Monte Carlo simulations of the Partial Defect Tester for the detection of fuel pins diversion

Amongst the Non-Destructive Assay methods for spent fuel verifications being studied at the Belgian nuclear research centre, SCK•CEN, the Partial Defect Tester (PDET) is proposed for the detection of partial defects. The PDET detector deploys a set of small neutron and gamma detectors that can be inserted in the guide tubes of the fuel assembly. A removal of fuel pins alters the baseline neutron and gamma distributions and allows detecting the anomaly. In this work, a PWR LEU 17×17 fuel assembly was considered and 25 positions that can host the PDET instrumentation. The study was carried out with the Monte Carlo code MCNPX and focussed, in a first stage, on determining the impact on the detector response due to the spent fuel composition. The spent fuel composition was obtained with the Origen-ARP code by simulating different irradiation histories, according to different values of initial enrichment, burnup, and cooling time. In addition, the impact on the neutron detector response of the PDET due to the composition and radiation field of neighbouring fuel assemblies was also assessed. The partial defect capabilities of the PDET were then determined by studying the expected change in the detector response when replacing fuel pins with dummy pins made of stainless steel.