Abstract
Gamma-ray spectrometry using collimated detectors is a well-established examination method for 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, and the total spectrum count rate in the detector should not cause throughput issues. Methods were assembled for gamma spectrum prediction to optimize instruments for gamma emission tomography and to enable <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a priori</i> feasibility evaluation of determination of single peaks of irradiated nuclear fuel. The aim was to find reliable results (~10% accuracy) regarding total spectrum and peak count rates with faster computation time than a full-Monte Carlo approach. For this purpose, the method is based on depletion calculations with SERPENT2, a point-source kernel method for the collimator response, and a rig response matrix and a detector response matrix, both computed with MCNP6. The computational methodology uses the fuel properties (dimensions, materials, power history, and cooling time) and the instrumental setup (collimator and detector dimensions and materials) as an input. The prediction method was validated using the 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, showing a total count rate overestimation of 7% and discrepancies between 2% and 20% for the single peaks (the same order of magnitude of the uncertainty). This level of agreement is deemed sufficient for measurement campaigns planning and the optimization of spectroscopic instruments for use in gamma scanning and tomography of nuclear fuel.
Highlights
GAMMA scanning and Gamma Emission Tomography (GET) are techniques for non-destructive assay of irradiated nuclear fuel [1, 2, 3]
The response was modeled for photons of varied energy with MCNP6, using a monodirectional point source to mimic the collimated beam hitting the detector, resulting in a detector response matrix
Because the response of the detector depends on its characteristics and the incident gamma energies, this is convoluted with the detector response matrix, QQkkkk, to get a realistic prediction of a pulse height spectrum
Summary
GAMMA scanning and Gamma Emission Tomography (GET) are techniques for non-destructive assay of irradiated nuclear fuel [1, 2, 3]. This can be determined e.g. by Currie’s decision limits [11] This is further complicated in the case of nuclear fuel where the background may largely come from other nuclides present in the same fuel sample. The gamma-ray background can not be characterized only using a background spectrum recorded in the lab, but one must consider the multitude of gamma-ray emitters that are produced in the nuclear fuel itself This is in turn dependent on the power history and the cooling time, which is specific for each fuel sample. The method presented in this work aims to predict gamma-ray spectrum in a collimated detector system, based on the input of the fuel dimensions and material, power history, collimator dimensions, and detector type and geometry. Methods for spectrum prediction may be useful in general, in the planning of gamma scanning and GET campaigns at research reactors, commercial reactors, or hot cell facilities
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