As of late April 2011, the status of the reactors at the Fukushima Daiichi station is believed to be the following:
The reactor cores in Units 1, 2, and 3 have been severely damaged. Tokyo Electric Power Company (TEPCO) estimates that the cores have sustained damage (i.e., melting of the fuel elements) ranging from 55 percent (Unit 1) to 35 and 30 percent for Units 2 and 3.
For purposes of comparison, the reactor meltdown at Three Mile Island was between 50 and 75 percent.
The cores also have extensive buildup of salt as the result of emergency injection of seawater into them in an effort to cool the reactors. Fresh water is now being used to cool the reactors, so no new salt is being added. Nitrogen is also being injected to reduce the potential for additional explosions. The initial addition of salt water is expected to accelerate corrosion processes, and will be one factor complicating longterm management of the accident.
High radiation levels have been detected in the containment buildings and around the reactor site. It is believed one or more of the building explosions ejected radioactive material around the site.
The reactor spent fuel pools have high density racking are relatively full, with 5,042 assemblies out of a total capacity of 8,310 assemblies.
The freshly offloaded fuel in the Unit 4 pool was apparently closely packed, rather than being distributed in a “checkerboard” pattern intermingled with older fuel as is required in the United States.
The central storage facility at the site, which seems to have escaped serious damage, is nearly full—it only has additional space for some 465 assemblies, only a fraction of the assemblies in the reactor pools that will need to be removed. 408 assemblies are stored in dry casks.
The entire core of Unit 4—548 assemblies—had been unloaded into its spent fuel pool for reactor maintenance about three months before March 11th.
Following the hydrogen explosions, radioactive cesium and iodine were detected in the vicinity of the plant, a clear indication of fuel damage. These had been released via venting of gases from the reactors, including an apparent rupture of the suppression chamber at Unit 2.
France’s nuclear safety agency IRSN estimates the maximum external doses to people living around the plant are unlikely to exceed 30 milliseiverts (3 rem, or 3,000 millirem) in the first year. This is based on airborne measurements taken to date, and has not been confirmed by any other agency. Natural background levels between 2-3 milliseiverts (0.2-3 rem, or 200-300 millirem) would normally be expected in the region.
Gamma radiation measurements onsite close to the reactors decreased greatly when the Unit 3 fuel pool was replenished with water on March 19th.