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PostPosted: Jul 20, 2014 8:15 am 
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Moltex Energy from Britain is one the latest molten salt reactor initiatives. I don't know exactly to file this under 'fluoride reactor design', 'chloride reactor design' or 'salt-cooled reactors', because their concept uses molten chloride fuel in fuel tubes, using a fluoride salt coolant.

Their SMSR (Simple Molten Salt Reactor) concept was presented at an Institute of Chemical Engineers conference in Manchester last April, it is a little scarce on details, but for those who are interested in this concept, some information about this concept can be found here:

http://www.icheme.org/events/conference ... DE65FB.pdf


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PostPosted: Jul 20, 2014 2:40 pm 
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Thanks Camiel

That is interesting and novel, but I don't like it.

My concerns include:
- Salt freeze in the steam generator section;
- What looks like an extremely high idling temperature when heat sink is removed;
- Neutron leakage top and bottom;
- Having a potential moderator like water in any location where it could possibly provide moderation in MCFR;
- The pressure energy available from the steam generators in proximity to a thin walled MSR core providing energy to damage containment and eject molten material and FP's.


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PostPosted: Jul 20, 2014 11:53 pm 
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Boiler tubes are built to stand steam pressures and could be used in nuclear power production too.
I prefer the idea from Transatomic of using reduced moderation and dual mode, thermal and fast. I would prefer water as moderator-coolant for simplicity and low cost, rather like a boiling water reactor. Only the water can be in tubes in case of an MSR.


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PostPosted: Jul 21, 2014 12:39 am 
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Boiling water and molten salt cannot coexist on opposite sides of a thin metal barrier unless you are using low mp salts similar to those used in solar heating of molten salt. The salt that they cite will freeze if boiling water is on the other side of that tube wall.


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PostPosted: Jul 21, 2014 11:49 am 
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I wonder if supercritical water could work?
You could also have two tubes as in the Candu/PHWR but interspace filled with lead, which could be in solid or liquid phase.


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PostPosted: Jul 21, 2014 1:30 pm 
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Keep it on the topic rather than your speculations as to how you can change it.


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PostPosted: Jul 22, 2014 3:13 am 
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I'm worried about the high chloride fuel salt melting point. Quoted as 750C. That means very high risk of fuel seggregation - Pu rich phase and U rich phase, possibly. This seems like a recipe for unstable power across the core unless the coolant salt input is above 750C which seems too much "next gen MSR" IMHO. In fact chloride fuel being liquid in tubes and cooled by Na/K/Zr fluoride seems next-gen as well.

Still, it seems like a major advantage in fuel inventory. Just having a concentrated fuel core means you can have a huge coolant salt pool inventory without excessive cost or fissile inventory. Like lead cooled reactors. The coolant salt should be a reasonable fast neutron reflector, though it will no doubt steal many neutrons in the process, looks much worse than lead in this respect. Not a real problem in neutron budget but it could be tough to get an acceptable void coefficient.

In terms of easy to build, that is doubtful. The cladding requirements are severe, chloride on one side, fluoride on the other, high fast flux, high temperature.

In terms of steam compatibility, mp of coolant salt of 385C seems adequately low, though marginally so. The coolant salt will have normal operating margins hundreds of degrees above the melting point because of the fuel salt 750C melting point. Anything under 700C inlet is probably a bad idea, that does mean coolant freezing isn't an issue, though I imagine that large a temp difference across a thing SG tube would be unacceptable on other counts, maybe Lindsay can comment on that (is there some internal thermal stress limit induced by temp differential/mm even when components are free to strain?). Still the coolant salt will activate badly with all that Zr and K and Na, so steam going in there is not acceptable in terms of containment performance and cost, even with zero fuel failure (which seems impossible based on LWR experience).


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PostPosted: Jul 22, 2014 1:37 pm 
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Too hot - agree

Cladding, probably short-lived due to neutron damage, but Mo or TZM could work quite well, but maintaining the right redox potential is crucial.

Depending on wall thickness the dT will be low, but it could potentially be set up as film boiling regime, where the steam/metal temp at that interface is so hot that the metal surface never becomes wetted by liquid water so the boiling heat flux may be low enough to allow a high wall temperature. Regarding thermal stresses, simple tubes can tolerate a lot, when we tie them together in bundles and have a range of temperatures across the bundle where different tubes see different temperatures, then we should expect trouble.

Steam generators inside the primary containment - I really don't like this part, the ability to drive highly radioactive material out of the system as a consequence of a tube failure is too great to ignore IMO. I'd also like to see at least two physical barriers between fast spectrum fuel salt and any potential moderator.


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PostPosted: Jul 22, 2014 2:39 pm 
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Having a look on the presentation I have the impression that the authors did not spend a lot of time and efforts on the engineering side of their concept.

One point I would like to highlight is that corrosion is one of the main challenges of an MCFR. The melting temperature of the fuel of is given to 750°C. That means the operating temperatures need to be at min. at 820°C. Max. temp. in the upper reactor center might reach more than 1000°C. Putting thin fuel tubes in the center of the reactor with max. temperatures, some free chlorine, sulphur and a high neutron flux might even overchallenge molybdenum alloys. Filling the reactor with thin tubes of such a difficult to manufacture material is not really simple from the manufacturing point of view.

The name "Simple Molten Salt Reactor" is a joke.


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PostPosted: Jul 22, 2014 3:02 pm 
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Lindsay wrote:
Cladding, probably short-lived due to neutron damage, but Mo or TZM could work quite well, but maintaining the right redox potential is crucial.


High fast flux exposure is know to cause Mo, TZM, and likely also all other high Mo alloys, to embrittle. Basically the brittle transition temperature creeps up. Maybe that's not a limiting factor if you must keep the darn thing from getting below 750C, but that's not a good thing either. Mo welds also are difficult and usually suffer insufficient impact toughness and other mechanical downsides.

Quote:
Steam generators inside the primary containment - I really don't like this part, the ability to drive highly radioactive material out of the system as a consequence of a tube failure is too great to ignore IMO. I'd also like to see at least two physical barriers between fast spectrum fuel salt and any potential moderator.


Agree its pretty silly. Even with intact fuel the source term of activated Na, K, Zr and F is unacceptable for volatization and steam reactions. And we can't ever be 100% sure of intact cladding.


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PostPosted: Jul 22, 2014 3:07 pm 
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HolgerNarrog wrote:
Having a look on the presentation I have the impression that the authors did not spend a lot of time and efforts on the engineering side of their concept.


Ya. The drawings aren't very good, I could have made them and I ain't no digital artist (more like the opposite).

Quote:
One point I would like to highlight is that corrosion is one of the main challenges of an MCFR. The melting temperature of the fuel of is given to 750°C. That means the operating temperatures need to be at min. at 820°C. Max. temp. in the upper reactor center might reach more than 1000°C. Putting thin fuel tubes in the center of the reactor with max. temperatures, some free chlorine, sulphur and a high neutron flux might even overchallenge molybdenum alloys. Filling the reactor with thin tubes of such a difficult to manufacture material is not really simple from the manufacturing point of view.


Agree. This concept needs many years of RD&D and probably decades more to convince the anal regulators. In terms of free chlorine though, they claim using (mostly) UCl3 rather than UCl4 would provide sufficiently reducing conditions for the life of the fuel. So that at least they've tackled. Sulphur should also be less aggressive when reducing though Mo likely does not like even elemental sulphur at >800C (need to check this).

Quote:
The name "Simple Molten Salt Reactor" is a joke.


Yeah, something like ESBWR can make a claim for simplicty, but the MSR is among the more complicated reactor types.

That message may be hard to convey on a molten salt reactor enthusiast forum!


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PostPosted: Jul 22, 2014 7:42 pm 
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Cross-fertilization of ideas between various reactor designs could lead to interest solutions (and related problems).
A molten salt (or lead) link as secondary coolant in sodium cooled fast reactors could provide substantial safety against sodium fires.
A water coolant-moderator in the tubes, like water tubes in a boiler, could really simplify the SMSR further. However, the tubes will have to handle, beside the thermal shock, the neutron flux, and salt or lead corrosion.
Engineering could provide the key to economical nuclear power.


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PostPosted: Jul 22, 2014 7:46 pm 
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In addition to the other concerns you have:
Chlorine in the neutron flux so either it must be enriched (and there went your argument about the expense of Li-7) or get get significant neutron loss and sulfur generation.

Second, the fissile is separated from the coolant. That works if the coolant is also your moderator. Otherwise, in a loss of coolant accident won't your reactivity will go up substantially?


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PostPosted: Jul 22, 2014 10:44 pm 
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MCFR's can tolerate natural Cl, enriched Cl37 is preferable, but not strictly necessary.

Regarding the reactivity question, I think that there are multiple factors in play. The coolant may provide some reflection, reducing neutron loss, it may also be an absorber. I think that it's hard to predict the net effect of a loss of coolant. Whatever the effect it could be quite major and would require careful modelling.


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PostPosted: Jul 24, 2014 4:37 pm 
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Thanks Camiel for this interesting presentation & concept.

It seems rather like a version of the German DFR (Dual Fluid Reactor), but without fuel salt circulation and salt coolant instead of lead.

It looks like they have done a nice job of simulating the convective heat transfer inside the fuel tubes - allowing for much larger diameter (45mm) compared to conduction-only heat transfer (fuel boiling with tube diameter <2mm). This reduces the number of fuel tubes to a practical level.

Cyril R wrote:
I'm worried about the high chloride fuel salt melting point. Quoted as 750C. That means very high risk of fuel seggregation - Pu rich phase and U rich phase, possibly.
This seems to be taken care of with the convective heat transfer inside the fuel tubes.
The convection only stops when the fuel salt solidifies, so not much chance for segregation, it would appear.

Lindsay wrote:
Boiling water and molten salt cannot coexist on opposite sides of a thin metal barrier unless you are using low mp salts similar to those used in solar heating of molten salt. The salt that they cite will freeze if boiling water is on the other side of that tube wall.
According to one of the slides, the cooling salt is indeed low mp:
10% NaF/48% KF/42% ZrF4
Melting Pt 385°C, Boiling Pt ~ 1150°C


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