My natural inclination is to look at data to try and understand things in terms of physical processes: what's going on at the atomic and chemical levels, how hot are things in the reactor and where are these fires coming from, and what's going to happen if the reactors or spent fuel is left alone without intervention.
I don't have any actual data from the Japanese reactors, but here's what I've come across in my reading...
About The Nuclear Fuel
Wikipedia has great articles about a lot of relevant topics including light water reactors (LWR's), decay heat, spent nuclear fuel, and spent fuel pool.
When the fuel is spent, it's moved to a holding pool. Here's an image of what used to be in the top of the reactors, before the explosion. Other images are here.
At first, I thought the fuel was coated by a zinc alloy. Turns out to be "zircaloy". I heard a reference on the radio to the alloy melting, which happens at around 1850 degrees, according to an old 1984 article, Review of Zirconium-Zircaloy Pyrophoricity:
In another experiment, self-heating of Zircaloy, indicative of a sustained reaction, has been observed in tests involving the reaction of Zircaloy with steam at temperatures in the region of the melting point (1850 °C).
Spent fuel is apparently not normally undergoing fission, the the primary heat is from beta decay. It gets cooler over time, and I'm not aware of any public data on the temperatures involved, but if cladding is melting or the spent fuel is burning, it must be hot. Basically, they need to keep cooling it with seawater, perhaps for many months, before they can tackle moving it. The seawater likely will render the plants inoperable in the future, regardless of any earthquake, fire, or tsunami damage - it's doubtful these reactors will ever be used again.
At first, I couldn't quite figure out how hot the fuel or spent fuel gets if it's not cooled. A PDF, PEAK CLADDING TEMPERATURE IN A SPENT FUEL STORAGE OR TRANSPORTATION CASK, has interesting information, including this quote (apparently the cladding is important):
In the latter instance, the spent fuel may be exposed to air and oxidize if the cladding is damaged during handling and transportation. The resulting volume expansion of the fuel could “unzip” the cladding, thus greatly increasing the potential for release of radioactivity and contamination.
This PDF from 2003, Reducing the Hazards from Stored Spent Power-Reactor Fuel in the United States, has an excellent overview on the problems of spent fuel, including the information on temperature and spent fuel combustion I was looking for. Here's some quotes:
It has been known for more than two decades that, in case of a loss of water in the pool, convective air cooling would be relatively ineffective in such a dense-packed pool. Spent fuel recently discharged from a reactor could heat up relatively rapidly to temperatures at which the zircaloy fuel cladding could catch fire and the fuel's volatile fission products, including 30-year half-life 137Cs, would be released. The fire could well spread to older spent fuel. The long-term land-contamination consequences of such an event could be significantly worse than those from Chernobyl.
I added the bold in the quote below:
In the absence of any cooling, a freshly-discharged core generating decay heat at a rate of 100 kWt/tU would heat up adiabatically within an hour to about 600C, where the zircaloy cladding would be expected to rupture under the internal pressure from helium and fission product gases, and then to about 900C where the cladding would begin to burn in air.
It will be seen that the cooling mechanisms in a drained dense-packed spent-fuel pool would be so feeble that they would only slightly reduce the heatup rate of such hot fuel.
In 2001, the NRC staff summarized the conclusions of its most recent analysis of the potential consequences of a loss-of-coolant accident in a spent fuel pool as follows:
[I]t was not feasible, without numerous constraints, to establish a generic decay heat level (and therefore a decay time) beyond which a zirconium fire is physically impossible. Heat removal is very sensitive to . . . factors such as fuel assembly geometry and SFP [spent fuel pool] rack configuration . . . [which] are plant specific and . . . subject to unpredictable changes after an earthquake or cask drop that drains the pool. Therefore, since a non-negligible decay heat source lasts many years and since configurations ensuring sufficient air flow for cooling cannot be assured, the possibility of reaching the zirconium ignition temperature cannot be precluded on a generic basis.
We have done a series of “back-of-the-envelope” calculations to try to understand the computer-model calculations on which this conclusion is based. We have considered thermal conduction, infrared radiation, steam cooling, and convective air cooling.
Thermal Conduction
Conduction through the length of uncovered fuel could not keep it below failure temperature until the fuel had cooled for decades.
A paper from International Network of Engineers and Scientists Against Proliferation, Radiological Terrorism: Sabotage of Spent Fuel Pool also quotes the Alvarez article.
The light water reactor Wikipedia article has this quote, which pretty much sums up everything and explains the hydrogen explosions.
In event of a loss-of-coolant accident, the moderator is also lost and the active fission reaction will stop leaving just a 5% power level for 1 to 3 years called the "decay heat". This 5% "decay heat" will continue for 1 to 3 years after shut down, where upon it finally reaches "full cold shutdown". "Decay heat" while dangerous and strong enough to melt the core, is not nearly as dangerous as an active fission reaction. During this "decay heat" post shutdown period the reactor requires water pumped cooling or the reactor will overheat to above 2200 degrees centigrade where upon the heat separates the cooling water in to its constituent parts Hydrogen and Oxygen which can cause hydrogen explosions, threatening structural damage or even the possible exposure of highly radioactive stored fuel rods stored ready for use in surrounding nuclear storage pools(approx 15 tons of fuel is replenished each year to maintain normal PWR operation). This decay heat is the major risk factor in LWR safety record.
But from a watching-from-afar perspective, I can't really know anything, except that some of the fuel in the spent fuel pools must have been fairly recent if it's so hot. Other questions:
* how much fuel is in the spent pools?
* how old is the fuel and how much energy is it emitting?
* what is the fuel configuration?
* what's the fuel density (did they densepack the stuff)?
* what are the actual temperatures in the reactor with and without the water?
* how badly has the fuel and storage system been damaged already?
Even with no further information, it seems like IF there's new fuel on top of old fuel in storage, the fuel is damaged, AND they've densepacked things then that would be worse than if the case where there's less fuel, it's in good condition, and it's not crammed all together. I don't blame them for not being specific on this... a general "we're doing the best we can, please don't panic" is probably all we can hope for.
Fukushima, etc.
The Fukushima 1 reactors were brought online in the 70's. I don't know what's happened to all the spent fuel. In the US, most spent fuel is still in storage pools.
I was shocked to see there was a fairly recent scandal, according to Wikipedia.
Ironically, there was a recent symposium on nuclear power. The the tsunami session, the presentation on tsunamis called TSUNAMIS -DISASTERS AND
COUNTERMEASURES was generic and there's no specific mention of how to protect a nuclear power plant from a tsunami. According to other presentations, the Fukushima plants were positioned above what they thought was the high water line for a tsunami. In any case, the last slide is ominous.
A tsunami has its individuality.
Disaster evolves.
Wikipedia suggests the damage was done by the earthquake cutting off auxiliary power.
A 9.0 magnitude earthquake strikes off the coast of Honshu Island at a depth of about 24 kilometres (15 mi). Fukushima I power plant's nuclear reactors 1, 2, and 3 are automatically shut down by the shake. Nuclear reactors 4, 5, and 6 were undergoing routine maintenance and were not operating, (reactor 4 was defueled Nov 2010). The tremor has the additional effect of causing the power plant to be cut off from the Japanese electricity grid, meaning that power is lost to the cooling pumps.
According to this page, there were over 800 spent fuel rods stored at Fukushima in 2000.
I don't think the spent fuel storage was envisioned as a significant risk during the quake and tsunami analysis, but I haven't been through all the available information. There's going to be plenty of people looking at spent fuel storage in the future. It's been a known risk for a while, but I think it's got some additional attention.
I also didn't come across any discussions of the effect of earthquakes on the emergency diesel generators, however, they were seen as important. Actual failed inspections of emergency diesel generators resulted in shutdown of reactors. So... failure to treat critical redundant systems at the same level of criticality as the reactor cores might have contributed to the current problems.
e 2 ) Inoperability of two emergency diesel generators
- Tomari Power Station Unit-1 [ PWR; 579 MWe ] -
1. Event Description
Tomari Power Station Unit-1 had been under the rated thermal power
operation. On September 18 at 01:37pm, when one of two emergency diesel
generators, EDG-B, was started up for its periodic surveillance test, it stopped
automatically.
In response to this, the operability of another emergency diesel generator,
EDG-A, was checked as a measure required in the safety preservation rules.
However, on September 19 at 3:49pm, the concerned emergency diesel
generator failed to start up.
Thus, the reactor was shut down pursuant to the safety preservation
rules.
I'm wondering what's happening to all the water they're using to cool the fuel and reactors... is it contaminated? Where does it go?
This seems like a great opportunity to actually study what can go wrong with a nuclear reactor... I know they're focused on mitigating disaster, but I hope they drag in some instruments and get good data from this while it's still ongoing. Maybe it will help in preventing future problems.
I was going to include some of the stuff I found on radiation measurements rads, rems, seiverts, grays, and so on, but this is too long already. But after reading all this crap, I think they could have put more thought into handling of the spent fuel. This took a long-ass time to compile.