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http://www.bbc.co.uk/news/science-environment-12773350
17 March 2011 Last updated at 11:02 ET, by Richard Black
Choppers and cannons bring no nuclear relief
The attempt to use helicopters to dump seawater on to the Fukushima power station is almost certainly unprecedented in more than half a century of nuclear power operations around the world.
And the long-range video images coming in indicate why it is not a method in general use: it does not appear to work.
The helicopters flew in some way above the reactor buildings, and went past without hovering - presumably because of fears of radiation.
Later, at a news conference given by the Tokyo Electric Power Company (Tepco), a spokesman said they were not able to tell whether any of the water had been successfully delivered.
In the absence of hard facts from the ground, the video footage is the only evidence we have on how much of the 60 tonnes of seawater delivered onto the site actually hit their target - the building containing reactor number 3.
Do watch, and judge for yourself how much fell on the square building and how much floated away on the wind.
While many reports have suggested the water was intended to cool the reactors, the actual target was the cooling pool (or pond) in building number 3, just below what remains of the roof ripped apart by a hydrogen gas explosion three days previously.
The reactors sit within containment systems designed to be sealed tight, and which appear to be intact, with the possible exception of a crack in a vessel attached to number 3 reactor; so dumping water from the air would not have any impact.
Earlier reports said the pond in number 4 building would also be targeted. This probably contains many more fuel rods than the pond in number 3, because at the time of the earthquake reactors 4, 5 and 6 were shut down for scheduled maintenance, which involved removing all the fuel.
The worst-case scenario would see the ponds starting to emit serious amounts of radiation, with some of the reactors going into a meltdown phase”
The company said that because steam was rising from building 3, they targeted that one first - though it is not clear whether the steam was coming from the cooling pond or the reactor, which has been suffering from cooling problems ever since emergency power ran out following Friday's tsunami.
The company noted that at 1000 local time, radiation levels were four times higher around building 3 than building 4.
However, the building 4 pond still clearly has a problem.
On Wednesday, the US Nuclear Regulatory Commission (NRC), which has a team of 11 experts advising in Japan, said the pond was completely dry - though this assessment has been challenged by the company.
The International Atomic Energy Agency (IAEA) released a bulletin giving temperatures in the ponds measured on different days.
Readings for building 4's pond on Tuesday and Wednesday were both 84C - way above normal, and about 25C higher than the equivalent ponds in buildings 5 and 6.
Thursday's entry for building 4 reads: "no data".
The tops of the rods are supposed to be about 5m (16ft) below the water surface.
Without sufficient water around the rods, they get hot, increasing the chances of radioactive substances being released. Already it appears that the cladding around some of the fuel rods has been damaged - possibly by fire - releasing hydrogen.
The helicopters' water-lifting gear is more commonly deployed on forest fires
"If this is damaged - and we suspect it must be - you've got radionuclides being produced and going upwards because of the fire," said Andrew Sherry, director of the Dalton Nuclear Institute at the University of Manchester.
"[Without water] the immediate outlook is that the fire and the generation of hydrogen will continue, so we've got quite an unstable situation."
The absence of water also reduces the extent to which workers are shielded from radiation, with gamma rays able to pass into the air unobstructed.
More remarkably, Tepco warned on Wednesday: "The possibility of re-criticality is not zero".
This meant that in the company's view, it was possible that enough fissile uranium was present in the cooling pond in enough density to form a critical mass.
In other words, a nuclear fission chain reaction could start in a pond that lies outside parts of the building designed to contain radioactive materials.
So how much material is there in the pond in building 4?
Clues come from a presentation given by a Tepco employee at a conference on fuel storage last November.
It confirms that the cooling ponds had been "re-racked" - in other words, they were storing more fuel rods than allowed for in the original design.
This is neither uncommon nor unsafe, provided that the rods are properly spaced - although it may mean that additional measures are needed to prevent criticality, such as the use of solid boron sheets or the addition of boric acid into the water, to absorb the neutrons that sustain a chain reaction.
The presentation shows an abundance of spent fuel rods at the plant, taking up 84% of the available space in the various storage facilities.
The dry storage facility was completely full, while a big cooling pool away from the reactors contained 6,291 rods - the maximum allowed being 6,840.
This left a further 3,450 rods distributed between the pools in the six reactor buildings.
How they were distributed is not known.
But it suggests that the pools in buildings 4, 5 and 6 may have been very full at the time of the earthquake, given that any older stored rods would have been supplemented by those taken from the reactors for maintenance.
If "re-criticality" did materialise, it would lead to the enhanced and sustained release of radioactive materials - though not to a nuclear explosion - with nothing to stop the radioactive particles escaping.
While the authorities are keen to re-fill the pools with water, this, paradoxically, would increase any risk of criticality, as water without boric acid in it aids the chain reaction.
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?
Core task
Away from the fuel cooling ponds, the most important task remains to get enough water flowing into reactors 1, 2 and 3 to cool the cores.
There have been no reports of explosions or sudden spikes of radioactivity since Wednesday morning - possibly suggesting that technicians have had some success in keeping water flowing into those three reactors.
Even at its worst, Fukushima will not be a replay of Chernobyl
Low radiation levels are also essential for allowing technicians to continue their work around the site.
The most optimistic note for a long time is the growing prospect of restoring electrical power to the station.
Both a grid connection and portable generators are due to arrive - possibly on a timescale of hours.
That should allow technicians to re-start the main pumps - provided that the site's internal electrical circuitry is intact, and that pumps have not been damaged by the earthquake, the tsunami or the explosions.
"It's clear that this is going to help significantly, because it'll allow them to start the pumps to re-circulate water both in the reactors and in the fuel ponds, because the water is constantly re-circulating," said Dr Sherry.
Best and worst
The most optimistic scenario now is that electrical power is restored, all the pumps work, and seawater mixed with boric acid can be pumped up to proper levels in all the reactors and all the cooling ponds.
There would still be some release of radioactivity, because steam would probably need to be vented from the reactors - and if there is a crack in the containment system of reactor 2, that might continue to leak steam as well.
But that would in all probability be all, with all the problematic issues under control within a few days.
On the other hand, the worst-case sequence of events would see the electrical connection not working for some reason, and further problems cooling the reactors, leading to more damage to containment systems.
That would mean more heat in the reactors, more heat in the fuel ponds, more release of radioactivity - perhaps keeping workers from doing what is needed now to stabilise the situation.
Sir John Beddington, the UK government's chief scientific adviser, said on Thursday that the situation had recently changed - which is why the UK issued fresh advice to people in Japan based on a new assessment of the worst-case scenario.
"The worst-case scenario would see the ponds starting to emit serious amounts of radiation, with some of the reactors going into a meltdown phase," he told BBC News.
"We put that together with [a possible scenario of] extremely unfavourable weather conditions - wind in the direction of Tokyo, for example.
"Even in that situation, the radiation that we believe could come into the Tokyo area is such that you could mitigate it with relatively straightforward measures, for example staying indoors and keeping the windows closed."
It' already a global disaster with two! If they don't get the generators fired back up today, the other two could also blow. There sure is alot of farm land world wide that will have radiation dust coming down on it. Look at all the contaminated food, and meat that people will eat and die. Are you still sure you feel this way?
