Japanese workers have entered the third reactor building hit by nuclear fuel meltdowns at the Fukushima Daiichi plant.
Tokyo Electric Power Co (TECPO) is trying to stabilise the plant which has been leaking radiation for more than two months after being hit by the Japan earthquake and tsunami.
Workers in protective gear have started to inspect the No.3 reactor building, the first time anyone has entered since fuel meltdowns were triggered by the earthquake and tsunami, as well as a hydrogen explosion that blew off the No.3 reactor building's roof.
"Workers being able to enter the reactors is perhaps the biggest improvement [since the Tepco announcement of its timetable for stabilising the plant]," an anonymous nuclear energy professor told Reuters.
"They have made the ventilation of the reactor building possible and it also enables Tepco to install heat exchangers and pumps."
TEPCO is keen to prevent further hydrogen blasts and set up a sustainable cooling system that will stabilise the reactors, although high radiation levels have hindered their efforts.
Leaks in the three reactors' pressure vessels discovered this week have been a further setback, but TEPCO is determined to stick to a timetable for bringing the Fukushima Daiichi reactors to a stable state by January.
Each of the two TEPCO employees that entered the No.3 reactor building was exposed to less than three millisieverts of radiation during their 10 minute stay, compared with the government-set upper limit of 250 millisieverts per worker for the duration of the Fukushima stabilisation project.
TEPCO has already sent workers into the buildings that house the No.1 and No.2 reactors, the two other units in operation at the six-reactor plant at the time of the massive quake and tsunami.
The prolonged nuclear crisis has sparked a national debate over nuclear safety and boosted support for anti-nuclear groups' calls to ditch plans, unveiled last year, to build nine new reactors by 2020 and at least 14 by 2030.
National Strategy Minister Koichiro Gemba said it would be impossible to go ahead with building all 14 reactors, while Prime Minister Naoto Kan has called for a clean-slate review of Japan's nuclear policy.
The policy had planned for more than 50 per cent of the total electricity supply to come from nuclear power by 2030.
TEPCO also said a giant floating steel structure that will store some of the plant's massive volume of contaminated water would arrive at the complex later this week and will be able to hold 10,000 tonnes of water.
More than 90,000 tonnes of radioactive water has collected in basements and trenches in and around the reactor and turbine buildings at the plant, after the utility poured water into reactors to cool their fuel rods, creating a risk of seepage into ground water and the ocean.