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| The building housing reactor 4 (left) appears severely damaged - despite the reactor itself being shut down |
Over the days of the Fukushima crisis, attention has switched from reactor 1 to reactor 3, to 2, back to 3 - and now, to 4.
This is a surprise.
Reactors 4, 5 and 6 were shut down at the time of Friday's earthquake, with some or all of their fuel rods extracted and left in cooling ponds that each reactor building has under its roof.
Once a reactor is turned off, radioactivity and heat generation in the rods die away quickly; down to 7% of the original power within a second of switch-off, 5% within a minute, 0.5% within a day.
Transferred to the cooling pond, allowing technicians to do routine maintenance on the reactor, the rods are supposed to sit quietly until the time comes for their re-insertion or their journey towards disposal.
The tops of the rods are supposed to be about 5m (16ft) below the water surface.
The water keeps them cool and also blocks the release of gamma rays that would harm people in the vicinity.
So why is it that the Tokyo Electric Power Company (Tepco) has warned: "The possibility of re-criticality is not zero"?
If you are in any doubt as to what this means, it is that in the company's view, it is possible that enough fissile uranium is present in enough density to form a critical mass in the cooling pond - meaning that a nuclear fission reaction could start in the building, outside the containment shield that surrounds the actual reactor.
If it happened, this would lead to the enhanced and sustained release of radioactive materials - though not a nuclear explosion.
Chain reaction |
| Fuel rods in rapidly-drying pools may be the source of increased radiation levels |
The first event in this chain appears to be that the level of water in the pond fell.
Why that should happen is not entirely clear.
Was the fuel so hot that it caused an unanticipated amount of evaporation? Did the earthquake somehow crack the building's structure, allowing water to leak out?
Did the two fires in the building have an impact? Visually, the building housing reactor 4 is the most damaged on the site - suggesting that reports of the two fires being relatively minor were wide of the mark.
Or, did technicians at some point take water from the pond for use in reactor 4's cooling system?
There is nothing to say they did; but during the chaos of the weekend, with power systems and options disappearing before their eyes, it might have seemed like a good idea.
Whatever the reason, the company admitted on Tuesday that water was below its normal level - and Japanese news sources reported that it was boiling.
This is consistent with the idea that as in buildings 1, 2 and 3, hydrogen was released from a reaction between the zirconium fuel-rod cladding and steam, resulting in an explosion.
Except that in reactor 4 building, the rods were in the fuel pond, not in a reactor.
The government then ordered Tepco to put water back in the pool.
But either because of high radiation levels or broken pumps or some other reason, they could not - hence the decision to drop water in from a helicopter, a technique that is used to fight forest fires.
On Wednesday, this plan was scrapped because of concerns about radiation affecting the pilots. Presumably, there is no water going into the cooling pond.
'Rock and a hard place'Still, in principle this should not raise any possibility of resumed criticality.
According to Laurence Williams, professor of nuclear safety at the University of Central Lancashire, it could depend on how the rods are arranged in the water.
"In some fuel ponds, they dose the water with boric acid at low levels," he told BBC News.
"In some systems they've re-racked the fuel assembly making it possible to put more rods in the pond than it was originally designed for, and then you might put extra sheets of boron in between."
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| The incident has sparked fears of radioactivity in other Asian countries |
Boron and boric acid mop up neutrons, the particles that sustain the chain reaction.
In this sort of reactor, water is a crucial component of the fission process.
It acts as a moderator - it reduces the speed of the neutrons, meaning they can be captured by uranium nuclei in the fuel rods, inducing them to split.
Without water, the neutrons travel too fast, and are not captured.
Professor Williams raised a scenario that may be unfolding in the cooling pond in building 4. It is just a possibility, because information is scanty; but here it is.
If the fuel rods are dry and hot, there could be damage to the cladding and the release of light radioactive nuclei.
To prevent that, you would want to inject water.
But water on its own is a neutron moderator and would enhance the chances, however small, of criticality.
"You're caught between a rock and a hard place," he observed.
Now, Tepco is also talking about putting boric acid into the cooling pond of number 4 building.
How closely the rods were packed, whether any boron sheets were in place and if so whether they were damaged by one of the two fires in the building; these are among the many unanswered questions.
Core taskMeanwhile, the most important task remains to get enough water flowing into reactors 1, 2 and 3 to cool the cores.
Following Tuesday's apparent cracking of the suppression chamber in reactor 2 - a likely cause of the radioactivity spike then seen - there were concerns that the same thing might have occured on Wednesday in reactor 3.
However, the latest reports from Japanese news sources suggest reactor 3's containment system is intact.
If that proves to be the case, the source of Wednesday's radiation spike remains a mystery.
It was big enough to force technicians to leave the plant.
Clearly, their presence is crucial to stabilising the reactors. If they were forced away for long periods, the chances of containing the crisis would fall.
"If water is continuously pumped, they could stabilise the position, because the moment when fuel rods are covered with water, the situation is basically stabilised," observed Jasmina Vujic, professor of nuclear engineering at the University of California, Berkeley.
While this work continues, so do the questions over the building 4 fuel pond.
Re-criticality seems an extraordinary thing to contemplate; but if it is not a real possibility, why was such an idea floated by the company itself?
The bigger picture, though, is still one of a serious local incident, with minor impacts outside the plant.
At a news briefing with experts in London on Tuesday, the question was raised as to whether the incident should be upgraded from a four on the International Nuclear and Radiological Event Scale (INES).
INES four is defined as "accident with local consequences", five as "accident with wider consequences".
All the experts said that so far, four was the most apt category.
Japan nuclear alert at Fukushima - Q&A
There have been a number of explosions and fires at the Fukushima Daiichi nuclear power plant in Japan, following Friday's earthquake and tsunami.
Four of the plant's six reactors have been in trouble. How great a danger do these problems pose for people in Japan and further afield?
Has there been a leakage of radioactive material?
Yes. Harmful levels of radiation have caused at least one temporary evacuation of staff at the power plant. Higher than normal, but harmless, levels of radiation have been registered in Tokyo, 140 miles (220km) away.
How much radioactive material has been released?
The World Health Organisation's representative in China says there is no evidence of any significant international spread of radiation.
What type of radioactive material has escaped?
There are reports of radioactive isotopes of caesium and iodine in the vicinity of the plant. Experts say it would be natural for radioactive isotopes of nitrogen and argon to have escaped as well. There is no evidence that any uranium or plutonium has escaped.
What harm do these radioactive materials cause?
Radioactive iodine could be harmful to young people living near the plant. After the 1986 Chernobyl nuclear disaster there were some cases of thyroid cancer as a result. However, people who are promptly issued with iodine tablets ought to be safe. Radioactive caesium accumulates in soft tissue, while plutonium accumulates in the bone and liver. Radioactive nitrogen decays within seconds of its release, and argon poses no threat to health.
Q&A: Health effects of radiation
How did the radioactive materials escape?
In at least two ways. Some is known to have escaped as a result of steam and gas released from overheating reactors. There has also been a release from the fourth reactor's fuel storage pond, which was damaged in an explosion, and caught fire.
Reactor breach worsens prospects
Could radioactive materials have escaped by any other means?
The authorities have pumped sea water into three reactors. This water is likely to have been contaminated by its passage through the reactor, but it is currently unclear whether any of it has been released into the environment.
How long will any contamination last?
Radioactive iodine decays quite quickly. Most will have disappeared within a month. Radioactive caesium does not last long in the body - most has gone within a year. However, it lingers in the environment and can continue to present a problem for many years.
Has there been a meltdown?
The term "meltdown" is used in a variety of ways. Some of the metal encasing fuel rods has been damaged by heat, and may have partially melted (a "fuel-rod meltdown"). However, there is as yet no indication that the uranium fuel itself has melted. Still less is there any indication of a "China Syndrome" where the fuel melts, gathers below the reactor and resumes a chain reaction, that enables it to melt everything in its way, and bore a path deep into the earth. If there were to be a serious meltdown, the Japanese reactor is supposed to be able to handle it, preventing the China Syndrome from taking place.
Could there be a Chernobyl-like disaster?
Experts say this is highly unlikely. The chain reaction at all Fukushima reactors has ceased. The explosions that have occurred have mostly taken place outside the steel and concrete containment vessels enclosing the reactors. At Chernobyl an explosion exposed the core of the reactor to the air, and a fire raged for days sending its contents in a plume up into the atmosphere. At Fukushima the explosions have damaged mainly the roof and walls erected around the containment vessels - though it is feared the steel and concrete containment vessel of reactor number two has been damaged. Even if a reactor at Fukushima were to explode - according to the UK government's chief scientific adviser - it would send radioactive material only 500m into the air (rather than 30,000 feet) and the fallout would be concentrated within 20km or 30km of the site.
Could there be a nuclear explosion?
The explosions so far have been caused by hydrogen released from the reactors. At Chernobyl there may have been a nuclear explosion in the reactor, but this has not been confirmed.
What caused the hydrogen release from the reactor?
At high temperatures, steam can separate into hydrogen and oxygen in the presence of zirconium, the metal used for encasing the reactor fuel. This mixture is highly explosive.
How do iodine tablets work?
If the body has all the iodine it needs, it will not absorb further iodine from the atmosphere. The tablets fill the body up with non-radioactive iodine, which prevent it absorbing the radioactive iodine.
What kind of radiation levels have been recorded at Fukushima?
Levels as high as 400 millisieverts per hour have been registered at the plant, itself. A few hours of this kind of dose-level would cause radiation sickness. However, the level has generally been lower than one millisievert per hour. (A spinal X-ray delivers roughly one millisievert of radiation.) On Monday morning the level was as low as 20 microsieverts per hour - only a few times more intense than the level of radiation experienced on a passenger jet flying at 40,000 feet. Also, intensity of radiation falls off quickly as one moves away from the source, like light from a candle.
Is any level of exposure to radiation safe?
In some parts of the world, natural background radiation is significantly higher than others - for example in Cornwall, in south-west England. And yet people live in Cornwall, and many others gladly visit the area. Similarly, every international air flight exposes passengers to higher than normal levels of radiation - and yet people still fly, and cabin crews spend large amounts of time exposed to this radiation. Patients in hospitals regularly undergo X-rays. Scientists dispute whether any level of exposure to radiation is entirely safe, but exposure to some level of radiation - whether at normal background levels or higher - is a fact of life.
Source by BBC News
