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 cylinder with several flow holes is placed at the upper plenum adjacent to the core outlet. Liquid sodium heated at the core goes into the upper plenum and flows out to the hot leg piping by overflowing at the top of the inner cylinder and passing through the flow holes. When the reactor scram occurs, a cold coolant flows into the bottom of the upper plenum and the thermal stratification takes place at the upper plenum. And thus, the inner barrel may be damaged by a thermal stress due to the temperature difference of the stratification. In this study, a structural integrity of the inner barrel is evaluated numerically based on the cumulative damage rule. First, the upper plenum of fast reactor is numerically investigated to obtain the time history of the temperature at the inner barrel based on a three dimensional thermal-hydraulic CFD tool (FLUENT code) based on the IAEA benchmark analysis (Reactor scram from 40% power operation).Then, a thermal stresses at each location of the inner barrel is calculated using the time history. With regard to the thermal stress calculation, a simplified model is assumed in which temperature decay due to heat convection through the barrel and heat conduction inside the barrel and heat contraction due to temperature difference caused by the stratification are considered. In addition, a model is combined with the effects of transient heat conduction. Then a cumulative damage factor is obtained at each computational cell of the barrel and is summed up through the stratification phenomenon. It is concluded that no significant cumulative damage is found during the IAEA benchmark condition. It is also notified that a practical damage assessment can be examined numerically using the present method. (author)