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 Post subject: Zirconium fuel rod fire
PostPosted: Apr 24, 2014 4:20 pm 
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All of the official documents of spent fuel pool accidents state that zirconium will ignite and burn in air. This is based on theoretical reaction kinetics and the like.

This is odd, because experiments such as this one,

https://www.youtube.com/watch?v=x__2yWx9zGY

suggests this won't happen even at extreme temperature. What do we know really? We know the reaction with highly superheated steam occurs readily and the resultant hydrogen then burns readily as well. We know zirconium powder will burn in air. But what do we know of fuel assemblies burning in air when there's no steam around?


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PostPosted: Apr 27, 2014 2:41 pm 
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I think you already answered the question yourself. According to this safety data sheet, solid zirconium will not burn, but a high surface area form will.
http://www.angstromsciences.com/sites/d ... conium.pdf

In nuclear reactors no pure zirconium is used, but zircaloy.


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PostPosted: Apr 28, 2014 1:49 am 
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Burghard wrote:
I think you already answered the question yourself. According to this safety data sheet, solid zirconium will not burn, but a high surface area form will.
http://www.angstromsciences.com/sites/d ... conium.pdf

In nuclear reactors no pure zirconium is used, but zircaloy.


Thank you. What is the effect of zirconium not being pure? This should be beneficial right, since the additives are there to improve properties, among them oxidation resistance.

It is really strange that all the official and regulatory agencies talk about runaway oxidation in air at around 900-1200C. When a blowtorch (under oxidizing blue flame conditions no less) that is much hotter than that does not set off any runaway oxidation at all.


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PostPosted: Apr 28, 2014 7:01 am 
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From a very pratical point of view, you will never see zirconium without water (fresh fuel excepted) in a NPP. And with steam, it does one hell of a racket. I remember seeing a video of experiments conducted at Sandia National Labs (if I remember correctly) where they tested it, and thought they could extinguish it. They could not. (Proof, if any, that there was no actual fire in the Unit 4's SFP in Fukushima, otherwise there would not be much left).


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PostPosted: Apr 28, 2014 7:24 am 
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Perhaps it should be coated with more chemically inert Zirconium Nitride.
http://en.wikipedia.org/wiki/Zirconium_nitride


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PostPosted: Apr 28, 2014 8:20 am 
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Boris H wrote:
From a very pratical point of view, you will never see zirconium without water (fresh fuel excepted) in a NPP.


Actually there are some postulated accidents with spent fuel pools that lead to exactly that situation. One is a rapid drain of the spent fuel pool due to some earthquake or other external event. In that case when the fuel rods start to heat up they are already completely dry. Heatup would drive away any steam before it could react with the zircalloy. Reaction with air becomes very important for cases with fresh spent fuel, with the older spent fuel being able to air cool below the supposed air oxidation runaway reaction.

There are more likely scenarios such as station blackout with failure of makeup water. In that case the fuel will uncover at some point from boiloff loss, then it gets interesting. According to NRC analysis, steam cooling and the cold (100C) pool of residual water (heat sink) below the uncovered portion of the active fuel is sufficient down to about 10-30% of the active fuel length (depending on spent fuel age and type). Below that the fuel heats up but the residual water in the spent fuel pool blocks natural circulation flow. This can be worse than if there is no water at all in the pool, depending on fuel rack design. Air is assumed to react a lot with the zircalloy in this scenario with the fresh spent fuel, and lead to runaway oxidation reaction, but that clearly is now in question.


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PostPosted: Apr 28, 2014 8:26 am 
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Quote:
And with steam, it does one hell of a racket. I remember seeing a video of experiments conducted at Sandia National Labs (if I remember correctly) where they tested it, and thought they could extinguish it. They could not. (Proof, if any, that there was no actual fire in the Unit 4's SFP in Fukushima, otherwise there would not be much left).


Yes, we know the reaction with steam is appreciable at elevated temperatures. In the case of the spent fuel pool though, the pressure is low so the steam is cold and wet, and the decay heat is a lot less than in the reactor, plus there are significant heat losses to air (not a closed system like the reactor). In this environment, as long as significant steam is present there is just too much cooling for it to react with zircalloy. And when it dries out, clearly air is not going to ignite the zirconium up to at least 2000C, which is a lot more than what you'd ever get here (because of heat loss to the air and building).

There would be an exception here for the case of a lot of debris on top of the fuel assemblies - like a hydrogen explosion at Fukushima covering the spent fuel with debris. Then air cooling becomes poor and we might see very high temperatures for high density storage racks (though low density open racks would probably be ok).


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PostPosted: Apr 28, 2014 8:28 am 
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jagdish wrote:
Perhaps it should be coated with more chemically inert Zirconium Nitride.
http://en.wikipedia.org/wiki/Zirconium_nitride


Not sure, but I think the oxide is more stable than the nitride, so my guess is it would eventually react with air and possibly steam.


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PostPosted: Apr 29, 2014 3:54 am 
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Quote:
Uses[edit]
Zirconium nitride is a hard ceramic material similar to titanium nitride and is a cement-like refractory material. Thus it is used in refractories, cermets and laboratory crucibles. When applied using the physical vapor deposition coating process it is commonly used for coating medical devices, industrial parts (notably drill bits), automotive and aerospace components and other parts subject to high wear and corrosive environments.


Also Zirconium Nitride was suggested as a Hydrogen Peroxide fuel tank liner for rockets and aircraft
This is an extract from the Wikipedia that led to suggestion.


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PostPosted: May 01, 2014 3:00 am 
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I have used Zirconium in pyrotechnic compositions. When 100 to 300 mesh particle size Zirc. is mixed with an oxidizer like a percolate, or nitrate it will produce very hot sparks that will easily ignite hard to light compositions.

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PostPosted: May 01, 2014 6:15 am 
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Tomswift wrote:
I have used Zirconium in pyrotechnic compositions. When 100 to 300 mesh particle size Zirc. is mixed with an oxidizer like a percolate, or nitrate it will produce very hot sparks that will easily ignite hard to light compositions.


Yes, this is well known, any powdered metal, in fact any powdered oxidizable substance, can burn.

Just because iron powder can be used in fireworks, that doesn't mean a steel construction beam can burn.

It seems to me that all the official analysis (NRC, IAEA, etc.) simply assumes that a zirconium fire will start and spread. Without any realistic experimental evidence to support this... yet in a youtube video the University of California Berkeley people put a blowtorch (in oxidizing type flame) to zirconium cladding and it doesn't burn at all...


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