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PostPosted: Sep 11, 2017 9:31 am 

Joined: Feb 14, 2017 12:15 pm
Posts: 4
Hi, now I've been wondering about this for a while , from all the articles i have read I gather some of the biggest molten salt reactor problems are those that are associated with materials strength and durability, things like high heat, strong neutron flux and corrosive substances.
This I assume is/was the reason why most of the initial reactor designs like the graphite tube design failed because of the complexity of the reactor itself which meant that heat expansion and other deformations occurred.

Now let me explain my take on this.I gather the best form and shape of a nuclear fuel under criticality is that of a sphere due to the even distribution of neutron flux etc. Since we are talking about liquids not solids here we have the freedom of shape more than in a solid fueled reactor, but we still need some protection for the reactor vessel and all the apparatus that supports it from the heat and neutron flux.

I believe similar ideas have been proposed before but let me ask, why cant we simply use a large vessel similar to a kettle or a pot , spherical in shape and fill say half of it with some good thermal conductor but also good neutron reflector moderator liquid - some sort of metal etc (help me out here because materials science is not my strongest part) then put in the nuclear fuel itself and fill the top with some inert gas or otherwise then achieve criticality and ideally the hot and critical core would float atop the neutron moderator/heat conducting fluid.
Instead of using the fluid in which the critical core floats as a heat exchanger fluid we could design all this in such a way that it doesn't circulate but instead conducts the heat to the metallic walls of the reactor outer vessel which then would have fluid passages and would largely be like a big radiator/heat exchanger from outside and then we could use this fluid directly to power turbines and depending on the temperature we could choose whether to go with standard steam cycle or have the vessel at higher temps and use gas as the circulating medium etc.

the main idea is simple, instead of having problems with high neutron flux combined with high temps and materials strength we could make the reactor vessel a simple design with simply high thermal strength and then use a fluid or a solution which would both serve as the neutron moderator/reflector and also as the heatsink between the very hot molten core and the vessel, the core itself could be molten and ideally float inside the moderator/reflector fluid.
the idea is such that the core fuel solution should not mix with the moderator/reflector/heat exchanger solution and ideally should be enclosed from all sides by this solution.
But because I doubt there are ny such two fluids in this world which can stabilize one inside the other without either flowing up or sinking completely down then i'm adding and option that the core fluid could flow on top of the heat exchanger/moderator solution. on top of all this we could add an inert gas and some equipment pipe leads by which we could drain and refill the core fluid in order to purify it.

Also a question could in sch a molten solution the different elements that form with time in the fission process reside at different heights inside the fluid due to their different nuclear masses and hence weight? this would make things easier as we could know at which height to search for certain byproducts of the reaction that are poisonous to the fission reactor and need to be cleared out.

i've added a small and quick drawing of the idea.
hope to hear some comments, thanks.

reactor idea.png
reactor idea.png [ 50.67 KiB | Viewed 1806 times ]
PostPosted: Sep 12, 2017 12:16 am 

Joined: May 05, 2010 1:14 am
Posts: 129
If your core was liquid, and less dense than the moderator fluid, it wouldn't float in the middle, it would spread out in a layer from one side of the vessel to the other, and so contact the walls. This means you'd have to make the walls corrosion-proof anyway. This sketch actually brings to mind one of the proposals for a fusion reactor, where the liquid surrounding the active volume is molten lead/lithium metal, and the whole shebang is spun at high speed so that there is more or less a vacuum in the centre, and the molten metal shields the wall from high energy neutrons. ... y-schemes/
There is also a German group who want to build a fast neutron reactor using molten uranium chloride salt, with molten lead as the coolant. Lead is heavy enough to sink beneath a molten salt, but it is quite corrosive, and would react with the salt, so they would need a lot of very expensive molybdenum piping to keep the two separate.
Other than molten salts and metals, there aren't many liquids you can put in a reactor, except, of course, water. Carbon is a good moderator as graphite, and hydrogen as water or heavy water; the two can be combined into hydrocarbons with suitable boiling points and reasonable coolant behaviour, but under a heavy neutron flux the stuff disassociates and gunks up the plumbing. ... ed_reactor

PostPosted: Sep 12, 2017 1:15 am 

Joined: Feb 14, 2017 12:15 pm
Posts: 4
Well the idea is to have as little moving parts and plumbing and seals as possible, in an environment where you have high temps and large neutron fluxes , simplicity is what you want I believe.
Yes I realize myself that this would only work if the two liquids or materials aka the fissile core and the surrounding medium would not interact with one another so as to not mix otherwise it would ruin the critical mass and the fission reaction.

Technically I think if the outside medium layer can be kept at the walls and is a good heat conductor one doesn't need any pumps , one could just use passive heat conduction from the fissile core to the reactor vessel and the on the vessel surface have secondary loops which dont interact with the primary and can work easily without the risk of contamination.
the fissile core sphere is smaller than the outside vessel sphere so say every cm2 on the fissile core surface translates into some 10cm2 on the larger outer vessel surface.
also i think as long as the moderator material doesn't disintegrate in such an approach we can run the fissile core at high temps as long as it is sealed in that inert environment. Unlike in a typical molten salt approach where the salt/fissile solution has a limited temperature due to it being passed around metal pipes and pumps which can only take so much.

creating a vortex in the moderator fluid would likely require some large and heavy blades and motor at the bottom of such a reactor which creates sealing problems I believe etc, how about having the moderator fluid slightly magnetic and making it stick to the outside of the sphere using magnets? but then again im not sure how magnetic is a fissile liquid salt core , technically at such high molten temps it shouldn't be atleast not as magnetic as the lower temp moderator fluid.

PostPosted: Sep 21, 2017 6:30 pm 

Joined: Dec 14, 2006 1:01 pm
Posts: 379
For a very small reactor, spin the vessel.

For a medium to large reactor, use a pump with its output jetted to spin the moderator/reflector. Put the drain in the bottom. The pump has to be there anyway to control the moderator temperature by running it through a radiator or heat exchanger. If the moderator gets too hot or cold, the neutronics will be messed-up by doppler broadening.

One worry is that passive safety is trickier. If a freeze plug is used, the moderator would drain with the fuel and the safety equipment might have critical configurations. (In the MSRE, the moderator was graphite logs that stayed in the reactor vessel.)

Maybe there could be two freeze plugs, a high one that melts fast for the fuel salt, and a low one (or none) that melts slowly for the moderator?

Another issue is that its criticality could change if it's in a vehicle (i.e. a ship) being shaken or stirred.

The best thing is that the moderator doesn't have to be remanufactured. It can just be purified in a continuous process. The mdoerator replacement is a maintenance issue in the medium-neutron-speed MSRs.

The moderator/reflector has to be something heavier than salt, immiscible, and a neutron reflector. (I can't think of anything. Lead? But I think it would corrode.)

Maybe relayer the system? The vessel's material can be some well-studied nickel alloy like Hastelloy N. (Hastelloy N handles FLiBe salt just fine, BTW.) Have an organic oil in the center (e.g. deuterated benzene) as a moderator, the fuel salt on the bottom, and make the vessel the reflector.

When the pump spinning the thing has a power failure, the oil and salt separate into layers with a smaller interface. So, the thing becomes noncritical if unpowered. The freeze plug can be in the bottom, and drain the salt into a heat-losing tank to handle the residual reactivity.

Another configuration might be to bubble the moderator oil through the salt. This could make really big reactors without a lot of expensive plumbing or a need to replace the solid moderators.

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