Burning nuclear-grade graphites

[Azael]

I was reading a bit on General Atomics homepage and found this.

http://gt-mhr.ga.com/graphites_all.html
(bolds added by me)

It is often incorrectly assumed that the combustion behavior of graphite is similar to that of charcoal and coal. Numerous tests and calculations have shown that it is virtually impossible to burn high-purity, nuclear-grade graphites.

The oxidation resistance and heat capacity of graphite serves to mitigate, not exacerbate, the radiological consequences of a hypothetical severe accident that allowed air into the reactor vessel. Similar conclusions were reached after detailed assessments of the Windscale and Chernobyl events; graphite played little or no role in the progression or consequences of the accidents. The Òred glowÓ observed during the Chernobyl accident was the expected color of luminescence for graphite at 700°C and not a large-scale graphite fire, as some have incorrectly assumed.

I have always been told that the graphite at chernobyl was burning

 

[Astronuc]

Quick test - Take the graphite from a pencil and see how easy it is to burn even with a torch.

Better burn the wood off the graphite. Note that the wood burns, but the graphite does not.

It may be possible for graphite to burn if the appropriate oxygen level is obtained with the graphite at the appropriate temperature. In the case of Chernobyl there was considerable heat from the nuclear fuel and the oxidation of metal (Zirconium alloy), as well as the steam-graphite reaction.

 

[Azael]

Quick test - Take the graphite from a pencil and see how easy it is to burn even with a torch.

Better burn the wood off the graphite. Note that the wood burns, but the graphite does not.

What a simple idea that I never even considered. And yeah it was impossible to get the graphite to burn.

It may be possible for graphite to burn if the appropriate oxygen level is obtained with the graphite at the appropriate temperature. In the case of Chernobyl there was considerable heat from the nuclear fuel and the oxidation of metal (Zirconium alloy), as well as the steam-graphite reaction.

But did graphite burning play a big part in the dispersion of radionuclides into the atmosphere from Chernobyl or is graphite like GA states a mitigating factor?

 

[Astronuc]

After I posted that I reflected upon the Windscale fire of 1957.

http://news.bbc.co.uk/2/hi/science/nature/7030536.stm
http://news.bbc.co.uk/2/hi/science/nature/7030281.stm


I don't think one should dismiss the concern over the potential of a graphite fire. I do think we need to understand the physics of the phenomenon, and design appropriate protection.

At this point, I would say it is difficult to burn graphite, but not impossible. I do not want to be caught downt he road by the exception or something we didn't consider.

The graphite in Chernobyl had been irradiated and exposed to high temperature steam, so some of it had chemically reacted. The high temperature steam could oxidize graphite.

I do know that one of the major problems with the Fort St. Vrain, graphite-moderated, gas (He) cooled reactor was steam intrusion into the primary circuit. IIRC, it never got much above 33% of full rated power power, or if it did, it didn't for very long. The first company in which I worked had some involvment with Fort St. Vrain and the utility, but the plant was shutdown by the time I joined the company.

 

[Azael]

So general atomics are a bit reckless perhaps when they claim the GT-MHR is so safe that it doesn't need a containment building?

A related question, roughly how much of the capital cost of a NPP is due to the containment?

 

[vanesch]

After I posted that I reflected upon the Windscale fire of 1957.

http://news.bbc.co.uk/2/hi/science/nature/7030536.stm
http://news.bbc.co.uk/2/hi/science/nature/7030281.stm

It is funny that in the first BBC article, they say that Windscale was dwarfed by Chernobyl (ok) and TMI (??). At TMI, nobody got hurt !

I don't think one should dismiss the concern over the potential of a graphite fire. I do think we need to understand the physics of the phenomenon, and design appropriate protection.

At this point, I would say it is difficult to burn graphite, but not impossible. I do not want to be caught downt he road by the exception or something we didn't consider.

It's the kind of recklessness displayed by some that are to blame for the bad image that nuclear power has with the public.

Let us not forget that the two only reactors that did spread a lot of radioactive stuff in the environment were Windscale and Chernobyl, both with an (impossible) graphite fire.

 

[Astronuc]

So general atomics are a bit reckless perhaps when they claim the GT-MHR is so safe that it doesn't need a containment building?

A related question, roughly how much of the capital cost of a NPP is due to the containment?

The containment building is probably on the order of 10% of the capital cost. The reactor and primary circuit, and heat exchangers, and power conversion systems are also big chunks of the total cost.


Pressurized systems need some kind of containment. In nothing else, the containment serves to keep out missiles (which could be natural, e.g. objects hurled by a tornado, or manmade, e.g. aircraft or rockets) that would damage the reactor and primary system.


GA's claim is more cavalier than reckless. I just don't think we in the industry should be so arrogant as to claim that something is SO safe that we don't need the protection. The nuclear industry made some bold claims in the past, and it served to undermine our credibility and acceptability. We do need to be mindful and respectful of the technology, and the need for safety and security of such technology.

The accident at TMI-2 was contained, but it lead to widespread anxiety because of the uncertainties involved. I did some work as a consultant with the utility on behalf of the sister unit TMI-1 many years ago, and I visited the site a couple of times. It is quite safe. The folk there take their work very seriously. Afterall, their livelihood, and the safety and security of their families depend on that too!

Chernobyl was the worst nuclear accident followed by Windscale.

 

[Azael]

A little update on this. At the university today I got ahold of some reactor grade graphite and put a blowtorch to it for a good 7-10 minutes.

Clip

Nothing at all happened.

Astro you mentioned steam-graphite reaction. What happens in that reaction?

 

[Astronuc]

C + 2H₂O -> CO₂ + 2H₂, but I believe there is a temperature threshold. IIRC, this is one proposal for hydrogen generation by coal gasification.

Steam in the primary circuit was just one problem at Ft. St. Vrain gas-cooled, graphite moderated reactor.

 

[Azael]

I guess next test then will be to se how glowing hot graphite reacts with hot steam

 

[Astronuc]

I guess next test then will be to se how glowing hot graphite reacts with hot steam

Be careful if you wish to do an experiment. That's probably steam at 400°C or even hotter, and that implies high pressure or superheat. That is dangerous stuff.

 

[Azael]

Yes defenetly, when it comes to chemistry, steam and heat I won't touch anything without the engineer resposible for the labs closeby :)

 

[GTrax]

In a BBC2 Open University educational video. "So you thought diamonds were forever!" A diamond under Oxy-Acetylene torch, heated to glowing, then turn off the acetylene only. It shrunk, and ended up as CO2 in not many seconds. I forget what the support was made of.

Not graphite - but I would have thought at least equal or harder to oxidize than graphite. In an accident, the oxygen can come from leaking water. We just cannot assume a fire is "impossible".

 

[ironmike56]

I observed in my physics class that putting a current through pencil lead (graphite) will heat the and lead result in glowing. After looking at forms of propulsion, could graphite be used in a resistojet with a continuous clow of water??

 

[zeitghost]

The fire at Windscale was finally put out by pouring water on it... they tried tons & tons of Carbon Dioxide to no effect, but good old H20 did the trick.

 

[GTrax]

As I understood it, the reactor was basically a graphite lump with horizontal holes in it. Fuel was in metal containers with "fins", shoved down the holes. The primary purpose was to make weapons material, so the mix of ingredients varied. Also, the fins were cut smaller (twice!) to raise the temperature.

What then happened is the stuff of heroes. Let us not forget the men taken from the back rows of the cinema at nearby Seascale who were "volunteered" by the local policemen to help push the fuel out using scaffold poles. There is a very good BBC documentary made about it, including the disgraceful scapegoating that happened afterward.
"[URL
http://en.wikipedia.org/wiki/Windscale_fire

 

[zeitghost]

What really saved the day were the filters at the top of the cooling stacks.

These were added to the design and referred to as someone's folly...

That "folly" saved a lot of people from contamination...

 

[GTrax]

The "folly" was from Sir John Cockroft, who was not popular at the time for his insistence on huge concrete cased filters placed at the top of the stacks. The air was simply blown through the core, and up out a chimney. It was not supposed to include radioactives, but nobody gave too much thought to what might happen if a can broke, or burned, or simply got stuck and rotted away. The filters probably captured most of what went up the chimney, but not all. The stack contaminated the countryside for miles around.

As to graphite, the stuff in the pencil is very impure ground up mix of carbon and organic binders. It will just burn! Carbon melts at 3550 deg C but it will burn away in air way before that.

I have used vacuum furnaces up to 1600 deg C, and it is only carbon and the more exotic ceramics that can stand it. Of course, it is the vacuum that let's it survive, but I notice that the vacuum collapses somewhat with every hike in temperature, and it then recovers some. There is a continuous loss of carbon insulation. The graphite surfaces seem to remain unharmed. The vacuum system, capable of 5E-7 millibar when cold, could not improve on 2.5E-5 millibar, no matter how long it was baked at 1150 deg C.

Graphite used in reactors is a very special pure grade. Graphite is affected by neutron radiation in that it stores energy which can suddenly be released, known as 'The Wigner Efffect". Deliberately raising the core to 250 deg C had an annealling effect where the energy could be safely released. At the time, it was hard to tell whether the core heating was released Wigner energy, or if it was on fire somewhere.