Fukushima Meltdown

Japan generates 29% of its electricity from nuclear power plants. The facilities are designed to withstand earthquakes and tsunamis that are common in Japan, which generates its nuclear electricity from 54 nuclear power reactors at 17 plant. Nuclear reactors at the Fukushima Daiichi plant are of the Boiling Water Reactor (BWR) type which is one of the Generation II nuclear reactors.

A state of emergency was declared on Friday, March 11 2011 after a combined earthquake of magnitude 8.9 - 9.0 on the Ritcher scale (Click Here) near the east coast of Honshu and a tsunami event generating a 15 - 24m high wave. The earthquake event is designated as the Tohoku-Chihou-Taiheiyo-Oki earthquake.Official records dating back to the year 1600 inspired the mechanistic safety analysis design of the plant to withstand the strongest earthquake at the 8.6 magnitude level for the Fukushima prefecture.

According to the plant design, the maximum probable height at Fukushima was at just 5.7 meters compared with the actual 14m. The earthquake triggered a shutdown of the 3 operating reactors at the site as designed. The 3 others were already shutdown for maintenance. There were 6,415 people at the site of which 5,500 were subcontractor. When the reactors were shutdown and the remaining decay heat of the fuel was being cooled with power from emergency generators. The subsequent destructive tsunami with waves of up to 14 meters disabled emergency generators required to cool the reactors. Over the following three weeks there was evidence of partial nuclear meltdowns in units 1, 2 and 3.Visible explosions, suspected to be caused by hydrogen gas, in units 1 and 3, with a suspected explosion in unit 2, which that may have damaged the primary containment vessel and a possible uncovering of the units 1, 3 and 4 spent fuel pools. Radiation releases caused large evacuations, concern about food and water supplies, and treatment of nuclear workers. 

When case of an earthquake happen the automatic control systems first and foremost would kill the sustained fission reaction that is going in the fuel elements. This was done at the Fukushima plant immediately by inserting the control rods and the nuclear reaction stopped. During normal operation in a BWR, the control rods are used to maintain the chain reaction at a critical state. The control rods are also used to shut the reactor down from 100% power to about 7% power (residual or decay heat).

 

Control Rods

The problem is that during the fission reaction one also produces a lot of short-lived nuclear isotopes. At this point, the cooling system has to carry away the residual heat, about 7% of the full power heat load under normal operating conditions. During that time, water is still being circulated through the reactor core in order to take away the heat produced in the decays of those short-lived isotopes. This is done via pumps that are operated via electricity from power grid or diesel generators or batteries.

After the earthquake, the grid was knocked out and the diesel generators got damaged. When the diesel generators failed after the tsunami, the reactor operators switched to emergency battery power. The batteries were designed as one of the backup systems to provide power for cooling the core for 8 hours. And they did. After 8 hours, the batteries ran out, and the residual heat could not be carried away any more.  Unfortunately, without an active removal of decay heat the reactor was adding heat to the water faster than it was taking it out, and the temperature was rising.  Because this was a reactor that operated on water that was already at its boiling point, this also meant that the pressure inside the reactor was rising as well. 

Apart from that, it appears that there were two hydrogen explosions in Units 1 and, recently, Unit 3. The hydrogen come from the chemical reaction when the reactor temperature exceeds 1000°C, the reaction of the zirconium alloy in cladding tubes with water generates large amounts of hydrogen in the name of oxidizing reaction. This oxidizing reaction produces hydrogen gas, which mixes with the gas-steam mixture being vented.  This is a known and anticipated process, but the amount of hydrogen gas produced was unknown because the operators didn’t know the exact temperature of the fuel rods or the water level. This hydrogen leaked and collected near the ceiling of the reactor buildings, and since hydrogen gas is extremely combustible, when enough hydrogen gas is mixed with air, it reacts with oxygen it caused immense explosions. This explosion destroyed the top and some of the sides of the reactor building, but did not damage the containment structure or the pressure vessel. While this was not an anticipated event, it happened outside the containment and did not pose a risk to the plant’s safety structures.

Japanese Spirit After Fukushima Accident

Japan continues to deal with the enormous task of cleaning up and moving forward after the 9.0 earthquake and tsunami that devastated the northeast coast. Local authorities are still dealing with the damaged Fukushima Daiichi Nuclear Power Plant, and due to weather condition, it could increase the risk of disease as workers clear away the debris, is approaching.

Kaisei Kubota and his grandmother Yae pray for victims in an area devastated by a tsunami in Miyako, Iwate prefecture, northeastern Japan, on Saturday June 11. Kaisei's father, a voluntary firefighter manning a water gate of a coastal levee, was killed after being swept away by a tsunami on March 11. Source: Kyodo News/Associated Press

A resident, evacuated from Namie town, right, undergoes a screening test for possible nuclear radiation after a brief visit to her home in the 20-kilometer exclusion zone around Tokyo Electric Power Co.'s Fukushima Dai-Ichi nuclear power station, in Minami Soma, Fukushima prefecture, Japan, on Saturday, June 11, 2011. About 80 percent of the city is within a 30-kilometer restriction zone around the plant, while 4,100 households lived in a full evacuation zone set up by the government within 20 kilometers of the plant.

In this combo of two photos, a sea coast is filled with destroyed houses and debris on March 12, 2011, one day after the devastating earthquake and tsunami hit the area, top, and the same area, bottom, with the houses and debris cleared as photographed on June 3.

There is a phrase in Japanese "Makeji Damashi" which means  the Undefeated Spirit. And spirit is one thing that runs in no short supply in Fukushima.

Regards,

Nuclear Boy