The plant's best hope may be restoration of outside power, which could allow workers to restart cooling operations for the reactors and the cooling pools, provided they have not been too severely damaged. NHK TV in Japan said TEPCO hoped to run power lines into the site from another power plant Thursday. TEPCO wants to finish the installation "as soon as possible after reviewing the procedures in order to keep the workers' radiation exposure to a minimum," NHK reported.
The problem of spent fuel storage
Nuclear reactor operators must store spent fuel removed from reactor cores for several years at least, in large pools at reactor sites until the remaining heat from the uranium fuel cools sufficiently. In the United States, much of the fuel units remained stored underwater in pools but some are removed for storage in large casks.
A report to Congress in 2006 by a National Research Council panel investigating terrorist threats to spent fuel storage concluded that "under some conditions," if a pool were partially or completely drained, that "could lead to a propagating zirconium cladding fire and the release of large quantities of radioactive materials to the environment."
The fuel rods in most cases consist of uranium dioxide pellets encased in zirconium alloy tubes or cladding. Heat from uncovered fuel could ignite the zirconium cladding, and the super-heated metal could then oxidize steam, releasing hydrogen and oxygen. Leaks of hydrogen from damaged reactors at units Nos. 1 and 3 is blamed for explosions at the tops of the outer, secondary containment structures, and an explosion within the No. 2 primary containment structure. Officials said hydrogen released from the spent fuel pool at No. 4 may have caused a fire there.
The National Research Council report said that as pool water levels drops, through a leak or other causes, temperatures of the fuel rods increase, accelerating oxidation of the cladding and the production of hydrogen gas. The reactions can become self-sustaining at high temperatures, if there is sufficient water or oxygen present, causing the cladding tubes to rupture. "The result could be a runaway oxidation reaction" and the release of radioactive fission gases" and some of the radioactive fuel material.
The mix of radioactive particles released into the atmosphere would vary with the length of time expired since the fuel units were moved from reactors to the pools. A particular concern is the spread of Cesium-137, which can enter the body or contaminate agricultural products, Lyman said.
Neff said the consequences of exposed spent fuel at the Fukushima Daiichi plant are not clear. If a pool is completely dry, the oxidized fuel units may be lying on the pool's bottom. "I think the zirconium fire [at unit No. 4] is probably over."
'Doomsday' scenarios
But the resulting radiation could be so high, "it would be almost impossible to get anyone in there," to continue supplying water to the damaged reactors, he said. That could put the cores in a meltdown scenario that could lead to an explosion within the core or a leak of puddle fuel from the bottom of the reactor -- "doomsday" scenarios that create the ultimate test of a reactor's designed defenses, he said.
"There is a possibility if the fuel is in the right configuration, has been out of the reactor long enough, and is sufficiently air cooled that a fuel fire would not start," said one U.S. expert. "If there were a fuel fire, the radiation levels off site would go off the charts -- which they have not," he said.
Tom Clements, southeastern nuclear campaign coordinator of Friends of the Earth, said the fuel in pool No. 4 was hotter than in the plant's other pools because it had more recently been transferred into the pool. "So, it may well have boiled faster, and it had more fuel in it."
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11 Comments
Add CommentWhat would happen if we froze the fuel rods with liquid nitrogen. It has been a long time since Chemistry class, would this be possible or would it create a worse problem chemically speaking.
Reply | Report Abuse | Link to thisLou
I beleive the extreme heat of the spent fuel would quickly evaporate the liquid nitrogen as it has a much lower boiling point than seawater so it's not really an option. Plus, I doubt they could get a tank of liquid nitrogen to the plant with all the infrastructure damage.
Reply | Report Abuse | Link to thisinteresting thought but if you can get that large quantity of N2 then getting water back to the system would be much easier and more feasible.
Reply | Report Abuse | Link to thisMight have a heat transfer problem. Super cold liquid on very hot metal might cause fracturing, maybe explosively. How hot is containment vessels? Would hate to cause fracturing in those! Presence of seawater might mitigate this risk though. Still, would take a lot of N2 and there would be a significant increase in pressure....who knows....at this point might be worth the risks however. Good question.
Reply | Report Abuse | Link to thisMy complete guess is that liquid N2 would rapidly cool and embrittle the zircalloy casing. An uneven cooling would probably cause cracking either of the zircalloy or the cooling pool itself...
Reply | Report Abuse | Link to thispaddleman007: If you could get close enough to try to use liquid nitrogen, what would prevent you from just using something much easier to supply and transport, like sea water? I honestly have to ask what in the world you are thinking? Do you think they are leaving the rods sitting around heating up for a lack of an idea of what to put on them? Honestly, what?
Reply | Report Abuse | Link to thisCould they build up a large temporary wall around the plant and then flood it? With enough sandbags and portable temporary walls they maybe able to build the walls tall enough. I'm sure the military has ways of putting up walls rapidly for all sorts of purposes.
Reply | Report Abuse | Link to thisIn Fukushima the two back-up generators to replace power failure in the cooling system were swallowed by the tsunami.
Reply | Report Abuse | Link to thisThis was the source of the problem in the reactors.
But a startling fact has emerged in this nuclear accident in Japan The fact is that all nuclear plants in the world
contains a basic error in design, which is just using ONLY ONE hydraulic cooling circuit.
As in Fukushima. The most modern plants use the so-called external circuit of COOLING, which is an improvement over the old mills.
But this is only ONE external circuit, and in case of failure would cause the same phenomenon of Fukushima.
The principle is basic in engineering: if you have one, IN FACT you have none.
The aeronautical engineering widely used this principle, using two engines on airplanes, instead of just one.
In addition of two independent cooling hidraulic circuit, would be needed: two or more electric
stand-by generators to move them.
And two fuel tanks, diesel, independent to ensure supply.
All confined in a screened room and waterproof.
Recent expert analysis indicate that the generators for cooling can not be at the same level of the sea, to prevent flooding or invasion of the sea on the generators. Should be built on the highest level. Most nuclear power generators in the world has generators on the same level of the reactor, and are not shielded and not waterproof.
From NHK news, they said water is (probably) seen from reactor 4's roof, which looks like ripple reflections in the photo. So the priority is to cool reactor 3 first. The latest radiation level has come slightly down instead of going up.
Reply | Report Abuse | Link to thisThey intend to restore power to the cooling system now. Difficulties arise from the radiation which is preventing them to work effectively. Friday is the earliest date which they can restore the power.
From TV, it said they have 13 diesel power supplies as backup, several to each of the reactor. But 11 / 13 failed owing to the tsunami. The plant was designed to prepare for tsunami too but only to take water level of about 5m in height. (Need confirmation)
At this stage they should flood the sites with sea water. That permanently disables the plants some say? Well too f'ing bad. The Japanese government had no business leaving reactors online since the 1960's on the most unstable earthquake subduction zone on Earth.
Reply | Report Abuse | Link to thisNow, it comes out that TEPKO has been lying about safety for years!
Reply | Report Abuse | Link to thisDoes anyone know -- are TEPKO and BP connected?