Study of unfolding method to determine pin-wise source strength distribution

An unfolding method has been developed to determine the pin-wise source strength distribution based on 16 measured dose rates obtained from an inside assembly gamma measurement of 14×14 PWR spent fuel. In the inside assembly gamma measurement method, the spent fuel assembly is measured using an ionization chamber inserted in each spent fuel guide tube. To calculate the pin-wise source strength distribution, an iterative method is used. At the beginning, the dose rate contribution of the pins in the first layer surrounding each guide tube is calculated using a flat source distribution. An MCNPX simulation is used in the calculation of the dose rate contribution. The calculated contribution is then used to calculate the source strength of the pins in the first layer based on the measured dose rate. The source strength at each guide tube positions is then calculated by averaging the source strength of the pins in the first layer. The unfolding method developed herein is based on the assumption that the pin-wise source strength distribution can be approximated by fitting the source strength at each guide tube position with a parabolic function. The new pin-wise source strength distribution is then calculated using the parabolic function and the new dose rate contribution of the pins in the first layer is calculated using the new pin-wise source strength distribution. The iteration is performed until the maximum relative error of the contribution and the pin source strength between iterations are less than 1%. The unfolding method has been validated by simulating the measurement with MCNPX using the calculated pin-wise source strength and correlating the tally result with the measured dose rate. The correlation result shows that the calculated pin-wise source strength is a good approximate solution with the coefficient of determination (R²) of more than 0.95 and 0.93 for the assemblies studied here. The pin-wise source strength is verified through a chemical assay burnup, and the result shows that the maximum difference of pin burnup is less than 1.3 %. The pin-wise source strength is also compared with declared burnup, and the result shows that the difference between the calculated and declared burnup is 4.2 % and 2.6 % for the assemblies studied here. The results suggest that the unfolding method can be used to determine the spent fuel burnup.