03/14/2011
Japan's Chernobyl
Fukushima Marks the End of the Nuclear Era
Photo Gallery: 46 Photos
AFP/ NHK
Part 3: Countdown to a Nuclear Disaster
When the earth quaked, machines reacted more quickly than any people could have. Seismic sensors at the Fukushima Daiichi nuclear power plant detected the devastating shock waves on Friday within seconds. Two minutes later, at 2:48 p.m. local time, the reactor control system triggered a rapid automatic shutdown of the three reactors that were then in operation.
Everything went smoothly at first. Within seconds, the control rods were inserted between the fuel rods, thereby interrupting the nuclear chain reaction. This is precisely the way the system should operate. But then a serious problem occurred, initiating the countdown to a nuclear disaster.
Even after an emergency shutdown, a nuclear reactor still produces massive amounts of heat as the radioactive materials created during nuclear fission continue to decay. Unless engineers cool down a reactor for several days after it has been shut down, a core meltdown can occur, as was the case at the Three Mile Island nuclear power plant near Harrisburg, Pennsylvania, and at Chernobyl.
To prevent this from happening, pumps continued to put water through the cooling system at Fukushima. But then the power grid collapsed, as a result of the earthquake. The backup generators then went into operation.
'Like Trying to Drive a Car with No Engine'
Each reactor has three or four of these diesel generators. But when the tsunami arrived, the generators failed in two of the reactor units at Fukushima. The entire power plant site was flooded.
The engineers eventually managed to connect emergency batteries to the system. But these batteries are only designed to bridge a period of a few minutes so that, for example, the power supply can be switched from the grid to an internal source. These weak power sources managed to avert an immediate nuclear disaster on Friday evening.
It was an act of desperation, "like trying to drive a car with no engine solely using the battery," says Michael Sailer, the CEO of the Freiburg-based Öko Institut, an independent research institute. Sailer was chairman of the German Reactor Safety Commission for many years. "The batteries represent absolutely a last-ditch attempt," says Lothar Hahn, the former managing director of the Society for Reactor Safety.
While the Japanese engineers were struggling to avert looming disaster, reactor safety experts around the world were sitting in front of their computers and monitoring the progress of the chain reaction in horror. They sent each other emails, spoke on the phone and discussed the problem in special forums closed to the public. There was hardly any official information, but they all had their contacts with experts in Japan. "The situation is very serious," Hahn concluded immediately after learning that the cooling system had failed. "If this continues," an employee with the Japanese nuclear energy agency admitted on Friday evening, "we could, in a worst-case scenario, see a meltdown."
Apparently this is precisely what happened. Because the cooling pumps failed as a result of the loss of power, the water level fell in the reactor vessel. The fuel rods were reportedly only half-submerged in the cooling water, protruding from the water by almost a meter. As a result, they were partially destroyed and became overheated, just as an immersion heater can become overheated when it is removed from water.
Hopeless Struggle
In their desperation, the authorities authorized a controlled release of radioactively contaminated steam into the environment. Radioactivity levels within the plant rose to 1,000 times normal values, and radioactivity also became elevated on the entire site.
Reports that the pressure in the reactor container in Unit 1 had risen to six times atmospheric pressure seemed to herald impending disaster, because the reactor's protective shell can only withstand a pressure level amounting to eight times atmospheric pressure.
The situation at Fukushima escalated dramatically late Friday night. German nuclear expert Sailer likened the situation "to a disaster movie," as engineers desperately fought to gain control over the reactors. In the end, it was apparently a hopeless struggle.
The fuel elements had melted, at least partially, and apparently only the steel container housing the reactor and the containment layer were left, preventing the most highly radioactive materials from escaping. On Saturday evening local time, the plant's operators announced that they intended to flood the reactor with seawater, a last-ditch attempt to prevent the reactor vessel from melting. "They're basically trying to sink the reactor," says nuclear expert Mycle Schneider, who compiles the annual "World Nuclear Industry Status Report."
Echoes of Three Mile Island
The Fukushima accident resembles what happened at the Three Mile Island nuclear power plant near Harrisburg, Pennsylvania in 1979. On the morning of March 28, 1979, a blocked valve and various operating errors led to the loss of vast amounts of fluid from the cooling system for the plant's second reactor unit.
An automatic emergency shutdown stopped the chain reaction in the reactor core, as was the case in Japan last week. But the loss of cooling water resulted in a buildup of residual heat coming from the core material, melting some of the fissile material. Radioactive gases escaped into the environment, and it took experts five days to regain control over the reactor.
The Harrisburg accident was the first reactor catastrophe to generate worldwide questions about the safety of nuclear energy. But it was only after the Chernobyl disaster, the 25th anniversary of which is coming up, that many nations turned away from the high-risk technology.
Deadly Legacy
The nuclear core in one of Chernobyl's reactors also melted on that fateful day, April 26, 1986. Ironically, it was during a safety inspection that operators lost control over reactor number four of the Chernobyl nuclear power plant, located near the city of Pripyat in present-day Ukraine.
As a result of various operating errors, the output of the reactor core rose to about 100 times its rated output. The resulting extreme heat destroyed the channels for the reactor's control rods, eliminating precisely the mechanism that is vital to preventing a nuclear fire. A disastrous series of chemical reactions led to the accumulation of an explosive mixture of gases beneath the roof of the reactor pressure vessel, which eventually ignited.
When the 1,000-ton concrete roof of the vessel was blown into the air, the reactor core caught fire. Large amounts of radioactive material, like iodine 131 and cesium 137, were released into the air and dispersed across large parts of the western Soviet Union and Western Europe.
The fallout descended onto about 200,000 square kilometers (77,220 square miles) of land. Because the Soviet government was unwilling to acknowledge the disaster for several days, valuable time was lost for such tasks as evacuating the nearby city of Pripyat. Many of the cleanup workers, known as "liquidators," were exposed to high doses of radiation in the first few days. The incidence of thyroid cancer has been elevated in the region surrounding the plant for years. The concrete containment shell that was hurriedly built around the reactor is beginning to crack and crumble.
'Historic Relic'
Human error was to blame for the reactor accident in Ukraine. Fukushima could now serve as a warning that nuclear reactors cannot be protected with absolute certainty against the forces of nature, especially not when it comes to aging plants like Fukushima.
The Japanese reactor is "a historic relic," says Shaun Burnie, a British nuclear expert for Greenpeace who is very familiar with the reactors on Japan's east coast.
Burnie has visited the Fukushima reactors several times and has repeatedly worked in Japan. Reactors 1 and 2 at Fukushima Daiichi went into operation in the early 1970s, when safety standards were significantly more lax than they are today. They were built in an era when Volkswagen was building its Beetle without safety belts, airbags and headrests. The reactor that exploded on Saturday was in fact slated to be shut down soon.
Because the new construction of nuclear power plants is so expensive and difficult to defend politically, energy utilities in more and more countries are convincing governments to approve operating-life extensions that are much longer than those planned for German reactors. However, the renaissance of these aging power plants is now proving to be a dangerous game.
Limited Chance of Upgrade
Plant operators are trying to keep their reactors on line beyond their original 40-year life spans. The United States has extended licenses for many of its nuclear plants by 20 years, and European countries are following suit. But the safety technology in older plants can only be upgraded to a limited extent.
Eleven reactors in Japan had to be shut down on the day of the earthquake. Five were in a state of emergency because they could not be cooled properly. "This is a traumatic event. The international nuclear industry has tried to delay its demise with massive life span extensions," says nuclear expert Mycle Schneider. "The ancient systems at Fukushima have now illustrated the consequences. The industry will not survive this."
Burnie takes a similarly critical stance. "Never in a thousand years would you get a license for Fukushima today," he says. In the second-generation boiling water reactors that are still being used in the plant, the fuel rods float directly in the reactor vessel. Germany also has nuclear plants in the same category, including Brunsbüttel in the northern state of Schleswig-Holstein. Most of all, says Burnie, earthquake safety can only be improved to a limited extent. "The foundation consists of thousands of tons of concrete. That can't be upgraded."
