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  1. Hi, Rod,

    Thanks for posting the updated version of your 1995 Adams Engines paper. I placed a copy in my database. Brought back 1958 memories of my Monroe, Michigan, Fermi I days as an electronic instrumentation grunt on that pioneering liquid sodium reactor. Learned never to run a thermocouple wire straight through the shielding into the core.

    I’m still running under the flag.

    1. Mr. Holm,

      Do you have any details about the Fermi I&C systems that you’d be willing to share? I’d love to see anything you have. Particularly items that are specific to liquid sodium instrument ation.

      I’m an I&C engineer working on a couple of projects where this history would be great to have.

  2. No. Combustion gas turbine technology continues to improve and to make a huge contribution in world energy markets.

    Supercritical CO2 is an interesting, laboratory-scale technology development that might have some utility with nuclear heat but isn’t worth doing with any other form of heat. That limits learning-by-doing and evolutionary improvements.

  3. So, off-the-wall question:

    Why throttle the main flow for power control?  If flow is restricted too much, the compressor will have stability issues.  (Drove a turbo car for enough years to understand this.)  Why not bypass flow past one or both of the turbines?  That dumps heat without restricting gas flow.

    1. Yes, compressor stability needs to be maintained. It’s one of the reasons why I tend to prefer centrifugal flow compressors over the axial compressors common among aeroderivative compressors.

      I like varying coolant flow as a means of controlling both the turbine power output and the reactor power output. As I understand it, “bypass flow” keeps fission rate relatively constant, so valuable actinides get used up in producing heat that is wasted.

      I realize some have concerns about Xe transients. But they can be handled by varying other reactivity factors like control rods or operating temperature. With HTGRs, I envision having a very wide “green band” for temperature to accommodate most Xe transients without any control rod motion.

      1. Note that centrifugal compressors are volume machines. Another way to throttle a closed loop centrifugal compressor/system is by adding or removing the working fluid from the system. A storage volume that can receive fluid by valve control on the discharge of the compressor and then return fluid on the suction side can provide mass flow control. The compressor/expander operate at the same speed at similar ratios but at varying pressure. This principal is used for cryogenic refrigeration (nitrogen expander LNG liquefiers)

        1. The method of control that you suggest has a long history of discussion in relation to closed cycle gas turbines. But it adds considerable complexity and cost to the system. High pressure tanks, piping and valves are not cheap components.

          It also takes a long time to remove or add a substantial amount of gas to a system, so the power response rates are not as fast as directly adjusting flow rate via a throttle in the working fluid path or by changing compressor speed.

  4. I am a little surprised you never mention the potential of nitrogen as a supercritical fluid. This is actually preferred over CO2 as an eluant because it is chemically inert and more easily diffused in gas filters because of smaller molecular size. The chief problem with SC is that after turbine expenditure, there is a sharp pressure drop back into a hot gas: a straight vertical drop probably, on the phase diagram. There is apparently no current assurance that a recuperator/heat exchange system will provide consistent safety or reliability for SC-CO2, but maybe a staged turbine at high temperature for N2 will help, even if there is a threat of a gas-liquid condensate — similar to wet steam. So perhaps just jetting all fluid into a heat sink which then boils steam, is the answer.

    N2 might be useful in scrubbing out Xe-135, the king of neutron poisons. To do this, I might suggest liquefying plutonium packed in rare earth – nitride granules. The net result will be a very small core filled mostly with solid material, and efficiently ma n.v raged neutrons, that will transmit its coolant with vanishingly little to e friction loss.

    1. whew – that last paragraph .

      Anyway, we all know that PB reactors work as designed – they have their benefits and their drawbacks. What is needed to further this conversation is input from a turbine manufacturer (GE, PW, Siemens, Saturn, etc.). Personally, I’d like to know how you get reasonable efficiency out of the Brayton cycle when the hot temperature is less than 1000 degrees – does this make the turbine very big? Is efficiency below 20%? Most of us are nuke guys – need to bring some of those guys that work in the basement at GE in Evendale Ohio into the conversation.

      1. Supercritical is not really a Brayton cycle, and for CO2 at least, 560 centigrade is considered the optimally efficient temp. But we do need nuke guys to examine my theory that better neutron management is the key to better fuel economics, and a more compact but heat radiant core with less space needed for coolant and more space devoted to neutron reflection and absorbance is the way to do it. As to integrating the reactor with natural gas, I think that is rather passe. So much work has been done on high temp thermal storage, such as a DOE financed experiment with MN-Mg oxide.

      2. @kalendjay

        Although I replied to your comment, I was was not discussing supercritical CO2 when I said we need to involve gas turbine designers in the conversation. AAE doesn’t use SCO2 (does anything?). I also made no mention of natural gas or thermal storage. With regards to ‘Neutron Management’, at least two of the concepts/ characteristics you mention are contrary to it: compact and radiant.

        Once all the compromises are made such that the fuel and core structural materials provide whatever longevity they can (~5 years at 110kw/liter), neutron economy is most sensitive to core size (bigger is better – ALWAYS). There are economic trade-offs that sacrifice neutron economy for ease of operations and increased heat rate… the reactors we have could use the fuel/neutrons more efficiently, but they would be even less competitive if they did so. With light water reactors the fuel is discharged due to depletion coincident with end of mechanical life of the fuel rod.

        We don’t need any new tech like temperamental vaporware SCO2 turbines – the tech we have works, well and it has room for growth, refinement, cost reduction, etc.

  5. Well, let’s just see how compact compact can be with microtubes and supercritical fluid, HALEU, thin and radiant fuel elements, etc. Also, numerous concerns are pushing lead as a secondary coolant, a non-absorber of neutrons and a gamma shield — and what does not pass through as gamma becomes heat anyway. I agree you did not mention natural gas and SC- CO2 may be a dead end.

  6. Rod, something that occurred to me:

    950°C is sufficiently hot to drive a number of useful thermochemical reactions.  For instance, steam methane reforming and gasification of solid carbon by both water and CO2 move along pretty well at 950°C.  You could probably base a chemical plant on syngas feedstock generated using a reactor like this.

    Imagine nuclear heat straight to “green” fuels.  You could go one step further and use the water-gas shift to convert CO and steam to H2 and a sequestration-ready stream of CO2.  Start with biomass waste, get a carbon-negative stream of fuel and clean ash.

    1. @E-P

      Sorry for the delayed reply. I’ve been a little busy.

      Yes, you are seeing one of the many reasons I have been attracted to reactor fuel technologies that offer a path to very high temperatures – 950 °C and up. There are some under development at USNC and other companies that show promise of achieving coolant temperatures in the 1200-1500 ℃ range. They offer the chemical processing concepts you describe but also offer exceptional heat engine efficiencies.