Citation:
G. Long, G. Gabrieli, M. Ruch, U. Steinitz, A. Pesach, and I. Levi “Simulation and Experimental Validation of the Effect of Neutron Absorbing Control Elements in Used Fuel on Advanced Experimental Fuel Counter Measurements”, Institute of Nuclear Materials Management Annual Meeting, 21-25 July 2024, Portland, Oregon.
Abstract:
The Advanced Experimental Fuel Counter (AEFC) is a nondestructive assay (NDA) instrument designed to quantify the mass of residual fissile material present in the used fuel following irradiation, which is important for safeguards declaration and verification purposes. This goal is met by performing active and passive measurements (with and without a Cf interrogation source) using six 3He tubes to measure the total neutron count rate (Singles) and the neutron coincidence count rate (Doubles) at different axial positions along the length of a used fuel assembly. Previous collaborative measurement campaigns at the Israel Research Reactor-1 (IRR-1) established a calibration curve relating the AEFC neutron coincidence counting measurement results and the residual fissile mass in used Material Test Reactor (MTR) standard fuel assemblies (FSs). However, there are special fuel assemblies with control blades (FCs), with different geometry and with elements containing neutron-absorbing materials. Without measuring multiple FCs and creating a dedicated calibration curve, it is desired to apply the FS calibration curve to the FC assemblies, and that requires further analysis.rnrnTwo computational analyses were conducted to study the expected effect of a neutron-absorbing control element on AEFC measurements. Both approaches performed MCNP simulations of an MTR-type fuel assembly with and without a neutron-absorbing control element inside the AEFC fuel funnel. The first analysis calculated the expected contribution of neutrons generated in each fuel plate to the AEFC detector response. This value, known as the adjoint flux, was calculated by simulating a fission source in each fuel plate position. The adjoint flux and the expected fission reaction rate in each fuel plate were used to predict the AEFC detector response for an FS and an FC. The second approach used F4 tallies to track the neutron flux and fission reaction rate in each fuel plate, and an F8 capture tally was used to simulate the expected neutron Singles and Doubles rates measured by the AEFC in an FS and an FC. In each computational approach, the ratios of the expected Singles and Doubles responses with and without the cadmium absorber were used to predict how the presence of the control element affected AEFC measurement results, defining an effective absorption ratio used to convert the FC measurements to the scale of the FS, and then applying the calibration curve of the latter. In addition to the simulations, physical measurements of an MTR-type fuel assembly with a removable cadmium absorber blade were performed at LANL using the AEFC. The results of the physical measurements were found to be in agreement with the absorption ratios predicted by the simulations. Refining this absorption ratio approach could allow for AEFC measurements of MTR control assemblies to be corrected such that they can be used alongside the previously established MTR calibration curve to predict the FC residual fissile mass.