Safety assessment of U–Mo fuel mini plates irradiated in HANARO reactor

Abstract

Neutronic and thermal hydraulic characteristics of U–Mo fuel mini plates irradiated in the HANARO reactor were analyzed for the safety assessment of these plates. A total of eight fuel plates were double-stacked; each stack contained three 8.0gU/cc U–7Mo fuel plates and one 6.5gU/cc U–7Mo fuel plate. The neutronic and thermal hydraulic analyses were carried out using the MCNP code and TMAP code, respectively. The core status used in the study was the equilibrium core, and four Control Absorber Rod (CAR) locations were considered: 350mm, 450mm, 550mm, and 650mm away from the bottom of the core. For the fuels in the lower stack, the maximum heat flux was found at the CAR located at 450mm. For the fuels in the upper stack, the maximum heat flux was found at the CAR located at 650mm. The axial power distributions for the upper and lower stacks were selected on the basis of thermal margin analyses. A mock-up irradiation target assembly was designed and tested at the out-of-pile test facility to measure the flow rate through the irradiation site, given that the maximum flow rate through the irradiation site at the HANARO reactor is limited to 12.7kg/s. For conservative analyses, measurement and correlation uncertainties and engineering hot channel factors were considered. During normal operation, the minimum ONB temperature margins for the lower and upper stacks are 41.6°C and 31.8°C, respectively. This means that boiling does not occur. However, boiling occurs during the limiting accidents. Nevertheless, the fuel integrity is maintained since the minimum DNBR are 1.96 for the Reactivity Insertion Accident (RIA) and 2.10 for the Locked Rotor Accident (LRA). From the neutronic and thermal hydraulic analyses of the U–Mo fuel mini plates irradiated in the HANARO reactor, it is concluded that boiling does not occur during normal operation and that the fuel integrity is maintained during limiting accidents.