The following summarizes the main risks as Japanese engineers scramble to deal with the worst nuclear crisis since the 1986 Chernobyl meltdown:
WHAT’S THE OVERALL SITUATION?
Two of the plant’s reactors are seen as safe, but the other four occasionally emit steam and smoke. The nuclear safety agency said on Saturday that the temperature and pressure in all reactors had stabilized.
In recent days fears have grown that the core of one of the reactors, No.3, had been damaged by the quake and tsunami and was leaking radiation. Three workers there sustained injuries on their legs after stepping into a puddle of water earlier this week and were shown to have been exposed to radiation levels 10,000 times higher than what is usual in a reactor.
Another challenge is how to store this radioactive water — found in the buildings of three of the six reactors — in a secure location after it has been collected.
A separate priority has been switching the source of water, used to cool the reactor cores and pools containing spent fuel, to fresh water from sea water. Sea water is more corrosive and leaves deposits that prevent water from circulating around the rods to cool them.
WHAT ARE THE RISKS IN EACH REACTOR?
— REACTOR No 3: 784-MW (Manufacturer Toshiba)
On March 18, TEPCO said the situation at the reactor had been labeled Level 5 severity on the International Nuclear and Radiological Events Scale. Level 7 is the most severe.
Preventing radiation leaks from reactor No. 3 is particularly crucial. It is the only one to use plutonium in its fuel mix. The others only use uranium, which is less toxic.
Following the incident in which workers were contaminated by highly radioactive water, Japanese officials have said damage to the reactor core is unlikely, though they have not ruled it out. Nuclear Industrial Safety Agency senior official Hidehiko Nishiyama said there was no data showing damage.
Officials have cited other possible sources of the unusually high radiation in the standing water, such as steam-venting operations or water leakage from pipes or valves.
The U.N. International Atomic Energy Agency (IAEA) said on Friday that it had received information the two hospitalized workers were likely to be released on Monday.
“From my medical perspective, if they got something serious, they wouldn’t be discharged on Monday,” IAEA Human Health Director Rethy Chem told a news conference.
On the corrosive water issue, TEPCO said it had started injecting fresh water into the reactor pressure unit on Friday.