Energy From Thorium Discussion Forum

Avoiding the Heat Exchanger Wall
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Author:  Cyril R [ Feb 26, 2011 5:22 pm ]
Post subject:  Re: Avoiding the Heat Exchanger Wall

Actually small reactors were shut down for economic or technical reasons, sometimes more like political reasons, but most of the big PWRs get new steam generators. Newer ones are often 10% more efficient so you get a hundred million bucks more each year in electricity sales. AFAIK the limiting factor is the reactor vessel itself. That's why Areva designed a heavy neutron reflector for the EPR (with some fuel savings as a bonus).

Making plants last 4x longer won't make them 4x cheaper, I'm afraid those darned bankers, entrepeneurs and economists don't agree with zero discounting. In fact according to those terrible financial people, the entire world ain't worth much in 40 years.

Lets look at that steam generator. Its high pressure and has a high pressure difference (>70 atmospheres) with the secondary side. It has hot pressurized water and steam not that far from the triple point which is corrosive. Of course that leads to design issues! One of the reasons the boiling water reactor was developed.

Now lets look at the MSR. Even with your direct contact primary heat exchanger, it also needs a steam generator with a high pressure difference because the intermediate salt will be low pressure and the steam will be at a higher pressure (temp) than the steam in the PWR. And fluorides aren't compatible with water, you know. This is in fact much more challenging than PWR steam generators!

Even if you have a helium turbine, you probably have even higher pressures to deal with.

Compared to that, the primary heat exchanger is so much easier. Pressure difference less than 1 atmosphere, operate under chemically reducing conditions all the time... so much easier than pressurized water. Yes it is radioactive as hell. That's why it will be in the hot cell in its own compartment where spills can be drained from when necessary and robots can lift in new HX modules. Having a physical barrier between the primary radioactive loop and the secondary, intermediate salt helps a lot with safety. But it really is mostly a materials choice issue; many materials are extremely inert to fuel salt, and there is no hard nuclear (neutron flux) requirement. Given the low mechanical stresses, if you have inert materials, and the right chemistry control, the hx should last a long time.

I suggest to use HXs, more of them not less. There should be a third loop with KNO3-NaNO3 eutectic for steam generator integration, tritium trapping and thermal storage/transient buffering. Another nice MS to MS HX. There's innovation on all sorts of compact HX designs as well.

I'm sorry, don't want to be ornery, but I fear to take one step forward and two steps back.

Author:  rgvandewalker [ Mar 05, 2011 3:18 am ]
Post subject:  Re: Avoiding the Heat Exchanger Wall

It's OK with me if you are ornery, as long as you are right....

Author:  USPWR_SRO [ Mar 05, 2011 4:33 am ]
Post subject:  Re: Avoiding the Heat Exchanger Wall

I am with Cyril on this one.

Why throw away all that HX engineering experience? Makes little sense to me.

Author:  Cyril R [ Mar 05, 2011 5:43 am ]
Post subject:  Re: Avoiding the Heat Exchanger Wall

Dr. Per Peterson, who is orders of magnitude less ornery than me, and knows a gazillion times more about engineering, has admitted that the big challenge from an engineering viewpoint is actually the salt to gas heat exchanger. According to Dr. Peterson, the challenge comes from high pressure, high temperature, and high heat shock potential (apparently coming from the relative difference in Prandtl numbers of the two HX streams - but don't ask me about it!).

Compared to that, the first HX actually looks easy. It has to be compact and there is severe radiation but this will be the case for any heat transfer concept from the fuel salt. Dr. Peterson's group is developing further experience with molten salt heat exchangers, that would be a good starting point for the MSR (despite the more stringent requirements for compactness and the radiation). High heat capacity fluid, low pressure, this is great!

Author:  Axil [ Mar 05, 2011 2:09 pm ]
Post subject:  Re: Avoiding the Heat Exchanger Wall

the big challenge from an engineering viewpoint is actually the salt to gas heat exchanger

If the gas is supercritical CO2, isn’t the gas in a liquid state when it goes through the heat exchanger?

Author:  Cyril R [ Mar 05, 2011 2:16 pm ]
Post subject:  Re: Avoiding the Heat Exchanger Wall

Yes, but the pressure at the gas side is high (20 MPa) and carbon based materials are incompatible; CO2 is oxidising. The CO2 cycle may make for a more compact IHX but it sure won't make it easier to build the darn thing!

Author:  Luke [ Mar 05, 2011 7:41 pm ]
Post subject:  Re: Avoiding the Heat Exchanger Wall

Wouldn't rgvandewalker's constant volume pump work for the secondary exchanger? Match the pressure of the salt to that of the gas. Still has to handle 20 MPa, but the pressure boundary doesn't have to pass heat. so it can be thick, and carbon reinforcement on the salt side is OK.

Author:  Cyril R [ Mar 06, 2011 2:56 am ]
Post subject:  Re: Avoiding the Heat Exchanger Wall

Yes that may be a better application of that idea. Is CO2 compatible with fluorides? It would certainly be compatible with molten carbonate.

Author:  iain [ Mar 06, 2011 4:57 pm ]
Post subject:  Re: Avoiding the Heat Exchanger Wall

I really like the idea of a gas-air HX done with droplets.

Note that there is more than one way to put a pressure boundary in a flowing loop. Constant-volume pumps are one, gravity is another. FLiBe is 2 g/cc at 700 C. You can get 20 atmospheres of pressure from 100 meters of height. Add density to reduce height. While this is not going to help a primary loop, it could be used in a secondary loop. I admit that putting the secondary heat exchanger down a well seems bad for servicability.

It seems to me that the secondary loop pump can be removed, as the helium blast will carry the droplets up some amount, say, 10 m, and the rest of the height can be used to recover energy from pressure in the secondary loop.

The thing that bothers me with these systems is what happens when it breaks. Does turbine pressure blow into the secondary loop, pressurizing it and blowing into the primary (fuel) loop through the primary HX? That seems bad, but then the primary HX has to deal with whatever pressure is transmitted, and you need some sort of expansion tank in the secondary loop to deal with a secondary HX blowout. I guess you need that in all systems.


Author:  rgvandewalker [ Mar 11, 2011 11:00 pm ]
Post subject:  Re: Avoiding the Heat Exchanger Wall

Using a carbonate is clever. Perfect compatibility.

Making a trompe that uses molten salt is more clever. Trompes are reliable. With cheap salt, it could even be affordable. I don't think the salt needs to be especially dense, as long as it is cheap (carbonates?) and the piping is well-insulated. Water trompes need high flow volumes to entrain air, but supercritical CO2 in a carbonate... maybe you want lower density salt, LiCO? (melts at 723C, vaporizes at 1310, Dilithium.... heh) The fluid CO2 produced would be very uniform, perfect for a turbine.

The mismatch of Prandtl numbers is exactly what a dual-phase Hx is supposed to solve. It gets very intimate contact between the media.

A trompe is even better, because it is passively safe, but safety is one thing to like about the constant-volume pumps. The walls are thick. If the pumps jam, you get a slow leak of immiscible gas into the secondary loop, and it has to be designed for that, anyway, even with a Hx (they can leak, too). A pressure relief valve or a burst diagram solves the problem, but if that goes wrong, a slow leak of immiscible gas into a salt loop is easier to fix than a burst Hx tube carrying radioactive crud into turbines.

Author:  Cyril R [ Mar 12, 2011 3:46 pm ]
Post subject:  Re: Avoiding the Heat Exchanger Wall

There is a carbonate analog to FLiNaK, called CLiNaK, using LiCO3 - NaCO3 - KCO3 in equal portions. Melting point around 400 degrees Celcius. Cheap and good.

Author:  J Perry [ Jun 25, 2013 6:26 pm ]
Post subject:  Re: Avoiding the Heat Exchanger Wall

Probably crazy idea:

I was reading about the General Fusion steam piston reactor design, which uses has a spinning sphere of molten lead and lithium with gas filling the vortex in the center.
Would it be possible to create a vortex in the heat exchanger or even in the reactor core of a LFTR for coolant gas to flow through? or is FLiBe too viscous

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