THE RISK of a nuclear reactor meltdown has come to dominate ongoing coverage of the Japanese earthquake/tsunami disaster.
Yet a full meltdown would likely represent a minor threat to human life compared to the current estimated loss of life from the combined catastrophe, ranging between 10,000 and 50,000 dead.
The world has never seen a full meltdown, said Dr Ciara McMahon of the Radiological Protection Institute of Ireland.
Chernobyl was complicated by an explosion and fire, while about 30 per cent of the Three Mile Island core melted. Even so, reactor engineers have built in a response to meltdown.
“Core melt, either partial or total, is something they have planned for. It is one of the scenarios they plan for,” said Dr McMahon.
“A core-melt accident involves the melting of fuel. Temperatures at the reactor core get so hot that the metal structure and the fuel begins to melt.”
Workers at the Fukushima Daiichi nuclear power plant have been struggling to keep three of its reactors cool after the failure of the usual cooling systems.
Although the reactors are fully shut down, the three cores are still too hot and are being cooled using sea water. Their approach is working well, as evidenced by the fact radiation levels have not risen and the plant remains at the same emergency level as it has since the earthquake.
But what would happen if the unthinkable occurred – a full meltdown of the fuel rods?
Surprisingly little, Dr McMahon suggests. The nuclear reactions that produce heat in the fuel rods are considered “critical” when they are self-sustaining. They can only do this if they are close together, with the right “geometry” essential to achieving criticality.
If cooling stopped, the rods would begin to melt at 1,900 degrees and burn a hole in the base of the reactor. The blob of molten uranium would then drop down into a flat empty space.
“It basically gets spread out like on a tray,” she said. The blob would not have the correct geometry. “You couldn’t get criticality.”
It would be for the containment vessel to deal with any steam or hydrogen produced.
DICK AHLSTROM