This is a reprint of NEI's update. I am supplementing in italics.
The hydrogen explosion on March 11 between the primary containment vessel and secondary containment building of the reactor did not damage the primary containment vessel or the reactor core. To control the pressure of the reactor core, TEPCO began to inject seawater and boric acid into the primary containment vessels of Unit 1 on March 12 and Unit 3 on March 13. There is likely some damage to the fuel rods contained in reactors 1 and 3. (i.e. hydriding that I discussed earlier - zirconium rust)
At both reactors 1 and 3, seawater and boric acid is being injected into the reactor using fire pumps. On reactor 3, a pressure relief valve in the containment structure failed to open, but was restored by connecting an air pressure to the line driving valve operation.
The water level in the reactor vessel of reactor 2 reactor is steady.
Personnel from TEPCO are closely monitoring the status of all three reactors.
The highest recorded radiation level at the Fukushima Daiichi site was 155.7 millirem at 1:52 p.m. on March 13. Radiation levels were reduced to 4.4 millirem by the evening of March 13. The NRC’s radiation dose limit for the public is 100 millirem per year. (Note that this is the level allowed from a nuclear facility. Also note this was the max level on site at a particular instance in time, not at the periphery and certainly not where residents have been evacuated to. Periphery levels have and remain to be less than 1 millirem/hr. Another note: if you smoke a pack of cigarettes a day your radiation dose is on the order of 8,000 millirem/yr!)
Japanese government officials acknowledged the potential for partial fuel meltdowns at Fukushima Daiichi Unit 1 and 3 reactors, but there is no danger for core explosion, as occurred at the nuclear power station at Chernobyl in 1986. Control rods have been successfully inserted at all of the reactors, thereby ending the chain reaction. The reactor cores at Fukushima Daiichi and Daini power stations are surrounded by steel and concrete containment vessels of 40 to 80 inches thick that are designed to contain radioactive materials. (This is an unfortunate misnomer. There was no 'core explosion' at Chernobyl; it was water flashing to steam creating a steam explosion.)
The Fukushima Daini plants remains in a state of emergency. There is electricity available at all four of the reactors at Fukushima Daini, although there is limited availability of the cooling water pumps at reactors 1, 2 and 4. (They are venting the built up steam from containment to atmosphere. Remember, to suddenly and completely stop something so big takes a LOT. Just like a train needs several miles to stop, a nuclear reactor needs a few days to cool.)
TEPCO is working to maintain constant cooling in the primary containment vessels of those reactors. No radioactivity has been recorded outside of the secondary containment buildings at Fukushima Daini, according to TEPCO.
Two other nuclear power plants in the Tohoku region, Onagawa Nuclear Power Station and Tokai Nuclear Power Station, were automatically shut down in response to the earthquake. The four reactors at these plants have functioning cooling systems and are being monitored by plant operators. (This is incredible. We've just experienced the 5th largest earthquake in recorded history. I am hoping we're on the right track and this is all to there is to it. Kudos to all those out there making the difference and making sure the rest of the country has the critical electricity it needs!)
The Rokkasho Reprocessing Plant and accompanying facilities, located far north of the tsunami zone in Rokkasho Town, is operating safely on backup power generation systems.
Japanese nuclear facilities are designed to withstand powerful seismic events, such as earthquakes. In this earthquake—the strongest recorded over the past 100 years in Japan—the containment structures of Fukushima Daiichi maintained their structural integrity. These facilities were designed to withstand tsunamis within a range of assumed strength; however, the force of the tsunami on March 10 exceeded the assumed range and flooded diesel generators at Fukushima Daiichi power station. This precipitated the loss of power for the reactor cooling systems. (The plants were designed to handle 6.5 meter tsunamis (21.33 feet!); the tsunami was 7 meters (22.97 feet).)
The automatic shutdown of the 11 operating reactors at the Onagawa Nuclear Power Station, Tokai Nuclear Power Station, Fukushima Daiichi and Daini, represents a loss of 3.5% of electric generation capacity for Japan.