Numerical study on structural integrity of inner barrel caused by thermal stratification in upper plenum of Monju

In Japanese prototype fast reactor, Monju, an inner barrel with several flow holes is placed at an upper plenum adjacent to a core outlet. When the reactor scram occurs, a cold coolant flows into the bottom of the upper plenum through the core outlet and thermal stratification will appear at the upper plenum. And thus, the inner barrel may be damaged by a thermal stress due to thermal stratification. In this study, a structural integrity assessment method is developed based on fluid-structure interaction analysis and cumulative damage rule. First, a three-dimensional thermal-hydraulics analysis is conducted to simulate a turbine trip test from 40% power operation. Full power output conditions are also simulated by modifying conditions of 40% power output conditions. Next, the thermal stress analysis is modified by adding a practical condition, such as a bending stress. Then, the thermal stress is calculated at each location of the inner barrel. Finally, cumulative damage is evaluated by using the present method. It is concluded that a main factor of cumulative damage is a stress near flow holes that causes stress concentration. It is also found that thermal transient within several hundred seconds after the reactor scram is an important factor. (author)