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how bad is the meltdown?

how bad is the meltdown?

how bad is the meltdown?

(OP)
i dont trust the news much coz of them exaggerating things. so guys here, worst case scenario "meltdown" happens.. how bad is this?
i assume this is absolutely nowhere near as bad as US droping the bomb in hiroshima, no?

RE: how bad is the meltdown?

Definitely nowhere near as bad as an atomic bomb. Hiroshima killed hundreds of thousands of people, and that is not plausible here. A nuclear explosion of the reactors is a physical impossibility. At worst, we are talking about steam explosions, hydrogen explosions, fires, and release of toxic pollutants into the environment that may have a public health impact.

I recommend World Nuclear News as the most technically accurate source of news on this event:
http://www.world-nuclear-news.org/

RE: how bad is the meltdown?

Define "Meltdown".

rmw

RE: how bad is the meltdown?

I know that different people have different definitions of meltdown, but I've never found it useful to split hairs over this. If the fuel gets too hot, it can burn and/or melt. Molten fuel could hypothetically drop to the bottom of the building and form a puddle. Melting of a portion of the fuel has happened a few times in various reactors, and this is usually where you get the legal wrangling over how much melting you need to really qualify as a "meltdown."

Those linguistic contortions only matter to lawyers and public relations firms. What really matters to public health is the environmental releases, not the final geometry of the core.

RE: how bad is the meltdown?

Thanks Trottiey,

This is an engineer's website and I assume that by joining you are presenting yourself as one.

It is not uncommon for engineers to define terms before setting out to access a technical issue, especially a sticky one like this one.

I may should have said; "Define 'worst case scenario' "meltdown".

rmw

RE: how bad is the meltdown?

Fair enough. The original poster does not sound like a nuclear engineer, and I guessed she was looking for a plain answer.

And yes, I am an experienced nuclear engineer.
 

RE: how bad is the meltdown?

Trotiey,

Thanks for your response.  I wanted to make sure that this thread nor any thread in this forum denegrated to a "China Syndrome" level.

But more to the OP and the question posed:

I found this article on the news wires this AM.  How close is it to being accurate.  I did notice that the author did not differentiate between a BWR and the type of reactor at Chernobyl which puts a "jaundice" in my mind on other things stated.

rmw

Paste follows:

What is a Nuclear Meltdown?
 

--------------------------------------------------------------------------------
 
Mar 16, 2011 -- Voice of America News/ContentWorks  
 
 
Japanese officials and nuclear experts have said they cannot rule out the possibility of a nuclear meltdown at a Japanese nuclear power plant that was badly damaged by last week's earthquake and tsunami. Here is a quick guide to the nuclear process, what can go wrong, and how to prevent catastrophe.

-- Nuclear power is produced by harnessing the heat produced by the splitting of atoms inside uranium -- a process known as fission.

-- Rods packed with uranium are submerged into water, and the heat produced by the nuclear reaction creates steam, which is used to power turbines that produce electricity.

-- The nuclear reaction can be controlled utilizing rods made of neutron-absorbing material, such as boron, essentially shutting down the fission process. But the rods still produce heat, even when control rods are in place, requiring a cooling system to maintain temperatures.

-- When the cooling system failed at Japan's Fukushima Dai-Ichi reactor Number 2, the fuel rods boiled through the available water and were for a period of hours exposed to the air.

-- If the rods get too hot, they can eventually melt, thus the term "meltdown."

-- In the event of a complete meltdown, the still-burning hot nuclear fuel could drip to the floor of the reactor. If the containment structure around the reactor is not strong enough, the fuel potentially could be exposed to the outside environment, and can have devastating consequences for nearby communities.

-- The world's worst nuclear power disaster was in Chernobyl, Ukraine in 1986. After an explosion at the plant, a cloud of radioactive dust spread for hundred of kilometers and was blamed for a surge of cancer deaths and birth defects. It has left some nearby towns uninhabitable to this day.

-- People also can be exposed to radiation poisoning through contaminated food and water. A recent U.N. study estimates the Chernobyl disaster caused 6,000 cases of thyroid cancer in children, largely through contaminated milk.

-- Workers at the Fukushima plant are pumping seawater, treated with boron, to try to cool the overheating reactor cores. This process, if successful, will completely shut down and destroy the reactor.

-- After the reactors are brought under control, nuclear technicians will either have to remove the spent fuel, or try to bury the remnants in a concrete "sarcophagus" that will prevent the excess radiation from leaking out, until they can be safely removed.

 
 

RE: how bad is the meltdown?

Gentleman,
A couple items to be added to the definition of  "melt down" that are paramount in reactor safety are:

1 - When zirconium melts is gives off hydrogen gas - hence the explosion.

2 - If enough uranium pellets fall to the bottom of the reactor there is the potential for self criticality, where the the fuel geometry becomes such that fission reaction is self sustaining and potentially beyond the immediate control of the operator.

But more important then what is a melt down is perhaps an explanation of why these evens are occurring in Japan, at least as I see it:

If I remember correctly, after a reactor is shut down, it still generates approximately 12 - 18% of its operating power in heat from the decay of the fission products generated in the fuel, but this level dies off quickly, around 30 days, as those products decay away and in nuclear engineering this is called decay heat.
Now, trying to remove this large amount of heat from the reactor, even after it is shut down, is the problem that is occurring in Japan.  Nuclear power plants are thermally very inefficient.  A BWR at around 1,100MWe produces 3,400MWt, or it converts about 32% of its heat into electrical energy.
But, now take 12% of 3,400MWt and we get 408MWt, and that is about the amount of heat that needs to be removed form the reactor to prevent this overheating.
How much water and at what rate of circulation is required to remove this amount of heat?  I don't know - but the Owners and Operators of that reactor should of known, and as long as events like this occur people will not trust nuclear energy.  This is especially sad since we have recently been on a up swing developing nuclear energy and I do believe the new generation of reactors address this problem.  But now the industry has a greater burden of gaining the public trust.  

RE: how bad is the meltdown?

Errata

Gentleman,

My decay heat values given above are incorrect.
The decay heat value initially after shut down is about 7% and after a week 0.2%.
So the initial heat generated is 238MWt.

http://decay-heat.tripod.com/

My apologies.

RE: how bad is the meltdown?

1) Zirconium is an element and cannot produce hydrogen just by melting. It does react with steam to produce hydrogen and heat. The reaction rate increases with temperature and may become self-sustaining somewhere around 1000˚C, long before melting. Zirconium can outright catch fire if exposed to air at much lower temperatures, and that is probably what happened in the spent fuel bays. Is that a "meltdown?"

2) If molten fuel falls to the bottom of the reactor, it will melt or burst its way through the pressure vessel, drip onto the core catcher, and spread out into a puddle. That geometry has a large ratio of surface area to volume, so you will lose too many neutrons to sustain the reaction. Perhaps some intermediate stage might form a ball at the bottom of the vessel, or a stalagmite on the core catcher, and it might approach criticality that way. But as it approaches criticality, the heat generation would rise, more stuff will boil or melt or generate pressure, and the molten control rods or other inhibitors will mix with the molten core. Even if the control rods float on top, the mixture would just sputter itself apart or accelerate its descent to the core catcher. It might go "plop," but it can't go "boom," which is what an out-of-control self-sustaining reaction would be.

3) Units 4, 5, and 6 were in cold shutdown long before the earthquake, so there were only 2028 MWe up and running. Divide by 32% and multiply by 7% and you get 444 MWth. Water's latent heat of evaporation at sea level is 2.26 MJ/kg. So if you're just hosing them down and letting the steam carry away the heat, you're going to need a flow rate on the order of 200 kg/sec, say 3000 gpm on day 1. If you do it in a more traditional way with heat exchangers and modest temperature rise of ocean water, you would need easily a hundred times that. Those numbers will drop exponentially with time due to radioactive decay, with some hickups if it the core flirts with criticality as per item #2 above. Keep in mind that the plant is designed for much greater flows to reject all the waste heat during full power operation.

RE: how bad is the meltdown?

Dear Experts,

I am just giving a wild thought, sea water salts if decomposed in the typically unknown environment under discussion

may yield water of hydrogen through sodium metal route I assume.

Please guide!

Best Regards
Qalander(Chem)

RE: how bad is the meltdown?

It's not a completely unknown environment. We have data from a number of intentional meltdown experiments, as well as a few accidental ones. The basic laws of chemistry continue to hold.

It's not clear what you mean by the "sodium metal route." Pure sodium will react with water to form hydrogen at room temperature, just like zirconium does at higher temperatures. But obviously sodium chloride (NaCl, sea salt) does not; you can confirm this at home by dropping some salt in a glass of water.

Going out on a limb a bit, I imagine that there exists a reverse reaction at higher temperatures or in the presence of radiation that would convert some of the salt into hydrogen chloride and sodium hydroxide. The acidic hydrogen chloride would accelerate corrosion, but that would take years to have any effect on stainless steel. I don't see a mechanism for significant hydrogen production from salt.

RE: how bad is the meltdown?


I had a heat transfer professor in college who evaluated core damage at TMI. His analysis concluded that it was impossible for fuel to melt through the reactor vessel. (Otherwise it would have happened when the operators turned off the ECCS)

I don't know the details of his evaluation, but I took it in a practicle way: Did you ever try to solder copper pipe when it still had water in it? It can't be done.  You can't melt tin-antimony solder in the presence of water.

Can U235 sustain a nuclear chain reaction without the presence of a moderator? No, U235 is a thermal fuel moderated by water.

Can U235 pellets magically precipitate out of molten metal to fall to the bottom into criticallity - Not in this universe.

Thus, the following statement is INCORRECT:
"2 - If enough uranium pellets fall to the bottom of the reactor there is the potential for self criticality, where the the fuel geometry becomes such that fission reaction is self sustaining and potentially beyond the immediate control of the operator."

 

RE: how bad is the meltdown?

Right conclusion, but only after a couple of wrong turns:

1) With enough heat input, you can weld underwater, especially if the pressure is high enough to reduce boiling. People do it all the time.

2) If U235 must have a moderator to sustain a chain reaction, could you explain why the Trinity test worked? I seem to have missed something.

3) Magic isn't real, it's true. I didn't quite follow the rest of that logic, but I'll trust it since you arrived at the same conclusion I did.  

RE: how bad is the meltdown?

Good discussion Gentlemen,

1 - Welding or soldering a pipe with water inside of it can be done, it only requires a greater heat source than that the little propane tank and soldering torch you purchase at your hardware store - but it just does not produce the ideal joint.

2 - For U235, the minimum critical spherical mass dimension  is diameter of 17cm and mass of 52kg (http://en.wikipedia.org/wiki/Critical_mass). Now a BWR has about 155,000 kg of fuel at about 4% U235 enrichment, so that's about 6,200kg of U235 inside the core.
Now it is highly unlikely that the U235 could form a critical mass in the reactor core, but maybe only a little more unlikely than a 7m wave could wash away the standby generator at a coastal power station and leave them high and dry!



 

RE: how bad is the meltdown?

http://www.latimes.com/news/science/la-sci-japan-quake-nuclear-20110319,0,7524053.story?track=rss&utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+latimes%2Fnews%2Fscience+%28L.A.+Times+-+Science%29

Quote:

The French nuclear agency IRSN said Friday that the Fukushima Daiichi plant had already released 10% as much radioactivity as Chernobyl, though the agency has been accused by some as being alarmist.
That 10% sounds way high to me as well. What do you guys think?

=====================================
(2B)+(2B)'  ?

RE: how bad is the meltdown?

Criticality is not as easy as dclarku58 says, otherwise everybody would be doing it. Just ask Iran or North Korea. The Little Boy device (I got it confused with the Trinity test earlier) needed 80% U-235 enrichment. You have to run centrifuges for years to get that; what would be an equivalent enrichment mechanism in a meltdown?

Now an activated core that's been in the reactor for a while will have a modified composition, (e.g. plutonium production,) but it's still going to be dominated by U-238 that parasitically absorbs neutrons. If you agree that it won't spontaneously enrich, that means the molten mess still needs a porous structure filled with moderator (water) in order to go critical. And the control rod debris can't be part of the mix.

Even if by absurd chance, your core starts to approach that configuration, the rise in heat generation will just boil and sputter the stuff apart or accelerate the descent to the core catcher. Nuclear weapons have to fire the pieces together at sonic speeds to avoid a pre-detonation fizzle. What would be your equivalent fuse in a reactor? A perfectly timed hydrogen explosion at just the right angle?

RE: how bad is the meltdown?

Nuclear weapons are designed with implosives in order to get a bigger nuclear explosion- They want far more than the minimum critical mass to explode. They do that by forcing lots of nuclear material together. They have lots of moderator- water (not as good as heavy water).  The minium critical mass is not a simple calculation. Anyway the problem is more likely to one of a mess of nuclear material which is more difficult to be clean up and more likely to leak into the environment.  

RE: how bad is the meltdown?


My analogy was not welding material, but the process of melting solder.  And yes, a greater heat source is required, but my point was that there is not sufficient heat available from decay heat alone.  Conduction through the pressure vessel to ambient and boiling water will prevent the fuel from melting through the RPV.

If it was possible - it would have happened at TMI after the operators turned off the emergency cooling systems.
 

RE: how bad is the meltdown?

"boiling water will prevent the fuel from melting through the RPV" - until the water runs dry, that is.

In any case, pure melting is the wrong condition to look at. The fires in the WTC weren't hot enough to melt steel, but they were hot enough to weaken it to the point of collapse. Same idea applies to pressure vessels.

TMI had inadequate cooling for 16 hours, and they melted half the core. Fukushima has been going for a bit longer than that.

RE: how bad is the meltdown?

I don't whether you have read the other threads on this topic but a nuclear expert involved with consulting with the Fukushima presented at a physics institute (link is in the other threads) but his information is that the spent fuel rods at Fukushima Reactor have collapsed (melted) obviously without nuclear explosion or significant fission (which would be too much their current cooling effort) so no worry about that. However their cooling effort is going have to go on long time as one the reactor is real mess (salts, broilled sea biological matter, broken reactor parts) and two they can't get close to top safely to clean it up and put a permanent cover. Just accidentally I found out that the Chinese construction giant Dany is sending the world's biggest/longest reach concrete supplying crane to Fukushima for free so they can direct a great flow of water right into the reactors quickly and efficiently.    

RE: how bad is the meltdown?

There is a summary of radiation levels in surrounding areas below (see "download the slides")
http://blog.energy.gov/content/situation-japan/

One caveat I would mention is that we need to be careful not to focus exclusively on radiation levels when there is also potential for breathing/ingesting contamination, which can deliver higher dose to certain organs over a period of time even after a person leaves the area.  

=====================================
(2B)+(2B)'  ?

RE: how bad is the meltdown?

If there is no water, then fission cannot be sustained (subcritical).

Decay heat is not sufficient to "...melt or burst its way through the pressure vessel" [unlike Chernobyl which became prompt-critical and blew up the fuel (deflagration), setting the graphite moderating rods on fire]

Like TMI, the fuel should be contained within the RPV and within the primary containment. [I haven't seen any reports stating that the earthquake actually broke something open]

There are four fission product barriers; the fuel rods, the RPV, the primary containment, and the secondary contaiment (blew up).

Once the 1st & 4th barriers failed, there are many ways for long lived isotopes to get out of the RPV & primary containment.

The fate of the nuclear industry stands on their success in containing the long lived fission products.

The fact that fuel rods may have been damaged in the spent fuel pool is very very serious threat to the public.

RE: how bad is the meltdown?

Its good questions as to whether complete structural analysis including NDT was done on Fukushima Daiichi between the earthquake and reactor destabilizing.  

RE: how bad is the meltdown?

At least she admits she's a lawyer- even American knows their big-headed morons.  

RE: how bad is the meltdown?

Try searching for "opinionated Nancy Grace" on Google. You get hundreds of hits. I have only seen about 5 minutes of her but it was enough.

HAZOP at www.curryhydrocarbons.ca

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