Radioactive release from Japan plants could last months

So far, Japanese officials have said the melting of the nuclear cores in the two plants is assumed to be "partial", and the amount of radioactivity measured outside the plants, though twice the level Japan considers safe, has been relatively modest.

But, the New York Times quoting Pentagon officials reported yesterday that helicopters flying 60 miles from the plant picked up small amounts of radioactive particulates — still being analysed, but presumed to include cesium-137 and iodine-121 — suggesting widening environmental contamination.

Japanese reactor operators now have to periodically release radioactive steam as part of an emergency cooling process for the fuel of the stricken reactors that may continue for a year or more even after fission has stopped.

The plant's operator must constantly try to flood the reactors with seawater, then release the resulting radioactive steam into the atmosphere, several experts familiar with the design of the Daiichi facility was quoted as saying by the paper.

The essential problem is the definition of "off" in a nuclear reactor. When the nuclear chain reaction is stopped and the reactor shuts down, the fuel is still producing about 6 per cent as much heat as it did when it was running, caused by continuing radioactivity, the release of subatomic particles and of gamma rays.

Usually when a reactor is first shut down, an electric pump pulls heated water from the vessel to a heat exchanger, and cool water from a river or ocean is brought in to draw off that heat.

But at the Japanese reactors, after losing electric power, that system could not be used. Instead the operators are dumping seawater into the vessel and letting it cool the fuel by boiling. But as it boils, pressure rises too high to pump in more water, so they have to vent the vessel to the atmosphere, and feed in more water, a procedure known as "feed and bleed", the paper said.

When the fuel was intact, the steam they were releasing had only modest amounts of radioactive material, in a non-troublesome form. With damaged fuel, that steam is getting dirtier.