Reactors

Reactors

Unit 1

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 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.

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 MPa, which was many times greater than the limit of the DDFP which could only operate below 0.8 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 MPa) exceeded design specifications (0.528 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 MPa at 02:30 on 12 March, after which it stabilized around 0.8 MPa. The decrease in pressure was due to uncontrolled vent via an unknown pathway. The plant was notified Okuma town completed evacuation at 9:02 on 12 March. The staff subsequently began controlled venting. Venting of the PCV was completed later that afternoon at 14:00.

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 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 (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. Power was restored to unit 1 (and 2) using a mobile generator at 15:30 on 12 March.

At 15:36, a hydrogen explosion damaged the secondary confinement structure (the RB). The cause was unknown to the workers at the time, most of whom evacuated shortly after the explosion. The debris produced by the explosion damaged the mobile emergency power generator and the seawater injection lines. The seawater injection lines were repaired and put back into operation at 19:04 until the valve pit was nearly depleted of seawater at 01:10 on the 14th. The seawater injection was temporarily stopped in order to refill the valve pit with seawater using a variety of emergency service and JSDF vehicles. However, the process to restart seawater injection was interrupted by another explosion in the unit 3 RB at 11:01 which damaged water lines and prompted another evacuation. Injection of seawater into unit 1 would not resume until that evening, after 18 hours without cooling.

Subsequent analysis in November suggested that this extended period without cooling resulted in the melting of the fuel in unit 1, most of which would have escaped the reactor pressure vessel (RPV) and embedded itself into the concrete at the base of the PCV. Although at the time it was difficult to determine how far the fuel had eroded and diffused into the concrete, it was estimated that the fuel remained within the PCV.

Computer simulations, from 2013, suggest "the melted fuel in Unit 1, whose core damage was the most extensive, has breached the bottom of the primary containment vessel and even partially eaten into its concrete foundation, coming within about 30 cm (1 ft) of leaking into the ground" – a Kyoto University nuclear engineer said with regard to these estimates: "We just can't be sure until we actually see the inside of the reactors

Unit 2

Unit 2 was the only other operating reactor which experienced total loss of AC and DC power. Prior to blackout, the RCIC was functioning as designed without the need for operator intervention. The safety relief valves (SRVs) would intermittently release steam directly into the PCV suppression torus at its design pressure and the RCIC properly replenished lost coolant. However, following the total blackout of unit 2, the plant operators (similar to unit 1) assumed the worst-case scenario and prepared for a LOC incident. However, when a team was sent to investigate the status of the RCIC of unit 2 the following morning (02:55), they confirmed that the RCIC was operating with the PCV pressure well below design limits. Based on this information, efforts were focused onto unit 1.   However, the condensate storage tank from which the RCIC draws water from was nearly depleted by the early morning, and so the RCIC was manually reconfigured at 05:00 to recirculate water from the suppression chamber instead.

On the 13th, unit 2 was configured to vent the PCV automatically (manually opening all valves, leaving only the rupture disk) and preparations were made to inject seawater from the valve pit via the FP system should the need arise. However, as a result of the explosion in unit 3 the following day, the seawater injection setup was damaged and the isolation valve for the PCV vent was found to be closed and inoperable.

At 13:00 on the 14th, the RCIC pump for unit 2 failed after 68 hours of continuous operation. With no way to vent the PCV, in response, a plan was devised to delay containment failure by venting the reactor vessel into the PCV using the SRVs in order to allow for seawater injection into the reactor vessel.

The following morning (March 15, 06:15), another explosion was heard on site coinciding with a rapid drop of suppression chamber pressure to atmospheric pressure, interpreted as a malfunction of suppression chamber pressure measurement. Due to concerns about the growing radiological hazard on site, almost all workers evacuated to the Fukushima Daini Nuclear Power Plant.

Unit 3

Although AC power was lost, some DC power was still available in unit 3 and the workers were able to remotely confirm that the RCIC system was continuing to cool the reactor. However, knowing that their DC supply was limited, the workers managed to extend the backup DC supply to about 2 days by disconnecting nonessential equipment, until replacement batteries were brought from a neighboring power station on the morning of the 13th (with 7 hours between loss and restoration of DC power). At 11:36 the next day, after 20.5 hours of operation, the RCIC system failed. In response, the high-pressure coolant injection (HPCI) system was activated to alleviate the lack of cooling while workers continued to attempt to restart the RCIC. Additionally, the FP system was used to spray the PCV (mainly the SC) with water in order to slow the climbing temperatures and pressures of the PCV.

On the morning of the 13th (02:42), after DC power was restored by new batteries, the HPCI system showed signs of malfunction. The HPCI isolation valve failed to activate automatically upon achieving a certain pressure. In response, the workers switched off HPCI and began injection of water via the lower pressure firefighting equipment. However, the workers found that the SRVs did not operate to relieve pressure from the reactor vessel in order to allow water injection by the DDFP. In response, workers attempted to restart the HPCI and RCIC systems, but both failed to restart. Following this loss of cooling, workers established a water line from the valve pit in order to inject seawater into the reactor alongside unit 2. However, water could not be injected due to RPV pressures exceeding the pump capability. Similarly, preparations were also made to vent the unit 3 PCV, but PCV pressure was not sufficient to burst the rupture disk.

Later that morning (9:08), workers were able to depressurize the reactor by operating the safety relief valves using batteries collected from nearby automobiles. This was shortly followed by the bursting of the venting line rupture disk and the depressurization of the PCV. Unfortunately, venting was quickly stopped by a pneumatic isolation valve which closed on the vent path due to a lack of compressed air, and venting was not resumed until over 6 hours later once an external air compressor could be installed. Despite this, the reactor pressure was immediately low enough to allow for water injection (borated freshwater, as ordered by TEPCO) using the FP system until the freshwater FP tanks were depleted, at which point the injected coolant was switched to seawater from the valve pit.

Cooling was lost once the valve pit was depleted, but was resumed two hours later (unit 1 cooling was postponed until the valve pit was filled). However, despite being cooled, PCV pressure continued to rise and the RPV water level continued to drop until the fuel became uncovered on the morning of the 14th (6:20), as indicated by a water level gauge, which was followed by workers evacuating the area out of concerns about a possible second hydrogen explosion similar to unit 1.

Shortly after work resumed to reestablish coolant lines, an explosion occurred in the unit 3 RB at 11:01 on March 14, which further delayed unit 1 cooling and damaged unit 3's coolant lines. Work to reestablish seawater cooling directly from the ocean began two hours later, and cooling of unit 3 resumed in the afternoon (approximately 16:00) and continued until cooling was lost once more as a result of site evacuation on the 15th.

Unit 4

The unit 4 reactor building after the explosion. The yellow object is the reactor's removed PCV head. The removed black RPV head with its lifting frame attached is to the left. Both had been removed to allow refueling at the time. The green gantry crane carries fuel between the RPV and the spent fuel pool.

Unit 4 was not fueled at the time, but the unit 4 spent fuel pool (SFP) contained a number of fuel rods. On 15 March, an explosion was observed at the unit 4 RB during site evacuation. A team later returned to the power station to inspect unit 4, but were unable to do so due to the present radiological hazard. The explosion damaged the fourth floor rooftop area of Unit 4, creating two large holes in a wall of the RB. The explosion was likely caused by hydrogen passing to unit 4 from unit 3 through shared pipes.

The following day, on the 16th, an aerial inspection was performed by helicopter which confirmed there was sufficient water remaining in the SFP. On the 20th, water was sprayed into the uncovered SFP, later replaced by a concrete pump truck with a boom on the 22nd.

Unit 5

Unit 5 was fueled and was undergoing a RPV pressure test at the time of the accident, but the pressure was maintained by an external air compressor and the reactor was not otherwise operating. Removal of decay heat using the RCIC was not possible, as the reactor was not producing sufficient steam.   However, the water within the RPV proved sufficient to cool the fuel, with the SRVs venting into the PCV, until AC power was restored on March 13 using the unit 6 interconnection, allowing the use of the low-pressure pumps of the residual heat removal (RHR) system. Unit 5 was the first to achieve cold shutdown in the afternoon on the 20th.

Unit 6

Unit 6 was not operating, and its decay heat was low. All but one EDG was disabled by the tsunami, allowing unit 6 to retain AC-powered safety functions throughout the incident. However, because the RHR was damaged, workers activated the make-up water condensate system to maintain the reactor water level until the RHR was restored on the 20th. Cold shutdown was achieved on the 20th, less than an hour after unit 5.