Development of plant dynamics analysis code Super-COPD, 2; Validation of natural circulation analysis function by MONJU experimental data

Ohtaki, Akira 

In this paper analyses of natural circulation simulation tests in Monju were conducted to study the applicability of a plant dynamics analysis code (Super-COPD) to the evaluation of decay heat removal by natural circulation. At first basic characteristics of both primary and secondary heat transport systems were examined by separate analyses and then integral system analyses with primary and secondary systems were conducted to confirm the whole system behavior. A modeling of the reactor core and pressure loss characteristics were selected as analysis parameters in the primary system analyses. The effect of the overflow line on natural circulation characteristics were examined in the secondary system analyses. By the separate analyses it was indicated that a precise modeling of core flow channels and input data of axial height for calculating natural circulation head were important for the evaluation of natural circulation characteristics. In addition in the case of low flow condition (about 1-2% of rated flow), the effect of thermal stratification in the horizontal pipe on natural circulation head was observed during the transition to natural circulation condition. Therefore, the need of modeling for thermal stratification in the piping system was pointed out. The switching of flow direction from forward flow to back flow observed at the evaporator side in the secondary system was successfully simulated by the consideration of the overflow line. The results of synthesis analyses with primary and secondary systems were agreed with the measured data in Monju within the accuracy of 10%. Thus it was confirmed that Super-COPD is applicable to the evaluation of decay heat removal by natural circulation.