Q+A: Risks at each reactor of Japan's stricken plant

TOKYO (Reuters) - The Fukushima Daiichi nuclear power plant at the center of Japan's crisis has six reactors. The plant is operated by Tokyo Electric Power Co (TEPCO).

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.

-- REACTOR No 2: 784-MW (Manufacturer: GE, Toshiba)

Level 5 severity.

TEPCO officials said on Saturday that engineers had begun to pump fresh water into the reactor building instead of sea water on Friday.

The No. 2 reactor's primary containment vessel, designed to prevent radiation leaks, may have been damaged when there was an explosion in the building on March 15. The portion of the vessel damaged was the suppression pool, into which steam is vented from the reactor to relieve pressure. The IAEA said the blast "may have affected the integrity of its primary containment vessel."

However, beyond the primary containment vessel is the containment building, which is also designed to prevent radiation from escaping.

The reactor's fuel rods were exposed fully at one point. When fuel rods are not covered in coolant, they can heat up and start to melt, raising the risk of a radiation leak.

-- REACTOR No 1: 460-MW (Manufacturer GE)

Level 5 severity.

This is the only reactor where engineers have begun collecting the radioactive water from parts of the building and containing it in a vessel.

Nuclear safety agency officials said on Saturday that the water was being stored inside the building for now. They said it was being collected into a vessel that was made to house the water resulting from cooling vapors given off by the reactors.

The officials said the next challenge for this as well as the other reactors was to devise a safe way of transporting this water to a place where it could not contaminate people or the environment.

-- REACTOR No 4: 784-MW (Manufacturer Hitachi)

Level 3 severity.

At the time of the earthquake and tsunami on March 11, the reactor was undergoing maintenance, Kyodo has reported.

This is currently the only reactor building that is using sea water instead of fresh water as a cooling agent.

Four days after the quake, the spent fuel pool caught fire and caused an explosion. Japan's nuclear safety agency says the blast punctured two holes around 8 meters square in the wall of the outer building of the reactor.

The pools contain racks that hold spent fuel taken from the reactor. Operators need to constantly add water to the pool to keep the fuel submerged so that radiation cannot escape.

Exposing the spent fuel to the atmosphere will release radiation.

-- REACTOR No 5: 784-MW (Manufacturer Toshiba)

The No 5 reactor has been stable since it was safely stopped on March 20, when the temperature of the water inside the reactor fell low enough for it to achieve a so-called cold shutdown.

At the time of the earthquake and tsunami on March 11, the reactor was undergoing maintenance

-- REACTOR No 6: 1,100-MW (Manufacturer GE, Toshiba)

The No 6 reactor has been stable since it was safely stopped on March 20, when the temperature of water inside the reactor fell low enough for it to achieve a so-called cold shutdown.

At the time of the earthquake and tsunami on March 11, the reactor was undergoing maintenance.

(Compiled by Chizu Nomiyama, Editing by Alan Raybould)