After the magnitude 8.9 earthquake rocked Japan, a "state of emergency" was declared at one of the country's nuclear power plants.
The Fukushima reactor, around 30 miles inland from the coast in north-east Japan, suffered a failure in its cooling system.
Chief Cabinet Secretary Yukio Edano, the top government spokesman, said the nuclear power plant developed a mechanical failure in the system needed to cool the reactor after it was shut down. He said the measure was a precaution as there was was no radiation leak and the facility was not in immediate danger.
Professor Tim Abram, a nuclear fuel technology expert at Manchester University, said that as long as a reactor is shut down, it is considered "benign" until bosses decide it is safe to be turned back on.
He said: "All nuclear facilities are designed to withstand seismic events. The magnitude of the seismic event that they are designed to withstand varies from country to country. It's not done on a case of a particular point on the Richter scale, but instead on the basis of probability of earthquakes in particular countries. In somewhere like Japan, the probability will be much, much higher."
The professor said although a failure in the cooling system of a nuclear power plant was "unexpected", once a reactor is shut down, the heat levels plummet anyway.
He said: "Reactors shut themselves down automatically when something called 'ground acceleration' is registered at a certain point, which is usually quite small. It will instantly drop control rods into the core."
At that stage, he said, the heat of a nuclear station drops dramatically in a matter of seconds, and within a couple of minutes, it is down to under 5% of its normal temperature.
He said: "That's a tiny, tiny percentage of the usual power output of the core. You still need to get rid of the decay heat, but the system is very capable of doing that. It's a bit like a braking system on a car failing when it is traveling at 3mph, when it is designed to slow it down from 120."
Meanwhile, Japan has issued an evacuation order to more than 2,800 residents close to the plant.
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