Energy From Thorium Discussion Forum

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PostPosted: Feb 23, 2011 5:24 pm 
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Heat exchanger walls are so inconvenient. They plate-out materials, they interfere with efficiency, they corrode and leak (the major mid-life service on LWRs is steam generator replacement and repair.) So, let's get rid of them.

See the attached illustration.

I've wanted to use a bubble plenum for a long time, but the problem was that it requires the reactor to be pressurized. This brings back many problems and expense. But what if only the bubble plenum were pressurized? I think this is actually possible with a pair of constant volume pumps, geared together with a differential.

The differential between the CV pumps would control the plenum's volume of liquid. The differential's torque would be actively controlled with a negative feedback loop from the fluid level in the plenum. The fluid level could be sensed by ultrasound, temperature, or IR. Or all of the above. The plenum's pressure would be controlled by the injection pump.

The major wear item would be the bearings on the constant-volume pumps, but these could be fluidic, so very reliable, and outside the salt loop, never wetted with salt. So repair and refurbishment should be easier.

If bubbles are entrained, they reduce the efficiency of the reactor, so no reactivity problems. (Turn down the pump!) At temperature, the gas and salt are both very fluid, and should cleanly separate, preventing radioactive turbines. So, multiple heat-exchanger stages seem unnecessary.

Just for care's sake, centrifugal separators on the exits are cheap, with no moving parts. The salt would run back into the plenum. The salt exit is the logical place to put a cold trap. The Xe would be entrained in the turbine gas. Maybe it can be extracted by cryodistillation.


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BubbleHx.PNG
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Last edited by rgvandewalker on Feb 24, 2011 4:41 pm, edited 2 times in total.
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PostPosted: Feb 23, 2011 6:38 pm 
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We have a massive heat flow so first thing to check is if you can move enough gas to hold the heat. I don't think bubbles conveys the right mental image for what would happen if you moved enough gas to transport the heat. So calculate the volume of gas per second required to move the heat, then its velocity going through the salt. I think a more accurate picture might be hurricane force winds going up and high pressure salt jets squirting down.

Second, there is tritium in the fuel salt and we need to move from a production of 2400 Curies/day to a leak to the biosphere << 10 Curies/day. I was figuring we would trap most of the tritium in the off gas system. Here, you are going to extract it into the helium gas right away. It would be great for removing fission products from the reactor very promptly but it puts the fission products one step closer to the turbines.


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PostPosted: Feb 23, 2011 7:27 pm 
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Back-of-envelope I get 175 m^3/sec of helium at 200 bar for a 2 Gw(th) reactor. Since you get at most 10 m^3 of salt out of core, the salt needs to encounter 20 times its own volume of gas each second.

He heat capacity =(3/2) * gas constant (joules/mole-K)

Volume/sec = (thermal power/(heat capacity * temp rise)) * gas constant * temperature/pressure

Flow = (2 * Thermal power * temperature) / (3 * temp rise * pressure).

This concept looses two of the usual barriers - primary and secondary heat exchangers - between the core salt and the environment, which the regulators aren't going to like.


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PostPosted: Feb 24, 2011 3:38 am 
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What's wrong with heat exchanger walls? MS to MS HX works very well. Low pressure, high power density. Just make sure to control the salt redox, remove particles from fission and corrosion products, and maybe use a little copper coating on the HX walls.


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PostPosted: Feb 24, 2011 1:56 pm 
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Darn, there goes a little bit of IP I had tucked away. No idea if ORNL or others looked at this approach.

I think it a very promising idea Ray, I was thinking along the same lines, that as long as you couple the in and out parts of the pump it takes virtually no energy to "briefly" pressurize the salt. Now that the cats out of the bag, the way I thought it likely a more effective heat transfer mechanism is to spray the liquid salt from a nozzle into the high pressure chamber and have a large volume of counter flowing gas coolant absorb the heat from the falling droplets of salt. On the bottom, the salt collects and goes through the energy recycling part of the pump. This should also very effectively strip out noble metals which would then have to be collected out of the gas. It is a lot of salt per second to spray of course so any chamber is not going to be tiny. Lots of other ways to approach things as well, like letting the salt run down the sides of many individual tubes while gas flows up the center.

Heat exchangers, especially the primary ones are a pretty big challenge even if we do have conventional designs to go by. This could represent a major cost savings and less the needed R&D time to get a working product.

Yes tritium is an issue but if you are going to any sort of gas turbine (CO2, He, N2) then stripping out tritium later is much easier (as well as the option of using salts that don't produce tritium). Using only a single stage and going right to a working turbine might be a bit optimistic though, primarily with the noble metals and actual salt that might entrain with the gas. However, even if you need a gas-gas heat exchanger afterward you still are helping things a lot by getting rid of the highly radioactive primary heat exchanger.

David LeBlanc


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PostPosted: Feb 24, 2011 3:20 pm 
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Spraying radioactive molten salt is interesting.

Whenever a highly viscous liquid also having a high vapor pressure is sprayed, extreme physical conditions become manifest: vapor temperatures up to 1,000,000C and pressures up to 1,000,000 atmospheres might occur.

The collapse of small cavitation bubbles occurs after they are formed within just ten nano-seconds after the salt exits the orifice of the spray nozzle in the low pressure area just beyond the orifice.

One significant consequence of these extreme conditions of the cavatation process might be decomposition of the salt into its constituent elements. The fluorine component in the salt may be carried away in the high velocity flow of the helium gas leaving nano-particles of solid material to settle out.

What those radioactive nano-particles will do as they condense out of the plasma also promises to be fascinating.

Maybe new nuclear physics reactions will manifest.

I can’t wait to see what happens.

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PostPosted: Feb 24, 2011 4:29 pm 
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Probably crazy idea:-
Molten lead (or lead bismuth) in contact with the fuel salt as an intermediate loop. Flow would be ~7-10 m^/s (=70-100 Te!), something like 3 times the fuel flow rate rather than twenty times (by volume), and low pressure. A bit of PbO in the circuit could trap tritium. Noble metals would dissolve. Some lead would be bound to get back to the core, but this should be tolerable for fast spectrum designs. Pumping power and lead corrosion problems would be a serious drawback.


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PostPosted: Feb 24, 2011 4:39 pm 
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I apologize about offending anyone who loves and studies conventional Hxs.
Hxs make trouble for reactors, that's all. It's just good design to eliminate problems, right?
With spraying or bubbling immiscible fluids, the big advantage is very high heat transfer rates across very large areas, in small volumes, with less expensive, more reliable strictures.

There's still a substantial design problem, it's just a different problem.

Good points about the volume of the gas; I hadn't got that far. Spraying the salt sounds like a good idea. Maybe it can be dispersed by ultrasound or a gas jet, just to get the particles small and the area really high.

Yeah, lead is nice. Bismuth can transmute from delayed neutrons, which is bad, because the activation product is a strong emitter. There are other low-temp. metals like wood's metal, etc. that could widen the heat exchanger range, but they also activate. Another problem with the lead and bismuth is corrosion in the pipes and impellers. That's what's nice about the direct to gas approach. The gas (whether He or CO2) is not corrosive. If it were, nobody would want it running the turbine.


Last edited by rgvandewalker on Feb 24, 2011 4:51 pm, edited 1 time in total.

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PostPosted: Feb 24, 2011 4:45 pm 
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The english looked at lead flow heat exchange in their chloride salt reactor. Certainly has some attraction but also brings in new problems. In the end, the tone of their report is that they did not think this was a good choice.


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PostPosted: Feb 24, 2011 7:44 pm 
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Image

The concept of greatly increasing the surface area of the molten salt exposed to a flow of helium as a heat transfer mechanism is a good one.

I wonder if a Radial Rotary Fiberizer might not best support this concept.



A rotary fiberizer (10) includes a spinner (12) for centrifuging fibers (30) from molten material (26) along a path generally coplanar with the spinner, and a pair of opposed annular blowers (18, 20) positioned on opposite sides of the path (32) of the centrifuged fibers, with the blowers having an interior face (34) which is oriented toward the path of the centrifuged fibers, and the blowers having an exterior face (36) which is oriented away from the path of the centrifuged fibers. An induced helium conduit (40) associated with each of the blowers is adapted to supply helium to the path of centrifuged fibers, and the exterior faces of the blowers contain blower openings to discharge attenuation gases into the induced helium conduits to attenuate the centrifuged fibers.

The process of making fibers will transfer large amounts of heat to the helium flow through a phase transition from the liquid to the solid state.

The solid fibers would be recombined with an alternate molten salt flow to be reintroduced into the input leg of the reactor as slurry.

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PostPosted: Feb 24, 2011 8:03 pm 
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Luke wrote:
Probably crazy idea:-
Molten lead (or lead bismuth) in contact with the fuel salt as an intermediate loop. Flow would be ~7-10 m^/s (=70-100 Te!), something like 3 times the fuel flow rate rather than twenty times (by volume), and low pressure. A bit of PbO in the circuit could trap tritium. Noble metals would dissolve. Some lead would be bound to get back to the core, but this should be tolerable for fast spectrum designs. Pumping power and lead corrosion problems would be a serious drawback.


Certainly not crazy. In fact many groups including ORNL have looked at direct contact cooling with lead in many different forms (both internal and external the core). ORNL thought it just had too many issues, especially the entrainment of lead through the core (not sure about the other way?). The other big problem is finding metals that are OK in contact with both lead and molten salt. Molybdenum is the obvious choice but tough to work with. I certainly haven't given up on lead cooling though, just not sure what sort of chance it might have. I started a thread on this a year or two ago I think, maybe we can start discussing it there again.

David L.


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PostPosted: Feb 25, 2011 5:26 am 
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Let me get this straight. You guys worry about some simple metal plates or tubes and suggest lead contact cooling with high speed jets, extremely narrow materials choice and unknown track records. Well you guys have humor, that much needs to be said.

There are good reasons why all industries and power plants use heat exchangers and none of them use direct contact lead or helium cooling. Heat exchangers work very well. Lets look at today's reactors which have thousands of little heat exchanger tubes in the worst possible environment: lots of radiation, no redox control, no gas sparging leading to tremendous pressure in the rods, very high peak temperatures and temperature differentials. Quite horrible compared to the MSR heat exchanger which has none of the aforementioned issues. Outside the high neutron flux, fine tuned redox control, sparging of gas as we see fit make the pressure low, the temperature differentials are lower, peak temperature is lower, this looks pretty good to me. If fuel rods can last 4 years in terrible environments we should have a good shot at making the MSR HX last much longer. The MSRE showed little corrosion under the right circumstances. They had a salt cooler which worked fine too. Very reliable.


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PostPosted: Feb 25, 2011 3:29 pm 
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Quote:
Let me get this straight. You guys worry about some simple metal plates or tubes and suggest lead contact cooling with high speed jets, extremely narrow materials choice and unknown track records.



Jeremiah Johnson Script: “You got old and scary since you growed hair on your head?”

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PostPosted: Feb 25, 2011 3:41 pm 
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More like, the name goes in before any quality goes in.


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PostPosted: Feb 26, 2011 4:57 pm 
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Heat exchangers work very well. Lets look at today's reactors ...


OK. The thing that actually forces most real reactors into retirement is when the operator cannot afford to replace a primary heat exchanger when it finally becomes necessary to do so (whether from safety or economics). Most real reactors have plugged Hx tubes from galvanic corrosion, stress corrosion, crud (a technical term) build-up, fatigue from mechanical movement, thermal fatigue, galvanic corrosion, or other blockages (piece of equipment, foreign objects...). The most expensive commonplace reactor failure is a heat-exchanger blow-out. Likewise, it's one of the most significant classes of radiological incident. Operators are at some pains to try to prevent these issues, and that brings substantial expense from coolant additives, deionizing towers, resin replacement, filter replacement, clean-up and disposal of same, etc, etc, etc.

Nuclear power is capital intensive, so increasing a plant's life decreases the cost. So, it helps to make the most common failure impossible by design. The example in my life is when I was working on a big impact line printer. The team found out that the first failure was usually the gear box that drove the ribbon(!). Substituting a small (cheaper) stepper-motor and a bit of software doubled the printers' service lives.

For LFTRs, eliminating Hx failures might quadruple reactor service lives. The next big failure in LWRs after the Hx replacement is stress cracking in the main reactor vessel. The LFTR already solved that one, by eliminating the core's pressure vessel, substituting an easily repaired weldment. So, if Hx's can be eliminated, or made truly maintainable, with replaceable parts, a LFTR could have a 100 year service life. Or more. if the plant is already equal to the cost of coal, this would drive it to 1/4 the cost or less.

That's my motivation. I don't doubt that Hxs for salt are possible and thermodynamically efficient. But Hx's are proven trouble spots.


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