(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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Early Sunday afternoon, the electricity source used to power the No. 1 unit's cooling systems was switched from a temporary diesel generator to a more permanent, external power supply, the International Atomic Energy Agency noted on its website.
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In addition, some (but not all) lighting has been restored in the No. 1 unit's turbine building, where large amounts of pooled, highly radioactive water had been found in the basement.
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Hidehiko Nishiyama, an official with Japan's nuclear and industrial safety agency, said Sunday that the parameters appear stable in the No. 1 reactor. Temperatures are gradually going down, he added.
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This is in sharp contrast to earlier in the crisis, when temperatures in the No. 1 reactor once topped 2,700 Celsius (4,800 Fahrenheit), according to an estimate from Areva, one of the world's top nuclear energy companies based in France. In such intense heat, much of the water used to cool the reactor's nuclear fuel rods may have boiled away, contributing to the full or partial exposure of the fuel rods (and little to cool them) and the significant release of radioactive vapor.
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The No. 1 reactor is considered to be in shutdown mode, according to its owner, the Tokyo Electric Power Company.
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Radioactive water that had been pumped out of the No. 1 unit's turbine building into a condenser was emptied on Sunday afternoon into a storage tank, reports Tokyo Electric.
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Nitrogen is slated to be injected into the No. 1 reactor, an attempt to minimize the possibility of the dangerous buildup of hydrogen, after Tuesday. A hydrogen explosion -- an indicator of possible core damage -- blew the roof and upper walls off the building housing the reactor on March 12.
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A camera was installed Sunday in the exposed maintenance tunnel leading to the No. 1 unit's turbine building, where highly radioactive water had been found last week. The goal is to pinpoint how water got into the tunnel, which had been used for electrical cables but wasn't supposed to contain liquid.
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Nishiyama on Saturday knocked down a claim made a day earlier by U.S. Energy Secretary Steven Chu that 70% of the No. 1 reactor's core has suffered severe damage. Noting that sensors have been unreliable, Chu said the calculation was based on the fact that radiation levels have been too high for workers to get inside. But Nishiyama said that Japanese authorities' data indicates only 3% damage to the unit.
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Tsunehisa Katsumata, chairman of Tokyo Electric Power Company that runs the nuclear power plant, said last 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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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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Tests from Saturday on water in the concrete pit outside the No. 2 unit's turbine building -- the same spot where water is rushing directly into the Pacific Ocean through a roughly 20-centimeter (8-inch) crack -- showed radiation levels 7.5 million times the regulatory limit, a Tokyo Electric official said. More recent findings showed a slight drop to 5 million times the norm.
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But the utility company also noted Tuesday that the radiation levels diminished sharply as one moved away from the leak, consistent with their assessment that the spill might have a minimal effect on sealife.
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For 2.5 hours on Monday, water was injected into the No. 2 unit's nuclear spent fuel pool, Tokyo Electric reported Tuesday on its website. That water cooling process was powered by what the utility called "a temporary, motor-driven pump."
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Attempts failed Saturday and again Sunday to plus the cracked concrete shaft outside the No. 2 reactor's turbine building. Pictures showed highly radioactive water gushing through the crack and directly into the ocean.
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Plan C is to install a silt fence along a damaged sea wall surrounding the plant, Hidehiko Nishiyama of Japan's nuclear and industrial safety agency said Monday. This will take several days to complete, a Tokyo Electric official said Tuesday.
The aim of this screening, which is usually used to halt erosion at construction sites, is to prohibit the spread of radioactive particles into the sea. Workers also have injected a dye tracer into the water to help them track the dispersal of such particles.
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Japanese Chief Cabinet Secretary Yukio Edano said Monday that authorities opted to dump 11,500 tons of that's collected in and around the Nos. 3 and 4 units for the "safety" of the No. 2 reactor. He called the decision "unavoidable," saying that while the Nos. 3 and 4 units' is radioactive, these levels are nowhere near as high or as dangerous that from the No. 2 unit.
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As of Sunday afternoon, an external power source was being used for the No. 2 unit's cooling system. This replaces the temporary diesel generator in use in recent days.
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Cameras have been set up monitor areas in and around the No. 2 unit's turbine building, an attempt to get more information about water levels and potential leaks, said Nishiyama, the nuclear safety official, on Sunday
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Conditions in the No. 2 reactor core appear to be stable, he added.
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Areva estimates that the temperature in the No. 2 reactor core at one point in the crisis soared as high as 1,800 Celsius (3,200 Fahreinheit).
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The No. 2 reactor is officially shut down, reports Tokyo Electric.
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As with the Nos. 1 and 3 units, there is a plan to inject nitrogen into the No. 2 reactor in order to prevent a buildup of hydrogen that might cause an explosion. One such blast occurred at the No. 2 unit on March 15.
A Tokyo Electric executive said last week 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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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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The level of water in the No. 3 unit's turbine building as well as in an exposed maintenance tunnel is high, a Tokyo Electric official said Tuesday.
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On its website, the utility company claimed that the water level in the tunnel had increased by 15 centimeters on Sunday. The theory is that water from the No. 4 unit's turbine building was somehow reaching the No. 3 unit's trench. The water level stabilized by Monday, after workers stopped transferring water in the No. 4 unit's turbine building.
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From 5 a.m. to after 7 p.m. Monday, water was sprayed using a concrete pumping truck to cool down nuclear fuel at the No. 3 unit, Tokyo Electric reported Tuesday.
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Loose water found in and around the No. 3 complex has shown high levels of radiation, but nowhere as high as that in the No. 2 unit, said Edano.
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Recent data on key parameters, such as pressure levels and temperature, suggest that the No. 3 reactor is relatively stable, the nuclear safety official Nishiyama said Sunday.
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An outside, more permanent electricity supply began powering the cooling system used to cool nuclear fuel in the No. 3 reactor on Sunday afternoon, according to the International Atomic Energy Agency.
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This cooling system -- including back-up and primary power sources -- had broken down earlier in the crisis. At one point, temperatures reached as high as 1,800 Celsius (3,200 Fahreinheit) in the No. 3 reactors, according to an analysis from Areva.
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Tokyo Electric said Monday on its website that the No. 3 reactor is considered to be in shutdown mode.
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On Sunday, workers were in the process of setting up a monitor in the exposed maintenance tunnel outside the No. 3 unit's turbine building, into which radioactive water had been leaking.
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Japan's nuclear safety agency earlier announced plans to pump in nitrogen -- a non-flammable substance -- into the No. 2 reactor and two others in a bid to prevent an explosion caused by the buildup of hydrogen. Eleven people were injured on March 14 when one such explosion occurred at the No. 2 unit.
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Like the Nos. 1, 2 and 4 reactors, the No. 3 reactor is likely to be 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 last week 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.
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Freshwater has been 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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The level of water in the No. 4 unit's turbine building as well as in an exposed maintenance tunnel is high, a Tokyo Electric official said Tuesday.
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For five hours Sunday night, freshwater was sprayed from a concrete pumping vehicle into the No. 4 unit's spent nuclear fuel pool, according to details on Tokyo Electric's website.
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The utility company announced Sunday that two workers, ages 21 and 24, had been found dead in the No. 4 unit's turbine building. The men, who suffered multiple traumatic injuries, had been missing since the March 11 earthquake and tsunami. Their bodies were found Wednesday, though Tokyo Electric said it waited several days to notify the public in order to let the victims' families know first.
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Some lights in the No. 4 unit's turbine building were turned on for the first time Thursday since the natural disaster.
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This reactor was offline in a scheduled outage when the earthquake hit. Still, it has had several major problems since then, including a March 15 fire that 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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Water has pooled up in the basements of the Nos. 5 and 6 units' turbine buildings, Edano said Monday.
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Tokyo Electric reports on its website that this amounts to about 1,500 tons of this water.
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The radiation level of this discharged water is about .6 millisieverts per year, according to Tokyo Electric. For reference, a person in a developed country is naturally exposed to 3 millisieverts per year.
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Beginning Monday night, that water was pumped out and dumped directly into the Pacific Ocean. The cabinet minister said there was "no choice" but to dump such water into the sea, rather than putting it in storage tanks, because that space was needed for more highly contaminated water from around the No. 2 unit.
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While those reactors' remained stable, Edano said this water had to be pumped out soon because it threatened to derail the electricity that is being used to power the systems' cooling the fuel rods in the Nos. 5 and 6 units' spent nuclear fuel pools.
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These two reactors were not in operation at the time of the earthquake and had been 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.
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