(UPDATE: The maximum radiation that the workers received by braving the Reactor 1 reactor building exceeded 3 millisieverts. It was 3.16 millisieverts, as announced by TEPCO in the morning press conference on May 6.)
For the first time since the Reactor 1 blew up on March 12, human workers entered the reactor building to install pipes for the air filtering system that they are building outside the reactor building. TEPCO hopes to lower the radiation level using this system to the level low enough for more human workers to work in the reactor building long enough to set up an external cooling system for the reactor.
Points from Asahi Shinbun (in Japanese; 5/5/2011):
2 TEPCO employees entered the reactor building at 11:32AM on May 5 to measure the radiation level.
13 workers installed pipes for the air-filtering system by 3:08PM.
The workers were equipped with air tanks so that they wouldn't breathe in radioactive materials. The work was scheduled so that each worker would not receive more than 3 millisieverts. The maximum radiation exposure was 2.8 millisieverts (provisional number).
TEPCO will run the air-filtering system for 24 hours. The air inside the reactor building goes through the pipes, through the air filters to reduce the amount of radioactive materials and circulate back into the reactor building.
If the radiation level is lowered according to the plan, they will start assembling the external cooling system in the reactor building on May 8.
The article is a bit puzzling because it talks about the cooling system using water. But the talk was to build an external cooling system with air, as Yomiuri reported (in Japanese).
That aside, if the max exposure limit was 3 millisieverts per worker and the radiation level inside the Reactor 1 reactor building was between 10 and 1,120 millisieverts per hour, the time a worker could have spent inside the reactor building was anything from 9.6 seconds (1,120 millisieverts/hr location) to 18 minutes (10 millisievert/hr location).
How can the air-filtering system filter out radioactive materials from the air inside the reactor building, when there is practically unlimited source of radioactive materials - i.e. reactor core which has been melted, at least partially if you believe TEPCO and the Japanese government, completely if you believe Mr. Ishikawa of the Japan Nuclear Technology Institute?