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.
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End Quote Jasmina Vujic University of CaliforniaThe moment when fuel rods are covered with water, the situation is basically stabilised”
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 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?
Fuel rods in rapidly-drying pools may be the source of increased radiation levels 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."
The incident has sparked fears of radioactivity in other Asian countries 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 task Meanwhile, 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 - up to 1 sievert per hour (1Sv/hr) - 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.
http://www.bbc.co.uk/news/science-environment-12762608
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