Following a major earthquake, a 15-metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, causing a nuclear accident on 11 March 2011. The cores of reactors 1, 2 and 3 largely melted in the first three days while unit 4 was written off by a hydrogen explosion. After two weeks units 1-3 were stable, by July 2011 they were being cooled with recycled water from the new treatment plant and official 'cold shutdown condition' was announced in mid December. Major engineering projects have seen airtight buildings erected around units 1 and 4. All nuclear fuel was removed from unit 4 by the end of 2014. No cases of radiation sickness resulted from the nuclear accident. The health effects, if any, on the population are estimated as too small to detect.