The Tokyo Electric Power Company, Incorporated, would like to make comments on subjects that are reported by the newspapers, TV or websites.
May 10, 2012
Isolation Condenser System (IC) is a standby system that cools and condenses the steam instead of the Main Condenser when the Main Condenser that normally cools and condensates the steam generated at the reactor is not available for some reason. Within the TEPCO's facilities, it is only installed in the Unit 1 at Fukushima Daiichi Nuclear Power Station.
At 14:46 on March 11, 2011, the Unit 1 reactor automatically shut down in response to the intense seismic motion. At this moment the Main Steam Isolation Valves (MSIV) were also closed. These valves are installed on the piping where the steam generated from the reactor goes to the turbine. The steam which continued to be generated was trapped inside and the reactor pressure rose gradually. On reaching to a certain pressure level, the IC automatically started up, opening its valve to release the steam, and the reactor started to reduce its pressure. From then, until the tsunami hit the station, the operators k ept operating the IC, keeping the reactor pressure within a certain range while avoiding extreme depressurization, as stipulated in the operating procedures. It is presumed that the IC had lost almost all function with the arrival of the tsunami. When the tsunami struck, the IC control signals were lost and as a result the function to isolate the IC automatically was initiated, which means that the IC almost lost its function.
Other than operating the IC, another way to depressurize the reactor by cooling and condensing the steam from the reactor is to open the main steam safety relief valves (SRVs). In the TEPCO's operating procedures, two of these operations are written side by side. When the reactor pressure rises, SRVs are used to release the steam from the Reactor Pressure Vessel (RPV) to the Primary Containment Vessel (PCV), thus suppressing pressure rise. However, before the tsunami struck, the SRVs were not actuated, since the reactor pressure was adjusted by the IC. After the tsunami struck, it is presumed that the IC lost its function, and instead, the SRVs were actuated and an abnormal reactor pressure rise was avoided. In addition, in either case, operating the IC or the SRVs, by keeping the reactor pressure within a certain range, it will not immediately cause a sharp reduction in pressure and boil and empty the water in the reactor. In the accident investigation further analysis is under way to obtain a clearer picture of the plant conditions during the accident by reproducing the operation of the IC and the SRVs at the maximum.
note: Both the IC and the SRVs are designed to start the operation automatically when the reactor pressure rises, although the IC starts up earlier than the SRVs because the set pressure is lower. Depressurization by the SRVs needs to be followed by water injection to make up for the steam that goes to the Suppression Chamber (SC) of the PCV, whereas depressurization by the IC does not require such procedures. The steam is condensed into water at the IC and returns into the reactor.
© Tokyo Electric Power Company Holdings, Inc.
