Fukushima update: cracked fuel rods threaten meltdown
(Image: DigitalGlobe)
Andy Coghlan and Michael Marshall, reporters
The situation at Japan's embattled Fukushima-Daiichi nuclear plant has grown worse. One of the plant's four reactors is now close to meltdown, and while that's bad, it's not as apocalyptic as it might sound. New Scientist asked nuclear scientists to explain what has happened, and what the risks are.
First, "there is no possibility of a nuclear explosion," explains Richard Wakeford of the University of Manchester's Dalton Research Institute. He says any such fears are "science fiction", as there is nowhere near enough radioactive uranium 235 in the reactor to create a nuclear explosion.
The real fear is that harmful radioactive material will escape from the reactor core.
Although all four reactors automatically shut down immediately after Friday's earthquake, engineers have struggled to cool down the reactor cores, because pumps that should have driven cooling water into the reactors failed. This meant that the reactors overheated, turning the water into steam.
The buildup of steam meant the pressure inside the reactor increased, making it impossible to pump more cooling water in. So the engineers vented the steam, carrying some radioactive caesium-137 and iodine-131 (both of which are produced by the uranium in the fuel rods) into the environment.
Michael Bluck of Imperial College London explains that the fuel rods are tubes of zircoloid stuffed with uranium dioxide. When these are not cooled enough, they swell up and can crack. At that point, radioactive caesium and iodine gases can escape. Bluck says:
When caesium was detected, that indicated that some of the rods had ruptured.
As the zircoloid heats up, it reacts with the cooling water to form hydrogen, which is a highly explosive gas. This was to blame for the dramatic explosions that damaged the outer buildings of reactors 1 and 3.
However, it is now reactor 2 that is causing the most concern. Replacement pumps intended to inject cooling water have repeatedly failed, meaning that water levels fell and the fuel rods overheated still further. According to Kyodo News:
Water levels sharply fell and the fuel rods were fully exposed for about 140 minutes in the evening as a fire pump to pour cooling seawater into the reactor ran out of fuel and it took time for workers to release steam from the reactor to lower its pressure, the government's nuclear safety agency said.
Then within the last few hours a further accident occurred. Kyodo News reports:
Tokyo Electric Power Co. said Monday fuel rods were fully exposed again in the No. 2 reactor of the Fukushima No. 1 nuclear power plant as of 11 p.m. TEPCO said a steam vent of the pressure container of the reactor that houses the rods was closed for some reason, leading to a sudden drop in water levels inside the reactor.
Meltdown?
This series of coolant failures has increased the chance that the fuel rods will start to melt. Bluck says:
If you fail to cool it, the uranium can melt and it will all fall to the bottom as a big soup.
But even if the rods do melt and sink to the base of the reactor vessel, this shouldn't be a problem unless the vessel itself breaks open. "The big question is whether the containment holds," says Wakeford. "There was a meltdown at Three-Mile Island in New York, but the vessel remained intact."
Wakeford says there is no chance of a "China syndrome" scenario, with the fuel burning its way right through to the earth's core with potential to blow up the planet.
The repeated coolant failures have made the situation much worse, because temperatures and pressures will have risen much more. The pressure vessel that contains the fuel rods will have some threshold beyond which it cannot cope, and will break open.
Health
If the pressure vessel does burst, radioactive gases would be released, mainly caesium-137 and iodine-131.
Radioactive iodine is the biggest problem, because if it contaminates drinking water or milk, it can be taken up by the thyroid gland, potentially leading to thyroid cancers as seen in the wake of Chernobyl. Wakeford says:
Children continued to drink heavily contaminated milk and got big doses. That's why we ended up with an excess of thyroid cancers after Chernobyl.
But Wakeford says that the Japanese authorities have not taken any chances, evacuating people to 20 kilometres away from the stricken reactors, dispensing iodine tablets which prevent take-up of the radioactive form of the element, and making sure they do not eat any contaminated fluids or food. Iodine 131 is short-lived, with half of it decaying away every eight days, so it will be gone "in three or four months," he says.
Much more long-lived, with a half life of 30 years, is the other potentially dangerous element that could be released: caesium-137. This has been the most problematic legacy of Chernobyl, as it was carried throughout western Europe by the wind. Wakeford says that the Japanese authorities need to do whatever they can to prevent it escaping.
Stopping it
The key to stopping radioactive material escaping is to cool down the reaction, and thus ensure that the pressure vessel remains intact. Bluck says:
You have to keep pumping water in to cool that mass, for as many years as it takes.
This is why the engineers at the plant have kept pumping water in, even though (as mentioned above) they had to keep venting the steam off to relieve the pressure, thus allowing some radioactive caesium and iodine to escape. He says that, based on the available information, the decision to keep venting the steam was "entirely reasonable", in order to avoid the far worse consequences of the pressure vessel bursting.
Japan has officially asked the IAEA to help with the crisis, Reuters reports.
This graphic illustrates where the affected power plants are:
1400 GMT, Monday 14 March 2011
Michael Marshall, environment reporter
A child evacuated from areas surrounding the Fukushima nuclear facilities damaged in Friday's massive earthquake is checked for radiation exposure with other residents Sunday, March 13, 2011, in Koriyama city, Fukushima prefecture, Japan. (Image:Wally Santana/AP/Press Association)
Japan is still battling to deal with the devastating consequences of Friday's magnitude-9.0 earthquake, the largest in the country's history, and the ensuing tsunami. The situation is being compounded by the threat of a nuclear accident - and now an erupting volcano is adding to the problem.
Engineers at the Fukushima-Daiichi nuclear power plant are struggling to stabilise two reactors and prevent a serious radiation leak. At present, things seem to be taking a turn for the worse.
The plant has four reactors. An explosion destroyed the building surrounding reactor 1 on Saturday, but the reactor core apparently remains intact.
Reactor 3 is a greater threat because it uses a plutonium-uranium fuel mix, which poses a greater radiological risk. At 11:01 this morning (local time), the outer building exploded, but again the core appears to remain intact.
The Tokyo Electric Power Company (Tepco), which runs the plant, is injecting seawater into the reactors to cool them down. But it is struggling with reactor 2, which has stopped cooling properly.
Reuters is now reporting that falling levels of seawater exposed at least some of the fuel rods to air, allowing them to heat up by several hundred degrees and possibly begin to melt. Tepco says it has begun pouring water into the reactor again to try to cool the rods down, but it cannot yet rule out a meltdown.
A meltdown could release significant levels of radiation into the environment, as the BBC explains:
This opens the possibility of a serious meltdown - where molten, highly radioactive reactor core falls through the floor of the containment vessel and into the ground underneath.
Three other nuclear power plants have reported problems, but as previously reported these appear to be under control.
Meanwhile, the New York Daily News reports that seventeen US navy crew members have been exposed to low-levels of radiation during disaster relief missions in Japan:
They were treated with soap and water and their clothes were discarded... The helicopters were also decontaminated. The US 7th Fleet... moved its ships further away due to "airborne radioactivity" and contamination found on its planes.
However it seems any danger is minimal:
The military noted, however, that the level of contamination was very low, and the ship movement was merely a precaution. "For perspective, the maximum potential radiation dose received by any ship's force personnel aboard the ship when it passed through the area was less than the radiation exposure received from about one month of exposure to natural background radiation from sources such as rocks, soil, and the sun," the Navy said.
Elsewhere, on Japan's southerly island Kyushu, the Shinmoedake volcano has begun erupting again. The volcano is part of the Kirishima volcano cluster. According to the LA Times:
Sunday's eruption, which was the biggest volcanic activity in Shinmoedake in 52 years, caused widespread destruction and panic. The blast could be heard for miles, and shattered windows four miles away. Hundreds of people fled the area as the volcano spewed debris, including hot ash and rocks, more than 6,000 feet in the air.
Shinmoedake erupted several times between 26 January and 4 February. After the eruptions ceased, heavy rain turned the accumulated ash to mud. On 17 February residents were urged to evacuate because of the risk of mudslides. Then a "lava dome" grew inside the volcano's crater, prompting volcanologists to warn of a further eruption.
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