Gamma-ray spectrometry using collimated detectors is a well-established method to acquire information about the state of irradiated nuclear fuel. However, the feasibility of examining a particular nuclide of interest is subject to constraints; the peak must be statistically determinable with the desired precision, which is governed by the peak count rate, and the continuum background in the region of interest. Furthermore, the total spectrum count rate in the detector will pose constraints, by potentially causing pile-up or dead time issues, that may paralyze the detector. Therefore, the prediction capability of these spectral parameters is needed.
Methods were assembled for spectrum prediction with the intended usage in the optimization of gamma emission tomography, and to enable a priori feasibility evaluation of determination of the intensity of a peak in an energy spectrum of gamma-rays emitted from an irradiated nuclear fuel rod. The focus was on finding reliable results regarding total spectrum and peak count rates with faster computation time, as compared to exclusively using Monte Carlo. For this purpose, the method is based on depletion calculations with SERPENT2, a point-source kernel method for the collimator response, and a detector response matrix pre-computed with SERPENT2. The computational methodology uses as input the fuel properties (dimensions, materials and power history and cooling time), and the instrumental setup (collimator and detector dimensions and materials).
The prediction method was validated using measured data from a high-burnup, short-cooled test fuel rodlet from the Halden reactor. Absolute count rates and ratios of characteristic peaks were compared between predicted and measured spectra. The comparison showed a total count rate underestimation of 6%. The trend is also confirmed for the single peaks, showing a minimum discrepancy of 4-5% for the characteristic gamma lines of 137Cs and 140La, and a maximum discrepancy of 30% for the peaks present in the energy interval between 400-600 keV.