Where did UCS/All Things Nuclear find their information for the number of fuel elements in reactor 4's spent fuel pool? From memory, I believe their first number referenced two newspaper articles in Japanese. Again, from memory, I believe this was the number picked up by the NY Times (1479):
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http://www.nytimes.com/interactive/2011/03/16/world/asia/reactors-status.html
The most recent figure of 1331 fuel elements from UCS/All Things Nuclear does not explicitly furnish the references:
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http://allthingsnuclear.org/post/4008511524/more-on-spent-fuel-pools-at-fukushima
After a couple of days of interrogation, the French agency IRSN said last week that it had a "confirmed" number of 1500 fuel rods in reactor 4's spent fuel pool. This seems like a suspiciously round number, given that its "confirmed" counts for reactors 1 to 3 were 292, 587, and 514 respectively.
There is more information about the fuel pools in this recent documentation:
| http://resources.nei.org/documents/japan/Used_Fuel_Pools_Key_Facts_March_16_Update.pdf
The following slide presentation (2 different links to the same document) mainly concerns dry storage and plans for future off-site storage, but it also contains information about onsite storage at Fukushima Daiichi:
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http://www.nirs.org/reactorwatch/accidents/6-1_powerpoint.pdf
| http://www.zerohedge.com/sites/default/files/Containment%20Pools.pdf
The LA Times published a story with information from the NRC. The NRC believed it had "compelling evidence" that the reactor 4 spent fuel pool had a significant hole or crack and was empty. This evidence was "provided by key American contractors who were in the plant at the time".
| http://www.latimes.com/news/nationworld/world/la-fg-japan-quake-wrapup-20110318,0,1937413,full.story
The evidence to date, I believe, contradicts NRC's conclusion.
The French agency IRSN concluded that reactor 3's spent fuel pool was compromised, leaving as little as one meter of water (worst case) over the top of the fuel rods. In addition, I suppose all the pools could have some kind of cracks or slow leaks.
Last week IRSN was calculating how long it would take for the spent fuel pools to start boiling. Because reactor 4's pool has the most uranium, their calculations had it boiling first (on Monday of this week, I believe). But I suspect these calculations assumed a full pool. Boiling would start earlier if the pool water level were low.
Various reasons for fuel pool low water level have already been discussed in this thread -- here are some related links and short quotes.
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http://www.nytimes.com/2011/03/18/world/asia/18spent.html?pagewanted=all
Sloshing in California:
"… much of the water may have sloshed out during the earthquake. Much smaller earthquakes in California have produced heavy water losses from sloshing at storage pools there, partly because the pools are located high in reactor buildings."
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http://spectrum.ieee.org/tech-talk/...amaged-nuclear-plant-spread-radiation-worries
Sloshing in Japan:
"Masashi Goto, a former Toshiba Corporation design engineer of nuclear containment vessels of the kind used in the Dai-1 Plant, said another possibility was 'sloshing': the water may have sloshed out of the storage pool due to the earthquake's shaking. Goto said this kind of splashing happened in 2007: 'This is what happened during the Kashiwazaki (Nuclear) Plant accident after the earthquake struck it and sloshed water outside the pool.'"
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http://allthingsnuclear.org/post/3964225685/possible-source-of-leaks-at-spent-fuel-pools-at
| http://www.ucsusa.org/nuclear_power/nuclear_power_risk/safety/nuclear-crisis-japan-telepress-transcript-03-19-11.html
Gate seal deflation:
"At the Hatch plant, it took, I think, about four or five hours for the inflatable seal to deflate, and 141,000 gallons of water, about half of the water of the spent fuel pool, leaked out through that little gap between the gate and the walls."
Here is a reference to general gate damage, which says there are in fact two gates in the pool inclosure. I suppose the second gate is used for removing spent fuel (and also for bringing in new fuel, although the water cover would be unnecessary for this).
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http://www.nytimes.com/2011/03/19/world/asia/19japan.html?pagewanted=all
"The steel gates at either end of the storage pool are also vulnerable to damage during an earthquake and could leak water if they no longer close tightly."
The explosion in the reactor 4 building blew out the metal roof panels. The French agency IRSN noted that this explosion also blew out the east and south concrete walls, and inferred that the explosion occurred (also?) underneath the service deck.
For the explosion in the reactor 4 building, most of the discussion I've seen on the internet concludes that it was a hydrogen explosion, with the the presence of hydrogen requiring the exposure of fuel rods in the pool. The French agency IRSN was the first source I saw that concluded the hydrogen was from radiolysis in the pool.
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http://en.wikipedia.org/wiki/Spent_fuel_pool
"Radiolysis, the dissociation of molecules by radiation, is of particular concern in wet storage, as water may be split by residual radiation and hydrogen gas may accumulate increasing the risk of explosions. For this reason the air in the room of the pools, as well as the water, must permanently be monitored and treated."
That little half-page Wikipedia discussion is not a great reference, but I have not found much else. A couple of online discussions I've read did mention the possibility of radiolysis as the source of the hydrogen, but dismissed this possibility. I wonder if hydrogen/oxygen bleeding into the building would be enhanced if the pool level were low and the water boiling (i.e., in this case would there be less recombination in the pool and thus more gas escaping into the air?).
While searching for info, I did run across this off-subject suggestion of using the pool water to generate hydrogen fuel:
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http://www.freepatentsonline.com/y2010/0072074.html
For the hydrogen explosions in reactors 1 and 3, most explanations refer to the venting of hydrogen (formed by the reaction of steam with hot cladding). Pool radiolysis could also contribute, although there is much less fuel in these pools than in the reactor 4 pool. I do not understand exactly where this gas would vented. The following discussion explains more about the venting process than I want to know as well as the role of
carbonized coconut shells in nuclear reactors (without explaining exactly where the gas would be vented):
| http://www.electronicspoint.com/see-if-you-think-might-dangerous-t178032p2.html
(See the long post, the final post in that thread as I write this.)
There is an interesting alternative source of hydrogen:
" … my view a far more plausible explanation is that hydrogen routinely injected in the Make Up Water System to control the corrosives (mainly O2) produced by radiolysis was released suddenly and catastrophically from outside the containment and within the reactor building. In reacting with oxygen from the atmosphere within the building at the correct concentration of hydrogen (4-74%) only a spark is required to detonate a hydrogen oxygen explosion."
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http://bravenewclimate.com/2011/03/14/fukushima-more-technical-info/
Follow this link for additional discussion of this possibility. I wonder if the reactor 4 building also contained a hydrogen inventory, despite the fact that the reactor was unloaded?
A couple of general references:
Here is a discussion of containment structures, with diagrams of the GE Mark I confinement structure. This document has been very recently posted or updated, for it already contains a reference to the Fukushima incident. Note that Table 4 says that 2 US Mark I systems have reinforced concrete confinement structures, while 22 have steel confinement structures. I suppose the steel structures also are surrounded by concrete for biological shielding. I wonder which design is used in Japan?
| https://netfiles.uiuc.edu/mragheb/www/NPRE%20457%20CSE%20462%20Safety%20Analysis%20of%20Nuclear%20Reactor%20Systems/Containment%20Structures.pdf
"A venting system was in place at the Fukushima plants to help relieve built-up pressure. With electrical power cut off in the aftermath of the earthquake and backup sources of power either failing or exhausted, workers injected seawater mixed with boron into the reactor to maintain control reportedly using fire engines pumps. They had difficulty venting the resulting steam with a report that the pressure relief valves were operated manually."
I was once a research physicist (not nuclear). Now I am particularly interested this problem because I am an investor in would-be uranium miners (at least, since last week when their stock prices fell, I am an investor). Besides wanting to know how probable a real disaster is at Fukushima, I'm interested in the reaction of the Chinese and Indian public and government (these countries have the largest planned reactor building programs), the reactions of other investors, etc. The following is a long rapidly assembled report which takes a high level view of this incident (such as power generation and electricity shortages in Japan, Asian reactions, etc.)
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http://www.nautilus.org/
| http://www.nautilus.org/publications/essays/napsnet/reports/SRJapanReactors.pdf
