Emergency crews resumed work at Japan’s stricken Fukushima Daiichi nuclear power station on Thursday after smoke cleared from its No 3 reactor, prompting authorities to lift an evacuation order issued a day earlier.
The evacuation, which lasted about 12 hours, delayed efforts to restore vital electrical systems knocked out by Japan’s March 11 earthquake and tsunami.
The magnitude 9 quake and 10m-high wave cut off the backup electricity supply to the plant’s cooling systems. When they stopped working, uranium inside the plant’s six reactors and fuel-storage tanks overheated, causing explosions, fires and releasing radiation.
It was the second time in three days that grey smoke from the No 3 reactor building brought emergency work to a stop. Tokyo Electric Power, the station’s operator, has been unable to identify its source, but said radiation readings at the plant have not risen noticeably.
However, soon after the resumption of work at the plant, authorities announced that two members of the emergency crew had been taken to hospital with radiation contamination. The electrical engineers were repairing underground power lines in a turbine building attached to the station’s No 3 reactor when radioactive material penetrated their protective clothing and contaminated skin on their legs.
The two workers and another technician working one floor above them received airborne radiation doses of more than 170 millisieverts, the nuclear and industrial safety agency said. That is higher than the usual 100 mSv legal limit for emergency workers at nuclear facilities, although Japan has raised its limit to 250 mSv to allow work to continue at Fukushima.
Exposure to more than 100 mSv per year increases the likelihood of cancer, according to researchers. However, the single-dose threshold for acute radiation sickness is ten times that level.
Before Thursday, 14 technicians and emergency personnel had been hurt at Fukushima Daiichi since the quake, according to Tepco.
Beyond the plant itself, contamination from the accident has now spread beyond the immediate Fukushima area, about 240km north-west of Tokyo. The government says radiation levels outside a 20km evacuation zone are not high enough to harm humans in the short term.
Tokyo’s metropolitan government on Wednesday warned residents not to give tap water to infants after water in a purification plant was found to contain elevated levels of Iodine 131, a radioactive element. A ban has been imposed on the sale of some vegetables and milk from Fukushima and several surrounding districts. On Thursday Australia joined the US, Hong Kong and other countries in banning the import of milk and fresh vegetables from the four prefectures surrounding the plant.
Restoring Fukushima Daiichi’s internal cooling systems remains the best hope for stabilising the plant and moving beyond the ad hoc emergency measures – from helicopter water drops to spraying with riot-police water cannons – that have been tried so far.
Technicians have succeeded in attaching new external power lines to all the reactor units at the plant, but the equipment inside has been damaged by the quake, explosions and thousands of tonnes of seawater that have been poured inside, and there is no guarantee that it will still work.
THE FUKUSHIMA DAIICHI REACTORS IN DETAIL
Number one A hydrogen explosion destroyed the top of the reactor building on March 12. The spent fuel pool is seen as relatively safe because it contains less uranium than those at the plant’s other reactors. The temperature at the reactor’s core rose to around 400 degrees centigrade on Wednesday, 100 degrees higher than normal, prompting technicians to add more seawater coolant. That succeeded in bringing the temperature back down, but the extra steam it created increased pressure inside the reactor vessel. Around 5am Japan time on Thursday, pressure was 0.4 megapascals (MPa), nuclear safety authorities said, up from 0.25 MPa on Wednesday but below the reactor’s safety threshold of 0.52 MPa. Electricity has been partially restored to the control room, though it will take time before internal cooling systems can be revived.
Number two A water tank under the containment vessel was damaged by an explosion on March 15, and the vessel itself may be damaged. Heat and temperature inside the reactor are stable, authorities said on Thursday. Seawater is being pumped into the unit’s spent fuel pool from fire hoses attached to its internal cooling pipes.
Number three A hydrogen explosion destroyed the top of the reactor building on March 14. Uranium in a spent fuel pool adjacent to the reactor has overheated, releasing radiation. Firefighters are spraying the pool from outside. Electricity has been partially restored to the control room, though it will take time before engineers can attempt to revive its internal cooling systems. On Wednesday, unidentified grey smoke was seen rising from the unit, triggering a second evacuation in three days. By Thursday morning the smoke was gone and repair work resumed. Technicians are working to attach fire hoses to the spent-fuel pool’s built-in coolant pipes and to fix a separate pump to circulate fresh water inside the reactor core. Concerns are mounting that salt residue from days of seawater injections may be making it more difficult to cool the core.
Number four A fire damaged the reactor building on March 15-16, and uranium in the spent fuel pool has overheated, releasing radiation. The reactor itself was off line when the earthquake hit, but the spent fuel pool is especially dangerous because it was storing more uranium than any of the other units’ pools. Electricity has been partially restored to the control room but internal cooling systems remain off line. An extra-long-armed spraying machine, normally used to spray concrete at construction sites, was deployed on Wednesday to pump water into the spent fuel pool. Technicians are aiming to attach fire hoses to the pool’s built-in coolant pipes by Friday.
Number five and six Both reactors were off line when the quake struck, and have experienced less severe problems than units 1-4. The reactor cores have been cooled to safe levels and outside power has been restored. However, cooling systems inside unit 5 failed on Wednesday when engineers attempted to switch them to external power.