FUKUSHIMA, MARCH 11, 2011 How the Accident Happened

A huge natural disaster hit Japan at 2:46 p. m., local time, on March 11 as a 9.0 earthquake occurred at sea 95 miles from the Daiichi nuclear power plant near Fukushima. There were six reactors at the Fukushima plant. Units 1-3 were oper­ating and they immediately shut down and went to emergency cooling with diesel generators after the earthquake disrupted electrical power to them, as they were supposed to do. Unit 4 was undergoing a fuel change so it had no nuclear fuel in the core. Units 5 and 6 were in cold shutdown and were not operating.

The huge earthquake, the largest ever to strike Japan, caused two enormous tsunamis that devastated coastal cities in northern Honshu about an hour later. By June, the toll of dead and missing was above 24,000. The final number of dead and missing a year later was about 19,000 with 27,000 injured. Four hundred thousand buildings were destroyed and nearly 700,000 more were damaged (36). The tsunami caused a wall of water about 45 feet high to surge over the reactors at Fukushima, topping the 20 foot wall that was designed to protect the reactor. The flood covered the diesel generators with water and they quit working. Battery backup power was activated to run an additional emergency cooling system, but the batteries failed after a few hours and there was no more electric power to run the pumps to cool the reactors. As a result, a nuclear accident developed on top of the enormous devastation of the earthquake and tsunami (37-39).

The reactors at Fukushima are boiling water reactors, different from those at Three Mile Island but the same general type as those at Chernobyl. However, they did not have graphite moderators but were cooled by water. The reactor core is contained in a steel reactor vessel which sits within a concrete primary con­tainment structure. A torus surrounds the reactor and is connected to the core (Figure 10.3). It is designed to provide emergency cooling even in the event of total loss of external power, but it can only do this until water gets to the boiling point. With the total devastation caused by the tsunami, after the water in the core reached boiling, the nuclear crisis began.

The water in the cores of reactors 1-3 began to turn to steam and, as pressure built up, relief valves opened up to vent the pressure. The water level dropped in the reactor cores, like what happened at TMI but even worse. As about three-fourths of the cores were exposed, the temperatures rose to over 1,200°C and the Zircaloy cladding started to oxidize and fall apart, fission products were

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released, and hydrogen was produced from the reaction of zirconium with steam. Most of the core melted in unit 1 by 16 hours after the tsunami. It was later learned that the melted core had burned its way through the bottom of the steel reactor pressure vessel (RPV) and ate its way about 70 centimeters into the concrete pri­mary containment structure. Much of the fuel in units 2 and 3 also melted a day or two later but did not melt through the steel RPV (40).

Hydrogen and volatilized fission products such as xenon, iodine, and cesium built up excessive pressure in the reactor primary containment structure and had to be vented into the secondary containment buildings. These buildings have only about 1-inch thick walls and could not contain much pressure, so they were vented, releasing hydrogen and some radioactivity. In spite of this, on March 13 hydrogen explosions occurred in reactor units 1 and 3 and blew apart much of the secondary containment building, though the reactor primary containment struc­tures were still intact. Reactor 3 burned reprocessed MOX fuel, so it had a higher proportion of plutonium in it, but the plutonium and uranium remained in the reactor cores. Reactor 2 had damage to the torus, and this led to an uncontrolled leakage of fission products to the environment (38, 41, 42). By October, a cover was constructed for reactor building 1 to contain any further releases of radiation and by December 16, all three reactors were in cold shutdown with temperatures at around 70°C. Construction of covers for buildings 3 and 4 began in 2012 (40, 42-44).

Reactor 4 was shut down before the tsunami and did not have any fuel in its core, so there was no problem with a core meltdown. However, the reactor build­ings housed the cooling pools that contained the spent fuel rods from the reactors,
and the cooling pool for reactor 4 was unusually full, with a full core load of fuel, thus generating a lot of heat. Cooling pools have to be cooled with circulat­ing water, and the tsunami also shut down those pumps. As water boiled out of cooling pool 4, part of the fuel rods became exposed and became hot enough to partially melt and release hydrogen gas and fission products. Building 4 also exploded from a hydrogen gas explosion, the gas apparently coming from shared vents with reactor 3. Water was pumped into the fuel pools after the accident to keep them from boiling out the water. At the present time, the cooling pools are being cooled to normal temperature with a closed loop water circulation system, and the fuel pool 4 building has been reinforced (40).

Not all of the reactors that were caught in the earthquake and tsunami had nuclear accidents. Reactors 5 and 6 were in cold shutdown and did not have any problems. Three of the four reactors at the Daini nuclear power plant about 7 miles south of Daiichi were running at full power, and their diesel engines were also shut down by the tsunami but they were able to go into a full cold shutdown without experiencing a meltdown or release of radioactivity. Seventy-five miles north of Daiichi and even closer to the epicenter of the earthquake, three reac­tors were operating at the Onagawa nuclear power plant, and they were built to withstand 9-meter tsunamis. They went into a normal cold shutdown and did not release any radioactivity (42, 45).

In contrast to the other nuclear power accidents that were caused by operator error and design problems, the Fukushima accident was the result of an enormous natural disaster for which the reactors were not designed. The entire infrastruc­ture of a huge area of northern Japan was destroyed by the earthquake and tsu­nami—roads destroyed, power out, buildings shattered, equipment trashed, boats sitting far inland, nearly 20,000 people dead. This made it extremely difficult to bring equipment in to deal with the accident, and workers had to deal with a nuclear crisis even though many of them had lost their houses and perhaps fam­ily members. It was truly a heroic effort on the part of these workers to bring the reactor crisis under control under these horrific conditions.

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