Atomic Accidents, page 51
In March 2011, Units 4, 5, and 6 were down for refueling and maintenance. Unit 4 was in the middle of refueling, with the dry-well lid and the reactor vessel cover unbolted and placed aside using the overhead crane in the reactor building. The refueling floor was flooded, and all the fuel had been moved to the adjacent fuel pool for a cool-down period. The only things turned on in the Unit 4 building were the overhead lights and the coolant pumps for the open fuel pool, keeping water moving over the spent fuel as its residual heat tapered off exponentially. Units 5 and 6 had just finished refueling, and Unit 5 was undergoing a reactor-vessel leakage test. Units 1, 2, and 3 were running hot, straight, and normal, and three 275-kilovolt line-sets were humming softly.
UNITS 5 AND 6 AT FUKUSHIMA DAIICHI WERE IN THE MIDDLE OF REFUELING when the Tohoku earthquake struck Japan. Both reactors were completely inert, with no fear of a meltdown or a hydrogen explosion. The fuel had been emptied from the reactor vessel and transferred to the storage pool using the refueling machine.
On March 9, 2011, 70 miles offshore, the Pacific Plate tried to slip under the Okhotsk Plate, 20 miles under the ocean floor. A magnitude 7.2 earthquake hit Japan. It caused the reactors on the northeast coast to scram due to indications from the ground-motion sensors, including Units 1, 2, and 3 at Fukushima, and it made the news, but nobody was hurt. Three more earthquakes the same day shook the ground. It was just another day in Japan, and life resumed a normal path after the bothersome disturbances. The reactors immediately restarted and resumed power production.
Earthquake prediction is a science in Japan, explored with more enthusiasm than anywhere else on Earth. Accelerometers are spread over Japan and out into the sea floor, and tsunami warning buoys are anchored offshore. These sensors can detect ground or ocean floor movement and send signals back to the Japan Meteorological Agency (JMA) at the speed of light over electrical cables. The earthquake shock travels much slower, at the speed of sound through rock (about 3.7 miles per second), and, depending on how far out the disturbance is, there can be minutes of warning issued by JMA. That is enough time to crawl under something solid or race out the front door of a building. The entire country is wired with earthquake alarms, designed to go off upon ground-movement detection from the array of accelerometers.
On Friday, March 11, at 2:46:43 P.M. Japan Standard Time, two days after the four minor earthquake shocks, Mikoto Nagai, head of the Emergency Response Team in Sendai, was at his desk on the third floor of an earthquake-proof building, sipping coffee. A lot of engineering thought had gone into how to make a building withstand ground accelerations. As Japan rebuilt after having been bombed to the topsoil during World War II, most of the new structures were constructed to sway without the foundation crumbling and the vertical support beams splintering. The early-warning earthquake alarm went off. Nagai put down his cup and looked up at the LED display bolted to the wall. It flashed 100 followed by a 4. In 100 seconds, a hit from a magnitude 4.0 earthquake was expected. The display quickly changed its mind. Make that a 6.0. No, an 8.0. Nagai stood up, and his coffee cup bounced sideways off the desk. Bookshelves collapsed, the internal wall in front of him came down, and people started screaming.
The Pacific plate had successfully relieved the east-west tension and hit Japan with its biggest earthquake ever recorded. It was 9.0 on the dimensionless Moment Magnitude scale.271 In three minutes, the eastern coastline of Japan fell 2.6 feet, and Japan moved 8 feet closer to California.272 The rotational axis of the Earth tilted by 10 inches. Roads were churned, high-voltage power lines were downed, and 383,429 buildings were destroyed.
The point in Japan nearest the epicenter of the earthquake was Onagawa in the Oshika District, and on a point of land jutting out into the Pacific Ocean was constructed the Onagawa Nuclear Power Plant by the Tohoku Electric Power Company, down on the beach. It consists of one BWR/4 and two BWR/5s, built by Toshiba under contract with General Electric. The last one started operation on January 30, 2002. As it was the newest reactor of the group, it has the most updated earthquake hardening techniques applied to it, and it has a substantial, 46-foot tsunami wall between it and the surf. The earthquake rolled through Onagawa, scrammed all three reactors, and subsided without doing any damage to the power plant. All the workers’ homes within driving distance of the site, however, were leveled to the ground.
About 22 seconds after it hit Onagawa, the ground-shock hit Fukushima I, which was twice the distance from the epicenter. An inspector for the Nuclear Industrial and Safety Agency, Kazuma Yokata, was permanently stationed in the office building at Fukushima I, in the no-man’s land between Unit 1 and Unit 5. He heard the alarm go off, but he was not overly concerned until the ceiling appeared to be coming down on him. He cringed as the L-shaped brackets holding up the bookshelves ripped out of the wall and his thick binders containing rules and regulations started flying.
There were 6,413 workers on the Fukushima I site that day. One of them, Kazuhiko Matsumoto, was in the turbine building for Unit 6, finishing some work on air ducts. He suddenly found that it was impossible to remain standing on the sparkling clean deck, and he had to cling to a wall to keep from being dribbled on the floor like a basketball. The lights went out, and the windowless expanse of the turbine hall went black. In a few seconds the emergency lights turned on, and over the loudspeaker came a simple instruction: “Get out.”
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.
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.
