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PostPosted: Nov 04, 2014 3:57 pm 
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Thanks a lot Cyril.

In short if the cost of the fuel cycle with liquid fuel is far lower than with solid fuel, as Dr Leblanc said, and if it is a decisive economic advantage, so let's go with Molten Salt Fueled Reactors. If it is not possible I would prefer the molten salt cooled reactors.


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PostPosted: Nov 05, 2014 11:45 pm 
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Cyril R wrote:
Quote:
I know that criticality excursions and core meltdowns are managable but maybe we can go back at your first idea and imagine designs sufficiently moderated to avoid or diminish these issues, that's may be good for convincing the public, the regulators and for reliability. But I think we will need flibe.

I think NaF-BeF2 is good enough if Li7 is unavailable. Be and F are good enough and Na is not a bad poison for epithermal/fastish spectrum (no bad resonances).
Quote:
The idea is to have high conversion ratio (possibly isobreeding) with a solid fuel reactor with not very high power density, a good doppler and negative void coefficients and no very hard spectrum ( for decreasing the issues raised before with fast spectrum). It looks like a PWR but with high conversion ratio, low pressure and high temperature.

Yes, well said, it is basically the molten salt version of RBWR.

The molten salt will not be generally used for boiling and it is best to call it Reduced Moderation Water Reactor. RMWR with a low pressure primary coolant is, I agree, a great idea. Molten salts other than FLiBe will be poor moderators and high boiling organic moderators, if stable compounds can be found, could be used.


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PostPosted: Nov 11, 2014 4:12 pm 
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Hi Cryl,
I read some papers of the posibility of using fisile salts (molten salts) as fuel rods in designs similar to CANDU reactors. The paper I read conclude that the research effort to design such a reactor v/s incease breeding is not justified economically.
The idea to use solid fuel is to form dense fisile material and the molten salts does not achieve that.


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PostPosted: May 27, 2015 3:18 pm 
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I found a little doc which speaks about the subject of this thread.

Some people in CEA did a study about replacing the sodium in the Sodium Fast Reactor by a clean molten fluoride salt. It seems that they kept the steel cladding, the UO2-PuO2 oxide fuel and the classic homogeneous core ( the geometry is inspired by an old abandonned project, the EFR ( for European Fast Reactor )).

The paper is in french and was made in 2007.

http://www.gedeon.prd.fr/ATELIERS/29_30_mai_2007/exposes/THEVENOT_Sels_liquides_RNR_29.pdf

With the fluoride salts the multiplication factor is lower, the void coefficient is higher and the reactivity drop is also bigger. So the sodium metal is better than fluoride salts in terms of neutronics for an uranium fast reactor.

They selected the salt NaF-KF-ZrF4 (the eutectic I think) for its relative low melting point (385 °C) and good neutronic performance in this fast spectrum and they studied several configurations. The maximum temperature of the salt is beetween 545°C and 600°C (depending of the configuration).

Their conclusion was that breeding is still possible with a molten salt and that they didn't see major issues for this concept, however the voiding reactivity is high but it is maybe not a big problem with this kind of reactor with an high boiling point coolant.

Sadly it seems that they didn't went a lot further ( I didn't find other studies ) I guess that the concepts on paper look weak against the experience gained on real sodium reactors.

I guess that it is possible to decrease the void reactivity by using the heterogeneous core that they are planning to use for ASTRID ( in ASTRID the global void coefficient is negative ).

It is maybe also possible to decrease the amount of salt in the core by using a more conductive fuel ( like discussed previously in this thread ) although this idea has some limitations since we need to ensure a sufficient flow on natural convection to remove the decay heat and the viscosity of the salt can be a problem.

The use of a Th232-U233 fuel would also maybe decrease the void reactivity since the number of neutrons emitted per neutron absorbed is nearly constant for U233 in the 10^4 eV - 10^6 eV range while it increases rapidly for Pu239 and U235 ( see this paper : http://thoriumenergyconference.org/sites/default/files/pdf/Fast%20Reactor%20Physics%20-%20K%20Mikityuk%20-%20PSI%20-%20ThEC13.pdf).

I think that it is an interesting concept.


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PostPosted: May 28, 2015 4:19 am 
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The NaF-ZrF4 could used as a secondary coolant with very little effect on nutronics. Such use could develop confidence in handling of the salts in commercial reactors.


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PostPosted: Aug 15, 2015 9:28 pm 
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I found an other paper which speaks about a molten salt cooled reactor with traditional fuel rods.

http://www.uxc.com/smr/Library%5CDesign%20Specific/ENHS/Presentations/2005%20-%20Molten%20Salt%20Cooled%20ENHS-Like%20Reactors.pdf

It is a small fast reactor with the primary and secondary coolant which are molten salts and the third loop is the working fluid (steam).

The primary and secondary loops run entirely on natural circulation.

It is the molten salt version of the ENHS (Encapsulated Nuclear Heat Source). The original ENHS concept is cooled by lead-bismuth eutectic.

http://www.uxc.com/smr/Library%5CDesign%20Specific/ENHS/Papers/2001%20-%20The%20ENHS%20for%20Proliferation-Resistant%20Low-Waste%20Nuclear%20Energy.pdf
http://www.uxc.com/smr/Library%5CDesign%20Specific/ENHS/Presentations/2009%20-%20Improvement%20in%20the%20ENHS%20Reactor%20Design%20and%20Fuel%20Cycle.pdf


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PostPosted: Aug 16, 2015 7:31 am 
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Water coolant can double as moderator in reactors. The salt coolant comes in only in high temperature reactors including the unmoderated reactors. If found too corroding in the reactor vessel, it could be used as secondary coolant, minimizing the risk of sodium contact with air or water. You have to balance the risks of sodium fire or salt corrosion. Nobody seems to have done it so far. It could be done for studies on salt handling in experimental fast reactors. The salt based fuels could be next step.


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PostPosted: Aug 16, 2015 2:20 pm 
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Quote:
Water coolant can double as moderator in reactors. The salt coolant comes in only in high temperature reactors including the unmoderated reactors. If found too corroding in the reactor vessel, it could be used as secondary coolant, minimizing the risk of sodium contact with air or water. You have to balance the risks of sodium fire or salt corrosion. Nobody seems to have done it so far. It could be done for studies on salt handling in experimental fast reactors. The salt based fuels could be next step.


Corrosion seems not a big problem with fuel salts so it is not a problem with clean salts.

For graphite I think that it is the best moderator for MSRs (cooled or fueled): it is fully compatible with the salts, it doesn't melt or boil, it doesn't have to be cladded, it doesn't have to be cooled (it depends of the design of course), and the moderating ratio is very good.

The only big problem is that you have to replace the graphite it if the power density is too high. For salt cooled reactors it should be possible to do it on site.


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PostPosted: Aug 18, 2015 2:20 am 
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fab wrote:
For graphite I think that it is the best moderator for MSRs (cooled or fueled): it is fully compatible with the salts, it doesn't melt or boil, it doesn't have to be cladded, it doesn't have to be cooled (it depends of the design of course), and the moderating ratio is very good.

The only big problem is that you have to replace the graphite it if the power density is too high. For salt cooled reactors it should be possible to do it on site.

That is only one of the problems. It also adds tremendously to core volume and building costs. It is best to go for unmoderated or water moderated designs.


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PostPosted: Aug 18, 2015 10:12 am 
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If you use water you will have to implement several cooling systems of safety grade and hydrogen recombiners, and cleaning systems. And you add volatile matter in your reactor which is not good for safety, and water is not fully compatible with the salts (precipitation of actinides, HF production ?). And you will have to design your containment in case of a lot of water boils. And you also loose in neutronics (light water absorbs more neutrons, plus the absorptions in the cladding).

Graphite replacement is a problem for fuel salt MSRs, water can maybe have an advantage here.

But for now I still prefer graphite.

Fast reactors have their own issues (high starting actinide load and high damages to the materials for fuel salt MSRs).


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PostPosted: Aug 18, 2015 11:34 am 
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Or we could just build a core with a low enough power density that replacing the graphite is not a major issue.

Sixty year graphite core or what not.


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PostPosted: Aug 19, 2015 4:28 am 
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Quote:
Or we could just build a core with a low enough power density that replacing the graphite is not a major issue.

Sixty year graphite core or what not.


This is a solution. You will have a big core. It will not be a SMR but a large and powerful reactor like the current LWRs. It is a trade-off between simplicity and compactness, like the choice between graphite and water for the moderator.

There are advantages and drawbacks, the choice depends of the qualities you favor.


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PostPosted: Aug 21, 2015 5:30 am 
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Candu tubes are a current technology for separating hot and cold fluids. Moderator can be in double tubes and cooled like the
main drum water in candu. It will be kept liquid at moderate temperature.
A few unmoderated fast reactors are already working and one in India nearing commissioning. Molten salt secondary cooling will be just an evolutionary step. It will further reduce the risk of sodium fires. The best place to start molten salt cooling.


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PostPosted: Aug 23, 2015 7:18 am 
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Quote:
Candu tubes are a current technology for separating hot and cold fluids. Moderator can be in double tubes and cooled like the
main drum water in candu. It will be kept liquid at moderate temperature.


And you will need pumps, and cooling systems, a cleaning system, a system to recombine the dissociated water molecules, the I&C associated to these systems, and maybe valves etc.

And you have to deal with volatile matter in the core.

Also a CANDU has water in both sides, so we can use zirconium alloys. Here zirconium is dissolved by the salts so you have to find something that resists to the corrosion of the salts, resists to the neutron flux and doesn't absorb too much neutrons. Same problem than Transatomic (maybe silicon carbide can work but we still don't know, if I am not mistaken).

If graphite replacement (or reactor replacement) is too difficult or uneconomic, maybe we should consider water otherwise I personnaly prefer graphite for now.


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PostPosted: Aug 30, 2015 4:48 am 
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http://www.moltexenergy.com/thessr/thes ... tordesign/
The Moltex MSR design found favourable by the UK is a fast MS reactor. I think that coolant and fuel tubes being parallel to each other, either vertical or horizontal will be better. 1 generally agree with their view.


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