Abstract
The thermal stratification is a phenomenon in which layer is formed by density difference when high temperature fluid of pressurizer and low temperature fluid of hot leg nozzle are mixed in a surge line. A deflection and load of the surge line occurs due to thermal stratification. Therefore, the thermal stratification load is a major factor for the LBB (Leak-Before-Break) evaluation of surge line like the safe shutdown earthquake load. However, if a large amount of leakage occurs due to surge line break, the resulting load decreases because the crack itself and the plasticity around the crack region affect the load distribution of piping system. This load reduction effect is able to secure additional margin in LBB evaluation. The object of present study is to evaluate load reduction effect under thermal stratification due to the presence of a through-wall crack in surge line of nuclear power plant. Firstly, heat transfer analyses were performed under thermal stratification of surge line using FE (Finite Element) solid model. Secondly, the elastic-plastic analyses of FE model with a through-wall crack were performed by using the temperature distribution of surge line, and the reduced LBB loads due to the thermal stratification were derived. Finally, effect of load reduction and thermal stratification disappearance due to the leakage crack were evaluated and compared.
