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Japan nuke plants risks explained

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

world Updated: Mar 18, 2011 13:02 IST

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

The following summarises what is happening at each unit, and the major risks from the worst nuclear crisis since the 1986 Chernobyl fallout:

WHICH REACTORS ARE MOST AT RISK?

REACTOR No 3: 784-MW (Manufacturer Toshiba)

-- What is happening:

Japanese Defence Minister Toshimi Kitazawa says on Friday fire trucks are to keep dousing the reactor, the one emitting the most radiation and the only one with plutonium in its fuel mix.

Plutonium is considered more hazardous than uranium.

The Japanese nuclear agency says the key aim is to get water into spent fuel pools. It also acknowledges that a "Chernobyl solution" of burying the reactors in sand and concrete "is in the back of our minds", but cooling the reactors is the main task.

Smoke or steam was seen rising Friday morning from this reactor along with the other crippled reactors 2 and 4. The agency said there was still water in the spent fuel pool.

There was an explosion at reactor 3 on Monday.

-- What are the risks:

The major concern is that any steam coming from the plant will carry radiation into the atmosphere. It's not clear where this could be coming from.

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

However, the Japan nuclear agency noted the steam could be coming from the spent fuel pool. That would indicate that water covering the spent fuel is evaporating, which in turn could mean the vapour 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 4: 784-MW (Manufacturer Hitachi)

-- What is happening:

The top US nuclear regulator said the cooling pool for spent fuel rods at this reactor may have run dry and another was leaking.

Gregory Jaczko, head of the US Nuclear Regulatory Commission, told a US congressional hearing earlier this week that radiation levels around the cooling pool were extremely high, posing deadly risks for workers still toiling in the wreckage of the power plant.

He said it could take weeks to successfully cool down the reactors. Japan's nuclear agency said it could not confirm if water was covering the fuel rods.

However, TEPCO said on Thursday that as of Wednesday the spent fuel pool still had water in it.

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

On 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 2: 784-MW (Manufacturer: GE, Toshiba)

-- What is happening:

Teams are trying to restore a power cable to reactors 1 and 2 at the plant to restart pumps needed to pour cold water on overheating fuel rods, but that work would stop to enable the fire trucks to proceed with spraying.

TEPCO acknowledges on Friday that preparatory work to reconnect the power "has so far not progressed as fast as we had hoped". Workers are constantly checked for radiation levels.

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. An estimated 33 percent of the nuclear fuel rods have been damaged at the No 2 reactor, Kyodo quoted TEPCO as saying on Wednesday.

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 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)

-- What is happening:

Japan's nuclear safety agency said on Thursday the reactor, along with units No 5 and No 6, was relatively stable for now.

Earlier on Thursday, it said that TEPCO planned to run a cable to reactors No 1 and No 2 to try to restore power to the water cooling system.

An explosion occurred at the reactor on Saturday. Kyodo quoted TEPCO as saying on Wednesday that an estimated 70 percent of the nuclear fuel rods have been damaged.

The Japan nuclear agency said on Wednesday the pumping of sea water into the reactor 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 (Manufacturer Toshiba)

-- What is happening:

The reactor is now being powered by a diesel generator shared with unit No 6.

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.