Engineers in Japan are currently battling to prevent a major nuclear incident at the Fukushima Daiichi power plant. So Ian Eddy asks: What exactly is the problem, what caused it and what are the risks?

How did the crisis start?

Many nuclear power stations work by using the heat from nuclear reactions to boil water. Then the steam is used to power turbines and create electricity. 

The recent 9.0 magnitude earthquake in Japan cut the power lines to the Fukushima Daiichi plant, which stopped the flow of coolant water from electric pumps. 

The plant’s first backup power supply comes from diesel generators, which kicked in as the electricity supply was cut. However, the tsunami caused by the earthquake came over the power station wall and flooded the generators. A second backup power supply was provided by batteries, but these only lasted for seven to eight hours.

The problem then was that some of the plant’s six reactors started overheating. The extreme temperatures caused some water to be split into hydrogen and oxygen, but some of the hydrogen exploded and started fires.

What’s a meltdown?

As the nuclear fuel rods overheat, there is a danger that this could cause a nuclear meltdown. This is when the fuel rods become too hot and melt through the reactor’s steel and concrete containment shields, possibly leading to a release of radioactive material into the atmosphere and water supply.

There are also used fuel rods stored in containment pools near the reactors. The heat from the reactors has started to boil off the water from these pools, which could expose the still-warm fuel rods. This could lead to more fires and a widespread release of radioactive material into the air.

So far, the amount of radiation released has been very small. Only the emergency workers at the plant are in any danger, although they are working in shifts to try to limit their exposure to radiation. One expert said that the workers would have a 1% higher risk of getting cancer in later life. 

The government has evacuated everyone within a 20km radius of the plant, and advised everyone living between 20-30km away to stay indoors. Some people are confused because other governments, such as those in the UK and US, have advised their citizens to stay over 80km away.

Should we be worried about radiation?

The UK’s Health Protection Agency says that there is absolutely no risk to people here. The Government Chief Scientific Adviser, Sir John Beddington, has said that even in the worst possible scenario of a nuclear meltdown and unfavourable wind and weather conditions, even people in Tokyo could keep safe by simply staying indoors and keeping their windows closed.