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PostPosted: Mar 17, 2010 2:45 pm 
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Yes, I'm new here, but I thought I would jump right in with a bit of a technical essay. I'm familiar with Kirk's blog and have a background in nuclear engineering, but I'm not an expert on the LFTR technology. I want to talk about something that always seemed to bother me.

Two Fluid Design
Summary: This picture shows a two-fluid LFTR but the heat exchanger loop is not shown.

I want to start this discussion from the design proposed for the two fluid design involving a fissile sale and a fertile Th salt.

Image

I'll echo as accurately I can the popular proposed design from the Google talks. I welcome any corrections to my understanding, this is all nothing more than my current mental impressions.

A separate Th blanket is necessary so that there is no need to separate the Th from the fission products, among other reasons. But for the design above, we have the added advantage that there is no particular need to remove the Pa continuously, as the large volume of the Th blanket will keep it at a low concentration so that neutrons won't often disturb it before it decays. Then the useful U-233 is continuously processed out.

In terms of desired attributes of a reactor, we want a low volume of fissile material. This is a cited reason for a thermal reactor, but a fast reactor could be much more sustainable. In terms of the core critical configuration, we have an engineering limitation on the volumetric heat rate; in particular the wall separating the reactor and blanket has a limitation of the flux passing through it. Since we are surrounding the core with the blanket, the only rational option is to increase the surface area of the reactor (I will offer another option later), thus a strong proposal is to elongate the critical configuration and employ basically a cylindrical design. Thus the core will have a set diameter but can be as long as the power output dictates.

The heat is produced in the fissile salt and both the fissile salt and the blanket salt are continuously processed. But in addition to the fissile salt being processed, it is pumped into the steam generator where it exchanges heat with the thermal cycle plant. Now here is where I feel like there should be room for improvement. The proposal is to have a fissile salt / coolant heat exchanger, but this requires a larger volume of the fissile salt.

"Cauldron" Design
Summary: Hyperion had an excellent design with a bubbling liquid fissile core that had tubes going through it to remove the heat

I've often pondered what is the best theoretical design for a nuclear reactor given different objectives. The original design of the Hyperion reactor provided some valuable reflection for me.

Image

This is what I would dub a "cauldron" design. Why? Because it is as nearly as possible a big blob of fissile material - close to a sphere. The fuel is also a liquid, which is advantageous for many reasons everyone here understands. Liquids don't loose integrity by melting down, there are no structural concerns other than the container itself, the volume of fissile material is minimized, and convection allows for better heat distribution and flux distribution. But there's a problem - if you make a blob of nuclear fuel, how do you remove the heat?! Note closely the rods coming out of the fuel pool in the image above. Those are cooling channels. They run cooling channels through a critical mass, thus the fissile mass never needs to go anywhere and doesn't need any processing until the standard reload procedure.

The other advantage of the cauldron design is that gases can be easily boiled off or dissolved in. Hyperion understood this and called for Hydrogen gas (if my memory serves me correctly) to be mixed in with the fissile salt as well as another sink that allowed material transfer as the temperature changed, giving a negative feedback that was the mother of all negative feedbacks. This is a very good design (that Hyperion itself abandoned), but like any design it's not perfect.

Mainly, because the coolant material is very different from the core material, we should expect a major event if one of the pipes failed. The pipes contain water/steam and the core contains liquid fuel. There is a risk of the water/steam leaking out into the core giving greater moderation or the fuel leaking into the pipe, causing a more compact shape and thus adding reactivity. I'm not sure which one increases power actually but I see this as a problem with the design because it has the potential to constitute a major contingency.

Fissile Salt + Fertile Salt cooling tubes?
Summary: Use a cauldron of fissile liquid fuel but run tubes through it containing fertile salt, exchange heat using the fertile salt, profit

Now I'll pick and choose my favorites to make a proposal. I like the cauldron design and I like the two-fluid design. Starting from the two-fluid LFTR design, why not have pipes containing the fertile salt permeating the fissile salt, allowing the heat exchanger to use the fertile salt?

The volume of the fertile salt can be large, thus it presents no added proliferation risk by having a large heat exchanger. The core itself, however, is a heat exchanger and is a more challenging environment. The barrier between the fissile salt and the fertile salt must withstand large neutron fluxes, and in this design, a temperature gradient as well. We would also require a large surface area between the two fluids (for the heat transfer).

However, given this design the reactor shape would logically be roughly spherical, like a cauldron. The fertile salt acts as a poison so adjusting the total volume as well as the ratio of fertile to fissile fluids would allow it to match whatever power requirement was demanded. Consider an infinitely large reactor. There will be a set ratio of the fertile to fissile materials giving k = k_infinity = 1. This infinite reactor would have pipes with the fertile material in the tubes and the fissile material outside the tubes with the engineered limit of neutron flux and heat flux going over the pipes.

The pressure of the fertile salt would have to be kept greater than that of the fissile salt so in the event of failure reactivity would decrease - just like in the current two-fluid proposal. There would still be a need for a blanket in order to use the leaked neutrons, so in addition to the pipes the fertile salt would be flowing through a downcomer in parallel.

The fissile salt would still have the "Chemical Separator" loop as shown in the first picture, but that would be the only loop and my understanding is that this loop can have a very low volume. This is fantastic because there is scarcely any more fissile material more than what's needed to have the core critical. A severe reactivity event could still occur if the fissile salt for some reason leaked into the fertile salt tubes, giving a tighter configuration with less poisons. But it's the same thing that can be said for the two-fluid design as it stands, and there would be less fissile material present at the site due to the heat exchanger using the benign fertile salt.

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Well I hope that kicks things off well. I just always thought to myself but it makes so much sense to use the fertile salt in the heat exchanger! Maybe there's something I haven't considered which kills the idea. Who knows? I thought this forum look cool, like people were actually discussing what the best possible design for a nuclear reactor would be. I would like to hear how this possibility fits into the picture.


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PostPosted: Mar 17, 2010 3:09 pm 
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ORNL did look at putting the heat exchanger inside the reactor and decided that it killed the breeding.
Your idea may be better in that the fluid being fertile would mean that absorptions in the "secondary" fluid would not count as a lost neutron. Still the structure will steal some of your neutrons.

I am worried about the wall lifetime and any true two fluid design where the fissile has no fertile mixed in with it means you have to pass 1/2 of all the neutrons through the wall. That is why I prefer the 1 1/2 fluid design where 90+% of the neutrons stay in the core and only 10% or so pass through the wall. But it does mean I take on the task of a more difficult fuel processing regime.

Using thorium of for the secondary salt means we will have some of it converted to u233. It also means some of the u233 in the secondary salt will get fissioned so we will have fission products in the secondary salt. That concerns me as then we either
a) have to add a third HX with its cost and drop in high temperature for the turbines OR
b) have radioactive material in the second HX so that there is only 2 barriers not 3 between radioactivity and release to the environment.


I could imagine some possible improvements (from my perspective) in your idea:
a) by adding fertile to the fuel salt and placing the HX piping on the outside of the core
This will dramatically reduce the neutron exposure of your piping.
b) using a pump to circulate the fuel within the core
Just a guess that this will mix up the thermal gradients better. The offgas system would want roughly 10% of the pump volume that the HX needs so perhaps just the offgas system pumping action will induce sufficient circulation of the fuel salt for the HX.

Doesn't solve for the radioactive material just a thin walled HX away from the turbine working fluid and then another thin walled HX away from the biosphere.


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PostPosted: Mar 17, 2010 3:59 pm 
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I'll address that post in some detail here.

The structural material will eat more neutrons, yes. The important discussion regards the surface area (SA) of the boundary between the fissile and fertile salts. Consider an ordinary heat exchanger. In order to have a heat flux that is reasonable, we will have to have a very large SA, and that will consume a lot of neutrons. The flux will be lower in this design because the gross # of neutrons crossing the barrier would still need to be the same (though slightly more due to those that enter and then escape). This neutron to heat flux issue will continue to plague us.

Doesn't the current design call for the active fissile salt to be used in the Heat Exchanger (HX)? That would mean that the thermal cycle would be one barrier away from the delayed neutrons (but not the FP). Even that is considered acceptable as things stand, so I don't think it would be a problem to make the HX primary side less active. Consider a BWR. The thermal cycle fluid itself is radioactive and they manage somehow.

Also, I'm really uneasy about the problems with flux shaping. In the ordinary cylindrical design, you simply have a large current going over that barrier, but if you increase the SA to satisfy the heat removal requirement, you must have a lower neutron current over the barrier. This is a problem since the only way to do that is to increase the ratio of flux to current on the pipe. This means you have to make the tubes of the fertile salt less self shielding. Qualitatively, you could see that this was already necessary since the path length of a fast neutron is a set quantity (that we know to be a foot +) and the pipes will probably have to be much smaller than that dimension (way smaller). So we've established that the # of neutrons passing over the barrier must be much greater than the gross current going over the pipes for the thermal requirements to be met, and your 1/2 figure represents the current needed.

That means that the pipes must withstand several times the total neutron production rate. But since the SA is greater, this might be doable with the pipe materials. But that doesn't means the reactor can take the loss of neutrons.

Interesting thought experiment.


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PostPosted: Mar 24, 2010 6:55 am 
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Well, that certainly is a cool idea, welcome to the board! (I'm not a nuclear engineer)


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PostPosted: Apr 05, 2010 9:44 pm 
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The heat exchanger is a relatively sensitive component; the most sensitive component in the reactor complex. The fewer irritants that go through it, the longer it will last.

Ideally, if only pure lithium is sent through the heat exchanger, that component would last a very long time. When you send U233 through the heat exchanger, fast neutrons from fission will erode the thin walls through atomic displacement causing premature heat exchanger failure.

This is one reason I like the Lithium Homogeneous Reactor (LHR) concept. The slurry can be removed from the lithium flow into the heat exchanger by a centrifugal separator. The U233 can be sent directly back into the core. In this way, U233 does not transit through the heat exchanger at any time. Redbat has stated that U233 fissions very slowly after it captures a neutron; up to one minute. That’s a very long time. It is possible when U233 is sent through the heat exchanger substantial fission activity will occur in those thin sensitive heat transfer channels; not good.

IMHO, this is another advantage that the LHR has over the Lftr; you can control what goes through the heat exchanger.

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PostPosted: Apr 06, 2010 7:20 am 
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Certainly the blanket then would be better for the heat exchanger than the core salt...


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PostPosted: Apr 06, 2010 12:13 pm 
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You can think of the Lithium Homogeneous Reactor (LHR) as a big Hyperion reactor with lithium added.


This gives the Hyperion reactor design all the features and benefits of the Lftr while still retaining it own feature set. Lithium use also mitigates many of the Hyperion reactor flaws.


By adding lithium, you increase the Hyperion operating temperature from 500C to 1000C. Uranium hydride decomposition temperature limits the Hyperion reactor operating temperature to 500C.


Lithium deuteride can get you to 1000C because lithium holds on to it hydrogen far better than Uranium does.


Lithium will give a more responsive (faster) negative void control response than Hyperion.


Like the Lftr, Lithium provides excellent heat removal capability via an external heat exchanger. You need this in a large reactor.


I believe that both a lithium heat pipe and internal heat exchangers are also possible to configure.


So a LHR can have all three types of heat exchangers going if necessary. Hyperion is limited in size because it can only use potassium heat pipes to transfer its heat to the outside.


Lithium used in heat pipes or internal heat exchangers won’t steal many neutrons.

If an internal heat exchanger leaked lithium into a lithium core, it would not have much impact. The slurry would not enter into the heat exchanger; it is too big to fit through a newly developed crack.


Using lithium gives the Hyperion design the ability to reprocess its waste continuously on line like the Lftr does. And this reprocessing is more far more efficient than the Lftr because of the fission recoil separation effect (FRSE). This comes from the progenitor The Aqueous Homogeneous Reactor(AHR).



Of course, the LHR uses thorium-232 instead of U-238. It is a thermal spectrum reactor which is very easy on the reactor structure. And it can breed fuel like the Lftr.



I must confess that I only see good things from the LHR. I really need help with the downsides. Can anyone think of any?

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