Details of the Japanese plant accident are sparse and, at the best, sketchy but a few facts have been established.

First, the accident occurred at a nuclear fuel fabrication plant. That is where the uranium metal is converted to a uranium dioxide and then to a solid, sintered pellet of about 15mm diameter and 30mm length.

It is this pellet that is sheathed in a thin metal cladding to form a fuel pin that is used in a nuclear reactor core - overall about 200,000 such pellets make up a reactor core fuel load.

Second, the accident resulted in:

• what witnesses described as a violent blue flash
• a radioactive release that reportedly resulted in initial radiation levels in the local area exceeding 15,000 times background or natural radiation levels.

Criticality

These facts all suggest a criticality incident - that is where too much fissile material, either uranium or plutonium, probably uranium in this case, collects together and fissions.

That means it undergoes a nuclear reaction as if it was fuel within a nuclear reactor.

Since the loose uranium 'fuel' does not have the same containment in the fuel processing plant as it would in a reactor, then the energy liberated during criticality results in a small explosion - usually expected to be equivalent to about 2kg of TNT.

The sequence that follows first forces the critical uranium apart. Criticality ceases, but the neutron irradiation of the uranium and any other materials nearby create a host of fission products and radioactive isotopes that are available for release to the atmosphere.

Worldwide technologies

So, a criticality incident has occurred in a relatively small Japanese plant, but could it happen elsewhere ?

About 150 people living near the plant were evacuated

Yes, of course. This is because the manufacturing and fabrication processes for nuclear fuel are global technologies.

Much the same equipment, techniques and process are deployed at all nuclear fuel fabrication plants - these plants are to be found in the United States, Russia, France, Germany and Britain, all of which are major manufacturers of both uranium and mixed oxide plutonium fuels.

There are other 'smaller player' plants in countries such as Canada, Argentina, South Africa and elsewhere.

Incredible accident

Generally, for their design and containment these plants share the same assumption as the Japanese plant - simply, that a criticality incident would be an incredible accident, that is, it could not foreseeably happen.

Similar technologies are used in plants around the world

Well it has happened in Japan, so all other similar plants will have to review this Japanese accident scrupulously.

They will have to satisfy their own state nuclear safety regulators that adequate safety systems are in place, that wholesome containment exists, and most importantly, that it is now recognised that such an incredible accident can occur.

Dire consequences

What are the potential consequences of such criticality accidents? All depends of the extent of the criticality incident itself and, of course, if the containment building is punctured in the immediate aftermath of the incident.

Obviously, for the 20 or so workers caught in the neutron 'flash' the immediate consequences could be very dire - the symptoms reported suggest a very large radiation dose indeed.

For the general public, the major source of radiation exposure is not necessarily immediate, say from the radiation shine of any escaping material, but more in the form of internal uptake into the body, where the radiation is directed to certain organs and reconcentrated.

If the human metabolism is unable effectively to clear this accumulating radiation quickly then, rather like a battery, the individual carries the packet of accumulated dose around within the body for a considerable period of time - hence, the total dose received over time can be very significant indeed.