Abstract
At the Fukushima Daiichi Nuclear Power Plant Unit-1 (1F1), three attempts were made to open the small Suppression Chamber (S/C) vent valve (AO valve) at 10:17, 10:23, and 10:24 on March 12th. The monitor near the main gate recorded a series of high dose rates from 10:20, seemingly in response to the vent operation. However, no clear decrease in the Drywell (D/W) pressure was observed, suggesting a potential failure of this venting. At present, it remains unclear whether the small S/C vent valve was indeed opened, and in case it was, whether it was the main cause that triggered the rise in the dose rate near the main gate at 10:20 on March 12th, 2011. These two issues were attempted to be clarified in this study by the following two steps.
As the first step, the possibility of the opening of the small vent valve was assessed by reproducing the behavior of the D/W pressure using GOTHIC v8.4 code. The boundary conditions (gas inflow, heat source, the opening of the small vent valve, etc.) were estimated based on the 1F1 accident scenario events (Primary Containment Vessel (D/W) leak, Molten Core-Concrete Interaction (MCCI), etc.), which was estimated based on the plant measurement data and the on-site observations. These estimated conditions were adjusted until the simulation results aligned with the D/W pressure measurement data. As a result, the potential opening status of the small S/C vent valve was evaluated based on the criterion of D/W pressure reproducibility.
Based on the result from the first step, the second step aims to discuss the relationship between the opening of the small S/C valve and the rise in the dose rate near the main gate at 10:20 on March 12th. Although the wind direction and speed were recorded by the monitor in the plant field, the information was insufficient to draw a conclusive inference. Therefore, this study planned to clarify this issue from another approach by comparing the amount of radioactive material released through S/C vent lines and the Reactor Building (R/B) to identify the major radiation source that led to the rise in the dose rate. The results of the D/W pressure reproduction simulation at the first step make it possible to estimate the leakage conditions through the reactor building and ventilation lines. The discussion on the primary cause responsible for the increase in the dose rate near the main gate will be discussed based on the results of this simulation and previous relevant studies.
It should be noted that only the results of the first step are included in this paper, as the second step is still ongoing.
