Earthquakes

Magnitude 5+ Earthquakes – Global

None 5.0 or higher today.

Fukushima Nuclear Power Plant

A task of extraordinary delicacy and danger is about to begin at Japan's Fukushima nuclear power station. Engineers are preparing to extract the first of more than 1,000 nuclear fuel rods from one of the wrecked reactor buildings. This is seen as an essential but risky step on the long road towards stabilising the site.

The fuel rods are currently in a precarious state in a storage pool in Unit 4. This building was badly damaged by an explosion in March 2011 following the Great Tohoku earthquake and tsunami. Moving the rods to safety is a high priority but has only become possible after months of repair work and planning.

The fuel rods are four-metre long tubes containing pellets of uranium fuel and the fear is that some may have been damaged during the disaster. When the tsunami struck the Japanese coast, the flood swamped the diesel generators providing back up power to the reactors. Three of the reactors went into a state of partial meltdown.

By coincidence, Unit 4 was undergoing maintenance, so all of its fuel rods were being stored. But the meltdown of a neighbouring reactor led to a build-up of hydrogen which is believed to have led to the explosion in Unit 4. In the days after the tsunami, there were fears that the blast had damaged Unit 4's storage pool and, in desperation, the authorities used helicopters and fire hoses to keep it filled with water.

A guiding principle of nuclear safety is that the fuel is kept underwater at all times - contact with the air risks overheating and triggering a release that could spread contamination. So the operation to remove the rods will be painstaking.

The rods will be lifted out in batches of 22 and in casks filled with water. This will be done with a new crane, recently installed in the wrecked building, after the original one was destroyed. The task of removing each batch will take 7-10 days.

Two critically important issues are whether the rods themselves are damaged and therefore likely to leak and whether the casks remain watertight to ensure the rods have no contact with the air. The risks include a possible "release of radiation" from the fuel or if the casks holding the fuel are dropped. "Countermeasures" have been prepared - including back-up wires to hold the loads and mechanisms to hold the fuel in the event of a power failure.

The fuel rods will then be deposited into a new "common" pool with a cooling system.

Tepco will not confirm the precise timing of the fuel rod operation but after so much public outrage at the company's handling of the crisis so far, scrutiny of this latest episode will be intense.