Image caption A large explosion was seen at the plant with debris blown out from the building

Two days after the alarm was first raised about safety at Fukushima Daiichi plant, uncertainty still surrounds the situation on the ground and the status of the three reactors that were functioning at the time of Friday's earthquake and tsunami.

It appears that a partial meltdown did occur in reactor 1.

On Sunday, officials said the same thing was suspected in reactor 3 - although later, they appeared to retract this statement.

What is certain is that engineers are still struggling to pump enough water past the reactors to keep the cores cool.

At noon local time (0400 GMT), Tokyo Electric Power Company (Tepco), which operates the plant, gave this status report:

Reactor 1 - shut down, under inspection because of Saturday's explosion, sea water and boric acid being pumped in

Reactor 2 - water level "lower than normal", but stable

Reactor 3 - high pressure coolant injection was "interrupted"; but injection of sea water and boric acid were under way.

Later, officials said seawater and boric acid were also being pumped into reactor 2.

Media playback is unsupported on your device Media caption Richard Black describes what is happening inside the reactors on BBC News Channel

They were still encountering problems - among them, a stuck valve. Its exact purpose was not revealed.

Venting of mildly radioactive steam continued at reactors 2 and 3, and officials warned that an explosion was possible in reactor 3's building.

The official line is that the reactor 1 explosion was caused by a build-up of hydrogen originally produced in the reactor, though this remains to be confirmed.

Although visually spectacular, these explosions are not necessarily dangerous in terms of releasing radioactivity. The buildings are an external shell, with the task of sealing radioactive materials falling to a metal containment vessel constructed inside the concrete shell.

"The explosion... wasn't a terribly important event," according to Malcolm Grimston from the Energy Policy and Management Group at Imperial College, London.

"The building was designed to fall outwards" - preventing damage to the thick steel containment vessel inside.

Meanwhile, monitoring around the site was picking up higher levels of radioactivity than expected, above safety limits; hence the evacuation of an estimated 170,000 people.

What materials were involved in this is not yet entirely clear, although earlier reports referred to isotopes of caesium and iodine; and the company's bulletin also suggests the route out of the stricken reactors is not known.

"We will continue to monitor in detail the possibility of radioactive material being discharged from exhaust stack or discharge canal," it reads.

Speculation warning

In the middle of such a confused and changing picture, what can safely be said?

Firstly, the reactors involved will not operate again, even if there has not been a meltdown.

Some of the speculation in which some commentators have indulged is just that - speculation Prof Richard Wakeford, University of Manchester, UK Timeline: Japan power plant explosion

Seawater is corrosive. But it clearly appears to the operators that it is the only available medium for keeping the cores cool.

Boric acid, meanwhile, is used because it absorbs neutrons, slowing down the residual nuclear activity. The term "acid" is not really relevant - it is the atoms of boron in the acid that do the job.

Secondly, the release of radioactive materials, whatever the route, is so far of only local importance.

Russian authorities, with territory to the north and west within 1,000km of the plant, say they have detected nothing abnormal.

"If the explosion at the Fukushima nuclear power station has resulted in a significant release of radioactive material then this will soon be readily apparent from the radiation monitoring that is undoubtedly under way around the plant," noted Richard Wakeford, visiting professor in epidemiology at the UK's University of Manchester.

"Until we have reliable information on the results of such monitoring from Japan, some of the speculation in which some commentators have indulged is just that - speculation."

Thirdly, levels of radioactivity - although above safe limits - are far lower than were detected during the Chernobyl accident in Ukraine, for example.

So far, there is nothing to indicate that the 170,000 people displaced will not be able to return once the immediate danger has passed.

No fanning for flames

Although the term "meltdown" is hugely emotive, the main threat to the public from nuclear accidents has been through plumes of radioactive gas rising into the air.

In both the Chernobyl accident of 1986 and the UK's Windscale fire of 1957, reactor components smouldered for days - in the case of Chernobyl, preceded by an explosive release of gas.

This meant radioactive substances could be carried huge distances depending on the vagaries of the wind - in the case of Chernobyl, initial detection was in Sweden, more than 1,000km (620 miles) away.

Scale of nuclear accidents Level 7 - Major release of radioactive material. Example: Chernobyl, Ukraine, 1986

- Major release of radioactive material. Example: Chernobyl, Ukraine, 1986 Level 6 - Significant release of radioactive material. Example: Kyshtym, Russia, 1957

- Significant release of radioactive material. Example: Kyshtym, Russia, 1957 Level 5 - Limited release of radioactive material. Example: Three Mile Island, US, 1979, and Windscale, UK, 1957

- Limited release of radioactive material. Example: Three Mile Island, US, 1979, and Windscale, UK, 1957 Level 4 - Minor release of radioactive material with at least one death from radiation. Example: Tokaimura, Japan, 1999

- Minor release of radioactive material with at least one death from radiation. Example: Tokaimura, Japan, 1999 Level 3 - Exposure in excess of 10 times the statutory annual limit for workers

- Exposure in excess of 10 times the statutory annual limit for workers Level 2 - Exposure of a member of the public in excess of 10mSv (average annual dose is 1mSv)

- Exposure of a member of the public in excess of 10mSv (average annual dose is 1mSv) Level 1 - Exposure of a member of public above statutory annual limit. Minor safety problems (Source: UN nuclear agency, IAEA) Japan fears second reactor blast

This type of explosive release has not happened at Fukushima, nor have there been reports of fires - meaning it is unlikely that contamination will go further than the immediate vicinity.

Instead, what does appear to have happened with the reactors is that portions of the core have been exposed for short periods to the air, as coolant levels fell too low.

If these periods are long enough, some melting will take place.

There is also potential for the cladding around the fuel rods to catch fire - a process that could have led to the hydrogen build-up - although the fires would be extinguished again once enough coolant arrived.

In the meantime, there have been suggestions that an incident at reactor 3 would inherently be more dangerous than at reactors 1 and 2 because it burns "mixed oxide fuel" (MOX) containing plutonium.

Plutonium is produced during nuclear fission, so is present in all reactor cores - the longer the fuel has been there, the more plutonium will be present, up to about 1%.

In some countries, spent fuel rods are re-processed and the plutonium set to one side.

However, Japan - in an attempt to be more frugal with a valuable resource - has a programme that mixes the plutonium coming out of the re-processing facility back into new fuel rods that also contain uranium. This is MOX fuel.

So, reactor 3 fuel rods will contain more plutonium than those in reactor 1.

But this would only become an issue if there were an explosion or a catastrophic meltdown. The radioactive release so far has been of much lighter fission products and of short-lived nuclei generated in cooling water, which are identical no matter which fuel is used.

Cool heads

The big challenge for the authorities, then, is to keep the reactors cool.

This will eliminate chances of a major meltdown, a fire and the need to vent radioactive steam.

In this context, warnings of electrical power shortages coming from the government and from Tepco itself are important because electrical power is needed to run the cooling pumps.

Whether local generators are up and running is unclear. If they are not, then everything depends on the grid connection.

Ironically, one of the reasons for the power shortage is the automatic shut-downs triggered in nuclear stations when an earthquake is detected.

However, possible implications outside Japan are already beginning to emerge.

In Germany, scene of a big anti-nuclear protest on Saturday, Environment Minister Norbert Roettgen suggested that safety systems at nuclear plants would be analysed anew in the light of the Fukushima incident.

"This happened in a country with very high safety standards... the fundamental question of whether we can guard against all dangers is now open again, and we will address that question," he said.

In the UK, the Stop Hinckley pressure group has called for a halt to a proposed new reactor at Hinckley Point in southwest England, on safety grounds.

Environment groups are beginning to feature Fukushima in their energy communications - and whatever actually happens at the site, it is likely to become a major card in campaigns to promote renewable energy above nuclear.

Two days after the alarm was first raised about safety at Fukushima Daiichi plant, uncertainty still surrounds the situation on the ground and the status of the three reactors that were functioning at the time of Friday's earthquake and tsunami.