Following a major earthquake, a 15-metre tsunami disabled the power supply of the Fukushima Daiichi reactors, interrupting cooling and hence causing a nuclear accident on 11 March. The reactor cores at units 1-3 largely melted in the first three days.

The accident was complicated by the failure of the cooling systems in the used fuel storage pools in reactors 1-4.

Since the accident occurred Tepco have focussed on implementing new cooling systems, which now use recirculated decontaminated water to cool the reactors of units 1-3 and also provide cooling to the fuel ponds of units 1-4.

The WNA Fukushima Accident Information Paper reports on the steps take to achieve a cold shutdown condition at Fukushima Daiichi, and the on-going efforts to treat contaminated water and minimise releases of radioactive materials

Nuclear power plants are designed to withstand earthquakes, and in the event of major earth movement, to shut down safely. In areas where the frequency and magnitude of earthquakes is significant, such as in Japan, particular attention is paid to seismic issues in the siting, design and construction of nuclear power plants.

From the outset, there has been a strong awareness of the potential hazard of both nuclear criticality and release of radioactive materials. There have been three major reactor accidents in the history of civil nuclear power - Three Mile Island, Chernobyl and Fukushima. These are the only major accidents to have occurred in over 14,000 cumulative reactor-years of commercial operation in over 30 countries.

Natural sources account for most of the radiation we all receive each year. Up to a quarter of that received is due to human activity and originates mainly from medical procedures. Radiation protection standards assume that any dose of radiation, no matter how small, involves a possible risk to human health. This deliberately conservative assumption is increasingly being questioned.