Science & Environment

Choppers bring no nuclear relief - but current might

Satellite photo of plant
Image caption A satellite photo shows steam rising from the site around reactor buildings 1-4

Thursday's 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.

Long-range video footage indicates why it is not a more widely-used technique: it does not appear to work.

Water cannon - tried, with similar results - seemed a similarly desperate measure.

Far more orthodox was the plan to reconnect the stricken facility back to the national grid, enabling the delivery of electrical power.

Although power stations' main job is to generate electricity, their grid connections flow both ways - the return leg meaning that everything at the site can keep working even if all its reactors shut down.

The connection supplying Fukushima was presumably swept away on Friday by the same wall of water that submerged its back-up on-site generators.

Early on Friday morning in Japan, authorities announced they had connected a cable from a functioning power line about 1km away.

That should allow technicians to re-start the main pumps - provided that the site's internal electrical circuitry and the pumps themselves have not been damaged by the earthquake, the tsunami or the series of gas 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 Andrew Sherry, director of the Dalton Nuclear Institute at the University of Manchester.

Drying out

As the week progressed, the focus switched from reactors 1, 2 and 3 to the fuel storage pools, also known as cooling ponds, in reactor buildings 3 and 4.

Image caption Parts of reactor buildings 3 and 4, pictured during the helicopter mission, were severely damaged

They look like big, deep swimming pools, and are designed to store bundles of fuel rods - assemblies, as they are properly called.

Fuel assemblies are put in the pond when they come out of the reactor during its shut-down period.

If they are relatively new, they go back in the reactor again prior to start-up - if not, they may remain in the pond for months, even years, before going to a more long-term storage site and eventual re-processing.

At the time of the earthquake, reactors 4, 5 and 6 had been shut down for routine maintenance.

The tops of the rods are supposed to be about 5m (16ft) below the water surface. The water keeps them cool and prevents radiation.

In the absence of water, the temperature of the fuel can soar, increasing the chances of radioactive substances being released.

By Tuesday, it was clear that the pond in building 4 was short of water, for reasons that have not been completely explained; and on Wednesday, the US Nuclear Regulatory Commission (NRC), which has a team of 11 experts advising in Japan, said it was completely dry.

The International Atomic Energy Agency (IAEA) released data showing that on Tuesday and Wednesday, the temperature in pond 4 was 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" - possibly because instruments monitoring the pond had been destroyed by heat.

It appeared that cladding around some of the fuel rods had been damaged - possibly by fire.

Critical issue

What made the situation concerning the fuel pond in building 4 seem particularly serious was Wednesday's statement from Tepco saying: "The possibility of re-criticality is not zero".

Image caption The helicopters' water-lifting gear is more commonly deployed on forest fires

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.

This raised the issue of just how much fuel was stored in the pond, how densely it was packed, and what precautions were being taken to absorb neutrons, the particles that sustain a chain reaction.

Some clues are contained in a presentation given by an employee of the Tokyo Electric Power Company (Tepco), which runs the power station, 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 assemblies than allowed for in the original design.

This is neither uncommon nor unsafe, provided the assemblies are properly spaced - although it may mean that additional measures are needed to absorb neutrons and prevent criticality, such as the use of solid boron sheets or the addition of boric acid to the water.

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 fuel assemblies - the maximum allowed being 6,840.

This left a further 3,450 assemblies distributed between the pools in the six reactor buildings.

How they were distributed is not known.

But 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.

Wind factor

Anyway - when the helicopters eventually flew in, after a day's delay, building 4 appeared to be the likely target.

Media playback is unsupported on your device
Media captionProfessor Andrew Sherry explains the situation as helicopters spray water to cool nuclear reactors

However, they headed for building 3 - perhaps because at the time, radioactivity was four time higher there than around bulding 4.

The roofs of both buildings had been destroyed in explosions caused by a build-up of hydrogen from degrading fuel rods.

The helicopters flew in some way above the buildings, without hovering - presumably because of fears of radiation.

Tepco said later that they could not tell whether any of the water had been successfully delivered.

In the absence of such data, all you can do is watch the footage and judge for yourself how much hit the target - and how much floated away on the wind.

Core task

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 reactors 1, 2 and 3.

The arrival of electrical power now opens up the possibility that the flow can be stabilised and increased - and the fuel ponds topped up to their proper levels.

As well as cooling the overheated fuel, this should reduce levels of radiation around the plant.

Under this scenario, there wil still be some release of radioactivity, because steam will 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 workers will be able to move more freely around the site and finish the job they have been performing under what must have been extraordinarily arduous circumstances since Friday, of putting this power station to sleep once and for all.

That depends, however, on the electrical and pumping systems functioning - and for now, given the hits they have taken, that is not a given.

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