Workers at the Fukushima Daiichi Nuclear Power Plant entered the number one reactor building on Thursday for the first time since an explosion ripped through it on March 12. The workers are building a air purification system that should aid further recovery work.
Until Thursday, nuclear workers had been sending remote-controlled robots into the building to check for damage and measure radiation.
The robots found levels as high as 49 milliSieverts per hour - so high that a worker would hit their annual limit in just five hours inside the building.
The danger meant Thursday's work began under severe safety limitations, says Tokyo Electric Power Spokesman Junichi Matsumoto.
Matsumoto says two workers entered the building at 11:32 on Thursday morning. They made up a team of 12 people, each of whom had their time in the building limited to 10 minutes because of the highly radioactive environment.
They were sent in wearing protective suits, masks and air tanks similar to those used by firefighters.
Thursday's work involves laying ducts that will be used to suck air into a purifier in a neighboring building. The machine should remove radioactive substances from the air, which will then be pumped back into the reactor building through additional ducts.
It should take about three days for the system to bring the radiation levels down to a point where staff can enter the building to conduct other work.
That means checks on pipes and valves used in the water cooling system could begin as early as Sunday.
Tokyo Electric Power wants to restart the system as soon as possible, to reduce the need to pump in large amounts of water from outside.
The plant operator also wants to install and calibrate a new reactor water level gauge, so it can better monitor water levels around the fuel rods in the reactor core.