Development of computational model for a scintillator based partial defect detector to safeguard PWR spent fuel assemblies

IAEA Safeguards Criteria indicates that the nuclear material should be verified before entering “difficult-to- access” area. Partial defect of a spent fuel assembly implies some fuel rods or fuel pins are missing or replaced by dummy materials in a spent fuel assembly. The amount of nuclear material, which is going to be stored in “difficult-to-access” area, is continuously increasing. Spent fuel partial defect detection becomes important due to the reason. Conventional methods for partial defect detection include gamma/neutron spectrometer and Cerenkov viewing devices. Although radiation spectrometers have higher accuracy, they require longer detection times limiting their applicability. Cerenkov viewing devices have fast screening speed but it can only detect spent fuel assemblies stored in a cooling pool. This research proposes a scintillator based spent fuel partial defect detector (SPDD), which can detect partial defects within a short time and both in and out of a cooling pool. The SPDD consists of a number of unit generation systems. Each unit generation system consists of scintillator and photovoltaic cell, which generates electric current using spent fuel radiation. SPDD distinguishes defective assemblies from normal assemblies by comparing the amount of generated electric current. Due to the difficulty in performing experiments using a real spent fuel assembly, a computational model was developed to analyze the performance of a scintillator based partial defect detector using a reference and test case spent fuel assemblies. Since the amount of generated current is a function of the discharge burnup and cooling time of a spent fuel assembly, this research defined the lower discharge burnup and upper cooling time limit for applying an SPDD.