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

Smoke is seen coming from the area of the No. 3 reactor of the Fukushima Daiichi nuclear power plant in Tomioka, Fukushima Prefecture in northeastern Japan in this handout photo distributed by the Tokyo Electric Power Co. on March 21, 2011.

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 (9501.T) (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 LATEST?

Authorities said they had reconnected all five reactors at the complex to a power supply. The next step is to see if they can get power running in each reactor.

This would allow TEPCO to restore systems to monitor radiation and other data and light the control room of the reactors. Most importantly, they would be able to cool down the reactors and the spent fuel pools.

But their ability to power up the reactors will depend on the extent of any damage to each unit. For that reason, experts said it was premature to suggest the crisis had reached a turning point.

WHICH REACTORS ARE MOST AT RISK?

-- REACTOR No 3: 784-MW (Manufacturer Toshiba)

Level 5 severity on the International Nuclear and Radiologicial Events Scale. Level 7 is the most severe.

Smoke was reported as rising temporarily. Some workers were withdrawn, TEPCO said. The smoke rose from the spent fuel pool, Kyodo said.

Earlier Monday, the nuclear safety agency said pressure was rising at the reactor and workers were considering venting pressure from the unit.

Power connected to all six reactors, TEPCO said on Monday. Company checking for damage at the plant's cooling system before trying to power up.

There was an explosion at the reactor last Monday.

Last week, authorities said reactor 3 was emitting the most radiation and so was the biggest priority. A key aim of dousing was to get water into spent fuel pools, they said.

Reactor 3 is the only one that uses plutonium, which is considered more toxic than uranium, in its fuel mix.

-- What are the risks:

The major concern is that any steam coming from the plant will carry radiation into the atmosphere.

Chief Cabinet Minister Yukio Edano said on March 16 there was a "possibility" the primary containment vessel, the first line of defense against a radiation leak, had been damaged, Kyodo reported. The reactors also have a secondary containment building. (see below: CONTAINMENT -- WHAT IS IT?)

Steam from the spent fuel pool would indicate that water covering the spent fuel is evaporating, which in turn could mean the vapor is carrying off radiation.

The spent fuel pool presents a significant radiation risk if its contents are exposed to the atmosphere. When fuel rods are exposed to the air, zirconium metal on the rods will catch fire, which could release radiation contained in the fuel, said Arnie Gundersen, a 29-year veteran of the nuclear industry who is now chief engineer at Fairwinds Associates Inc.

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

Level 5 severity.

Power connected to all six reactors, TEPCO says on Monday.

Nuclear safety agency said on Monday that smoke was seen over the reactor, Jiji reported.

Last Tuesday, an explosion rocked the plant, 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 last week.

TEPCO said last Tuesday the fuel rods were fully exposed. An estimated 33 percent of the nuclear fuel rods have been damaged at the No 2 reactor, Kyodo quoted TEPCO as saying last Wednesday.

-- 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 and in a worst-case scenario a full meltdown.

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 (Manufacturer GE)

Level 5 severity.

Power connected to all reactors, TEPCO says on Monday. System tests are being carried out.

An explosion occurred at the reactor on March 12. Kyodo quoted TEPCO as saying on March 16 that an estimated 70 percent of the nuclear fuel rods had been damaged.

-- 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 4: 784-MW (Manufacturer Hitachi)

Level 3 severity.

-- What is happening:

Power connected to all six reactors, TEPCO says on Monday.

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

Last week, Gregory Jaczko, head of the U.S. Nuclear Regulatory Commission, told a U.S. congressional hearing the cooling pool for spent fuel rods at this reactor may have run dry and another was leaking.

Last Tuesday, the spent fuel pool 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.

-- 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 5: 784-MW (Manufacturer Toshiba)

The No 5 reactor was safely stopped on Sunday. Temperature of water inside the reactor fell below 100 degrees Celsius, achieving a so-called cold shutdown, Kyodo news reported.

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

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

The No 6 reactor was safely stopped on Sunday. Temperature of water inside the reactor fell below 100 degrees Celsius, achieving a so-called cold shutdown, Kyodo news reported.

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

WHAT IS THE WIND DIRECTION?

Wind and light rain are blowing from the northwest to the southeast, a direction out across the Pacific Ocean, Japan's weather agency said on Monday.

CONTAINMENT -- WHAT IS IT?

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

Should there be a breach, there is another, final line of defense 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.

(Compiled by Asia Desk)