As described in my first post, I reviewed the detailed data the Tokyo Electric Power Company (TEPCO) released, to understand the operation of Fukushima Units 1, 2, and 3.
The available information for Unit 2 does not extend long after the arrival of the tsunami, and does not extend to the point at which fuel in the reactor core was damaged by overheating. Much of the available information ends at 3:47 pm local time, about 60 minutes after the earthquake occurred at 2:46 pm.
The available information for the first hour following the earthquake shows:
- The reactor shut down around 2:46 pm local time and remained shut down.
- Normal power supplies to in-plant equipment were lost about a minute later. It is assumed that this occurred when the operators manually tripped the turbine/generator per procedure.
- Both emergency diesel generators on Unit 2 automatically started and connected to their in-plant electrical buses within seconds of the power loss, restoring power to essential plant equipment.
- The power interruption caused the main steam isolation valves to automatically close, disconnecting the reactor core from its normal heat sink and disabling the normal source of makeup water to the reactor vessel.
- A safety relief valve (SRV) automatically opened around 2:52 pm to control rising pressure inside the reactor vessel. This SRV automatically re-closed when reactor pressure dropped. This SRV cycled opened/closed periodically over the next hour to control pressure inside the reactor vessel.
- The operators manually started the reactor core isolation cooling (RCIC) system around 2:51 pm,when stopped it about a minute later. The RCIC system was restarted around 3:03 pm and provided makeup flow to the reactor vessel to compensate for the inventory being lost when the SRV opened.
- The operators manually placed one loop of the residual heat removal (RHR) system in the toruswater-cooling mode of operation. This step removed the heat being added to the torus water from repeated SRV openings and RCIC system operation.
- The operators removed the RCIC system from service around 3:28 pm. The water level inside the reactor vessel at that time was above normal, so continued makeup was not needed.
- Around 3:38 pm, one of the emergency diesel generators stopped running. About four minutes later, the other emergency diesel generator stopped running. It is assumed that the tsunami caused these failures.
- Around 3:39 pm, the operators returned the RCIC system to service. The system continued to provide makeup to the reactor vessel until the available data ends at 3:47 pm.
Details of my assessment of Unit 2 for the first 60 minutes after the March 11 earthquake are given here.