Tokyo Electric Power Company says debris scattered inside the No. 4 reactor building at the Fukushima Daiichi nuclear plant is posing an obstacle to work to bring the crippled reactor under control.

Workers entered the fifth floor of the building on Wednesday for the first time since an explosion on March 15th.

Photos taken by the workers show that most of the ceiling, except for a small part of the framework, has collapsed. Debris, steel frames, and other various things blown by the force of the explosion are scattered all over the floor.

The radiation level inside the building was less than one millisievert per hour, which TEPCO says is permissible for workers to carry out operations there.