Thursday, March 24, 2011
The Roots of Fukushima
The nuclear safety problems in Japan go back a long way to the start of the industry
When Chiro Okuba was a young engineer in his 20s, he went to work on the containment and safety systems of one of Japan's earliest nuclear power plants, Unit-1 of what would become a complex of six boiling-water reactors at Fukushima north of Tokyo, site of the unfolding nuclear emergency. Fukushima Unit-1 is one of the oldest nuclear power plants in Japan, indeed the world. It went into operation in 1971, exactly 40 years ago this month. It was expected to keep pumping out power. Just in February, it received permission from the Ministry of Economy, Trade and Industry to extend its working life.It is not known whether the plant's advanced age contributed to the disaster (although the New York Times reported this week that there may have been unreported cracks in the backup diesel generators), but the roots of the crisis go back a long way.
"We didn't take a tsunami into account” in planning for an emergency, says Okubo, who spent his career with Toshiba, one of Japan's principal nuclear power constructors. Only when he was near retirement, he says, did he hear some concerns being raised about impact of a tsunami on nuclear plant safety.
That is a strange lapse. When the plant was being designed and built in the late 1960s, the famous Chilean Earthquake in 1960 was fresh in people's mind. It was the largest earthquake of the 20th century, measuring 9.5 on the Richter scale. It set off a huge trans-Pacific tsunami that hit northeastern Japan, same area as this month's quake, killing 195 people.
The first unit was built by the American company General Electric, which pioneered the type of power plant known as a boiling-water reactor. Later units were built by Toshiba and Hitachi but using American plans. "We just copied the US blueprints,” says Okuba, without trying to adapt them to conditions specific to Japan.
The early designs presumably did not factor in a tsunami, since such an event is not widely feared in the US except in Hawaii, where there are no nuclear power plants. Early American designs were sited inland, and even today only two plants are sited on the Pacific coast in California.
Weaknesses in the early design, known as Mark -1, were suspected early on. The containment system was thought by some critics to be too weak to stand up to a hydrogen explosion such as the ones that occurred at three of the Fukushima reactors.
(It is correct to refer to this as a "system” since the containment at a boiling-water reactor is made up of the steel and concrete pressure vessels around the core and a suppression pool designed to absorb steam. The box-like structure one sees in photographs is not part of the containment.)
Nuclear safety regulators in the US have ordered utilities to make modifications and retrofit their existing plants, but it is unclear how much was done in Japan before the crisis. It is possible that no reactor could have withstood the one-two punch of an earthquake and tsunami.
Everyone understands that Japan, sitting right on the so-called Pacific Rim of Fire, is very prone to earthquakes. The country made a huge bet that it could build nuclear reactors that are strong enough to withstand even the strongest shaking. It has 54 such plants supplying roughly 30 percent of the nation's electrical energy.
In fairness it can hardly be argued that Japan has been cavalier about the prospects of an earthquake (some anti-nuclear groups might beg to differ). Indeed, even before last Friday's tremor, Japan had experienced two recent major quakes that severely impacted its nuclear program although without causing any radiation leakage.
In July 2007 a major quake struck of the coast of the Sea of Japan near where the Tokyo Electric Power Co. has seven huge reactors, the largest concentration of civilian plants in the world. Although they were only slightly damaged, the regional authorities have been very cautious about restarting them. Three units are still out of commission nearly four years after the quake.
Then in August 2009 another large quake struck in Suruga Bay south of Tokyo near where the Chubu Electric Power Co. has five nuclear power plants. One plant, which experienced unusually high shaking, has only recently returned to service. At one time some 12 nuclear power plants were out of service, contributing to Japan's miserable capacity factor, the percentage of time a reactor is operating.
The event on the Sea of Japan resulted in a general strengthening of seismic standards and some refitting. Chubu decided to decommission two of its older plants near Suruga Bay because they deemed it uneconomical to bring them up to current seismic standards.
But all of this attention was focused on ground movement and protecting the containment. That a tsunami might take out the entire emergency core cooling system at half a dozen plants at once apparently was not considered to be a credible threat.
It is an interesting but now largely academic question is whether the Fukushima plants would have survived the earthquake if there had been no tsunami. We will never know because the plant operators never had time to assess the damage before they were overwhelmed by the mighty tide.
It is apparent, however, that the emergency core cooling system did in fact activate as it was supposed to do immediately after the earthquake and functioned for several minutes before the surging wave took every one but one out of service.
There was nothing left to do but improvise. That set off a mad scramble to try to keep ahead of falling water levels, hydrogen explosions, radiation venting and a multitude of other obstacles in hopes of preventing a complete collapse of the reactors' overheated cores, a race which, as of this writing, is still touch-and-go. by Todd Crowell for The Asia Sentinel