent fuel pieces repacked into 72 canisters, following an incident in which 30 VVER-440 spent fuel assemblies became damaged and unusable. The HPGe detector was placed behind a collimator built into the concrete wall of the service pit of the reactor block. Calibration measurements were performed by intact spent fuel assemblies, whose burn-up was calculated and declared by the operator of the power plant. Canisters and reference assemblies were scanned longitudinally down and up, along their 3 and 6 sides, respectively. Spectrum acquisition was restarted each 100 s. More than 15 thousand spectra were acquired and evaluated. Special software was developed for extracting the relevant information from the spectra. The mass of nuclear material in the whole canister was then determined by integrating along its length. As a result, a new nuclear inventory was prepared for the U, U-235, and total Pu content of the individual canisters as separate batches.