Or more than 1 sievert/hour.
When 7 TEPCO employees and 2 NISA employees entered the Reactor 1 reactor building on early hours of May 9 (JST), the act that released mere 500 million becquerels of radioactive iodine and cesium into the atmosphere, they measured the radiation level on the ground floor which was 600 to 700 millisieverts/hour at the highest spot.
Then on May 10, someone went upstairs to the 2nd floor for the first time since March 11, and measured the radiation there. It was so high that the Geiger counter couldn’t accurately measure.
And TEPCO thinks the totally melted blob of fuel rods (uranium, plutonium) + cladding (zirconium alloy) + control rods (boron, cadmium, silver, indium) + stainless steel pipes + whatever was inside the Reactor Pressure Vessel = “corium” is being safely cooled at the bottom of the RPV.
From Mainichi Shinbun (5/11/2011):
経 済産業省原子力安全・保安院は１１日、東京電力福島第１原発１号機の原子炉建屋２階で、１時間当たり１０００ミリシーベルトを超える高濃度の放射 性物質を測定したと発表した。１号機は冷却装置の設置に向けた準備を進めている。この数値は短時間での作業すら難しくする高い線量で、保安院の西山英彦審 議官は「冷却のための配管のつなぎこみ作業に影響するかもしれない」との見方を示した。
Ministry of Economy, Trade and Industry’s Nuclear and Industrial Safety Agency (NISA) announced on May 11 that the radiation level exceeded 1,000 millisieverts/hour on the 2nd floor of the Reactor 1’s reactor building at Fukushima I Nuclear Power Plant. TEPCO has been preparing for the work to install the external cooling system inside the reactor building. This radiation level may be too high for workers to enter and work even for a short period of time. NISA’s spokesman Nishiyama said the high radiation “may affect the work to connect the pipes for the cooling system.”
The location that registered the high radiation level was near the valve of the emergency core cooling system. Measurement was done for 5 minutes starting 12:45PM on May 10 (JST), and at 1.6 meters from the floor the Geiger counter went overscale [and couldn’t measure beyond 1,000 millisieverts/hour].
The radiation level of 600 to 700 millisieverts/hour was detected on the 1st floor of the same reactor building on May 9.