(Newser) – Exposure to 4 sieverts of radiation would kill 50% of people, according to Japan's National Institute of Radiological Sciences, per Japan Times. That's less than 1% of the radiation now being detected inside a damaged reactor at the Fukushima nuclear power station, according to Tokyo Electric Power. The Fukushima operator says it has calculated radiation of 530 sieverts an hour—though with a 30% margin of error—inside the containment vessel of one of three reactors damaged in the 2011 meltdown. Such a level is "unimaginable," an expert tells Engadget, and hasn't been seen since the meltdown itself. Since March 2011, the highest radiation recorded has been 73 sieverts per hour.

A rep says a hole in metal grating beneath the reactor's pressure vessel, likely made by melted nuclear fuel, might be to blame but "it is very difficult to assume the actual condition inside." Five robots sent into the reactor to discover the location of the melted fuel haven’t reappeared, though officials hope to try again with a robot built to withstand 1,000 sieverts. The Guardian notes safely removing the fuel—previously believed to have been contained in the pressure vessel, where the reactor core is housed—"represents a challenge unprecedented in the history of nuclear power." Yet it will have to be overcome: The fuel is supposed to be removed by 2018 as crews work to dismantle the plant at a cost of $187 billion. (Read more Fukushima Dai-ichi stories.)

