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The Yomiuri Shimbun

Tokyo Electric Power Co. and outside experts have said that Saturday's blast at the No. 1 reactor at the Fukushima No. 1 nuclear power plant was caused by a so-called hydrogen explosion, in which hydrogen filled the reactor's outer container and combined with oxygen in the air.

A partial meltdown of the nuclear fuel in the reactor also is believed to have occurred. Why and how did the No. 1 reactor plunge into such a serious condition?

Every nuclear power plant is equipped with a number of protective systems to prevent radiation leaks in the event of an accident.

One such system is the reactor pressure vessel that covers the reactor core--it is designed to prevent radiation from being released should something go wrong with the nuclear fuel.

Even if the reactor vessel were damaged, radiation leaks are supposed to be prevented by another container surrounding it. Around these is the reactor's outer building, of which all but the framework was blown off in Saturday's explosion.

In an emergency press conference later Saturday, Chief Cabinet Secretary Yukio Edano said a hydrogen explosion had occurred inside the external container structure.

Oxygen explodes when it combines with hydrogen. This can be see in experiments in which a hydrogen-filled test tube is placed close to a flame, and a popping sound is heard as the two combine.

U.S. space shuttles have used the hydrogen-oxygen reaction to acquire the enormous energy required for space travel.

Why did hydrogen fill the external containment structure before the blast Saturday afternoon?

According to Professor Emeritus Keiji Miyazaki at Osaka University, an expert in atomic energy engineering, nuclear fuel is enveloped in a particular type of metal covering. When the fuel reaches an extremely high temperature and then comes into contact with the cooling water, the metal's properties break down the water and generates hydrogen.

The hydrogen can leak from the reactor containment vessel into the outer structure through valves and other gaps.

The hydrogen that filled the outer structure Saturday is believed to have exploded when it mixed with the oxygen in the air, Miyazaki said.

According to a nuclear technology expert with experience designing the same type of reactor as the Fukushima No. 1 plant, neutrons in the reactor's pressure vessel usually collide with water molecules and create hydrogen. It is quite possible that hydrogen derived from water molecules leaked out of the vessel, the expert said.

Miyazaki said TEPCO should have acted more promptly.

"It was an extremely serious accident. I have the impression that [TEPCO] spent too much time backing up the emergency electricity source, which had gone down," the professor said.

"I think they could have prevented the accident if they'd started putting water [into the reactor] with temporary fire pumps to lower temperatures inside the reactor earlier. They finally started doing that Saturday morning."

Though it turned out the explosion was caused by hydrogen, this does not necessarily mean that all the problems have been clarified.

"We can't be relieved just because the government said the reactor pressure vessel and container were safe," a former official of the government's Nuclear Safety Commission said.

"Though the level of detected radioactive substances has declined, we don't know what really happened to the nuclear fuel rods inside the pressure vessel," he said.


Reactor core overheated

A nuclear meltdown may have occurred before Saturday's explosion in the Fukushima No. 1 nuclear power plant's No. 1 reactor building, according to a key government nuclear safety panel.

The Nuclear and Industrial Safety Agency's statement indicates that the temperature in the reactor rose to an extremely dangerous level, analysts said.

Nuclear fuel rods normally are kept submerged in water inside reactor cores, thus preventing the rods' temperature from exceeding a certain limit. However, if the water level goes down and the fuel rods are exposed, the cooling system's efficiency rapidly deteriorates, causing the reactor core's temperature to rise.

If the core's temperature exceeds a certain level, the fuel rods melt--in other words, a meltdown occurs.

The detection of traces of a nuclear substance called cesium outside the Fukushima nuclear power plant led the agency to infer that a possible meltdown occurred.

Cesium is formed after uranium, which is used for nuclear fuel at the plant, undergoes fission. However, nuclear fuel is kept in a pellet form covered by a special metal. It is impossible for cesium to leak from the shell unless the metal melts--which occurs when the temperature reaches 2,700 C to 2,800 C, according to the agency, an affiliate of the Economy, Trade and Industry Ministry.

The detection of cesium indicates the temperature of the reactor core has reached a seriously high level.

If the water level does not rise enough to stop the reactor core from overheating, the reactor core's stainless steel cover could melt. In the worst-case scenario, the reactor core itself might explode due to a build-up of excessive pressure.

(Mar. 14, 2011)

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