The operating staff at Unit 1 knew that after being without a cooling system for several hours, the Mark I would have to be vented up the stack, but there was no power to open the main valve, AO-72. It was an air-operated valve, but it was possible to open it by hand if they could get to it. The entire reactor building was radiation-contaminated, which was a clue that the containment structure was already broken open, but men volunteered for the hazardous job of running down pitch-dark hallways, through a maze of doorways and passages, to the valve, open it by turning on a compressed-air line, and rush back, receiving the maximum allowable dose for the entire month in a few minutes. First, a gasoline-engine air compressor would have to be located and connected to the line. Every detail took time.
The entire area around Fukushima would have to be evacuated before it was legal to vent the containment, and government permission had to be verified. The TEPCO office in Tokyo finally gave the go-ahead at 9:03 a.m. on March 12, the next day. At 2:30 P.M., after heroic effort, the torus in Unit 1 was vented up the stack shared with Unit 2, but it was too late to prevent damage to the plant.
At 3:30 P.M., the men at Fukushima I had bucked all odds and installed external AC power to the standby pumps at Unit 2. With great effort, fire hoses had been attached to the outside access points for the condensate tanks in Units 1 and 2, and fire trucks were standing by to start pumping water and relieve the obvious heat buildup inside.
The men paused a moment to rest and admire their work. Six minutes later, at 3:36 P.M., the Unit 1 reactor building exploded in a spectacular geyser of debris, sending radiation-contaminated chunks of concrete and steel beams high in the air and careering through the newly installed equipment. Five men were injured, the wiring was ripped out, the generator was damaged, and the fire hoses were torn. Heavy debris came down all around for what seemed a long time. Radioactive dust from the Unit 1 fuel floated down out of the air and began to cover the entire power plant.[278] Not only had this explosion destroyed Unit 1, but from now on all work at Fukushima I would require heavy, bulky radiation suits and respirators, and now there was a new layer of movement-restricting debris on top of the already-established debris. It was a setback.
The next day, at 2:42 a.m. on March 13, the passive high-pressure coolant injection (HPCI) system in Unit 3, running on steam made from the afterglow in the fuel, finally gave out, and by 4:00 a.m. the fuel began to degrade, eventually collapsing into the bottom of the reactor vessel and generating a great deal of hydrogen gas. A fire engine was eventually able to inject seawater into the system, effectively closing the gate after the livestock had escaped. By 8:41 a.m., the operators had managed to open the air-operated torus vent valve and relieve the pressure that was building up. It was seen as a semi-miracle. Steam was seen coming out the vent stack, and the site boundary dose rate suddenly increased to 0.882 rem per hour.
At 11:01 a.m. on March 14, the day after the Unit 3 core structure melted, the Unit 3 reactor building exploded with a fireball, taking the lead over Unit 1 for the ugliest debris field. Hydrogen gas from the core deterioration had collected in the top of the building until it reached a critical concentration, somewhere over 4 % in the air, and a spark must have set it off. Two fire engines were put out of commission, 11 workers were injured, the portable generators that were now collecting in the yard were all damaged, the temporary wiring was torn out, and the fire hoses were ripped apart. The new debris on the ground, everything from dust to chunks of walls, was extremely radioactive. The dose rate in the Unit 3 airlock, not even entering the reactor building, was now 30 rem per hour. The absolute emergency dose allowed one worker at the plant was 10 rem. That meant that if a worker stood in the airlock for 20 minutes, he had to be relieved and sent away, and he could no longer work on the problems at the plant. Debris on the ground after the Unit 3 explosion caused a dose rate of 1 rem per hour in the yard, and all personnel outside the control room were evacuated to the Emergency Response Center, near Unit 5.
At 12:40 P.M. the Reactor Core Isolation Cooling System (RCIC) in Unit 2 had absorbed all the shutdown heat it could stand, and the coolant-pump turbine stopped turning. It had held out for 70 hours, outperforming its design. The water in the reactor vessel boiled away, overstressing the Mark I containment structure, and at 4:30 P.M. the fuel pins started to melt, eventually falling into the bottom of the vessel and vigorously making hydrogen. Fortunately for Unit 2, the explosion of Unit 1 had blown a large hole in the side of the reactor building, so all the hydrogen leaking out of the torus was able to escape freely and not collect near the ceiling. Unit 2 never exploded, but its radioactive steam, iodine, and xenon were able to escape into the environment along with the hydrogen. Plans to vent the torus were cancelled when the pressure inside was found to be too low to open the rupture disc on the vent stack.
As the situation at Units 1, 2, and 3 continued to deteriorate, Unit 4 remained serenely innocent. All its fuel had been removed and stored in the fuel pool on the top floor in the reactor building. The cooling water surrounding the fuel was at 80.6° Fahrenheit, the tops were off the reactor vessel and the dry well portion of the containment structure, and nothing was anywhere near a crisis condition. The electrical power was gone, but Units 1, 2, and 3 were in continuous crisis, and they obviously needed more attention than Unit 4. The operating staff pitched in to help the units that were in deep trouble.
The fact that there was no power meant that the Unit 4 vent-stack damper valves had no compressed air holding them closed. They were, in fact, hanging open. Unit 4, for reasons of economy, shared a vent stack with Unit 3. There was no backflow damper installed, so when the overpressurized torus in Unit 3, heavily invested with hydrogen, was vented up the stack, using the correct procedures by the book, half of the vented gas went up the stack and half went back through the Unit 4 vent pipe. The hydrogen and radioactive steam proceeded through the relaxed valves in the Standby Gas Treatment System filters, up two stories, and out the exhaust air ducts on the fourth floor of the Unit 4 reactor building, where it collected at the ceiling and awaited an ignition spark.
At 6:14 a.m. on March 15, four days after the earthquake, the Unit 4 reactor building exploded with a mighty roar, much to the surprise of everyone working at Fukushima I. Having no theory as to what had just happened, the operators at Units 5 and 6 quickly climbed to the tops of the reactor buildings and hacked large holes in both roofs to let out the hydrogen, which did not exist, thus inflicting the only damage that the two newer reactors sustained in the Tohoku earthquake.
The cross-contamination of hydrogen and radioactive steam from Unit 3 was not figured out until August 25, and on March 15 the only plausible explanation was that the water in the fuel pool must have leaked out through a crack caused by the earthquake. The spent fuel, removed from the reactor core only days before, must have overheated and caused its zirconium cladding to generate hydrogen from the remaining steam in the pool. A helicopter flyover confirmed a great deal of radioactivity in the remains of the upper reactor building. A great deal of effort and time was spent in vain, trying to reload the fuel pool with water using helicopter drops and water cannons. It turned out that the fuel in Unit 4 was in fine condition, and the high radioactivity over the building resulted from dissolved fission products delivered to the space above the refueling floor by steam from Unit 3.
After the upper floor on the Unit 4 reactor exploded, there was basically nothing left to happen that could further degrade Fukushima I. The three reactors that were operating when the earthquake hit had melted down and were left an enormous liability for TEPCO. It would be feasible to rebuild Unit 4, because only the roof and walls covering the refueling floor had been blown up, but the radioactivity spread all over the power-plant grounds would make it an impractical work environment. Units 5 and 6, the newest reactors in the plant, could be brought back online with some rebuilding of the seawater intakes, new outside pumps, and an enhanced tsunami wall.
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Fortunately, at 3:36 P.M. on March 12, the wind at Fukushima I was blowing out to sea. It is never good to let radioactive dust settle over the ocean, but it would have been much worse to have it settle over the inland territory to the west of Fukushima. Much nuclear mischief can be lost at sea and rendered much less harmful, because the dilution factor is so enormous as opposed to when it stays on land.