Spent fuel rods may have burned in blaze at nuclear plant (It's not panic time)

cnn ^ | 3/15/2011 | cnn

[janetjanet998]

Gov’t eyes using SDF choppers to cool spent fuel pool at nuke plant
TOKYO, March 15, Kyodo

The government is considering using Self-Defense Forces helicopters to pour water on the spent fuel pool of one of the troubled reactors at a nuclear power plant in Fukushima Prefecture to help cool it, Defense Minister Toshimi Kitazawa said Tuesday.

But the measure has been put on hold because the government has had trouble assessing the potential impact it would pose to the fuel rods underwater and SDF personnel involved in such an operation, Kitazawa told reporters.

The request for the measure came from a task force set up Tuesday by the government and Tokyo Electric Power Co., which operates the Fukushima No. 1 nuclear power plant, according to the Defense Ministry.

The power supplier is battling to bring reactors at the plant under control, including the No. 4 reactor, where a spent fuel pool was boiling and its water levels were feared to be receding on Tuesday. The plant was crippled by Friday’s devastating earthquake and ensuing tsunami.

The ministry began preparing to dispatch a Ground Self-Defense Force helicopter unit in Chiba Prefecture, seeing that it would be possible for the helicopters to apply the same technique used for putting out a forest fire — namely, dropping water from the air.

But for now, the spent fuel pool is being cooled by police and firefighters on ground, after the government judged that an aerial approach ran the risk of damaging the spent nuclear fuel underwater and exposing SDF personnel to radiation, according to Kitazawa.

‘’We will perform our duty when we reach the stage where (the temperature rise in spent fuel) begins to settle down and we decide to drop large amounts of water from the sky,’’ he said.

[SargeK]

A couple of things can happen. One, once the water is lost from the spent fuel pool, it pretty much inevitable that the spent fuel will heat up, likely to the melting and ignition point. Doubleplus ungood.

Two, even if the fuel pool water level is maintained, it needs to be circulated through some cooling system, else it will eventually heat to boiling, evaporate the water and then see # 1 above.

Three, if the racks holding the fuel are distorted by heat, we lose control of the geometry - one of the three ‘legs’ of criticality control (the other two being mass and moderation). Water is a moderator - it promotes criticality. I’m neutral on whether the risk of a criticality is increased or decreased with a loss of fuel pool water. Someone a lot better educated than me would need to do the calculations.

[meyer]

Please note that the reactor base in your picture is located inside the containment structure, which itself includes provisions to deal with a melt-through of the reactor core material.

[erikm88]

Hmm... If the fuel rods have been exposed to "outside air", that would mean the reactor vessel (huge thick steel-walled container), and the primary containment building (6-8 inch steel reinforced concrete) have been extensively damaged, along with the reactor building.... if that's the case, then, they have massively bigger problems than what is being reported. There are basically 5 lines of defense (defense in depth):

• Ceramic coating of uranium pellets
• Zirconium alloy cladding on rods that contain the uranium pellets (pellets are stacked in rods end-to-end)
• Reactor vessel (4-6 inch stainless steel)
• Primary containment building (6-8 inch concrete)
• Reactor building (varies in composition and construction)

If the fuel rods are slumping (due to extreme high temperatures) you can bet that zirconium cladding has lost all structural integrity (I believe zirconium has a melting point of approx. 3000 F)... This also means the ceramic coating has lost all structural integrity as well.... so... who knows what is really happening; however, if all of the containment structures have lost all integrity, then they've got very very serious problems.

[meyer]

I wonder where they stand in terms of bringing an off-site power source to the plant. I’m talking about rebuilding a transmission line from a nearby area that still has power to this site so that some of the cooling facilities can be repowered and brought into operation.

There are a lot of obstacles that would prevent this from working. Obviously, if the power distribution substations and control centers were damaged significantly by the tsunami/earthquake, then having power available would be of little use. Also, with radiation now an issue, linemen would have to suit up and work limited hours to keep exposure to a minimum.

However, they’ve got to be working on repowering at least some of the plant equipment so that they can utilize the assets that exist already.

[winoneforthegipper]

That has been my contention all along. At some point they will remove these people even and then....u have all six reactors in melt down.

There are no automatic functions at plant one from my understanding. Pumping in the seawater requires manually releasing the steam. They can not do that off-site, logic then dictates result.

[SargeK]

The hydrogen explosions seem to have blown the tops of the reactor buildings to pieces. That is everything from the fuel floor up. This would expose the fuel pool to the environment. If the fuel pool lost its water, then the spent fuel melts and maybe burns. Beautiful billowing clouds of fission products rising up into the sky.

The reactor itself may still be safely contained, but all of the fuel that is has disgorged over the past, oh ten or so years, is out there in the open air. Burning.

Maybe not as many curies released as Chernobyl, but bad, bad nonetheless. I once visited a GE BWR plant and stood there, looking down into the fuel pool, which the plant management was cheerfully explaining was 175 feet in the air. I wondered what would happen if the water were to suddenly be drained out. Now I know.

[erikm88]

Water acting as a moderator I thought decreased neutron velocity. I suppose it would decrease it enough to convert to thermal neutron velocity in order to maintain criticality....

Can they treat the water with boron or boric acid to absorb neutrons and reduce any reactions?

[janetjanet998]

they are concerned about plant 4’s spent fuel rods but what happened to numbers 1 2 and 3 spent fuel rods? Or are those stored with number 4?

[SargeK]

I believe the answer is yes to both questions.

Water is a moderator, making ‘fast’ neutrons ‘slow’ or themal, thus enhancing the capture cross section, resulting in fission. Good in a controlled environment like a reactor, bad in a system like gaseous UF6 processing.

Boronated water would help control fission by absorbing neutrons. However, the math involved in figuring out the consequences in this scenario is WAY over my head.

[SargeK]

All of the plant units would seem to be affected. At least three of them have been blown up by hydrogen explosions and that would degrade the ability of the operators to maintain water and cooling in their spent fuel pools, to say the least.

At the ones that are still more or less intact, loss of power, and maybe damage from the explosions at the adjacent units could eventually cause them to lose the ability to maintain a safe, cool level of water in their fuel pools. In that case, then they too are on a path to failure.

It is likely that there is some unbelievable heroism going on at those plants right now as they struggle to maintain control.

[NVDave]

Yep. Gotta agree with you. And what brings me to that conclusion is the amount of hysteria I see even among the ‘conservatives’ here on FR, especially females.

Never mind the actual numbers of coal miners dead vs. nuclear power deaths is a terribly lopsided loss for coal. Nah.

What matters here are “feelings.”

Welcome to the new, feminized America: A scant, hollow shell of what we used to be, but we’ll all be like Oprah and emote our way forward into riches.

[Attention Surplus Disorder]

“I heard that even uncovered spent fuel rods would melt down in 72 hours maximum if not cooled.

accurate or not?”

They certainly can. It depends on how long they have been out of the reactor. The “fresher” out of the reactor, obviously the more potent the decay products and thus the more heat the naked rods will generate. I do not know how long they have to sit in water before mere (air) convection will be adequate to allow them to cool without hazard, but it is definitely months and possibly a couple of years.

[Robert A Cook PE]

Sorry. Miss read your post. Gonna get some rest after this. Basically in Reactor #4 the temperature of the rod storage pool water got to 84 C which was twice normal. Apparently the cooling had been slowed or shut down. A rod got to hot, exploded and had a small fire. Then as you pointed out, they put the fire out within an hour or two. But you do not just put that fire out. And a couple of reports claimed the fire just put its self out. Well, those fires just do not leave that easy. And new reports are that a spent fuel pool in #4 is currently boiling.

No part of this report makes sense: If the water is boiling, then its at 100 C (212 F) and is being cooled by the simple presence of the boiling water. "Twice normal" temperature (at 84 degrees C) means only that the water has heated up -> It's not even boiling yet. Much less has boiled off 14 feet of water to expose any spent rods!

If they are discussing spent rods, then they cannot be talking about the reactor, but the rod storage pool, with is like a 36 ft deep open-air pure water pool. The spent rods create a little, but very little fuel, and cannot “burn” at all. Melting could occur, but only if the top of the entire rods were uncovered for much more time: 8 to 12 hours “might” damage the rod's integrity, but then the sagging rod would fall down into the water and immediately cool off. (The water would be contaminated - but the rod would not “burn”.)

In any case, the cooling water pool (spent rod storage) can be filled up with a fire truck if it ever gets low. Top of the regular spent water pool is about 12 feet above the top of the twenty foot long rods.

In the reactor? In at least two of the reactors rods were exposed and rods were damaged. This definitely released some radioactivity into the containment building, which is underneath the sheetmetal crane building above the concrete containment. The vented hydrogen that blew out the sheetmetal building did even more damage - going to be real hard to lift covers and get into the containment to assess damage.

The reactor rods ARE shut down, they can't react any more or go back to criticality. But they are much hotter (have more internal decay heat being generated from the rod internals than the spent fuel rods. They will need to keep adding more replacement water to the reactor to replace what is being boiled off. But as long as new water is added, and hot water boiled off, the reactor rods cannot get above 212 degrees either.

So they can't melt = get more damaged than what's already occurred. What's occurred is bad enough - those plants will never run again.

Will small amounts of radioactivity continue to get out? Probably.

Better avoid that ski vacation. The higher altitude will give you more radiation than staying near this power plant for a few weeks. 8<)

[TXnMA]

OK, I'm tired of all this speculation and guessing about the spent fuel conditions.

At the top, to the right of the reactor (and beneath the crane rails) you can see the spent fuel rods in their (water-filled) storage tank.

Any physical damage to that tank will cause the cooling water to leak out. Interference with the system that circulates cool water through the tank will allow the heat generated by residual nuclear decay to make the water temperature rise. If the water boils, water is lost as steam -- and the water level falls.

All of the above can expose the rods to air (oxygen), and allow rapid oxidation (burning) of the metals to occur.

~~~~~~~~

Anyone who thinks metals can't burn must have slept through the seventh-grade science demo where the instructor lighted up the entire room by igniting a bit of magnesium ribbon...

[TXnMA]

I see we both "skated around" having to explain how phase change involving the heat of vaporization is a much more efficient cooling mechanism than simple conduction cooling by chilled water... <GRIN>

I swear -- even English majors when we were young had a minimal grasp of such basics. Don't they even boil water in paper cups for kids anymore?

[TXnMA]

Even simpler explanation for the #4 unit fire:

" Experts: Fire at Fukushima #4 caused by lubrication oil in recirculation system and not hydrogen; fire is out."

[dynoman]

Now that explaination finally passes the common sense test, do you have a source?

[Palladin]

Thanks for the updates, jj. You seem to have reliable sources.

[TXnMA]

http://news.sky.com/skynews/Home/World-News/Video-Japan-Quake-And-Tsunami-Plea-For-US-Help-After-Second-Explosion-At-Nuclear-Plant/Article/201103215951706?lpos=World_News_Carousel_Region_0&lid=ARTICLE_15951706_Video_Japan_Quake_And_Tsunami%3A_Plea_For_US_Help_After_Second_Explosion_At_Nuclear_Plant

"djysrv: Experts: Fire at Fukushima #4 caused by lubrication oil in recirculation system and not hydrogen; fire is out [via Twitter] Tuesday March 15, 2011 3:23 "

(Sorry -- I couldnt find a longer URL...) /S ;-)