Japan earthquake shuts nuclear facilities

However, as yet there has been no reported leakage of radiation from any of the reactors and at this stage experts are not predicting any major discharge that would be hazardous to human health.

According to the International Atomic Energy Agency (IAEA) the Fukushima Daiichi, Fukushima Daini, Tokai and Onagawa nuclear power stations all automatically shut down soon after the 8.8 magnitude quake hit at 2.45pm local time. A fire broke out in the turbine building of the Onagawa plant while the Fukushima Daiichi plant reported problems with its cooling system.

Nuclear power plants have a series of strategically-placed sensors that detect peak ground acceleration (PGA) — which describes surface movement at particular places, rather than the cruder measure of the magnitude of the earthquake itself. Over a certain PGA threshold the sensors will trigger an automatic shutdown of the reactors.

‘Although this earthquake was massive, it was some way off-shore and the levels of shaking on land should not have been as high as in 2007, where the earthquake was smaller in magnitude but directly below the nuclear plant itself,’ said Julian Bommer, Professor of earthquake risk assessment at Imperial College London.

Geohazards are a key criteria when citing a nuclear power stations — plants are very unlikely to be built on soft or unstable soil. Then there will be a very detailed assessment of the potential seismic hazard — in other words, the probability of different levels of ground shaking at the site

Prof Julian Bommer, Imperial College London

In 2007 a 6.8 magnitude earthquake struck just 16km from the world’s largest nuclear power plant — Tepco’s Kashiwazaki-Kariwa facility. This caused a fire on an electric transformer and the leakage of 1.5 litres of mildly radioactive water, but the reactor was ’absolutely unscathed,’ according to Bommer.

Based on this event, Japan’s Ministry of Economy Trade & Industry (METI) set up a 20-member Chuetsu Investigation and Countermeasures Committee to investigate the impact of earthquakes on power stations and potential counter-measures.

‘Geohazards are a key criteria when citing a nuclear power stations — plants are very unlikely to be built on soft or unstable soil. Then there will be a very detailed assessment of the potential seismic hazard — in other words, the probability of different levels of ground shaking at the site,’ said Bommer.

He explained that nuclear reactors employ the same sort of engineering techniques and designs used in other structures that are built to withstand earthquakes — but with considerably higher tolerances built-in.

‘The starting point would be a very high ground acceleration with a probability of around one in 10,000. Then the reactor will be designed for that, but with a large safety margin — so even if that ground acceleration occurred, the probability of there actually being any elastic behaviour in the structure is about one in a 100,000. And because there are further redundancies in the system, the probability of going from elastic behaviour to release of radioactivity from the core is more like one in a million,’ he said.