The facility’s operator, 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 by a huge tsunami that struck the north-east coast of Japan in March 2011.
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.
The recent reading, described by some experts as “unimaginable”, is far higher than the previous record of 73 sieverts an hour in that part of the reactor.
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.
The presence of dangerously high radiation will complicate efforts to safely dismantle the plant.
A remote-controlled robot that Tepco intends to send into the No 2 reactor’s containment vessel is designed to withstand exposure to a total of 1,000 sieverts, meaning it would survive for less than two hours before malfunctioning.
The firm said radiation was not leaking outside the reactor, adding that the robot would still prove useful since it would move from one spot to the other and encounter radiation of varying levels.