Development of underwater gamma ray measurement system for spent fuel verification

A gamma ray measurement system (GAMESY) was designed and developed to measure the gamma rays of spent nuclear fuel nondestructively in pool, and the performance test on the system was carried out in the spent fuel storage pool of KAERI's post irradiation examination facility. As the results of the mechanical performance test, the GAMESY showed a good position reproducibility and precision within ±1 mm along full scanning length and a perfect watertightness of the detection chamber immersed in 9m deep water. The preliminary experiment for energy resolution and radiation shielding in detection part were performed both in air by using standard gamma ray source and in pool water by using PWR rods with the average burnup of 36GWD/tU. The results showed that the 10 extension cable between detector preamplifier and amplifier did not cause any deterioration of signal in its energy resolution, and the detector shielding thickness of about 15cm Pb equivalent seemed to be sufficient for minimizing the unwanted gamma rays coming from the off slit region. From these results, it is believed that the newly developed prototype GAMESY can be used for the verification and accounting for spent nuclear fuel material for the purpose of nuclear material safeguards by determining several characteristics such as burnup profile and cooling time that are obtainable from the measured gamma ray spectra