Estimation of the Gaseous Iodine Concentration in the Containment of Phebus FPT-1 using MELCOR1.8.5 with a Revised Organic Iodide Model

Under a severe accident, organic iodide starts to be generated in the late phase of a severe accident. Due to its difficulties in a removal by a spray system or a filtering, it is important to estimate the level of the equilibrium concentration of the gaseous iodine in the containment. The purpose of this study is to predict the gaseous iodine concentration in the containment using MELCOR and validate the calculation results against the Phebus FPT-1 data. The organic iodide formation is mainly due to the reaction of I2 with organic radical (·R) produced from the reaction of dissolved organic impurities with water radiolysis products. The organic iodide starts to be generated in the late phase of a severe accident. But it cannot be removed easily either by a spray operation or a filtering due to its physical features. Therefore, in terms of the source terms, it is important to predict the amount of organic iodide concentration in the containment well. The purpose of this study is to predict the variation of the gaseous iodine concentration in the containment for the Phebus FPT-1 test with MELCOR1.8.5. But the current MELCOR1.8.5 does not consider the production of organic iodide from the pool. Therefore, 43 relevant equations that were mentioned in MELCOR manuals were implemented. In the previous study, the implemented MELCOR under-predicted the gaseous iodine in the containment for the Phebus FPT-1 test. Especially the amount of organic iodide was largely under-predicted compared to the Phebus FPT-1 experimental data. However, in a previous study, there were two problems for simulating the production of organic iodide from Phebus FPT-1 test. The first one was that the vapor pressure for CH3I was not applied. The second one was that only CH3I was considered as gaseous iodine. In this study, these two problems was revised and how to use the buffer material to control the pH in the pool was introduced for an accident management