Energy From Thorium Discussion Forum

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PostPosted: Jul 15, 2012 7:43 am 
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I have come here after watching David LeBlanc's DMSR talks and reading his papers.

It seems that even if you have a long lifetime for the salt and the vessel materials, you have to replace the graphite several times during the reactor life because of neutron damage. Surely it would be better not to have to do this?

I'm new to this and this may be a silly question, but are there any more durable moderator materials? I'm wondering about beryllium (metallic? oxide?), but I can't find any references on how long solid moderator materials in general last or the theory of how to calculate this.


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PostPosted: Jul 15, 2012 9:39 am 
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Yes it would be nice not to have to replace the graphite.
The French TMSR uses no graphite, the only moderation comes from the fuel salt itself.
As a result, the neutron spectrum is rather fast. 232Th/233U works nicely in that it has around the same breeding performance regardless of spectrum (not quite as good in epithermal, and slightly better in fast than in thermal). The other common fertile/fissile combination (238U/239Pu) only works well for breeding in the fast spectrum. In a thermal spectrum it works as a converter (needs to get make up fuel periodically).

We should keep the waste flow in perspective. The waste graphite from a gigawatt plant would be around ten tonnes per year. Sounds like a lot. But remember that plant serves around a million people so this is around ten grams per person per year. The graphite will have radioactive particles embedded in it and a tiny bit of the carbon will be converted to carbon-14. Carbon-14 is also generated naturally and is part of the atmosphere we breathe. Science uses the amount of carbon-14 present to indicate how long ago a living organism died (since typically it stops absorbing carbon-14 from the atmosphere or the food it eats once it has died). However, the standards for radioactive releases from nuclear power plants is very strict (so strict that coal power plants release many times as much radioactivity as do nuclear plants). So, the graphite (all ten grams or about 1/3 of an ounce per person) must be buried deep underground. It will not be radioactive enough to create a heat or radiation problem so burial should not be difficult. Eventually, society might deem it worthwhile to find a way to recycle the graphite back into the reactor rather than bury it. This will cost more and likely entails more risk of radiation release than burial but as a society we set some pretty tough goals for nuclear power.

Other moderators are tough to find. The lighter the element the more effective it is at slowing down the neutrons. That is why hydrogen (the lightest element) is currently used as the moderator in today's light water reactors (in the form of H2O). The other factor to consider in choosing a moderator is how easily it absorbs a neutron. No chemical link can survive a hit by the particles flying out of a nuclear fission at near light speed. The thing that makes the salt survive so well is that the ionic bonds get broken but then reform very quickly so no lasting damage is done. I don't know of any molecule with hydrogen that shares this property. The next element, helium is a noble gas and doesn't form bonds with anyone. It is a gas so the molecule density is very low - even under high pressure so that doesn't work for a molten salt reactor. The next two elements are lithium and beryllium - which are used in the molten salt reactors.


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PostPosted: Jul 15, 2012 12:18 pm 
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So is graphite definitely the most durable solid moderator?


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PostPosted: Jul 15, 2012 2:56 pm 
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It is said that the lifetime of graphite as a moderator is limited due to swelling. But what exactly is the problem with that? If the graphite does not serve any structural or plumbing purpose I would say, let is swell. Moderation could be done by just submerging pebbles or rods in the salt. What would the lifetime be in this case?


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PostPosted: Jul 15, 2012 7:56 pm 
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swelling beyond the original dimensions is tough to engineer for (to simultaneously keep the graphite from moving around despite tonnes of fluid per second flowing past while the graphite itself changes size). In addition, eventually the integrity of the graphite comes into question. We don't want to go anywhere near when pieces of the graphite flake off and clog the heat exchanger. Using graphite rods or pebbles won't change their lifetimes much (you do gain a bit because you can move them around to even out the neutron exposure). It comes down to hassle in changing the graphite but I'm not sure the easiest isn't the long graphite logs.

PR wise having tonnes of waste per year is unattractive. Technically, I think it is reasonable to recycle the graphite (the carbon isn't spoiled by being in the neutron field) but it does require development and will be more expensive.


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PostPosted: Jul 15, 2012 8:11 pm 
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Why not just re-sinter them?

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PostPosted: Jul 15, 2012 9:41 pm 
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That is the approach suggested by ORNL. But I suspect this will be more expensive than working with fresh carbon where you don't have to deal with radiation.


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PostPosted: Jul 15, 2012 11:15 pm 
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So, how hot will this graphite be? Is it high level waste or low level waste? If its low level waste then whats the problem?


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PostPosted: Jul 16, 2012 12:45 am 
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The ARE used BeO in a molten-salt reactor.


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PostPosted: Jul 16, 2012 4:57 am 
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Graphite is durable, but a potential graphite fire might not go down well in the public perception. I believe there were problems with the graphite in the Windscale and Chernobyl incidents.


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PostPosted: Jul 16, 2012 7:21 am 
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camiel wrote:
Graphite is durable, but a potential graphite fire might not go down well in the public perception. I believe there were problems with the graphite in the Windscale and Chernobyl incidents.
Indeed - there is a similar perception problem with sodium coolant in SFRs.
Technically, both can be managed.


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PostPosted: Jul 16, 2012 9:15 am 
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camiel wrote:
Graphite is durable, but a potential graphite fire might not go down well in the public perception. I believe there were problems with the graphite in the Windscale and Chernobyl incidents.

Windscale was indeed a graphite fire. It is well understood. When operated at low temperatures the radiation can induce strains in the graphite. A strain is stored energy. Heating the graphite will release the strain releasing the stored energy. If too much stored energy gets released at once you have a major heat source to feed the fire. This is known as the Wigner Effect. If you have graphite in a cool reactor then you need to periodically heat the graphite to release the stored energy before it builds up too much. In graphite this occurs at 250°C.

However, the stored energy is not a problem for molten salt reactor as it operates at 650C. It simply can not build up this kind of energy.

Nuclear graphite is tough to get to burn. You can see various you tube videos where they are using a blow torch to try to get it to light. It gets hot enough to glow red but won't burn.

(Sodium on the other hand makes for pretty dramatic demonstrations).


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PostPosted: Jul 16, 2012 9:25 am 
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jaro wrote:
camiel wrote:
Graphite is durable, but a potential graphite fire might not go down well in the public perception. I believe there were problems with the graphite in the Windscale and Chernobyl incidents.
Indeed - there is a similar perception problem with sodium coolant in SFRs.
Technically, both can be managed.



These aren't quite on the same scale, though. You can put a blowtorch to nuclear grade graphite and nothing will happen. Try that with sodium. Yes, graphite will oxidise under more potent oxidizers (most likely steam/water). But nothing as violent as the same reaction of sodium with said oxidizer. Chernobyl was a water cooled graphite moderated reactor. So you have the chemical potential right there.

Graphite will not void until extreme temperatures, unlike sodium. Graphite won't go anywhere because it's solid. Sodium can leak and go somewhere you don't want it to go. So you need more engineered provisions to deal with that.

I would assert that dealing with graphite in a thermal reactor is much easier (cheaper) than dealing with sodium in a fast reactor. But I would like to see a more detailed breakdown of the cost of the engineered safety measures to deal with sodium in such a fast reactor (extra loop, double walled steam generators, sodium isolation valves, etc.). This would also be important, to a large extent, for a molten salt reactor. While the molten salt reactor has chemical inertness, even with graphite, the entire primary loop is very radioactive, and the liquid form makes it more easily leaked into the secondary loop.


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PostPosted: Jul 16, 2012 3:01 pm 
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The graphite should not be a real safety concern in a molten salt reactor, I think (although the replacement of the graphite after a couple of years can be undesirable, from a cost perspective). During operation the liquid salt is in the core, and air and water are not. If the core is drained, the liquid salt, with the fission products, goes from the core to the drain, removing the risk that a graphite fire can be triggered. Are any other scenarios possible, which would warrant safety concerns regarding graphite ?


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PostPosted: Jul 16, 2012 3:37 pm 
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The concern with graphite is waste. Cost while not zero should be minimal. I don't see safety as a concern at all.


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