Friday 24 February 2012

It was a really stupid Japanese nuclear blunder

The nuclear plants at Fukushima in Japan are ‘Boiling Water Reactors’ (BWR for short). A BWR produces electricity by boiling water, and spinning a turbine with that steam. The nuclear fuel heats water, the water boils and creates steam, the steam then drives turbines that create the electricity, and the steam is then cooled and condensed back to water, and the water returns to be heated by the nuclear fuel. The reactor operates at about 285 °C. This is generally the way it is done in all nuclear plants.

In the pressurized water reactor, the water which flows through the reactor core is isolated from the turbine. In the boiling water reactor the water which passes over the reactor core to act as moderator and coolant is also the steam source for the turbine.

The disadvantage of this kind of reactor is that any fuel leak might make the water radioactive and that radioactivity would reach the turbine and the rest of the loop the water passes through. However, if operated safely, it is a great way to generate electricity.

In the pressurized water reactor (PWR), the water which passes over the reactor core acts as moderator and coolant does not flow to the turbine, but is instead contained in a pressurized primary loop. The primary loop water produces steam in the secondary loop which drives the turbine. The obvious advantage to this is that a fuel leak in the core would not pass any radioactive contaminants to the turbine and condenser thereby making it to dangerous to be near it.

Another advantage is that the PWR can operate at higher pressure and temperature at about 160 atmospheres and about 315 C. This makes it more efficient than the BWR, but the reactor is more complicated and more costly to construct. Most of the U.S. reactors are pressurized water reactors.

For decades, Three Mile Island and Chernobyl have served as warning for the nightmare of nuclear power generation gone awry. In the wake of Japan's deadly earthquake and tsunami, the still-unfolding disaster of Fukushima Daiichi has come closer than any nuclear crisis in history to making it a fearsome trio.

It remains to be seen how much damage will be caused by the crisis at the Fukushima nuclear power complex, where four of the six reactors have seen a range of woes including three explosions in four days, damage to two containment vessels, possible overheating from spent fuel rods, and mounting peril for the last remaining 50 workers due to dangerous spikes in radiation emissions.

Yet it is already possible to outline key differences that set the current Fukushima situation apart from the 1979 Three Mile Island emergency near Harrisburg, Pennsylvania, and the disaster in Chernobyl, Ukraine, that unfolded seven years later.

Reactor Type

Japan's Fukushima Daiichi nuclear power complex, which began operating in the 1970s, is made up of six boiling-water reactors, or BWRs—a type of ‘Light Water Reactor.’ (Using ordinary water, it is distinguished from ‘heavy water reactors’ which use deuterium oxide, or D2O, instead of H2O.) Three Mile Island used another type of ‘Light Water Reactor’ known as a pressurized-water reactor, or PWR.

Both of these reactors use water for two purposes. The water acts as a coolant, carrying heat away from the nuclear fuel, and as a ‘moderator’ that slows down the release of neutrons during fission reactions.

In a PWR, the water is kept under pressure. This means the temperature can be higher than the boiling point of water without generating a significant amount steam. So the reactor core operates at a higher temperature in these systems, and heat can be transferred more efficiently. Boiling-water reactors operate at lower temperatures, and they tend to be simpler, with fewer parts.

Chernobyl's reactors were a type called RBMK (for the Russian, "reaktor bolshoy moshchnosty kanalny"), which also used water for the coolant. But unlike the ‘Light Water Reactors’, the RMBK used graphite as a moderator. According to the World Nuclear Association, an industry trade group based in London, no other power reactor in the world combines a graphite moderator and water coolant as Chernobyl did, although Russia does have several RBMK reactors in operation.

Most nuclear reactors in the United States today use either BWR or PWR technology, which it is said to be equally safe. Both types of reactors have a kind of self-regulation or ‘negative feedback’ loop: As the reactor gets hotter, the fission reaction slows down, decreasing power. The RMBK design, on the other hand, could go into positive feedback where higher temperature begets more power, which in turn increases the temperature, and so on.

Cause of the accident

At this point in the Fukushima disaster, the tsunami appears to be the immediate culprit, since the plants shut down as they were designed to do following the earthquake. When the tsunami hit an hour later, it damaged the site infrastructure. So while the earthquake had cut the reactors' external power supply, which is needed to keep coolant pumps doing their job, the tsunami killed the diesel backup generators needed to provide power for the cooling system. Batteries provided power for only up to eight hours. Mobile generators were brought in to take over. Still, it's too early to know for absolute sure what sequence of events led to what outcome.

My personal opinion is that the diesel backup should have not been at ground level. Because it was at ground level, the water from the tsunami covered the generator thereby rendering in inoperative. This resulted in no electricity being brought to operate the cooling system.

According to the 1979 Kemeny Commission report on Three Mile Island—the definitive document of that disaster—‘equipment failures’ initiated the event but ‘operator error’ was the fundamental cause of the accident. Emergency cooling systems were shut down, with dire consequences. Three Mile Island would have been a ‘relatively insignificant incident’ the commission found, if the plant operators (or those who supervised them) had kept the emergency cooling systems on through the early stages of the accident.

At the Chernobyl reactor in Ukraine, an ‘ill-conceived, badly executed safety test’ initiated the disaster. A sudden surge of power triggered a steam explosion that ruptured the reactor vessel, according to a report from the United Nations. This allowed further violent fuel-steam interactions that destroyed the reactor core and severely damaged the reactor building. As a direct result of that blunder, hundreds later died and an entire city was evacuated and now that city and its buildings are off limits to everyone who lived there.

Understanding the Problem

The level of access to information about what is going on inside a reactor has increased in the decades since Three Mile Island and Chernobyl.

As Peter Bradford, who served on the U.S. Nuclear Regulatory Commission at the time of Three Mile Island, said, “At Three Mile Island, much of what we thought we knew on the third day turned out to be incorrect.” The extent of fuel melting, and even the fact that a hydrogen explosion had occurred in the containment on the first day, he said, did not become clear for years. There was all kinds of information that we didn't know about.”

During the first few minutes of the accident at Three Mile Island, more than 100 alarms went off, and no system was in place to filter out the important signals from the insignificant ones, according to the 1979 Kemeny report. “Overall, little attention had been paid to the interaction between human beings and machines under the rapidly changing and confusing circumstances of an accident,” the commissioners wrote.

By contrast, the level of computerization and information transfer available today could give Japanese officials much more insight to what happened in the four troubled reactors at Fukushima—at least in theory. They've got so much more going on in terms of the earthquake and the tsunami that the nuclear reactors at the Three Mile Island didn't have.

Radiation Containment

Like the Three Mile Island plant, the Fukushima reactors have three barriers designed to prevent radiation leakage, including metal cladding surrounding the nuclear fuel, a reactor pressure vessel, and the primary containment vessel. Chernobyl lacked a containment vessel.

Once radiation is released into the environment, it can contaminate vast areas like they did when the Chernobyl reactor exploded. Contamination levels are not linear. The further away you are doesn’t necessarily mean that you will get lower doses. That is because other factors such as prevailing winds can influence what areas are affected. In the Chernobyl accident, some areas 100 miles away from the facility had radiation levels that were higher than areas just 10 or 20 miles away.

The Chernobyl pattern was quite erratic because radiation was released very, very high in the atmosphere because of the nature of the reactor and graphite fire. Weather changed over a prolonged emission period, as a graphite fire burned for 10 days. So radioactive gases and particles were picked up by wind and carried high in the atmosphere over long distances before raining down on communities far from the source.

Ultimately, the radiation released as a result of Three Mile Island was not high enough to present detectable health effects in the general population. That accident rated as a level 5 of 7 on the International Nuclear Event Scale, and for this reason it was an accident with wider consequences rather than simply local consequences. At Chernobyl, the level 7 ‘major accident’ radiation exposure affected thousands of people.

Fukushima Daiichi has been elevated to level 4—"accident with local consequences. But it remains to be seen how much higher on the scale this incident will go. In Tokyo, 180 miles away from the plant, peak radiation levels were recorded at 23 times above normal at one point but they reportedly dropped to about 10 times above normal later in less than a day.

Exposure in Perspective

In the United States, the average radiation dose from natural background and man-made sources, such as medical procedures and consumer products, is 620 millirems (mrem) per year, according to the NRC.

One millisievert (mSv) is equal to 100 millirems. The Japanese Ministry of Health Labor and Welfare on Wednesday lifted the maximum allowable exposure for nuclear workers to 250 mSv, from 100 mSv, the Associated Press reported. According to the Nuclear Energy Institute, radioactivity at the plant hit a dose rate of 1,190 mrem per hour initially but dropped to 60 mrem per hour six hours later.

The Chernobyl accident caused acute radiation sickness in 134 of the 600 workers who were at the site on the morning of the initial explosion and received high doses of radiation—80,000 to 1.6 million mrem, according to the UN report and the U.S.

Nuclear Regulatory Commission. Of this group 28 people died within three months. Two others died due to injuries from the fire and radiation. And eventually as many as 4,000 people are expected to die as a result of radiation exposure from the Chernobyl plant, according to the World Health Organization.

From a public health perspective, Chernobyl's greatest impact was an epidemic of thyroid cancer (more than 6,000 cases so far) among children and adolescents exposed to radiation, often caused by them drinking contaminated cow's milk.

The global nuclear power industry today has come together to share information in an effort to help resolve the dangers at Fukushima. There is significantly more communication within the industry now than there was during the disasters at Three Mile Island and Chernobyl.

Communication during a nuclear crisis, of course, must extend beyond industry, and in this area plant operator Tokyo Electric Power (Tepco) is facing harsh criticism. The director general of the International Atomic Agency, Yukiya Amano, called for Japanese counterparts to facilitate stronger communication. According to the Kyodo News Agency, Prime Minister Naoto Kan admonished Tepco executives in a meeting after he learned about an explosion from TV news rather than receiving a call from Tepco. He reportedly demanded to know, "What the hell is going on?"

As the Three Mile Island emergency unfolded, officials attempted to reassure the public that the "danger was over," even as efforts to cool the reactor and stabilize the plant proved ineffective. And in Chernobyl, information hardly flowed at the speed of Twitter. In the view of the London-based World Nuclear Association, an industry group, Chernobyl "was a direct consequence of Cold War isolation and the resulting lack of any safety culture."

The U.S. Environmental Protection Agency wrote in a 1986 journal article on the accident that, "Chernobyl was a secret disaster at first." In fact, the earliest evidence for the international community that a major nuclear accident had occurred came from Sweden, where the discovery of radioactive particles on nuclear plant workers' clothing instigated a search for the source of radioactivity. The following day, the Soviet news agency confirmed the accident at Chernobyl plant but did not offer details, according to the EPA account. "The resulting information vacuum fueled rumors of all kinds, from fatality estimates to speculation about fires in adjoining reactors."

As the crisis in Japan intensified, officials came under fire for statements that in hindsight seem to underestimate the escalating threat. Arjun Makhijani, president of the Institute for Energy and Environmental Research, criticized Japanese authorities for “working from a standard nuclear industry playbook whose byline seems to be, “What, me worry?”

Makhijani is calling for "a frank appraisal of what is known and not known and the potential range of damage and consequences. This would afford the public more confidence in the pronouncements. As it is, verbal reassurances about low radiation levels stand in stark contrast to repeated increases in the radius of evacuations.
As the Wall Street Journal reports, Japan's government had complained about the slow release of information from Tepco. Edwin Lyman, a physicist in the Union of Concerned Scientists Global Security Program and former president of the Nuclear Control Institute, commented in a call with reporters Tuesday that Tepco's briefings are becoming “less and less transparent.”

“There's clearly a kind of erratic quality to the information coming out by the Japanese,” Lyman, said. But this could be explained, he said, by the fact that they're still struggling to find out what's going on. “There's a staggering amount of confusion on the ground,” added Union of Concerned Scientists nuclear expert Ellen Vancko.

“Our concern is that industry in United States and elsewhere doesn't try to whitewash this,” said Lyman. Fukushima Daiichi, he said, is “one of most serious accidents that has occurred in history of nuclear power.”

The unfolding disaster at the Fukushima nuclear plant follows decades of falsified safety reports, fatal accidents and underestimated earthquake risk in Japan’s atomic power industry.

The destruction caused by Japan’s which was a 9.0 earthquake and tsunami came less than four years after a 6.8 quake shut the world’s biggest atomic plant, also run by Tokyo Electric Power Co. In 2002 and 2007, revelations the utility had faked repair records forced the resignation of the company’s chairman and president, and a three-week shutdown of all 17 of its reactors.

With almost no oil or gas reserves of its own, nuclear power has been a national priority for Japan since the end of World War II, a conflict the country fought partly to secure oil supplies. Japan has 54 operating nuclear reactors which is more than any other country except the U.S. and France to power its industries, pitting economic demands against safety concerns in the world’s most earthquake-prone country.

Nuclear engineers and academics who have worked in Japan’s atomic power industry spoke in interviews of a history of accidents, faked reports and inaction by a succession of Liberal Democratic Party governments that ran Japan for nearly all of the postwar period.

Katsuhiko Ishibashi, a seismology professor at Kobe University, said that Japan’s history of nuclear accidents stems from an overconfidence in plant engineering. In 2006, he resigned from a government panel on reactor safety, saying the review process was rigged and ‘unscientific.’
Nuclear Earthquake

In an interview in 2007 after Tokyo Electric’s Kashiwazaki nuclear plant was struck by an earthquake, Ishibashi said that fundamental improvements were needed in engineering standards for atomic power stations, without which Japan could suffer a catastrophic disaster.

Despite what has been said above, Japan’s record isn’t the worst. The International Atomic Energy Agency rates nuclear accidents on a scale of zero to seven, with Chernobyl in the former Soviet Union having been rated seven, the most dangerous. Fukushima, where the steel vessels at the heart of the reactors have so far not ruptured, is currently a class five, the same category as the 1979 partial reactor meltdown at Three Mile Island in the U.S.

The key thing here is that Fukushima is not another Chernobyl. said Ken Brockman, a former director of nuclear installation safety at the IAEA in Vienna. “Containment engineering has been vindicated. What has not been vindicated is the site engineering that put us on a path to having such an accident.”

The 40-year-old Fukushima plant, built in the 1970s when Japan’s first wave of nuclear construction began, stood up to the country’s worst earthquake on record which occurred on March 11, 2011 only to have its power and back-up generators knocked out by the 7-meter tsunami that followed.

Lacking electricity to pump water needed to cool the atomic core, engineers vented radioactive steam into the atmosphere to release pressure, leading to a series of explosions that blew out concrete walls around the reactors.

Radiation readings spiked around Fukushima as the disaster widened, forcing the evacuation of 200,000 people and causing radiation levels to rise on the outskirts of Tokyo, 135 miles (210 kilometers) to the south, with a population of 30 million.

Basement Generator

Back-up diesel generators that might have averted the disaster were positioned in a basement, where they were overwhelmed by waves. That is the height of stupidity. This stupidity occurred in the country that invented the word ‘tsunami’. The Japanese nuclear scientists should have known better.

The cascade of events at Fukushima had been foretold in a report published in the U.S. two decades ago. The 1990 report by the U.S. Nuclear Regulatory Commission, an independent agency responsible for safety at the country’s power plants, identified earthquake-induced diesel generator failure and power outage leading to failure of cooling systems as one of the most likely causes of nuclear accidents from an external event such as an earthquake or a tsunami.

While the report was cited in a 2004 statement by Japan’s Nuclear and Industrial Safety Agency, it seems adequate measures to address the risk were not taken by Tokyo Electric.

Accident was foretold

It’s questionable whether Tokyo Electric really studied the risks. That they weren’t prepared for a once in a thousand year occurrence will not go over as an acceptable excuse for what had happened.

All six boiling water reactors at the Fukushima Dai-Ichi plant were designed by General Electric Co. (GE) and the company built the No. 1, 2 and 6 reactors, spokeswoman Emily Caruso said in an e-mail response to questions. The No. 1 reactor went into commercial operation in 1971.

Toshiba Corp. (6502) built 3 and 5. Hitachi Ltd. (6501), which folded its nuclear operations into a venture with GE known as Hitachi-GE Nuclear Energy Ltd. in 2007, built No. 4.

All the reactors meet the U.S. Nuclear Regulatory Commission requirements for safe operation during and after an earthquake for the areas where they are licensed and sited, GE said on its website.

Botched Container?

Mitsuhiko Tanaka, 67, working as an engineer at Babcock Hitachi K.K., helped design and supervise the manufacture of a $250 million steel pressure vessel for Tokyo Electric in 1975. Today, that vessel holds the fuel rods in the core of the No. 4 reactor at Fukushima’s Dai-Ichi plant, hit by explosion and fire after the tsunami.
Tanaka says the vessel was damaged in the production process. He says he knows because he orchestrated the cover-up. When he brought his accusations to the government more than a decade later, he was ignored.

The accident occurred when Tanaka and his team were strengthening the steel in the pressure vessel, heating it in a furnace to more than 600 degrees Celsius (1,112 degrees Fahrenheit), a temperature that melts metal. Braces that should have been inside the vessel during the blasting were either forgotten or fell over. After it cooled, Tanaka found that its walls had warped.

The law required the flawed vessel be scrapped, a loss that Tanaka said might have bankrupted the company. Rather than sacrifice years of work and risk the company’s survival, Tanaka used computer modeling to devise a way to reshape the vessel so that no one would know it had been damaged. He said that he did that with Hitachi’s blessings.

“I saved the company billions of yen,” Tanaka said in an interview March 12, the day after the earthquake. Tanaka says he got a 3 million yen bonus ($38,000) from Hitachi and a plaque acknowledging his “extraordinary” effort in 1974.

That changed with Chernobyl. Two years after the world’s worst nuclear accident, Tanaka went to the Ministry of Economy, Trade and Industry to report the cover-up he’d engineered more than a decade earlier. Hitachi denied his accusation and the government refused to investigate.

Kenta Takahashi, an official at the NISA’s Power Generation Inspection Division, said he couldn’t confirm whether the agency’s predecessor, the Agency for Natural Resources and Energy, conducted an investigation into Tanaka’s claim.
In 1988, Hitachi met with Tanaka to discuss the work he had done to fix the dent in the vessel. They concluded that there was no safety problem.

In 1990, Tanaka wrote a book called Why Nuclear Power Is Dangerous that detailed his experiences.

Tokyo Electric in 2002 admitted it had falsified repair reports at nuclear plants for more than two decades. Chairman Hiroshi Araki and President Nobuyama Minami resigned to take responsibility for hundreds of occasions in which the company had submitted false data to the regulator.

Then in 2007, the utility said it hadn’t come entirely clean five years earlier. It had concealed at least six emergency stoppages at its Fukushima Dai-Ichi power station and a “critical” reaction at the plant’s No. 3 unit that lasted for seven hours.

Ignored Warnings

Tokyo Electric ignored warnings about the tsunami risks that caused the crisis at Fukushima, Tatsuya Ito, who represented Fukushima prefecture in the national parliament from 1991 to 2003, said in a March 16 telephone interview.

The Fukushima Dai-Ichi plant was only designed to withstand a 5.7-meter tsunami, not the 7-meter wall of water generated by last week’s earthquake or the 6.4-meter tsunami that struck neighboring Miyagi prefecture after the Valdiva earthquake in 1960.

The dangers posed by a tsunami the size of the one generated by the 9.5-magnitude Valdiva temblor off Chile are described in a 2002 report by the Japan Society of Civil Engineers in which it said in part;.
“Tokyo Electric brought this upon itself. This accident unfolded as expected.”

Coming Clean

Ito said he has met Tepco employees to discuss his concerns at least 20 times since 2003 and sent a formal letter to then- president Tsunehisa Katsumata in 2005. “We are prioritizing the safety of the plant and are not at a point where we can reflect upon and properly assess the root causes,” said Naoki Tsunoda, a Tokyo Electric spokesman in Tokyo. He said he couldn’t immediately confirm the exchanges made between Ito and the company.

Kansai Electric Power Co., the utility that provides Osaka with electricity, said it also faked nuclear safety records. Chubu Electric Power Co., Tohoku Electric Power Co. and Hokuriku Electric Power Co. (9505) said the same.

Only months after that second round of revelations, an earthquake struck a cluster of seven reactors run by Tokyo Electric on Japan’s north coast. The Kashiwazaki Kariwa nuclear plant, the world’s biggest, was hit by a 6.8 magnitude temblor that buckled walls and caused a fire at a transformer. About 1.5 liters (half gallon) of radioactive water sloshed out of a container and ran into the sea through drains because sealing plugs hadn’t been installed.

While there were no deaths from the accident and the IAEA said radiation released was within authorized limits for public health and environmental safety, the damage was nominal since three of the plant’s reactors are still offline.

After the quake, Trade Minister Akira Amari said regulators hadn’t properly reviewed Tokyo Electric’s geological survey when they approved the site in 1974.

The world’s biggest nuclear power plant had been built on an earthquake fault line that generated three times as much seismic acceleration, or 606 gals, as it was designed to withstand, the utility said. One gal, a measure of shock effect, represents acceleration of 1 centimeter (0.4 inch) per square second.

After Hokuriku Electric’s Shika nuclear power plant in Ishikawa prefecture was rocked by a 6.9 magnitude quake in March 2007, government scientists found it had been built near an earthquake fault that was more than twice as long as regulators deemed threatening.

While Japan had never suffered a failure comparable to Chernobyl, the Fukushima disaster caps a decade of fatal accidents.

Two workers at a fuel processing plant were killed by radiation exposure in 1999, when they used buckets, instead of the prescribed containers, to eye-ball a uranium mixture, triggering a chain-reaction that went unchecked for 20 hours.

Regulators failed to ensure that safety alarms were installed at the plant run by Sumitomo Metal Mining Co. because they believed there was “no possibility” of a major accident at the facility, according to an analysis by the NRC in the U.S. The report said there were ‘indications’ the company instructed workers to take shortcuts, without regulatory approval.

In 2004, an eruption of super-heated steam from a burst pipe at a reactor run by Kansai Electric killed five workers and scalded six others. A government investigation showed the burst pipe section had been omitted from safety checklists and had not been inspected for the 28 years the plant had been in operation.
Unlike France and the U.S., which have independent regulators, responsibility for keeping Japan’s reactors safe rests with the same body that oversees the effort to increase nuclear power generation: the Trade Ministry. Critics say that creates a conflict of interest that could hamper safety.

“What is necessary is a qualified, well-funded, independent regulator,” said Seth Grae, chief executive officer of Lightbridge Corp. (LTBR), a nuclear consultant in the U.S. “When you have an independent regulatory agency, you can have a utility that has scandals and lies, but the regulator will yank its licensing approvals.
Overall, the situation at the Fukushima Daiichi nuclear power plant remains very serious, but there are signs of recovery in some functions, such as electrical power and instrumentation.

At a press conference held at 11:00 (Japan local time) on the 21st of April, 2011,the chief cabinet secretary, Mr. Edano, announced the establishment of a no entry zone around Fukushima Daiichi nuclear power plant, as well as basic policies concerning temporary re-entry. As of midnight (Japan local time) on 22 April 2011, the area within 20 km of Fukushima Daiichi nuclear power plant was announced as a no entry zone.

Chief cabinet secretary, Mr. Edano, also announced a re-designation of the evacuation zone around Fukushima Daini nuclear power plant. He announced that "the size of the evacuation zone around the station would be reduced from 10 km to 8 km," and that “the order to evacuate based on the incident at Fukushima Daini nuclear power station would be lifted from areas farther than 8 km around the station.”

The Japanese nuclear disaster and tsunami's effects on the environment and the Japanese people will not be known for at least 5 to 10 years. This is how long radiation can take to completely take hold in the body and do its damage.

This will likely also be the time frame in which it will take Japan to reconstruct its damaged cities. It's at that point that the world will be reminded of the infamous deadly tsunami that rocked Japan in 2011, because the poorest, most destitute people there will no longer be able to make a living as farmers. This is because the land will be poisoned. And, rich, affluent Westerners will not be able to indulge in Japanese cuisine as they once did.

Traces of radiation are contaminating vegetables and some water supplies, although in amounts the government says do not pose a risk to human health in the short term. Sale of raw milk, spinach and canola from prefectures over a swathe from the plant toward Tokyo have been banned. The government has just started to test fish and shellfish.

The troubles at Fukushima have in some ways overshadowed the natural catastrophe, threatening a wider disaster if the plant spews more concentrated forms of radiation than it has so far.

The accident at the Fukushima Daiichi nuclear power plant has drawn comparisons to the Chernobyl disaster in 1986. According to some experts, while they are very different, the consequences could be as bad or worse if authorities in Japan fail to prevent a meltdown.

A meltdown is when the nuclear fuel which is basically uranium or plutonium in the reactor core heats up enough to melt. Radioactive materials emit heat along with nuclear radiation, and it is that heat that turns the water in the reactor to steam to power the turbines and make electricity. Reactors are cooled by pumping water through the core. If that system fails, then the core gets hotter until the nuclear fuel melts and runs the risk of breaching the containment vessel.

The short-term radioactive emissions are not as bad as Chernobyl was, but the Fukushima reactor might release more long-term radioactive material.

At Chernobyl, a failure in the cooling system, coupled with a mistake on the part of the operator, meant that the water in the reactor vessel boiled and blew the top of the reactor's containment vessel off. In addition, the Chernobyl design used graphite to moderate the reaction and control it. Graphite works well in this role, but it is almost exactly the same substance as charcoal briquettes. Heated enough, it will burn.

When the Chernobyl reactor vessel exploded it sent out hot chunks of graphite, now exposed to the air, making an already bad situation worse. At Chernobyl, much of the radiation release was due to the fire and the escape of a plume of radioactive gas and dust into the upper atmosphere, mostly caesium-137, iodine-131 and strontium-90, all of which have half-lives that are measured in days or years.

The Japanese reactors use a very different design. The control rods are made of metals that absorb neutrons and would not catch fire. In addition, there is a huge containment building that surrounds the reactor, in part to address just this kind of emergency.

But the Fukushima plant has a lot of spent nuclear fuel in a pool of water above the reactor. Keeping the spent fuel cooled in addition to the active reactor core complicates the situation. Not only could the reactor core melt down, but if the spent fuel loses its coolant (the water) it too could melt down and possibly damage the containment building.

That's why the plant could be longer-term problem, Chernobyl didn't keep the spent fuel in the plant itself. Also, the kinds of radioactive materials that could be released if the containment building fails are a bit different, as one of the reactors at the Fukushima plant uses what is known as mixed oxide (MOX) fuel, which is made up of oxides of plutonium and uranium. Plutonium is considered dangerous even in small amounts. Japan is a much smaller country than the Ukraine, and much more densely populated, so a release of any radionuclide's into the air is that much more dangerous to the Japanese people.

At the Fukushima reactor, the cooling system for three reactors failed because the tsunami, which was higher than any of the surrounding walls had been designed for, submerged the generators that ran the water pumps. At first the plant workers thought they might be able to get more water into the reactor vessel and cool it down, but even with the control rods inserted all the way (which occurred immediately once the earthquake struck) it wasn't enough. The water boiled away in one of the reactors and for this reason, it exposed the fuel rods to the air. The casing of the fuel rods is made of zirconium alloys. Exposed to air and steam at high temperatures causes a reaction and one of the gases it releases is hydrogen.

Plant operators tried to relieve the pressure in the reactor by venting some of the gas. But something went wrong, causing an explosion and another two days later. In a desperate move, the operators flooded the reactor with seawater and boron, an element that absorbs neutrons. Seawater is corrosive, and ruins the equipment in the plant. Unfortunately there was no other choice.

Further problems developed at one of the reactors at the Fukushima plant as efforts to keep the core covered with seawater failed for several hours. That left the upper parts of the fuel rods exposed, and the possibility that there was a partial meltdown. By early the third day, another explosion occurred at reactor number 2, raising the possibility that the containment building had been damaged.

Chernobyl was eventually encased in concrete. The Japanese authorities should do the same with their reactor if they are going to continue with their stupidity.

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