Criticality Safety Analysis of Fuel Storage Under Fuel Assembly Dropping Accident

During fuel storage, lifting fuel assembly is an essential process. The fuel assembly dropping accident may lead to consequences, such as fuel assembly lying on the storage rack, fuel assembly deformation leading to out of control of fuel rod pitch distance, fuel rod breakage and other accidents, and then affect the critical safety of fuel storage system. Therefore, it is necessary to analyze the critical safety of fuel assembly storage under fuel assembly dropping accident. This study analyzes all kinds of accident conditions that may occur under fuel assembly dropping accident, and uses Monte Carlo particle transport code JMCT to simulate the neutron effective multiplication factor (keff) at typical conditions. The results show that the calculated keff of the fuel storage system will not exceed the criteria when the fuel assembly drops on the fuel storage rack without deformation. When fuel assembly is deformed due to dropping from a height, change of fuel rod pitch distance in fuel assembly will affect the moderator-uranium ratio of the fuel assembly storage system, which may lead to an increase in keff in the system. However, the fuel storage system can also will not exceed the criteria when 2000 ppm soluble boron acid is taken credit in the spent fuel pool. Based on the above analysis, the fuel storage system can ensure the subcritical state at different condition of fuel assembly dropping.