Fukushima update: No chance cooling fuel can breach vessels

Still nothing to get in a flap about


Nobody reported to have sustained a radiation dose with any health consequences

In a separate incident at the same time as the apparent suppression-chamber damage at Reactor No 2, a fire reportedly broke out at Reactor No 4 at the site, not previously considered a problem and which had been shut down at the time of the quake. The fire was at a pond used to store spent reactor fuel. WNN quotes government spokespeople as saying that the fuel was not the cause of the fire. The International Atomic Energy Agency (IAEA) indicates that it may have resulted from a hydrogen explosion rather than a problem with the spent fuel itself. In any case the fire was put out by 2am UK time (11am at Fukushima).

It is unclear how much, if any, of the heightened radiation at the site resulted from the fire and how much from the probable leak at the No 2 suppression chamber.

Meanwhile three other reactors at the separate Fukushima Daini site (the three worst hit reactors described above are at Fukushima Daiichi), which had previously been a cause for concern, have now been safely brought to "cold shutdown" status. Reactor No 4 at Daini is still in the process of being shut down, and local residents were previously evacuated from the area within 10km of that site. Reports of possible serious troubles at the Onagawa site appear to have been unfounded.

Thus far there are no reports at all of anyone receiving a radiation dose with measurable health consequences as a result of the Fukushima damage. The IAEA previously reported that one plant worker had sustained a dose equivalent to about a month's normal background radiation: the US Navy has also said that personnel returning aboard ship from relief work on the quake-stricken coast had sustained similar doses. The Japanese government has carried out a massive programme of radiation checks among evacuees and WNN has reported that so far nine people have been found to have sustained measurable levels of exposure.

Iodine pills intended to prevent radioisotopic iodine from being taken up by the thyroid gland have been distributed to centres in the area. However the pills have not been administered so far as there is no indication of a need to do so.

Analysis

A note on "meltdowns". The general view of nuclear experts is that a proper meltdown has only occurred when the fuel rods and their cladding melt to liquid in such a terrifically hot state that they can burn through the containment vessel. The danger of this has passed, as the main nuclear chain reaction was shut down at once by control rods automatically inserting into all the cores when the quake hit (this despite the fact that the older plants were not designed to withstand so strong a shock).

The heat in the reactors now is produced by residual isotopes decaying, not a full-blown fission chain reaction. It will by now be less than 1 per cent of the cores' normal output and chances remain good that the Fukushima crews will manage to keep the three hard-hit cores cooled such that they don't actually melt – though it is acknowledged that they are now probably damaged beyond economical repair, the more so as they were nearing the end of their lives anyway.

If they can't be kept cooled and the cores melt, this will drop the molten fuel into coolant remaining beneath, and it will solidify again in a less dangerous state. Cleanup will be more difficult if this happens, but provided the containment vessel remains unbreached there will be no massive release of radioactives. There is basically no chance of any Chernobyl-style incident*.

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