TEPCO Confirms Partial Meltdown of No.2 and No.3 Reactors

blau writes with an article in NHK World. From the article "The operator of the crippled Fukushima Daiichi nuclear plant says findings show that fuel meltdowns may have occurred at the No.2 and No.3 reactors within days of the March 11th earthquake. But it says both reactors are now stable at relatively low temperatures." TEPCO is also now blaming the tsunami for most of the damage rather than the earthquake.

The new news

[symbolset]

Japan has increased by 20 times the permissible level of radiation for schools, to the limits permitted a German nuclear worker. Thousands of parents are protesting, which in Japan is a pretty big deal. The Fukushima plant is out of places to store radioactive water, more storage is weeks away, and they still need to pump water to keep the fuel cool. The evacuation area may expand again. The slaughter and disposal of livestock in the evacuation zone has begun. Nobody really knows whether or not the fuel is burning through all three primary containment vessels on its way to massive contamination release.

Re:relatively low temperatures

[Mindcontrolled]

Well, if you followed the discussion over at physicsforum.org, which is populated by quite a lot of nuclear engineers, it seemed to be relatively clear from the onset that the cores at 1-3 had at least partially melted down. Reported water levels left not much room for speculation there. TEPCO is not exactly known for playing it straight, so yeah, I would call that downplaying.

Re:relatively low temperatures

[Mindcontrolled]

If you look at their temp gauges over at the TEPCO website, this is definitely not the case here. Especially at unit 3 there are still temperatures over 200 ÂC and they do not really get them down, even with constantly increasing water injection rates. For some reason, they started borating the water again last week - wonder why that is, if recriticality is not even remotely possible, as by their statements.

Re:Tepco's Just Looking for a Scapegoat

[Solandri]

The Tsunami knocked out the power, but if it knocked out the valve control systems and pumps, why didn't all three reactors melt down at the same time?

Unit 1 is a 460 MW reactor. Units 2 and 3 are 784 MW reactors. They have totally different ratios of heat generated to cooling capacity. This is why you're seeing reports for unit 1 coming separately, while reports for units 2 and 3 are (generally) coming concurrently. (The rest of your stuff about TEPCO being negligent, I agree with.)

Loss of power was the big problem.

[Animats]

Mod parent up.

Some pumps were still running after the earthquake and tsunami, and they continued to run until the backup batteries ran down. Loss of power was the real cause of the disaster. If they'd some backup power source that worked, the reactors would have reached cold shutdown in a day or two, there would have been no hydrogen explosions, and no core melting.

This is really important. A plant could lose backup power for many other reasons: fire, flood, hurricanes, terrorism, contaminated fuel, tank leakage, transformer damage, maintenance outages, or exhaustion of fuel supplies. Hospitals and data centers with backup power have at times lost power for all those reasons.

Read NUREG/CR-6890, "Reevaluation of Station Blackout Risk at Nuclear Power Plants " [nrc.gov], from 2005. Volume 2, page 22, has the line "Risk is evaluated only for critical operation, not for shutdown operation. External events, such as seismic, fire, or flood, are also excluded." That, as we know now, is an overoptimistic assumption. The NRC does a statistical analysis on backup power sources, assuming independent failure of separate units, and computes the odds accordingly.

Nuclear plants that need power to reach shutdown need power sources as tough as the containment vessel. That's now very clear.