Gamma-ray spectrometry analysis of pebble bed reactor fuel using Monte Carlo simulations

Monte Carlo simulations were used to study the gamma-ray spectra of pebble bed reactor fuel at various levels of burnup. A fuel depletion calculation was performed using the ORIGEN2.1 code, which yielded the gamma-ray source term that was introduced into the input of an MCNP4C simulation. The simulation assumed the use of a 100% efficient high-purity coaxial germanium (HPGe) detector, a pebble placed at a distance of 100 cm from the detector, and accounted for Gaussian broadening of the gamma-ray peaks. Previously, it was shown that Cs-137, Co-60 (introduced as a dopant), and Cs-134 are the relevant burnup indicators. The results show that the 662 keV line of Cs-137 lies in close proximity to the intense 658 keV of Nb-197, which results in spectral interference between the lines. However, the 1333 keV line of Co-60, and selected Cs-134 lines (e.g., at 605 keV) are free from spectral interference, which enhances the possibility of their utilization as relative burnup indicators. (C) 2003 Elsevier Science B.V. All rights reserved.