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

The following summarises what is happening at each unit, and the major risks:

For a PDF of this Q+A: http://r.reuters.com/wys58r

WHAT STOPS A RADIATION LEAK?

Each reactor is surrounded by the primary containment vessel. This is made of strengthened steel four to eight inches thick. It provides the most critical line of defence against leaking radiation from the reactor.

Should there be a breach, there is a final line of defence to prevent radiation leaks, a bigger containment building made of steel and concrete. A breach of the containment building would release radiation into the atmosphere.

WHICH REACTORS ARE MOST AT RISK?

REACTOR No 3: 784-MW

-- What is happening:

TEPCO said on Wednesday that resolving problems at this reactor was the top priority because it had the highest radiation levels. This reactor is the only one that includes plutonium in its fuel mix.

The operator has been pumping sea water into the reactor to prevent overheating. The building housing the reactor was hit by an explosion on Monday.

An attempt by a military helicopter to drop water on the reactor failed on Wednesday probably because radiation levels were too high, Kyodo news agency reported. The Japan nuclear agency had said earlier in the day that the pumping of sea water was proceeding smoothly.

TEPCO said the spent fuel pool may have heated up, producing steam. The temperature has risen to around 60 degrees Celsius from the usual 30-40 degrees, but the change was not critical, it said.

-- What are the risks:

The International Atomic Energy Agency (IAEA) said on Tuesday that the primary containment vessel, the first line of defence against a radiation leak, appeared intact.

However, Chief Cabinet Minister Yukio Edano said on Wednesday there is a "possibility" the vessel had been damaged, Kyodo reported.

If that is the case, authorities will be worried that radiation may leak through the first containment wall into the secondary containment building.

The spent fuel pools present a radiation risk if the spent fuel is exposed to the atmosphere. When a rod is exposed to the air, zirconium metal on the rods will catch fire, which could release radiation contained in the fuel, said Arnie Gundersen, a 39-year veteran of the nuclear industry who is now chief engineer at Fairwinds Associates Inc.

REACTOR No 4: 784-MW

-- What is happening:

TV on Wednesday showed smoke or steam rising from the facility after flames were seen earlier. The reactor had been shut down for maintenance when the earthquake and tsunami struck.

On Tuesday, a pool where spent fuel is stored caught fire and caused an explosion. Japan's nuclear safety agency says the blast punctured two holes around 8-metres square in the wall of the outer building of the reactor.

TEPCO has said it may pour water through the two holes within two or three days to cool spent nuclear fuel that is inside. Workers cannot prepare to pour water into the pool sooner because of high radiation levels, Kyodo said.

Instead, TEPCO plans to bulldoze a road to the reactor building so water-pump trucks can approach and hose water inside, said Kazuya Aoki, a director of Japan's Nuclear and Industrial Safety Agency.

-- What are the risks:

Exposure of spent fuel to the atmosphere is serious because there is more radiation in the spent fuel than in the reactor, said Gundersen. The spent fuel pool is not inside a containment facility either.

"They need to keep water in those pools because the roof over the building housing the pools is already damaged and radiation will escape," he said.

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 2: 784-MW

-- What is happening:

An explosion rocked the plant on Tuesday, damaging a suppression pool, into which steam is vented from the reactor to relieve pressure. The roof of the reactor building is damaged, Jiji news agency reported.

TEPCO said on Tuesday the fuel rods were fully exposed. Kyodo reported an estimated 33 percent of the nuclear fuel rods have been damaged at the No 2 reactor.

However, on Wednesday, Japan's nuclear agency said the pumping of sea water into the reactor was proceeding smoothly.

-- What are the risks:

When fuel rods are no longer covered in coolant they can heat up and start to melt, raising the risk of a radiation leak.

The suppression pool is part of the primary containment vessel, which is designed to prevent a leak, but the IAEA said the blast "may have affected the integrity of its primary containment vessel."

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

REACTOR No 1: 460-MW.

-- What is happening:

An explosion occurred at the reactor on Saturday. Kyodo reported on Wednesday an estimated 70 percent of the nuclear fuel rods have been damaged.

Authorities are pumping sea water into the reactor to prevent overheating, and pressure levels were stable, Edano said on Tuesday.

The Japan nuclear agency said on Wednesday the pumping was proceeding smoothly.

-- What are the risks:

The IAEA said on Tuesday the primary containment vessel appeared intact. If the fuel rods in the reactor are not covered by coolant, they can heat up and start to melt.

REACTOR No 5: 784-MW

-- What is happening:

The reactor had been shut down for maintenance at the time of the quake and tsunami.

TEPCO said on Wednesday water was being poured into the reactor and that temperatures in the spent fuel pool were rising slightly.

-- What is the risk.

Reactor 5 and reactor 6 are seen less at risk than reactors 1 to 4.

REACTOR No 6: 1,100-MW

-- What is happening:

TEPCO said on Wednesday water was being poured into the reactor and that temperatures in the spent fuel pool were rising slightly.

-- What is the risk.

Reactor 5 and reactor 6 are seen less at risk than reactors 1 to 4.

(Compiled by World Desk Asia)