Status report: Reactor-by-reactor at the Fukushima Daiichi plant

(CNN) -- Since March 11, the six reactors at the Fukushima Daiichi nuclear power plant have been in various states of disrepair after a 9.0-magnitude earthquake and subsequent tsunami struck the area.

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Here is the latest on each reactor and efforts to prevent further releases of radioactive material.

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Reactor No. 1

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Just after midnight Friday, a Tokyo Electric official said that iodine-131 levels in ground water from a pipe near the No. 1 reactor had 10,000 times the standard limit. But the utility later backtracked, promising to get more clarity later. Japanese Chief Cabinet Secretary Yukio Edano addressed this confusion in a press conference later Friday, noting that a "constant amount of radiation" appeared to be getting into the groundwater and noting that further tests are forthcoming.

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Water levels in an exposed maintenance tunnel leading from the No. 1 unit's turbine building had dropped 1 meter from its previous measure, a Tokyo Electric official said Friday. The authorities assume this relates efforts to pump water out of the building's basement, which had been flooded with radioactive water.

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There has been little appreciable change recently in the reactor's status, an official with Japan's nuclear and industrial safety agency said Friday. Temperatures rose slightly in one part of the unit and fell in another. Water levels in the reactor increased about two inches, and pressure appeared to decrease.

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Tsunehisa Katsumata, chairman of Tokyo Electric Power Company that runs the nuclear power plant, said Wednesday, "Looking at current conditions, ... there are no options other than decommissioning" the No. 1 reactor, as well as Nos. 2, 3 and 4 units. This would mean that the reactor would never be used to produce electricity again.

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This reactor's core has been damaged, but its containment vessel was not, according to the Japan Atomic Industrial Forum, an industry trade group that tracks information from government and Tokyo Electric officials. The containment vessel is a concrete and steel structure that keeps radioactive material inside the reactor.

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A hydrogen explosion -- an indicator of core damage -- blew the roof and upper walls off the building housing the reactor on March 12.

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Lighting has been restored to the No. 1 and No. 2 units' control room, though the overall power supply in both is subpar.

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Reactor No. 2

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Water levels in the exposed maintenance tunnel leading from the No. 2 unit's turbine building were down one meter, a Toyko Electric official said Friday.

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Earlier this week, the water in that tunnel had radiation levels of 1,000 millisieverts per hour -- which is more than 330 times the dose that an average individual living in a developed country receives per year and can result in vomiting and up to a 30 percent higher risk of cancer, according to the International Atomic Energy Agency.

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An official with Japan's nuclear and industrial safety agency on Friday reported little significant change in the status of the No. 2 reactor. Temperatures fell slightly, with pressure inches up.

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The temperature of the No. 2 unit's spent nuclear fuel pool measured 48 degrees Celsius (118 degrees Fahrenheit). This is well the 100-degree level at which water used to cool the nuclear fuel rods in the pool would boil off.

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Katsumata said Wednesday that, "looking at current conditions," the No. 2 reactor and three others would be decommissioned -- meaning it would never be used to produce electricity again.

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Japanese Chief Cabinet Secretary Yukio Edano has said that he has received a report that the No. 2 unit's containment vessel "is damaged and water is leaking."

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Workers were pumping freshwater Tuesday into the No. 2 unit's reactor core, which the Japan Atomic Industrial Forum says has been damaged. The building housing the reactor has only been "slightly damaged," according to the industry group.

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Lighting has been restored to the No. 1 and No. 2 units' control room, though the overall power supply in both is subpar.

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Reactor No. 3

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Water was sprayed Thursday night from a concrete pumping truck into the No. 3 unit's spent nuclear fuel pool, a Tokyo Electric official said Friday. This is part of the ongoing effort to prevent fuel rods there from overheating and releasing radioactive materials into the atmosphere.

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The water levels in the exposed maintenance tunnel leading from the No. 3 unit's turbine building has decreased by 1.5 meters, a Tokyo Electric official said Friday. Earlier, tests revealed that water in this tunnel had high levels of radioactivity -- prompting authorities to make it a priority to drain the tunnels, to prevent this water from overflowing and seeping into the ground. But by Friday, the utility company said the drainage had been largely effective.

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There have been small but not significant changes of late in the No. 3 reactor's condition, an official with Japan's nuclear and industrial safety agency said Friday.

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Some temperature readings have increased slightly, though they are still well below those at the Nos. 1 and 2 reactors. There was also a small increase in pressure.

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Like the Nos. 1, 2 and 4 reactors, the No. 3 reactor is likely to put out of service permanently even after the crisis resolves, Katsumata said Wednesday. Among other issues, the use of seawater in the post-crisis response has corroded the reactor, experts have said.

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The No. 3 reactor had been of particular concern because it is the only one to use mixed-oxide fuel that contains a small percentage of plutonium, which is also a byproduct in other reactors. A small amount of plutonium was detected in soil samples on the plant grounds last week, Tokyo Electric reported Monday. Edano said Tuesday that it was "likely" the plutonium came from this reactor.

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The cooling pool where spent fuel is stored may also have been damaged, the Japan Atomic Industrial Forum reports. Workers used a concrete pump to douse the spent fuel pool with water Tuesday, said Hidehiko Nishiyama of Japan's nuclear and industrial safety agency.

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Freshwater is being injected into the No. 3 reactor core in order to prevent overheating of nuclear fuel inside.

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The No. 3 reactor is believed to have suffered core damage, and a hydrogen explosion did extensive damage to the building surrounding the reactor March 14.

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Reactor No. 4

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Freshwater was injected into the No, 4 unit's spent nuclear fuel pool on Friday using a concrete pump truck, a Tokyo Electric official said.

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Workers restored power in the reactor's control room Tuesday -- a move that officials say could be a key step in efforts to bring cooling systems back online.

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This reactor was offline in a scheduled outage when the earthquake hit, but a March 15 fire damaged the building that houses the reactor.

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The nuclear fuel rods were in the unit's spent fuel pool, but not in the reactor itself. The reactor's pool of spent nuclear fuel was "possibly damaged," which is why authorities have made repeated efforts to pour water onto the structure.

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Reactors Nos. 5 and 6

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Reactors No. 5 and 6 were not in operation at the time of the earthquake and are in "cold shutdown," Japan's nuclear and industrial safety agency reports.

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The reactors were shut down for a scheduled outage when the quake hit and there are no major issues with the reactors and cores themselves. The cooling systems in the pools of spent nuclear fuel are thought to be functioning, though there are continued concerns about keeping power running to the systems.

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Three holes were punched in each building earlier to relieve pressure and prevent a feared hydrogen explosion.