Non-destructive analysis of spent fuel samples using a gamma-blind fast neutron detector

Abstract

This paper presents the capabilities of a measurement setup built to measure the axial distribution of neutron emitters in spent fuel pins from nuclear power plants. The measurement system uses a custom gamma-blind detector based on a novel combination of silver-activated zinc sulfide scintillators and wavelength-shifting fibers for direct fast neutron detection while having a low sensitivity for gamma rays even in the strong gamma fields present during the inspection of spent fuel. A collimator with an opening of 2 cm is used to create axial resolved information. The system has been tested using UOX and MOX spent fuel samples with burn-ups between 40 and 90 GWd/t and with long cooling times (20–25 y). Measurements of the total neutron emission of the samples relative to each other achieved a 1σ uncertainty of 2%. Absolute values were measured by comparison with a calibrated 252Cf source, achieving a 1σ uncertainty of 5%. The neutron emission of the samples was previously measured using a thermal neutron detector. The results from that previous measurement agree within 2.5σ with the new results. The reconstruction of the axial neutron emission profile was performed for one of the samples. Based on the edges of the sample, the resolution of the reconstruction was estimated to be 2.5 cm. These promising results are the basis for future development of the measurement setup which should not only improve the accuracy of the results but also enable the measurement of spent fuel with much shorter cooling times of ∼1 y.