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 Post subject: Graphite Fire Risk
PostPosted: Feb 21, 2013 1:41 pm 
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I searched in the forum but didn't find a thread on this, so excuse me if this has been discussed before.

I'm still in the middle of debating LFTRs on another forum and another strong proponent of them posted this;

Quote:
Concerning graphite fire. I found that powdered graphite has an ignition temperature of 730.00 deg C. It is conceivable that this temperature could be reached in a LFTR accident (operating temperature is about 600 deg C). So the danger of fire must be considered.

As mentioned, the containment can be kept full of inert gas. As a backup, in the event that somehow the containment is breached during a failure, inert gas cylinders within the containment could be designed to open when the temperature reaches some point (via melting plugs allowing slow release of gas), to assure that the graphite is kept in an inert gaseous environment until it has cooled well below ignition temperature.

I think this is necessary for a LFTR, but it is also a passive system that cannot fail due to loss of power, some type of mechanical failure or human error.


So I was wondering what is the risk of the graphite core in an LFTR catching fire. It would be immersed in molten salt so wouldn't be exposed to oxygen I was thinking and also the constant high temperatures of the reactors should anneal the graphite releasing the build up Wigner energy?

Is there a risk of graphite fire above 730 C in an operating LFTR or one that has had it's fuel drained in an emergency?


Last edited by DougC on Feb 21, 2013 1:48 pm, edited 1 time in total.

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 Post subject: Re: Graphite Fire Risk
PostPosted: Feb 21, 2013 1:47 pm 
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What LFTRs use is NOT powdered graphite. You can buy graphite crucibles that are good to 2500C. I don't think there is an issue.

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 Post subject: Re: Graphite Fire Risk
PostPosted: Feb 21, 2013 1:48 pm 
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KitemanSA wrote:
What LFTRs use is NOT powdered graphite. You can buy graphite crucibles that are good to 2500C. I don't think there is an issue.


That's what I needed to know, thanks.


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 Post subject: Re: Graphite Fire Risk
PostPosted: Feb 21, 2013 2:23 pm 
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Most anything powdered can burn. Powdered steel can burn too, that doesn't mean you can set a steel construction I-beam on fire! It's impossible to set a construction I-beam on fire even with a blowtorch.

Solid graphite chunks are like that. Fire is not a problem.

Moisture and water are problems. They will oxidise the graphite at elevated temperatures. Water is not compatible with the fluoride salt, though, so most designs arrange for no significant amounts of water inside containment, by design.


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 Post subject: Re: Graphite Fire Risk
PostPosted: Feb 21, 2013 2:34 pm 
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Cyril R wrote:
Most anything powdered can burn. Powdered steel can burn too, that doesn't mean you can set a steel construction I-beam on fire! It's impossible to set a construction I-beam on fire even with a blowtorch.

Solid graphite chunks are like that. Fire is not a problem.

Moisture and water are problems. They will oxidise the graphite at elevated temperatures. Water is not compatible with the fluoride salt, though, so most designs arrange for no significant amounts of water inside containment, by design.


I was thinking it could possibly be a risk due to the Windscale fire, where if I remember right the air cooled graphite moderated reactor caught fire during an attempt to anneal the graphite by heating it. Does solid graphite burn at some temperature that could conceivably be achieved by an LFTR?


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 Post subject: Re: Graphite Fire Risk
PostPosted: Feb 21, 2013 2:55 pm 
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I believe Windscale was releasing Wigner energy.
At our operating temperatures, we have no Wigner build up.


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 Post subject: Re: Graphite Fire Risk
PostPosted: Feb 21, 2013 4:42 pm 
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As Jack mentioned, the Windscale fire was due to irradiating the graphite at low temperatures. This causes a radiation damage type of energy to accumulate. But that actually wasn't the direct issue at Windscale; the issue was that the engineers had to anneal (heat) out this damage periodically, (think of ironing out rimples in your clothes with a hot iron) and the reactor wasn't designed for this as they were unaware of Wigner energy buildup during design. It had low melting point cladding and metallic uranium fuel which, due to a hot spot forming apparently in one of the annealing heatup runs, caught fire. This metallic uranium fuel was on fire, not the actual graphite itself.

The Wiki article is quite good:

http://en.wikipedia.org/wiki/Windscale_fire

Also like Jack mentioned, the Wigner energy only builds up significantly below 300 degrees Celsius, and is almost zero at 400 degrees Celsius. Minimum graphite temperature in a MSR is >550 Celsius so there's no Wigner energy building up. No metallic uranium to catch fire either. Fluoride salts don't burn. There not even something that can cause a pressure rise in containment.


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 Post subject: Re: Graphite Fire Risk
PostPosted: Feb 22, 2013 2:15 pm 
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Was it the presence of water in the reactor at Chernobyl that caused the graphite to burn there?

http://www.iaea.org/newscenter/features ... -faq.shtml

Quote:
In addition, the graphite blocks used as a moderating material in the RBMK caught fire at high temperature as air entered the reactor core, which contributed to emission of radioactive materials into the environment.


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 Post subject: Re: Graphite Fire Risk
PostPosted: Feb 22, 2013 5:49 pm 
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DougC wrote:
Was it the presence of water in the reactor at Chernobyl that caused the graphite to burn there?

http://www.iaea.org/newscenter/features ... -faq.shtml

Quote:
In addition, the graphite blocks used as a moderating material in the RBMK caught fire at high temperature as air entered the reactor core, which contributed to emission of radioactive materials into the environment.


It's very likely, as it was water cooled (boiling water in tubes). Lots of hot water and steam to react with graphite, forming CO and H2 gas (syngas) which can burn (and at the very least add more driving force to push out radionuclides into the environment). But not with just air. This is only possible if graphite gets so hot as to sublime, then you get carbon gas which can burn. This requires temperatures over 3600 degrees Celsius, far higher than the fuel melting point, so will only happen if fuel slumps onto graphite. Seeing how it was a vertical fuel channel reactor, this isn't very plausible.


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 Post subject: Re: Graphite Fire Risk
PostPosted: Feb 22, 2013 6:57 pm 
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Cyril R wrote:
It's very likely, as it was water cooled (boiling water in tubes). Lots of hot water and steam to react with graphite, forming CO and H2 gas (syngas) which can burn (and at the very least add more driving force to push out radionuclides into the environment). But not with just air. This is only possible if graphite gets so hot as to sublime, then you get carbon gas which can burn. This requires temperatures over 3600 degrees Celsius, far higher than the fuel melting point, so will only happen if fuel slumps onto graphite. Seeing how it was a vertical fuel channel reactor, this isn't very plausible.


It sounds like you need something extreme to break down the rigid structure of solid graphite before it will burn very well and by that point a graphite fire is probably not going to be the worst of your worries.


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 Post subject: Re: Graphite Fire Risk
PostPosted: Feb 23, 2013 6:42 am 
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DougC wrote:
Cyril R wrote:
It's very likely, as it was water cooled (boiling water in tubes). Lots of hot water and steam to react with graphite, forming CO and H2 gas (syngas) which can burn (and at the very least add more driving force to push out radionuclides into the environment). But not with just air. This is only possible if graphite gets so hot as to sublime, then you get carbon gas which can burn. This requires temperatures over 3600 degrees Celsius, far higher than the fuel melting point, so will only happen if fuel slumps onto graphite. Seeing how it was a vertical fuel channel reactor, this isn't very plausible.


It sounds like you need something extreme to break down the rigid structure of solid graphite before it will burn very well and by that point a graphite fire is probably not going to be the worst of your worries.


Absolutely, you've put it much better than I. Though I would make an exception with regards to steam and hot water: having these together in a tight reactor space is in my opinion bad design.

Sometimes I wonder. All the nuclear engineers I've talked to are absolutely brilliant people. Yet the major nuclear accidents of history are full of really dubious design and operational mistakes. Steam and graphite combined with strong positive void coefficient plus no good containment and then doing experiments with overriden control rods, which even a junior nuclear engineer knows will obliterate the reactor (Chernobyl). Running a reactor much hotter than it's design basis on a casually regular basis, and having air cooling for the graphite moderator, in a reactor that has pyrophoric uranium metal fuel alloy and a low melting point cladding (Windscale). Building a nuclear reactor in an extremely flooding prone location, with a flooding prone electrical system, that is completely dependent on electricity for most of the vital safety functions (Fukushima). These are outright stupid and irresponsible design and operational practises.


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 Post subject: Re: Graphite Fire Risk
PostPosted: Mar 14, 2013 5:09 pm 
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HTGR Graphite Design Handbook has steam/graphite and air/graphite
oxidation rates for three different reactor grade graphites.
You can get it by googling DOE-HTGR-88111

No idea how accurate they are.


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 Post subject: Re: Graphite Fire Risk
PostPosted: Mar 19, 2013 7:07 am 
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'Building a nuclear reactor in an extremely flooding prone location...'
I think Cyril is a little bit hard on Tepco. Until recently most plate tectonics specialists thought that a 9 level earthquake was only likely in Chile or Alaska, as that is where plate movement is fastest. The Indonesian earthquake and tsunami, followed by the Japanese one , were a major wake up call for them, that slower moving plate boundaries can still, over time, build up enough tension to generate what they call a ' great earthquake '. If Tohoku had conformed to what was then the dominant view, the flood wall in place at Fukushima would been big enough to protect the plant.


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 Post subject: Re: Graphite Fire Risk
PostPosted: Mar 19, 2013 8:13 am 
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Sorry, before plate techtonics was even a speculation, Japan had records of tsunamis way in excess of those planned for. It was simply bad decision making, bad requirement setting.

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 Post subject: Re: Graphite Fire Risk
PostPosted: Mar 19, 2013 11:58 am 
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Kiteman has said it already, large tsunamis with runup heights >>10 meters occur all the time in Japan. There was one a century ago that had a bigger tsunami even than the 2011 one and killed more Japanese than the 2011 tsunami. I don't think I'm very harsh on Tepco/the designers/the regulators; it's pretty hard to miss recent events that kill tens of thousands of your people. This is true for the designers, the owner (tepco) and the regulators. It's their job to take care of the big risks. If one of them screws up, that's pretty bad already. All three closing their eyes is unforgiveable. Even in 1960 it was well known that the biggest weakness (biggest contribution to core damage frequency) for BWRs was station blackout. It was also known that the biggest cause of station blackout was common mode failure, and the biggest cause of common mode failure was flooding.


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