Fukushima I was built to withstand a horizontal ground acceleration due to an earthquake of 0.447g (1g = 32 feet per second per second). Unfortunately, this 9.0 earthquake came in at 0.561g.[273] The reactors, particularly the three earliest units that were running at full power, were treated roughly. Some pipe runs ripped out of wall anchors, all external power lines went down, and anything not bolted down went flying. Fortunately, almost everything in a nuclear plant is bolted down. All 12 available emergency generators came on after a few seconds with the control rooms running on batteries. Over the next minutes, several aftershocks hit the island, with magnitudes up to 7.2.
With full AC power from the emergency generators, the three reactors that had been running at full power experienced orderly shutdowns, with the cores being cooled by the usual means, and everything was under control. At Unit 1, the completely passive isolation condensers were doing their job, cooling down the reactor core after shutting down from running at full power. There was no need to turn on the HPCI, at least not yet.
In the opinion of the reactor operators, the isolation condenser was doing its job too well. The temperature was falling too rapidly, and, with the steam condensing in the reactor vessel, a pipe could be collapsed from the vacuum it created. Over-thinking the simple, hard-wired digital logic that had turned it on, an operator put his hand on the switch handle that would stop the isolation condenser coolant flow and turned it off. Then, the remotely controlled flow valves MO-3-A and MO-3-B closed.[274]
In major commercial reactor accidents, there always seems to be a single operator action that starts the downward spiral into an irrecoverable disaster. In the case of Fukushima I, closing those two valves at Unit 1 was the turning point. With that simple action, overriding the judgment of the automatic safety system, an operator doomed Fukushima I to be the only power plant in Japan that suffered irreparable damage due to the Tohoku earthquake of 2011.[275]
At 3:27 P.M., 41 minutes after the earthquake, a tsunami hit the beach at Fukushima I with a towering wave, 13 feet high. The wall built in front of the plant kept the wave from harming anything. Eight minutes later, a second and then a third wave hit. At 49 feet high, they went over the 18.7-foot wall and inundated the entire plant.[276]
The water-intake structures for all six reactors were collapsed by the wave, the water pumps were blown down, and any electrical service outside the buildings was shorted out by the salt water and then torn away. In six minutes, all the underground diesel generators were flooded, and the emergency AC power failed. One diesel-powered generator, the air-cooled unit located above ground at Unit 6, remained online, providing power for Units 5 and 6. Units 3 and 4 were now on DC power, enabling operators to read instruments in the control room and manipulate remote-control valves until the batteries lost power, and now was a good time to make sure all the valves were in an open/close condition that would do the most good, keeping the core of Unit 3, recently operating at full power, from melting. In Units 1 and 2, the battery room was flooded, and the plant was in total blackout. No valves could be turned on or off, and the status of reactor systems was not available on the control panels. They were stuck with whatever configuration was in effect when the lights went off, and that meant that Unit 1 was coming down off full power with nothing to cool its 69 tons of hot uranium oxide fuel, continuing to generate megawatts of power. The isolation condenser was shut off. In Unit 2, at least the RCIC was left running when the power failed, but without some tweaking, it too would fail eventually. TEPCO advised the Japanese government that an emergency condition existed at Fukushima I.
The tsunami rushed inland, to the ancient tsunami warning stones and beyond, carrying everything with it and drowning the Earth beneath it. Fishing boats and ocean-going ships hit the beaches and kept going. Down came houses, factories, and entire towns. Cars, trucks, and trains were moved like toys in a fire-hose spray. Power transformers blew up as electrical lines touched the ground, gas lines broke, and fires broke out, taking out any last burnable structures that had not washed away.
The wave came in, and then it went out, taking everything that would float out to sea. An estimated 18,000 human beings were washed into the Pacific Ocean. The loss of life was devastating. Two operators drowned at Fukushima Unit 4, trapped in the turbine building as the water quickly rose in the basement.
The immediate crisis at Fukushima I was a need for AC power to manage the cooldowns in Units 1, 2, and 3. Unit 1 was in total blackout with no passive systems running, and in 2 and 3 the water circulated through the torus pool was eventually going to have absorbed enough heat to start steaming. All the reactor interconnection cables, allowing the units to share 6.9 kilovolt and 480 volt power, had been lost in the tsunami. An obvious solution was to bring in portable diesel generators and hook up to whatever wiring stubs were left sticking out of the buildings, but this was not going to be simple. All roads into Fukushima I were either completely washed away, blocked by collapsed buildings, or jammed by fleeing people. Appropriate generators were available, but they were too heavy to be flown in by helicopter. They could only be transported by wide trucks on a smooth highway.
The plant wiring was also a problem. Temporary cables would have to be installed, first running from the plant parking lot to the standby liquid-control pumps for Unit 2.[277] Cables were available, but they were four inches in diameter, 656 feet long, and weighed more than a ton. Unreeling the cables and running through debris field covered with collapsed buildings and newly established lakes would have to be done without any powered equipment. No trucks, cranes, or bulldozers were available, and hidden beneath the ground clutter were manholes with the covers blown off. Everything about establishing AC power involved tremendous adversity, and it was going to take time.
In Unit 1 there was no instrument feedback revealing the state of the systems and no lighting in the control room. The operators could only look at the dead instruments using flashlights. By three hours after the earthquake, all the steam-relief valves had pried open and the water had boiled out of the reactor core. An hour and a half later, the fuel, still generating power at a fractional rate but naked of liquid coolant, started to melt away the zirconium sleeves on the fuel pins. The red-hot zirconium began to react chemically with the steam around it, oxidizing and leaving hydrogen gas in place of the steam. The zirconium core supports started to get soft and sag, and entire fuel assemblies started coming apart and tumbling down into the bottom of the reactor vessel. There were 400 fuel assemblies, and each one was 171 inches long. Compressed by the weight of the fuel, the wrecked mixture of uranium oxide, zirconium oxide, and melted neutron control blades increased its temperature. Fuel started to melt.
The combination of steam pressure and hydrogen gas pressure vented from the isolated reactor vessel exceeded the designed yield strength of the torus several times over. There was no electricity to open any valves, so the normal severe emergency action of venting the torus safely up the vent stack could not be initiated. General Electric’s Mark I containment, made of steel one inch thick, split open, and the soluble and volatile components of fission products, set free by the absence of any zirconium cladding, were sprayed into the reactor building. Included with it was hydrogen gas, mixing with the oxygen-containing air in the large space above the refueling floor. Unit 1 was now a bomb, set to go off and heavily contaminated with fission products.
273
The Diablo Canyon Nuclear Power Plant, the American equivalent of Fukushima I built on the coast in California, is built to withstand a horizontal ground acceleration of 0.750g.
274
The closing of these valves is surprisingly complex. The operators were “cycling” Isolation Condenser A, turning it on and off, trying to slow the temperature descent, with Condenser B turned off. Both condensers, A and B, had been turned on automatically by the scram-control logic. The last operator action was to cycle Condenser A down by turning off the cold-leg return valve, which was motor-operated.
275
There is a strange twist here. The operators at Unit 1 were desperate to restart the isolation condensers or to initiate the HPCI, but there was no DC current from the backup batteries. They stormed what was left of the plant parking lot, where the 6,413 workers had left their cars, and started pulling batteries out of the cars that remained on site, and, where available, they took jumper cables out of trunks. They stacked batteries on the floor in the control room, connecting ten 12-volt batteries in series using the jumper cables to make 125 volts DC for the valve-control motors. There was not enough current to keep the system alive for more than a few minutes, but for an instant the control panel lighted up and they could see that the isolation condenser valves MO-2A and MO-3A indicated closed. At 6:18 P.M. they were able to apply power to the motors and open the valves. A lookout reported steam coming from the condenser pool, indicating success. Seven minutes later, an operator reached for the switch and closed the valves, just as the automobile battery-pack died. No reason for this action has been determined. The isolation condenser valves could not be reopened.
276
The precise height of the second and third waves is not known, and it is estimated by the level of silt on the buildings at between 46 and 49 feet. The wave meter at the plant was capable of recording a wave as high as 24.6 feet, where it jammed right before it was carried away by the wave. The tsunamis that hit Fukushima I were the sum of several circular waves originating sequentially at large sea-bed movements in the quake epicenter and in the Japan Trench, running north-south between Honshu and the epicenter. The wave transmitted across the Pacific Ocean as the circular wave components increased in radius, eventually hitting the California coast. No damage was done on the eastern side of the Pacific. A tsunami of the type that hit Japan starts out with a wavelength of about 120 miles, moving at 500 miles per hour. When it approaches shallow water, the wavelength compresses to 12 miles, the wave-height expands, and the speed reduces to 50 miles per hour.
277
These pumps (SLC) were not flooded or damaged, and if they could be powered up, it would save Reactor 2 by cooling it down with seawater. Reactor 1 was in greater need of help, but starting the SLC pumps for Reactor 2 looked like something that could be accomplished. Unfortunately, the seaward intakes for the pumps were blocked.