SPENT-FUEL ASSAY PERFORMANCE AND MONTE-CARLO ANALYSIS OF THE RENSSELAER SLOWING-DOWN-TIME SPECTROMETER

The slowing-down-time method for the nondestructive assay of light wat er reactor (LWR) spent fuel is under development at Rensselaer Polytec hnic Institute. A series of assay measurements of an LWR fuel assembly replica were carried out at the Rensselaer lead slowing-down-time spe ctrometer facility by using U-238 and Th-232 threshold fission detecto rs and U-235 and Pu-239 probe chambers. An assay model relating the as say signal and the signals of the probe chambers to the unknown masses of the fissile isotopes in the fuel assembly was developed. The probe chamber data were used to provide individual fission counting spectra of U-235 and Pu-239 inside the fuel assembly and to simulate spent-fu el assay signals. The fissile isotopic contents of the fuel were deter mined to better than 1%. Monte Carlo analyses were performed to simula te the experimental measurements, determine certain parameters of the assay system, and investigate the effect of the fuel-assembly and hydr ogen impurities on the performance of the system. The broadened resolu tion of the system caused by the presence of the fuel was still found to be sufficient for the accurate and separate assay of the uranium an d plutonium fissiles in spent fuel.