By Kennedy Maize
Washington, D.C., March 11, 2011 – At this writing, 7:45 p.m. EST, it is hard to be optimistic about what is happening at Japan’s Fukushima 1 nuclear power plant. Matt Wald of the New York Times reports, based on Japanese accounts, that radiation levels in the control room are “1,000 times above normal” and there is some evidence of off-site radiation.
This suggests to me – and it is far too soon to offer definitive opinions – that there has been fuel damage in the 439-MW boiling water reactor. If that is the case, it seems likely that it resulted from one of the most feared accident conditions that can hit a functioning light-water nuclear plant – what’s known in the industry as “station blackout.” That’s when offsite power to the plant – power which runs the main and emergency cooling systems – fails and backup diesel generators on site for the purpose of keeping vital safety systems running also fail.
When a nuclear plant shuts down suddenly, the threat that exists is not radiation. The cooling water is also the nuclear moderator; without a moderator, the chain reaction ceases. Control rods would also automatically activate to stop an uncontrolled fission reaction. But there is a tremendous amount of heat left in the fuel – known as “residual heat” – that must be removed to prevent the fuel from breaking apart, melting and releasing radioactive products of the fission reaction.
At the point that the plant lost backup AC power, it would rely on batteries to keep the pumps running and the core covered with cooling water. But batteries have a limited life. The Times cites the International Atomic Energy Agency as saying that mobile generators have been moved to the plant to provide power for core cooling pumps. The radiation reports suggest that fuel has already been damaged, perhaps melted, releasing radiation on site.
Ed Lyman of the Union of Concerned Scientists, in a briefing paper today, explains the details of the emergency core cooling system at the plant: “The boiling water reactors at Fukushima are protected by a Reactor Core Isolation Cooling (RCIC) system, which can operate without AC power because it is steam-driven and therefore does not require electric pumps. However, it does require DC power from batteries for its valves and controls to function.
“If battery power is depleted before AC power is restored, however, the RCIC will stop supplying water to the core and the water level in the reactor core could drop. If it drops far enough, the core would overheat and the fuel would become damaged.”
The risk to the public, as Japanese officials have stressed, are minimal. The amount of radiation that is likely to escape the site is small. But boiling water reactors, including the 40-year-old Japanese unit, do not have the large, dry concrete-and-steel containment domes that protect pressurized water reactors, so a radiation leak is more likely for a BWR. This has been a contention of the UCS for decades. I won’t go into the intricacies of arguments over BWR versus PWR containments here, but the debate has been going on for a long time.
According to a fact sheet from the Nuclear Information and Resource Service, an anti-nuclear group, the Fukushima station consists of six General Electric BWRs. Unit 1 went into service in in 1971. Units 2,3,4, and 5 are all 760-MW boilers that went into service between 1974 and 1978 and Unit 6, at 1067 MW, went commercial in 1979. The five earliest units all have Mark 1 containments, known among industry folks, as “light bulb and donut” containments for the way they look, which are design to channel steam in an accident through water, reducing the steam pressure on the surrounding structures. All BWRs feature “pressure suppression” containments.
After initially evacuating a three-kilometer zone around the plant, Tokyo Electric Power Co. this afternoon (our time) expanded that to 10 kilometers. There have been uncorroborated reports that Fukushima 2 also lost emergency backup power. The other four operating units at the plant some 200 miles north of Tokyo reportedly shut down as designed as backup generators kicked on to supply cooling water.
I’ll update this blog as more details emerge, which, given the past history of serious nuclear accidents, is likely to occur over a long period of time.