Japan earthquake/tsunami

[Van]

After the earthquake and tsunami, there were difficulties powering the cooling system for unit 1 of the Fukushima Daiichi plant. After a buildup of hydrogen gas in the secondary containment structure at the plant, there was an explosion at that reactor on March 12.

The explosion caused a breach in the secondary containment. However, the primary containment that houses and protects the reactor vessel and fuel remains intact and is safe. This structure is made of steel and is extremely robust. The primary and secondary containment are designed to prevent radiation from being released into the environment in the case of an accident. However, TEPCO intentionally vented steam from the secondary containment building in an effort to reduce pressure in that building. For a diagram of the reactor type used at Fukushima Daiichi, click here.

It appears that as the level of coolant in the reactor vessel lowered, a portion of the top of the uranium fuel rods was exposed. This may have caused zirconium cladding of the fuel rods to react with water to create hydrogen. This hydrogen was vented, then somehow ignited, causing the explosion.

As the explosion did not occur inside the reactor core—and the primary containment was not breached—there has not been a significant public health impact from the release of radiation from the containment structure.

Reactors 2 and 3 at Fukushima Daiichi were shut down in response to the earthquake. Units 4, 5 and 6 had been shut down prior to the earthquake for inspections and scheduled outages.

[Van]

Starting to look better than expected.

UPDATE AS OF 10:20 A.M. EDT, TUESDAY, MARCH 15:
The level of radioactivity at the Fukushima Daiichi nuclear power plant has been decreasing, according to the International Atomic Energy Agency. At 8 p.m. EDT March 15, a dose rate of 1,190 millirem per hour was observed. Six hours later, the dose rate was 60 millirem per hour, IAEA said. About 150 residents near the Fukushima Daiichi site have been checked for radiation and 23 have been decontaminated. Japanese authorities have distributed potassium iodide tablets to evacuation center (see this page for more information on potassium iodide). If taken within several hours of ingesting radioactive iodine, potassium iodide can protect the thyroid gland.


UPDATE AS OF 9:15 A.M. EDT, TUESDAY, MARCH 15:
Fukushima Daiichi
Units 1 and 3 at Fukushima Daiichi are stable and cooling is being maintained through seawater injection. Primary containment integrity has been maintained on both reactors. The Japan Atomic Energy Agency (JAEA) reported an explosion in the suppression pool at Fukushima Daiichi Unit 2, at 7:14 p.m. EDT on March 14. Reactor water level was reported to be at 2.7 meters below the top of the fuel. The pressure in the suppression pool decreased from 3 atmospheres to 1 atmosphere. Radiation readings at the site increased to 96 millirem per hour. Dose rates at Fukushima Daiichi as reported at 10:22 p.m. EDT on March 14 were:

Near Unit 3 reactor building 40 rem/hr
Near Unit 4 reactor building 10 rem/hr
At site boundary 821 millirem/hr.
Kitaibaraki (200 km south of site) 0.4 millirem/hr.

We are working on getting updated information on radiation and dose rates at and near the plant. Station personnel not directly supporting reactor recovery efforts have been evacuated, leaving approximately 50 staff members at the site. Operators are no longer in the main control room due to high radiation levels. Safety relief valves were able to be re-opened and seawater injection into the reactor core was restarted around 1 a.m. EDT on March 15 and is continuing.
At Unit 4 on March 14 at approximately 8:38 p.m. EDT, a fire was reported in the reactor building. It is believed to have been from a lube oil leak in a system that drives recirculation water pumps. Fire fighting efforts extinguished the fire. The roof of the reactor building was damaged.

Fukushima Daini
All four reactors at Fukushima Daini are being maintained with normal cooling using residual heat removal systems.

[JohnR]

Originally Posted by Van

The Japan Atomic Energy Agency (JAEA) reported an explosion in the suppression pool at Fukushima Daiichi Unit 2, at 7:14 p.m. EDT on March 14. Reactor water level was reported to be at 2.7 meters below the top of the fuel. The pressure in the suppression pool decreased from 3 atmospheres to 1 atmosphere. Radiation readings at the site increased to 96 millirem per hour.

Not good for Reactor 2 to be at 1 atmosphere - would indicate a leak in primary containment - no? That's bad. Explosion of R2's spent fuel pool. Bad. Real bad.

Good news on Reactor 4 not being SFP.

Good news is 3 reactors fairly cool so they are working serious problems on only 3 and not 4 as discussed last night / earlier today.

Real bad for remaining personnel onsite, real bad Control Room is too contaminated to occupy.

If things go SWIMMINGLY well, they will be cooling those reactors for a long time to come. I've read that weeks / months of dealing with "Decay Heat" will be in the cards even though no fission is occurring.