Fukushima nuclear accident: Difference between revisions

The isolation condenser (IC) was functioning prior to the tsunami, but the DC-operated control valve outside of the primary containment had been in the closed position at the time to prevent thermal stresses on the reactor components. This status was uncertain at the time due to a loss of indications in the control room, who had correctly assumed loss of coolant (LOC). At 18:18 on March 11 March, a few hours after the tsunami, the plant operators attempted to manually open the control valve, but the IC failed to function, suggesting that the isolation valves were closed. Although they were kept open during IC operation, the loss of DC power in unit 1 (which occurred shortly prior to the loss of AC power) automatically closed the AC-powered isolation valves in order to prevent uncontrolled cooling or a potential LOC. Although this status was unknown to the plant operators, they correctly interpreted the loss of function in the IC system and manually closed the control valves. The plant operators would continue to periodically attempt to restart the IC in the following hours and days, but it did not function.<ref name="IAEA2015" />{{rp|29-34}}

The plant operators then attempted to use the building's fire protection (FP) equipment, operated by a diesel-driven fire pump (DDFP), in order to inject water into the reactor vessel. A team was dispatched to the reactor building (RB) in order to carry out this task, but the team found that the reactor pressure had already increased significantly to 7&nbsp;MPa, which was many times greater than the limit of the DDFP which could only operate below 0.8&nbsp;MPa. Additionally, the team detected high levels of radiation within the RB, indicating damage to the reactor core, and found that the primary containment vessel (PCV) pressure (0.6&nbsp;MPa) exceeded design specifications (0.528&nbsp;MPa). In response to this new information, the reactor operators began planning to lower the PCV pressure by venting. The PCV reached its maximum pressure of 0.84&nbsp;MPa at 02:30 Marchon 12th12 March, after which it stabilized around 0.8&nbsp;MPa. The decrease in pressure was due to uncontrolled vented via an unknown pathway. The plant was notified Okuma town completed evacuation at 9:02 Marchon 12 March. The staff subsequently began controlled venting. Venting of the PCV was completed later that afternoon at 14:00.<ref name="IAEA2015" />{{rp|34-37}}

At the same time, pressure in the reactor vessel had been decreasing to equalize with the PCV, and the workers prepared to inject water into the reactor vessel using the DDFP once the pressure had decreased below the 0.8&nbsp;MPa limit. Unfortunately, the DDFP was found to be inoperable and a fire truck had to be connected to the FP system. This process took about 4 hours, as the FP injection port was hidden under debris. The next morning (March 12 March, 04:00), approximately 12 hours after loss of power, freshwater injection into the reactor vessel began, later replaced by a water line at 09:15 leading directly from the water storage tank to the injection port to allow for continuous operation (the fire engine had to be periodically refilled). This continued into the afternoon until the freshwater tank was nearly depleted. In response, injection stopped at 14:53 and the injection of seawater, which had collected in a nearby valve pit (the only other source of water), began.<ref name="IAEA2015" />{{rp|37}}

Power was restored to unit 1 (and 2) using a mobile generator at 15:30 on March 12 March.<ref name="IAEA2015" />{{rp|37}}<ref name="Japan Earthquake Update (2210 CET)" />