A chimney for both the No. 1 and No. 2 reactors remains unrepaired at the Fukushima No. 1 nuclear power plant. At left is the No. 1 reactor building. (Asahi Shimbun file photo)

Tokyo Electric Power Co. plans to start work on May 20 to dismantle a 120-meter-tall, highly contaminated chimney that could collapse at the crippled Fukushima No. 1 nuclear power plant.

It will be the first highly radiated facility at the plant to be taken apart, the company said May 9.

The stack, with a diameter of 3.2 meters, was used for both the No. 1 and No. 2 reactors. TEPCO plans to remove the upper half of the chimney within this year to prevent the structure from collapsing.

The dismantling work will be conducted by remote control because the radiation level around the base of the chimney is the highest among all outdoor areas of the plant. Exposure to radiation at the base can cause death in several hours.

After the Great East Japan Earthquake and tsunami struck in March 2011, pressure increased in the containment vessel of the No. 1 reactor. Vapors with radioactive substances were sent through the chimney to the outside.

TEPCO also found fractures in steel poles supporting the chimney. The damage was likely caused by a hydrogen explosion at the No. 1 reactor building when the nuclear disaster was unfolding.

Since then, the chimney has been left unrepaired because of the high radiation levels.

Immediately after the nuclear accident, a radiation level exceeding 10 sieverts per hour was observed around the base of the chimney. In a survey conducted in 2015, a radiation level of 2 sieverts per hour was detected there.

TEPCO will use a large crane that will hold special equipment to cut the chimney in round slices from the top.

The company set up a remote control room in a large remodeled bus about 200 meters from the chimney. Workers will operate the special cutting equipment while watching footage from 160 video cameras.