Analysis on the ongoing nuclear reactor crisis by Keiji Takeuchi, a veteran Asahi Shimbun writer of nuclear issues
Crucial vents were not installed until 1990s
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A belated decision to install vents in the core containment vessels in the reactors at No. 1 Fukushima nuclear power plant may have helped avert a major nuclear catastrophe.
The situation at the plant continues to develop, but it is clear that gas release vents on the containment vessels at the heart of the damaged plant's reactor buildings have played an important role in trying to limit the scope of the disaster.
Those vents were not part of the original design for the reactors and were not installed when they were first built because officials believed a core meltdown would never occur in Japan.
Only after vents were installed at nuclear power plants abroad did Japanese officials eventually decide to install the crucial feature.
The vent releases gas in the containment vessel into the atmosphere through a filter designed to remove radioactive material.
Gas was released from the No. 1 reactor on Saturday. Because there was no electric power to the reactor, the vent had to be opened manually by plant employees with the help of a small compressor.
After the vent was opened, the pressure within the core containment vessel, which had risen to 8 atmospheres (atm), fell sharply.
The containment vessel is designed to withstand a pressure of only 4 atm.
Experts said the vent may have been crucial to preventing an explosion of the core containment vessel that could have led to a major disaster. As of the time of writing, there does not appear to have been an explosion of a containment vessel at the plant. The explosions that have occurred have been in the buildings housing the containment vessels.
The vent in the No. 3 reactor was opened on Sunday morning to release gas.
The six reactors at the No. 1 Fukushima nuclear power plant began operation in the 1970s, and the four reactors at the No. 2 Fukushima nuclear power plant came online in the 1980s. Originally, none of the reactors had a vent because officials believed a "severe accident," such as a core meltdown, would never occur.
The 1979 meltdown at the Three Mile Island nuclear power plant was stopped only just short of a dangerous explosion. In 1986, an explosion occurred at the core of the Chernobyl nuclear power plant in the former Soviet Union.
Following those accidents, France, Sweden, Germany and the United States, among other countries, began revising their reactor designs to include vents.
In Japan, many officials stuck with the line that a severe accident could not occur in the country and that there was no need to take such measures.
However, in 1992, the Nuclear Safety Commission of Japan recommended that experts give renewed consideration to whether to install vents.
Electric power companies eventually changed their position, saying that, while the probability of an accident was extremely low, such a measure would improve safety.
Vents were installed at the boiling-water reactors (BWRs) operated by Tokyo Electric Power Co. from the mid-1990s. The design was revised to allow gas to be released into the atmosphere through a filter located at the bottom of the containment vessel.
Kansai Electric Power Co. did not install vents because its pressurized-water reactors had larger containment vessels than TEPCO's BWRs. KEPCO took measures to increase its ability to cool gases within containment vessels.
Opening the vents entails major risks. After gas was released from the No. 1 reactor, a hydrogen explosion occurred within the building housing the reactor. The walls were blown away and four people were injured. An explosion also ocurred at a building that houses the No. 3 reactor on Monday.
It is believed that the release of the gas from the core containment vessel may have been connected to the accumulation of hydrogen within the building.
That explosion has raised a number of questions about TEPCO's disaster management. Had it properly considered the ramifications of opening up the vents? Was it sufficiently prepared?
Whatever the outcome of the current crisis at the No. 1 Fukushima plant, the facts are that a core meltdown has occurred, that a large number of people have been evacuated and that radioactive materials have been released into the atmosphere. That reality will decisively change how the Japanese people think about nuclear energy.
The issue cannot be explained by repeating the excuse, "It was beyond what we could foresee."
Keiji Takeuchi, an Asahi Shimbun senior staff writer, has covered nuclear-related issues, such as the Chernobyl nuclear power plant accident in 1986.
