CNN NEWS: Since March 11, the six reactors at the Fukushima Daiichi nuclear power plant have been in various states of disrepair after being battered by a 9.0-magnitude earthquake and subsequent tsunami.
Here is the latest on the status of each reactor and what was being done to prevent further emissions of radioactive material.
Reactor No. 1
Fresh water was injected both into the unit's reactor core and its spent nuclear fuel pool on Sunday, said an official with the Tokyo Electric Power Co., which runs the plant. This is in place of the saltwater that had been used for the same purpose, to cool nuclear fuel rods in the reactor and spent fuel pools. Besides its cooling purposes, experts hope the fresh water will help flush out salt to allow better operation of the cooling system.
Work resumed on Sunday to pump water out of the basement of the No. 1 unit's turbine building. Earlier tests indicated the presence of radioactive material in this pooled water, though not at nearly as high levels as those in the turbine building basements of the Nos. 2 and 3 reactors.
According to the Japan Atomic Industrial Forum, a nuclear trade group that has been keeping tabs on government and utility company accounts of the nuclear crisis efforts, the lighting is now working at buildings in and around the No. 1 reactor.
The same group has said the No. 1 unit's reactor core has been damaged, but its containment vessel was not. Saturday, the reactor's cooling systems were still not operational.
Still, the building was "severely damaged" by an earlier hydrogen explosion.
Reactor No. 2
Radiation levels in pooled water tested in the No. 2 nuclear reactor's turbine building are 100,000 times normal, utility company and government officials said Sunday, correcting an earlier finding of 10 million times normal. The reading applies to radioactive iodine-131, which has a half-life of eight days.
Iodine-134, which loses half its radioactive atoms every 53 minutes, was at less than a detectable amount, officials said, correcting an earlier figure of 2.9 billion becquerels per cubic centimeter.
There was no indication of harm done to the two people working in and around the No. 2 reactor when the radiation result became known. Those two subsequently left, and work in the turbine building has stopped until the government signs off on the power company's plan to address the issue, according to an official with the Tokyo Electric Power Co., which runs the plant.
A Tokyo Electric official noted Sunday that people continued to work in other buildings -- including a control room, which got power and light for the first time in weeks the previous afternoon -- in the No. 2 reactor's complex.
Also, fresh water was pumped Sunday into the No. 2 unit's reactor core as well as its spent nuclear fuel pool.
Damage is "suspected" in this unit's containment vessel, according to the Japan Atomic Industrial Forum. The reactor's core is also thought to be damaged, but the building has only been "slightly damaged," the group reports.
Even though the temperature and pressure levels are "unknown," the containment vessel pressure is considered "stable," the nuclear industry trade group reports.
Reactor No. 3
Work continued to be halted in the basement of the No. 3 unit's turbine building, as authorities tried to work around contaminated water pooled there. Plans are being devised on safely and efficiently removing that tainted water.
The water that three men stepped in while laying cable in this building's basement had 10,000 times the amount of radiation typical for that locale, Nishiyama had said. The workers -- each of whom tested positive for 173 or more millisieverts of radiation, including two with direct exposure on the skin -- were set to be released Monday after four days of observation at Japan's National Institute for Radiological Sciences, a research hospital in Chiba, according to Chief Cabinet Secretary Yukio Edano.
Despite the suspected damage to the reactor core -- something that isn't presumed at any of the other five reactors -- Nishiyama said there is evidence that pressure is somehow being maintained in the vessel, making it less likely there is a big gash.
Fresh water was being being pumped Sunday into the No. 3 reactor core as well as its spent nuclear fuel pool. This replaces the seawater that had been used previously. The aim of using fresh water is to cool down nuclear fuel and flush out accumulating salt that might hinder the reactors' existing cooling systems.
The No. 3 reactor has been of particular concern, experts have said, because it is the only one to use a combination of uranium and plutonium fuel, called MOX, considered more dangerous than the pure uranium fuel used in other reactors.
The Japan Atomic Industrial Forum, an industry trade group that is tracking official accounts of the cleanup efforts at the Fukushima Daiichi plant, said the pressure of the No. 3 reactor's containment vessel has been upgraded to "stable."
Whereas the group had stated Friday that damage was suspected in the reactor, on Saturday its assessment changed to "unknown" -- a further acknowledgment of uncertainty as to whether the contaminated water was the result of a leak in the nuclear reactor core or had some other cause.
The building of the No. 3 reactor was "severely damaged" after an explosion caused by the buildup of hydrogen gas, the Japan Atomic Industrial Forum reported. Its core reactor is also damaged and its fuel rods are either partly or fully exposed. As to its pool of spent nuclear fuel, reports are that the pool was "possibly damaged" and the water level has been low -- a reason for the repeated spraying.
Reactor No. 4
A concrete pump truck was used once again Sunday to inject seawater into the unit's fuel pool, said Nishiyama of Japan's nuclear safety agency.
Along with the No. 5 and 6 reactors, the No. 4 was offline in a scheduled outage when the earthquake hit, and as a result the reactor's water level and pressure are safe.
The reactor's pool of spent nuclear fuel, however, was "possibly damaged," which is why authorities have said its water levels are low and why they've made repeated efforts to fill it up with water.
Reactor No. 5
The No. 5 unit appears safe and in "cold shutdown state," according to Nishiyama. It is once again able to cool the fuel rods in the spent fuel pool, according to the Japan Atomic Industrial Forum.
As with units No. 4 and 6, this reactor was offline in a scheduled outage when the quake hit and there are no major issues with the reactor and core itself. The pool of spent nuclear fuel is thought to be functioning, though there are continued concerns about powering the reactor's cooling system to ensure the fuel rods contained within remain cool.
As with unit No. 6, three holes were punched in the building earlier to relieve pressure and prevent a hydrogen explosion.
Reactor No. 6
The No. 6 unit appears safe and in "cold shutdown state," according to Nishiyama. It is once again able to cool the fuel rods in the spent fuel pool, according to the Japan Atomic Industrial Forum.
The No. 6 reactor was offline when the tsunami struck, and there are no major concerns about the structure or safety of its core or containment vessel. The pool of spent nuclear fuel is thought to be functioning, though there are continued concerns about powering the reactor's cooling system to ensure the fuel rods contained within remain cool.
As with unit No. 5, three holes were punched in the building earlier to relieve pressure and prevent a hydrogen explosion.
