Nuclear gas turbines, again

[Genial Precis]

During the Cold War, the Good Idea Fairy set various US and Soviet workers to the task of building an aircraft engine using nuclear fission as the heat source. I haven't thought very carefully about how difficult such a development task might have been, but some workers at KAIST have some new interest in a nuclear gas turbine.

They appear to intend to use it in a closed cycle in a small modular reactor, and there's no indication that they've any interest in putting it on an aircraft, but the attached figure grabbed my attention.

To be fair, the main thrust of the linked article is to claim that a supercritical CO2 power cycle in which the turboexpander adds heat as the pressure drops in order to approximate isothermal expansion would be more efficient. Which is true and relevant. But that figure...

[john smith 19]

During the Cold War, the Good Idea Fairy set various US and Soviet workers to the task of building an aircraft engine using nuclear fission as the heat source. I haven't thought very carefully about how difficult such a development task might have been, but some workers at KAIST have some new interest in a nuclear gas turbine.

They appear to intend to use it in a closed cycle in a small modular reactor, and there's no indication that they've any interest in putting it on an aircraft, but the attached figure grabbed my attention.

To be fair, the main thrust of the linked article is to claim that a supercritical CO2 power cycle in which the turboexpander adds heat as the pressure drops in order to approximate isothermal expansion would be more efficient. Which is true and relevant. But that figure...

Well given that' where the molten salt reactor concept originated the fact they were prepared to devote serious funds to looking at something so radically different to existing system should have suggested that this task is non trivial 

Yes those stator blades have to be quite carefully shaped as well. So you've got machining/forging/casting of a uranium or uranium alloy.

One of the thinks I've always disliked about plans for uranium nitride fuels is the way they insist that it be N15 as  N14 will turn into C14. This looks like a C14 making system to me. IOW the whole system will be hot. Obviously it would be closed cycle but it doesn't look remotely simple.

[Genial Precis]

The funny thing is that the paper is about perfectly sensible sCO2 heat engine cycles. "Wouldn't it be great if you could use turbines for an isothermal compression/expansion process?" they ask. The fissionable stators are just weird and out of place, unless South Korea is planning on a 6th generation super stealth high endurance combat aircraft and hasn't told anyone. For any other purpose you'd just use a heat exchanger, as exemplified in any of the experimental examples of quasi-isothermal turbines...

[john smith 19]

The funny thing is that the paper is about perfectly sensible sCO2 heat engine cycles. "Wouldn't it be great if you could use turbines for an isothermal compression/expansion process?" they ask. The fissionable stators are just weird and out of place, unless South Korea is planning on a 6th generation super stealth high endurance combat aircraft and hasn't told anyone. For any other purpose you'd just use a heat exchanger, as exemplified in any of the experimental examples of quasi-isothermal turbines...

I take "isothermal" to mean the CO2 gas remains at constant temperature as it travels  through the turbine. As the the whole point of a turbine is to trade some part of the drive gases energy for power you have to "top up" the temperature during the pass through the turbine.

It did slightly put me in mind of the fission fragment reactor I saw as part of a workshop proceedings on NTP designs I saw.

[Genial Precis]

Yes, the practical work thus far consists of normal turbines with various methods of integrated heat exchangers. Lots of already-proposed sCO2 cycles use intercooling/reheat, where you have a compression(expansion) stage and then a discrete heat exchanger, and then another turbine stage, and so on.

At the less-integrated end of isothermal turbomachinery, there are proposals to integrate more of that into one casing, or try out different shapes of heat exchanger that fit the turbine better and try to minimize losses through the heat exchangers. At the more-integrated end, there are proposals for radial turbines with cooled walls, or axial turbines with cooled rotors and stators so that the turbine is itself a heat exchanger. These layouts are described in "An Investigation of Turbomachinery Concepts for an Isothermal Compressor Used in an S-CO2 Bottoming Cycle" with some of the same authors.

The alternating turbine-heat exchanger approach can yield a couple % efficiency gains, so it may be used with whatever number of stages. Also, if they're discrete components there are all sorts of head-spinning topologies with split flow and recuperation, limited only by thermodynamics and the designer's imagination. The integral approach might be more powerful, but it also asks for cooling/heating fluxes of 500 MW/m^2 before you redesign the turbine to account for this new trick.

The other advantage of this kind of technique would be closed cycle heat engines of amazingly high power density. You would definitely have military customers.

[Asteroza]

The former RamGen Rampressor rotary supersonic inlet compressor design could be an alternative for the compressor side, though that originally was for CO2 and not supercritical CO2, and most definitely has a high heat rise which may be unattractive. I believe there were mentions somewhere on NSF on heat pipe cooled turbine blades for conventional jet turbines, so ostensibly the fixed stator design would potentially be an good fit for using heat pipe methods bring heat from a separate reactor as a substitute for directly fissioning stators. It looks like a radial grid pattern though if following the pattern of Frontline Aerospace's IsoCool compressor stator vane cooling concept, so manufacturing a uranium fuel stator will either be casting or perhaps a 3D print via laser? I wonder if that becomes easier with just the ring component of the stator vane grid being uranium fuel in simple symmetric airfoil shapes, while the radial vanes (which need proper blade shaping to move flow to the next turbine blades) could be made of something else?

Digging a little into the papers, there is mention of a theoretical axial compressor design for supercritical CO2 (apparently that still isn't a thing) in the following report

Wang, Yong, et al. Aerodynamic Design of Turbomachinery for 300 MWe Supercritical Carbon Dioxide Brayton Power Conversion System. MIT-GFR-015, 2005

But that doesn't seem to be available online?

[Vahe231991]

The former RamGen Rampressor rotary supersonic inlet compressor design could be an alternative for the compressor side, though that originally was for CO2 and not supercritical CO2, and most definitely has a high heat rise which may be unattractive. I believe there were mentions somewhere on NSF on heat pipe cooled turbine blades for conventional jet turbines, so ostensibly the fixed stator design would potentially be an good fit for using heat pipe methods bring heat from a separate reactor as a substitute for directly fissioning stators. It looks like a radial grid pattern though if following the pattern of Frontline Aerospace's IsoCool compressor stator vane cooling concept, so manufacturing a uranium fuel stator will either be casting or perhaps a 3D print via laser? I wonder if that becomes easier with just the ring component of the stator vane grid being uranium fuel in simple symmetric airfoil shapes, while the radial vanes (which need proper blade shaping to move flow to the next turbine blades) could be made of something else?

Digging a little into the papers, there is mention of a theoretical axial compressor design for supercritical CO2 (apparently that still isn't a thing) in the following report

Wang, Yong, et al. Aerodynamic Design of Turbomachinery for 300 MWe Supercritical Carbon Dioxide Brayton Power Conversion System. MIT-GFR-015, 2005

But that doesn't seem to be available online?

The paper "Aerodynamic Design of Turbomachinery for 300 MWe Supercritical Carbon Dioxide Brayton Power Conversion System" can be downloaded for free at this link:
https://dspace.mit.edu/handle/1721.1/44765

I also found a paper from 2010 about the possibility of optimizing a nuclear jet engine for long-haul aircraft:
https://www.researchgate.net/publication/266503875_THE_POSSIBILITY_TO_USE_A_NUCLEAR_REACTOR_AS_SOURCE_OF_ENERGY_FOR_A_JET_ENGINE