On the hazards of ignorance of thermodynamics

The feasibility of non-combustion gas turbines in nuclear reactors

In a discussion about nuclear reactors, a discussion subthread about gas turbines as energy converters ended with this late-arriving statement:

Non-combustion gas turbines are not proven. They're mostly in pilot/research stages. You say that the conditions in non-combustion lower temp operation are more reasonable than in higher temp combustion gas turbines, but the fact that they are not commercially competing with Rankine steam cycles, even in the higher temperature regimes, should caution us not to trivialize the engineering/commercial issues.

The one-week period for comment on the post ended before I could write a response.

What's missing from this analysis?  Let me lay out the pieces:

  • One can already purchase simple-cycle combustion turbines achieving 46% thermal efficiency.
  • These are internal-combustion units running on open cycles, requiring neither hot-side nor cold-side heat exchangers.
  • A turbine using inert gas as a working fluid is not internal combustion, by definition.  The source of heat must be something outside the fluid.  If the heat source is combustion, this requires a hot-side heat exchanger.  This is an unnecessary capital expense.
  • Preserving the inert working fluid against loss requires a cold-side heat exchanger.  This is another unnecessary capital expense for a combustion system.
  • Reducing the operating temperature from ~1100°C to ~800°C would also reduce the thermal efficiency.  If the heat source is combustion, this increases fuel costs.
  • We can see from a relatively simple analysis that today's absence of inert-gas turbine generators has nothing to do with technical feasibility.  It is soley a matter of economics.

    How does a nuclear heat source change the economics?  Comparing to the points above:

    1. One cannot buy a steam turbine operating at 650°C and higher temperatures.  The most feasible option for taking advantage of the high temperature of molten-salt and pebble-bed reactors is gas turbines.
    2. The hot-side heat exchanger is either inherent (in a gas-cooled reactor) or required to separate the nuclear materials and the working fluid (molten-salt reactor).
    3. Nuclear plants do not chemically modify the working fluid of their heat engines, so are the equivalent of "external combustion".
    4. The cold-side heat exchanger is required (like the condenser in a steam turbine).
    5. The reduced operating temperature is a given, set by the nuclear heat source.
    6. Since the gas turbine can operate at a higher source temperature than a steam turbine and can thus achieve greater thermal efficiency, it improves the return on the capital investment in the rest of the plant.  This reduces costs relative to revenue.

    The thermodynamic properties of inert gases are well-understood.  Designing a fractional gigawatt gas turbine to run on e.g. helium would require design changes such as gas bearings (to eliminate petroleum lubricants or water which would cause corrosion or coking in the hot side), but these have already been proven in other applications.  The only reason we aren't running helium turbines today is that it would increase both capital and operating expenses.  If the heat source was a high-temperature nuclear reactor, the helium turbine would generate more revenue than a steam turbine for the same capital expense in the reactor.  This is why we can expect to see inert-gas turbines as part and parcel of any Pebble Bed Modular Reactor (PBMR) or Molten Salt Reactor (MSR) powerplant.

    Thanks, EP, for this good addition. I am too lazy to make any such comments to what struck me as a very uninformed remark. Non-combustion gas turbine not proven!. I remember good Prof Marble at Caltech going on and on about closed cycle gas turbines and their advantages and applications, including nuclear reactors, in 1962.

    He said something like - Obey the iron law of thermo that for high efficiency you try to add heat only at the highest temp, and release heat only at the lowest temp in the cycle. So in a closed cycle, you have to get from the low temp to near the highest temp without heat addition, either by a high compression ratio using a complex compressor, or by a counter current heat exchanger, in which the hot turbine exhaust heats the low temp compressor discharge to near the highest temp.

    That is, for high efficiency, you take your pick, high pressure ratio compressor , or simple low pressure ratio compressor but a complex exhaust heat exchanger. Either way, you can get good efficiency from some external heat source, whatever that might be, like nuclear or solar. And there is no combustion in the cycle gas, so call it non-combustion if you wish.

    Then you can pick the gas you like- helium, hydrogen, nitrogen, carbon dioxide or whatever suits your fancy.

    Or, heh, heh, you could go to some other cycle, like, Surprise! A stirling engine. This is what NASA is thinking about for a moon nuclear power source. They are also thinking about a closed cycle gas turbine, but there, they gotta be out of their minds.

    Helium has very nice thermodynamic properties for heat transfer. But what about Peak Helium?



    Is there any particular reason that Neon or Argon could not be used instead? (Neutron activation?)

    In any case, technically a nuclear reactor should produce helium as a by product, although goodness knws how you'd go about capturing it..

    Argon isn't quite as good. The heavier the atom, the poorer the heat transfer characteristics of the gas. I don't think Argon has much of a neutron cross section, but I'd be guessing on that.

    It does make argon useful in those insulating windows, though :)

    Cryogenic plants for Oxygen and Nitrogen produce Argon, Neon, Krypton, Xenon, and Helium, in that order. Argon has a high thermal neutron absorbtion, like Nitrogen, but is widely available. Neon has a low thermal neutron absorbtion, and is available but scarcer. For Fast Neutron Gas Cooled Reactors, Argon is perfectly acceptable and we aren't about to run out.
    If we have carbon sequestration rules, then there is also going to be a good deal of byproduct Neon arriving for Pebble Bed Thermal Neutron Gas Cooled Reactors.
    Since we have a limited production capability for Pressurised Water Thermal Neutron reactor pressure vessals, the only quick reactor ramp up is going to be Fast Neutron Gas Cooled Reactors. Pebble Bed Thermal Neutron Gas Cooled Reactors don't have to worry so much about pressure surges, so the pressure vessal is simpler and cheaper to build.

    Thermal-neutron molten salt reactors might be even faster to build than gas-cooled, because the fuel cycle might be started with plutonium from spent PWR fuel instead of enriching raw uranium.  MSR's don't merely dispense with forged reactor vessels, they operate at atmospheric pressure.

    Right. that's why, when I think of ways to save the world for heat engines after TSHTF, I restrict myself to nitrogen- (separated air ), or hydrogen (separated water) as the only sane choices for working fluid. Nitrogen is sluggish but ok, and hydrogen is just great, but has bad habits re alloys and such. So, nitrogen. Then you have a big sort of ugly engine, but works well enough. I am talking about the one you put on a tractor train to haul all the neighbors into town for the week end party, using moldy hay as fuel.

    Truth of it is that I still can't beat a plain old IC engine running on a wood gasifier. Anybody can do it. Does not take any superannuated dreamers like me.

    Hydrogen is a great coolant, but it reacts with graphite and causes hydrogen embrittlement in metals.

    Very true, not proven. Like most engineering fields, advances in power cycles are getting held back by materials issues.

    If you think that companies which currently build and sell combustion turbines running at 1380°C firing temperatures would have any difficulty cranking out inert-gas turbines taking a measily 850°C input stream, you're kidding yourself.

    Well the Helium one should be ok. But you have to remember that the people who sell these turbines, design them to break even after selling a lot of them. There's an inherent resistance to a new working fluid or different working conditions. I heard GE sells a new aircraft engine with the break-even time frame on the order of 5-10 years. I bet GE Energy has lost money on a couple of their combustion turbines. That requires a lot of confidence in the power plant design and market.

    A CO2 turbine in the 500-600 C range for a sodium cooled reactor would be a materials nightmare.

    The materials problem with the HTGR is the nuclear fuel. Fission product leakage plus the graphite in the fuel might turn the turbine into a combustion turbine if air ever leaked in.

    But you have to remember that the people who sell these turbines, design them to break even after selling a lot of them. There's an inherent resistance to a new working fluid or different working conditions.

    Offsetting this are the mild operating conditions (old, cheap materials and processes will be more than sufficient) and large potential volume.  Certifying one turbine as part of a power reactor (especially one which doesn't compete with other designs for fuel, using e.g. spent PWR fuel and/or thorium) could sell 100 GW of capacity or more in the USA alone.

    A CO2 turbine in the 500-600 C range for a sodium cooled reactor would be a materials nightmare.

    Sodium is a materials nightmare.  Molten fluorides appear to be much easier to handle.  Neon is more expensive than CO2, but unreactive and with a much more favorable set of thermodynamic properties (ratio of specific heats = 1.67).

    The materials problem with the HTGR is the nuclear fuel.

    Silicon carbide is sufficient to protect carbon-carbon against re-entry heat on the Shuttle.  It will keep HTGRs from going all Chernobyl on us even if the reactor fills with air.

    If you think you can get away with comparing apples and oranges like that on this site, you're kidding yourself.

    Uninformed remark? Hardly. The confusion here stems from a deviation in the definition of what is 'proven'.

    The definition of proven I use is not some talks and experiments in 1962, which appears to be the definition you use. There are still no commercial systems operating right now. Half a century later! It's still steam cycles. Proven means systems operating right now, providing real kWhs to real consumers, who are paying real money for them. Oh wait, that last one is a sensitive oxymoron these days.

    You can interpret the situation any way you want, but facts are facts and there is no proof in this thread. In the absence of real systems operating today, at the very least we need a detailed engineering/financial report that shows the engineering feasibility and commercial practicality and cost estimates etc, with detailed figures everywhere.

    Where is it? One of the topics of this thread was financing. Funny, I don't see any financing at all.

    The limit is the strength of materials under high pressure to transfer heat through the wall of the heat exchanger. e.g., See: Ultrasupercritical generation "A conventional supercritical unit operates at a steam pressure of 3500 psi or higher and steam temperatures of 1000 –1050F."

    (Note:  1050 F = 565 C.)

    There is also the chemical reactivity of water at such high temperatures.  At some point, water becomes too difficult to handle and other fluids are preferable.

    It seems to me you would need a 2-stage cooling system with parasitic losses from pumping liquids through narrow tubes. The first heat exchanger would bring liquid coolant X near the inert gas and the second stage would transfer heat from X to air or water. X would have to be somewhat unreactive across a range of substances and easily contained in the event of a leak.

    BTW I see from Big Gav's article on coal seam gas there will a gas fired air cooled thermal plant to add to Australia's two coal fired but air cooled supercritical steam plants.

    Even a steam turbine has such a 2-stage system:  turbine steam to condenser cooling water, condenser cooling water to air.  Open-cycle gas turbines dump directly to the atmosphere.

    Interesting points all.

    With uranium so cheap efficiency is only important where cooling water is limited and expensive. When a good heat sink is available, like the ocean, the most important factors are which technology can be built in the shortest time and at the lowest cost.

    A lot of these issues were addressed in the 1960s and 1970s at General Atomic in San Diego. I worked on the High Temperature Gas Cooled Reactor (HTGR) which used helium as an inert coolant,on the direct cycle gas cooled reactor and on the Gas Cooled Fast Reactor. We were working with really high gas temperatures, 1460F outlet gas temperature as I recall. GA built two HTGRs, with quite different designs but never got around to actually building the other designs. The thermal efficiencies were quite high.

    While the concept was brilliant the execution was not so hot (pun intended) and a commercial failure. The current pebble bed concept, I believe, also came out of General Atomic.

    While the concept was brilliant the execution was not so hot

    Alas, humans live in the world of execution and only hope to get to the land of what has been conceived.

    There is a long history of gas cooled reactors(CO2 cooled, graphite moderated) but only few are still operating. Graphite moderators are considered by many people to be dangerous--i.e. graphite is combustible and reactor temperatures are high.

    There were 11 French UNGG reactors(all closed), the 13 ancient British Magnox reactors(of which 3 are still running), 7 modern British AGR reactors(600 MWe) and the German THTR-500 thorium reactor(closed). All these reactors transfered heat to steam turbines(rankine cycle) and are much smaller than the standard 1 GWe commercial LWR.

    It seems that GCR even with gas turbines is still not commercially 'proven'.
    Of course gas turbine plants tend to be small as well( biggest one in the world is ~350 MWe). Economies of scale probably strongly favor LWR.

    Graphite is primarily dangerous if it is operated below the annealing temperature, allowing Wigner energy to build up.  Pebble-bed reactors run hot (well above the 250°C annealing point) and seal the graphite behind a layer of silicon carbide.

    I've been thinking about using amorphous synthetic diamond as moderator, but it's just too outrageous.

    So far, it appears the D2O (Candu-style arrangment) moderation is the most interesting.

    Of course good ol' graphite will do just fine, too.

    If you have an unlimitted supply of cold water then you don't need a condenser.

    If the heat source was solar thermal, we'd be looking at the same set of issues right? Minus the graphite moderator. Plus how to keep the molten salt from solidifying at night.

    Are you talking about a once thru steam cycle? Such a cycle would need an unlimited supply of extremely clean water to not leave deposits in the steam boiler.

    Dry cooling is cheaper for higher thermal efficiencies, and is advancing technologically as well (eg carbon/foam based condensers).

    If you use organic salts (ionic liquids) then the freezing problem is fixed, as many of them have strong subzero freezing temperatures. There's been some experimental work on producing stable ionic liquids lately, but the quantities were small and costs were quite high. Hopefully, scale-up will offer cost advantages.

    I hope this won't lead to a reawakening of the 1950s and early 60s Nuclear Airplane Project


    I know were short of jet fuel and all that...

    The high temperature reactors (HTREs) were only tested on the ground. A lower temperature non-propulsion version was actually test flown 47 times in a B36 bomber.

    Despite the project's secrecy, I can't help feeling that the 1956 film 'Forbidden Planet' has a reference to it where the crew actually take their nuclear reactor out of their flying saucer when they land. The nuclear airplane reactor had minimal shielding (to save weight) and had to be hastily buried in a pit when the plane landed.

    Also is it my imagination or is the sodium cooled HTRE3 reactor, with it's twin turbines, the inspiration for Star Trek's USS Enterprise?see:



    I hope this won't lead to a reawakening of the 1950s and early 60s Nuclear Airplane Project

    More likely the nuclear part will be on the ground, making nuclear kerosene.

    If, however, you make an A380 out of tungsten and then collapse it into a 3.5-metre ball, you have the makings of a real omnidirectional shield for a real nuclear airplane, but it would have to be one of three or four per plane, so these would be a tenfold scaleup from today's largest aircraft.

    Also is it my imagination or is the sodium cooled HTRE3 reactor, with it's twin turbines, the inspiration for Star Trek's USS Enterprise?see:


    There is a resemblance. But I don't believe it was sodium-cooled. Rather, it had liquid fuel -- UF4 dissolved in ZrF4 and some other fluorides -- and this fuel was its own heat-transfer fluid. It circulated between the neutron-reflected chamber at the top and the two air turbines at the bottom.

    --- G.R.L. Cowan, author of How fire can be tamed

    BobE -

    Ah, the atomic airplane ..... one of the most bone-headed ideas ever to come out the fevered minds of 1950s-vintage cold-warriors!

    As I recall, the idea was to have a B-36 size bomber that would be capable of staying airborne for weeks at a time (or as long as the food and water supply for the crew lasted). We would keep a whole bunch of these planes airborne at all times, thus making it nearly impossible for our bomber fleet to be destroyed on the ground.

    I don't recall all the details and I know there were several different design concepts, but in the version with the intake air coming in direct contact with the reactor surfaces it would seem that every single piece of particulate matter normally present in the air would be prone to becoming irradiated. If so, then this monstrosity would be laying down a steady stream of low-level radioactive pollution. However, such concerns were hardly a priority at a time when we convinced ourselves that we were in a life-or-death struggle with the Rooskies.

    As I recall, they never did solve the weight problem associated with providing enough shielding to keep the crew from being fried.

    The atomic plane is a perfect example of a seemingly attractive concept that turned out to be a total technological deadend.

    I was one of many back bench junior engineers of that era who whispered to each other- "hey, why not just carry that reactor/shield weight of kerosene, and refuel the damn thing every now and then with an identical plane fitted as a tanker".

    "Shut up, you guys, we are having fun playing "ATOMPOWER", go off and make supersonic fighters like you 'been told to do, and quit piddling on our party".

    In other words, it wasn't at all technically attractive, as simple calculations would show, it was just a plaything for people who liked to play that sort of game.

    Same as star wars, etc etc etc. Instead of real simple stuff. Sigh.

    Like, take the liquid and gaseous fuels away from buildings, replace that with biomass buried directly, and put the L&G into cars. Way too simple. Way too obvious. Way not talked about.

    OT, but there are several outgrowths of the various air/space (NERVA) efforts which could have some serious potential if we ever tried them.  I recall the Dumbo (porous-tube fuel elements heating hydrogen, thrust/weight very high), a more recent pebble-bed using ammonia as reaction mass, and the "light-bulb reactor" and dissolved-salt fission rocket concepts.


    There are a couple of problems with your "simple" approach. One is that the amount of energy represented by the "reactor/shield weight of kerosene" is not within a few orders of magnitude of the amount of energy that can be stored in a reactor with uranium, thorium or plutonium fuel. The energy density of heavy metals is about 2 million times that of a hydrocarbon fossil fuel and the weight of required shielding does not grow much in relationship to the quantity of heavy metal stored in the reactor.

    The second challenge is that kerosene costs have continued to rise over the years in relation to the cost of heavy metals. Large planes burn a vast amount of fuel, especially if they have to be refueled in the air.

    There are also many dangers associated with carrying massive quantities of fossil fuels - one of the main reasons that few people survive aviation accidents is that fires kill. In fact, history shows that a plane full of fuel is a potent weapon. A plane with a well shielded reactor would certainly kill anyone hit by it, but I very much doubt that the damage would be very widely spread.

    Your final point is probably the most questionable. Do you really believe that weather dependent, intermittently harvested biomass can be gathered and stored in sufficient quantities to provide the energy required to heat, cool, light and operate installed electrical equipment in buildings? Have you ever heated a home with wood and seen just how large the pile has to be at the start of winter? Have you ever computed how much noxious smoke such a system emits over the course of a few months?

    Atomic Power is really a far better choice. I have seen just how tiny the "pile" of uranium has to be to power a 9,000 ton submarine for 15 years and I have also seen just how tiny the waste pile is after that service has been completed. Either pile would easily fit in my spare bedroom with complete shielding. Without the shielding, the mass would fit under my office desk.

    The atomic plane is a perfect example of a seemingly attractive concept that turned out to be a total technological deadend.

    I wouldn't be so quick to say that--if it hadn't been for the atomic airplane, it is likely that the molten-salt reactor would have never been developed. Only the extreme demands of atomic flight could have pushed the extreme performance and simplicity that that reactor offered. And the molten-salt reactor may end up saving the world by allowing us to fully use thorium as a nuclear fuel.

    It seems obvious that if a reactor is gas cooled one should run a turbine with the coolant gas. This would lead to greater thermal efficiency than using a heat exchanger and a steam turbine. But I'm not convinced that a gas turbine is the best option for a molten salt reactor.

    The helium supply is very limited, so gas turbines running on helium would be very expensive to operate.

    Fun for all - Physical Chemistry at last on TOD!
    No one mentioned the fugacity of light gases, which is most problematic at high temps, when circulated through complex, multijointed pipe mazes around shaft seals and the like, in an environment that tends to generate microcracking through both vibration and neutron embrittlement.
    In the event of a seal failure, those hot gases will escape quicker than you can say "Scram!"

    What makes you think that these gas turbines would be exposed to neutron bombardment?  They wouldn't be located in the reactor.

    Large electric generators have been hydrogen cooled for a long time; it's safe to say that seals are a solved problem.  Helium is much safer to handle than hydrogen, and neon gets rid of the supply and leakage issues of helium at the cost of lower performance.

    Correct. There would be a primary salt loop, and high temperature heat exchangers transferring the heat onto the gas. In fact, some designs use two salt loops, a primary (radioactive) one and a secondary ('clean') salt loop.

    The open cyle has simplicity and cooling advantages. The downside is that it puts out a lot more radioactivity into the atmosphere. Still not a whole lot though, and seems fixable as Rod Adams points out.

    Engineer-Poet - thank you for the simple review of thermodynamics. Brayton cycle machines are excellent matches for reactors that can produce turbine inlet temperatures in excess of 650 C.

    There is a lot of discussion in the thread about the best choice for coolant/working fluid gas. I strongly favor N2 because of its similarity with air and its exceedingly low cost.

    There is a huge and vibrant market for turbomachinery that uses air at atmospheric pressure at the compressor inlet. Making use of that existing base of machinery is what led me to choose low pressure N2 as the coolant choice for the Adams Engine. I will never forget the moment in 1993 when I realized that helium turbines are simply not available for purchase but air turbines are widely produced in a variety of sizes.

    N2 might not be the very best gas in terms of neutron absorption or in terms of specific heat transfer coefficient, but it is a reasonable choice that provides significant economic and scale advantages.

    The only real challenge to its use is the need to capture the absorption product - C14 - out of the coolant stream and to isolate it from the environment. We believe this is not terribly difficult or expensive compared to the challenges of designing and building specialized helium turbo machinery.

    Rod Adams
    President, Adams Atomic Engines, Inc.

    Thanks for the kudos, Rod.  I've been a fan of your work for almost two decades now.

    With all due respect, it appears EP didn't get my main message. There are no large commercial operating non combustion Brayton cycles (I should have more explicitly stated non-CT braytons) operating today, to the best of my knowledge. They are simply not competing with super/ultracritical steam cycles in commercial power generation.

    That I find strange; considering the advantages such as high power density, related low materials use, relatively simple design (at first glance) and high thermal efficiency, and the fact that it's components are not exactly a new technology. More expensive or not, there should have been systems operating already (perhaps military). I wondered what was wrong, but EP offered only speculation, not proof, which isn't his style.

    Apologies for being incomplete and too brief. But now we still don't know what went wrong.

    At any rate, it is a minor issue altogether, swapping the non combustion brayton with an ultracritical steam cycle shouldn't be an insurmountable problem. About 550-600 degrees Celsius systems exist already. It looks like going slightly lower on core temp is favorable anyways - cheaper simpler core. Hardly worth an entire post I'd say - and it's not specifically about thermodynamics - but the thermodynamics overview is interesting.

    I think CO2 Braytons around the critical CO2 temperature are promising. Only mildly corrosive CO2 at that temperature, and high efficiency and simple design. Plus CO2 doesn't creep as much as helium. I'd rather see very high compression ratios in the power generation system than higher temperatures in the nuclear part of the system.

    With all due respect, it appears EP didn't get my main message. There are no large commercial operating non combustion Brayton cycles (I should have more explicitly stated non-CT braytons) operating today, to the best of my knowledge. They are simply not competing with super/ultracritical steam cycles in commercial power generation.

    Not true. There is the 300+ MW GE H frame unit that is commercially available now and hundreds of 175 MWs GE F frame units in operation today.

    However, these are all Brayton cycle engines designed for NG combustion. And they are all open cycle engines, not the closed cycle ones we are talking about for high temperature gas (He, CO2, N, etc).

    For an inert gas nuclear reactor, developing the turbine will be the easy part since all the technology exists. Obviously it's a case of tweaking and, scaling up, that will take some time. A big base load unit has to be over 1100 MWs these days. But the potential for the less-than 100 MWs unit is probably greater than the larger base load ones.