Extremely high radiation levels have been recorded inside a damaged reactor at the Fukushima Daiichi nuclear power station, almost six years after the plant suffered a triple meltdown. Tokyo Electric Power (Tepco) said atmospheric readings as high as 530 sieverts an hour had been recorded inside the containment vessel of reactor No 2, one of three reactors that experienced a meltdown when the plant was crippled. Even if a 30-percent margin of error is taken into account, the recent reading, described by some experts as unimaginable", is far higher than the previous record of 73 sieverts an hour detected by sensors in 2012. A single dose of one sievert is enough to cause radiation sickness and nausea; 5 sieverts would kill half those exposed to it within a month, and a single dose of 10 sieverts would prove fatal within weeks. Quantities of melted fuel are believed to have accumulated at the bottom of the damaged reactors' containment vessels, but dangerously high radiation has prevented engineers from accurately gauging the state of the fuel deposits. The extraordinary radiation readings highlight the scale of the task confronting thousands of workers, as pressure builds on Tepco to begin decommissioning the plant - a process that is expected to take about four decades. In December, the government said the estimated cost of decommissioning the plant and decontaminating the surrounding area ... had risen to 21.5tn yen (150bn), nearly double an estimate released in 2013.