TEPCO announced that 2 TEPCO employees entered the reactor building of the Reactor 3 on May 18 in the evening. 4 TEPCO employees entered the Reactor 2 reactor building in the morning on May 18.
From Asahi Shinbun (in Japanese; 12:41PM JST 5/19/2011):
It was the first time that human workers entered the Reactor 3 reactor building since the accident started.
The radiation level was 160 to 170 millisieverts/hour at the maximum.
TEPCO is preparing for the nitrogen gas injection into the Containment Vessel of the Reactor 3 in order to prevent another hydrogen explosion. 2 TEPCO employees with radiation protection suits and oxygen tank entered the reactor building for 10 minutes, and inspected the pipes and valves to be used for the nitrogen injection and measured the radiation level. Near the valves, the radiation measured 50 millisieverts/hour.
10 minutes in 170 millisieverts/hour environment will result in 28 millisieverts radiation.
If I remember right, when TEPCO opened the double door of the reactor building of the Reactor 1 on May 8, the purpose was to reduce the amount of radioactive materials so that the workers' exposure is limited to less than 3 millisieverts. The radiation level didn't go down much at all, but the human workers entered the building anyway.
Now that they openly admit to the total core meltdown in all three reactors, the pretense seems to have gone at TEPCO. High radiation? So? They've got a job to do.