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:
REACTOR No 3: 784MW (Manufacturer Toshiba)
What is happening:
Helicopters and trucks were used to water down reactors as authorities reiterated on Thursday that resolving problems at the plant - the only unit to include plutonium in its fuel mix
- was the priority.
White smoke coming from the plant could be steam evaporating from the spent fuel pool, the Japan nuclear agency said on Thursday. It said pressure in the reactor was rising again.
Radiation readings at the reactor are the highest at the Daiichi complex, Tepco said on Wednesday.
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.
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.
Plutonium is considered more hazardous than uranium.
REACTOR No 4: 784MW (Manufacturer Hitachi)
What is happening:
There is no water in the spent fuel pool and radiation levels are extremely high, the chairman of the US Nuclear Regulatory Commission said in Washington on Wednesday.
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: 784MW (Manufacturer: GE, Toshiba)
What is happening:
Tepco plans to run a cable to reactors No 1 and No 2 to try to restore power to the water cooling system, the Japan nuclear agency said on Thursday.
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: 460MW (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.
REACTOR No 6: 1,100MW (Manufacturer GE, Toshiba)
What is happening:
The reactor is now being powered by a diesel generator shared with unit No 5.
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.
What are the radiation levels, wind direction?
Radiation levels were higher than normal but not dangerous, Japan's Nuclear and Industrial Safety Agency said on Thursday.
They were measured at 338 microsieverts per hour at the west gate at 5am (4am Malaysian time) on March 17. If a person stands outdoors for a year, they would be exposed to a radiation level of 400 microsieverts, the agency said.
The wind is blowing northwest-to-southeast, towards the Pacific Ocean, Japan Meteorological Agency said.
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 defence against leaking
radiation from the reactor.
Should there be a breach, there is another, 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.
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