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      1. Ad claims a potential of 200 MW. That’s actually a bit higher than the limit that I foresee for passively safe closed cycle gas cooled reactors with direct gas turbine power plants.

        They might not have been as interested in passive safety.

        1. The modern trend for passive safety in gas-cooled reactors (the “inherently safe” core configuration) started with a paper that was published in the early-to-mid eighties.

          1. @David Walters

            Early versions of commercial pebble bed reactors should be constrained to be less than 300 MWth for each reactor. Above that level, it becomes increasingly difficult to prove passive safety well enough so that I would be willing to actually run the physical demonstration.

            For the Adams Engine design that I believe should be the first to market, that implies an electrical output of something less than 100 MWe. I do not advocate recuperation, intercooling, or combined cycles initially. They add complexity and capital cost with insufficient ROI–IMO.

    1. @Cheryl

      Thank you for mentioning UHTREX. As you noted in your 2013 blog post, there is not very much information about the project on the web, but what I have found is fascinating so far. I think there might also have been a few additional papers uncovered since 2013 and published on sites like ResearchGate, but I have only read the abstracts so far.

      I’ll keep looking and may eventually produce a follow-up blog post.

      1. I would be interested in whatever you may find. I haven’t looked intensively, but I think there may be a bit more available.

        I worked in the Los Alamos Reactor Division when UHTREX was being built and operated, so I have some firsthand recollections, but mostly I was working on other things.

      2. Rod,
        Do you believe that the UHTREX concept is compatible with an Adams Engine?
        Or would the abundant fission fragments admitted to the coolant “sand blast” the blades of the turbine?

    2. That link gets me to a “Nothing could be found at this location. Maybe try a search?” page. So I search for “UHTREX” which gets me to a “The Sad History of High Temperature Gas Cooled Reactors” titled page with a short blurb, the title being hyperlinked to http://nucleardiner.com/2013/01/30/htgrs/ i.e. back where I started.

  1. I read somewhere that the IAEA considers gas-cooled graphite moderated reactors like this to be the only reactor concept which is (or can be) made truly ‘inherently safe’/’walkaway safe’. But I’ve never found the IAEA document in which this is stated and/or in which the reasoning for such a statement is explained.

    If anyone knows about this, and can post a link to the relevant document, I’d be much obliged.

    1. Well, they’re not like this one. The TRISO fuel particle, which is a key element to modern high-temperature gas-cooled reactors, was only being developed in the 1950s, so I doubt that a concept in 1956 would rely this type of fuel.

      The idea for the modern inherently/walk-away safe design was first conceived in the 1980s. For example, Fort St. Vrain, the US’s largest commercial gas-cooled reactor relied on active safety systems. Then someone realized that the fuel could be arranged in a geometrical configuration such that the core has a large thermal inertia, but with the fuel (and decay-heat generation after shutdown) located relatively close to the outside.

      The large heat capacity of the core means that an accident progresses on a much longer time scale — hours and days, instead of minutes — which allows the decay heat to escape the core via passive heat transfer before temperatures become high enough to cause significant fuel failure. Thus, a reactor such as this could lose all of its gas coolant, the operators could walk away from the plant, and the reactor core and the fuel would be just fine.

      There’s not one IAEA TECDOC that I’m aware of that covers this in a nutshell. This property of modern HTGRs has been demonstrated by calculations in numerous published papers and reports. There is one TECDOC, however, that was published a few years ago that describes some of the recent designs that have been developed.

      1. @Brian Mays

        Thus, a reactor such as this could lose all of its gas coolant, the operators could walk away from the plant, and the reactor core and the fuel would be just fine.

        If you look at this statement through the lens of someone seeking to provide competitive power plants for ship propulsion, you can see it as offering the potential for unmanned engine rooms with power plants that are controlled from the bridge of the ship. Through the lens of someone wanting to replace diesel engines or gas turbines on the grid or in remote areas, you can see the possibilities for remote controls without on-site operators.

        Note: I’m not suggesting eliminating operators, just allowing them to be in more comfortable, better supported locations.

        1. Russian Alpha Class subs had unmanned engine rooms. Lead cooled fast reactors. Very small plant, very small crew, lots of power.

          1. I wonder – I’ve heard most military reactors have used high-enriched uranium (20%+) – was that true as well of the subs you mentioned? It seems that HEU, despite technical advantages, is considered too much of a proliferation risk for civilian use?

    1. @Keith Pickering

      I’ve been following the progress of the HTR-PM as closely as possible; there is not too much news in English language publications.

      There is a major difference between it and the closed cycle gas cooled reactor described by the 1956 vintage ad – its twin reactors each have a steam generator (aka boiler) that uses the helium to create steam that is then used to drive a single steam turbine.

      It is thus an indirect use of the heated gas whereas the Ford Instrument concept was to directly use the gas that was heated in the reactor to drive a gas turbine. That is [was?] the plan for Adams Engines. The key enabler is using nitrogen, not helium, because it has virtually the same thermodynamic characteristics as air. That avoids the burden of developing helium driven turbomachinery.

      1. The ad also lists carbon dioxide as a possible coolant/working fluid. Were they missing the boat there? Could the necessarily lower operating temperatures dictated by CO2 still make for a decent engine?

        1. Carbon-dioxide was a popular choice of coolant in early gas-cooled reactors. It was used for the Magnox reactors (the last of which just recently shut down for the final time) and the British Advanced Gas-cooled Reactors (AGRs), which are still running.

          1. Right, Brian, but the Magnox and the AGR used (and use, in the case of the AGR) 2 loops: a core cooled by CO2 and that hot CO2 is used to make steam in a separate loop (the steam thereby generated is employed in a Rankine cycle plant).
            Here is the big difference: The Ford Instrument Company ad proposes an “Adams Atomic Engine” type plant.
            My question is directed at the feasibility of using CO2 (at, say, 650 degrees C) to spin a turbine in a closed Brayton cycle.

          2. Oh, you were asking about the power conversion system. There’s nothing remarkable or challenging about a carbon dioxide (Brayton) gas turbine.

          3. There’s nothing remarkable or challenging about a carbon dioxide (Brayton) gas turbine.

            Actually there is, but the problems are things like outrageous power density and other things which we’ll be delighted to have once we’ve worked around them.

          4. Engineer-Poet – Well, I wasn’t talking about supercritical CO2 turbines. With a direct cycle, you’d have S-CO2 going through your core, which I didn’t want to have to think about. I know that the Supercritical Water Reactor (SCWR) still has some serious challenges to overcome.

          5. Yes, E-P, I’m sure that Supercritical CO2 has great potential, but the Ford Instrument Company ad certainly doesn’t describe a SCCO2 system. It is a closed cycle Brayton that uses Helium, CO2, or Nitrogen. Rod has made a great case for Nitrogen — but Carbon 14 generated by the effect of the neutron flux on the N14 poses a problem (perhaps more of a regulatory probleme than a technical problem, but a problem nonetheless). If CO2 (in gaseous form, at about 650 degrees C) could work well in a closed Brayton, then maybe an alternate route for the Adams Atomic Engine can be found.
            I suggested to Rod that CO be used instead of N2, but Rod likened that to “jumping off of a cliff to avoid a bee sting”. A nice analogy. Still, the VERY serious dangers of carbon monoxide could be mitigated in certain siting applications (offshore and submerged, for example) and with certain containment choices (vacuum building, for example).

          6. I wasn’t talking about supercritical CO2 turbines. With a direct cycle, you’d have S-CO2 going through your core, which I didn’t want to have to think about.

            Nothing about an sCO2 cycle requires the working fluid to go through the core.  You could use heat exchangers with sodium, lead or molten salt just as easily (though molten salt might not be a good choice because of insufficient freeze margin below the coking temperature of CO2).

            If CO2 (in gaseous form, at about 650 degrees C) could work well in a closed Brayton

            As I recall reading, CO2 starts coking at around 550°C (I could be remembering wrong).

            I suggested to Rod that CO be used instead of N2

            Definitely NOT; 2 CO -> CO2 + C is very exothermic.

            1. @E-P

              Of course it’s possible to add heat exchangers. They are really easy to draw into a schematic system and to enter into system heat balance equations.

              My experience tells me they are heavy, large, expensive, and a maintenance burden, especially when the fluids on separate sides of the tubes have radically different pressures and chemical properties.

              I like direct, simple cycle machines.

          7. Nothing about an sCO2 cycle requires the working fluid to go through the core.

            I agree. However, in this case, the context was about a direct cycle system, and that was what I was commenting on.

            Read again the article and the comments here to see what I mean.

          8. @ E-P
            (From wikipedia on the AGR): “The mean temperature of the hot coolant leaving the reactor core was designed to be 648 °C.”
            From what I’ve seen elsewhere, I’m very sure that that is correct.
            NOW: would CO2 at such a temperature be a good working fluid in a closed Brayton? It is 50% or so heavier than N2, so could existing turbines designed to work with air still be used effectively?
            The ad from 1956 seems to assume that the answer to the first question is “yes”, but I’ve no guess as to the answer to the second question.

          9. Brian, I looked up the pressure and temperatures for the Dungeness B AGR.
            Pressure: 32 bar
            Inlet temperature: 320 °C
            Outlet temperature: 675 °C
            Could a “Modular AGR” (under 200 MWth, but with the same pressure and temperatures as Dungeness B) produce a worthwhile working fluid for an Adams Atomic Engine?

          10. It is 50% or so heavier than N2, so could existing turbines designed to work with air still be used effectively?

            I doubt it, because the ratio of specific heats is very different and this affects quite a few things about the turbine design, especially the pressure and temperature ratio between stages.  sCO2 designs like the recompression cycle analyzed by Dostal operate at a pressure ratio of about 3.  If you aren’t going to take advantage of the characteristics of CO2 near the critical point (the specific heat goes sky-high so there is little temperature rise in compression) there doesn’t appear to be a good reason to use CO2.

            Of course it’s possible to add heat exchangers…. My experience tells me they are heavy, large, expensive, and a maintenance burden

            Modern printed-circuit heat exchangers are much smaller, lighter and transfer more heat than shell-and-tube.  Whether modern materials eliminate the maintenance burden is a question I’d love to see addressed by thorough testing.

            I like direct, simple cycle machines.

            They’re attractive, but getting your reactor core and primary coolant out of the high-pressure circuit has its attractions also.  That’s one of the reasons why LWRs have their unique headaches.

            1. @E-P

              …getting your reactor core and primary coolant out of the high-pressure circuit has its attractions also. That’s one of the reasons why LWRs have their unique headaches.

              Agreed.

              Avoiding high pressures is part of the Adams Engine concept. It takes advantage of the single phase nature of nitrogen gas and emulates the pressures successfully used in combustion turbines to produce reasonably efficient, low capital cost machinery.

              The heat rejection/compressor inlet phase of my proposed closed cycle system takes place at atmospheric pressure (~100 kPa). For initial versions of the machine using currently proven fuel characteristics the compressor produces a pressure ratio of about 8:1, making the highest pressure in the system roughly 800 kPa.

  2. Yes…they are Keith, but I believe they are using Rankine cycle steam turbines attached to it.

    The Brayton closed-cycle gas turbine is the actual stumbling block all Gen IV reactor designers have come up against because the lack of a serious R&D program to develop one, Ford notwithstanding 60 years ago. It is what caused Westinghouse, in large part, to back out of the S. African joint venture there (thus killing the project). It is why the Chinese just said “screw it, we’ll convert the energy to steam” for their HTR reactor.

    I look forward to the “Frame” closed-cycle GT ASAP. But real money has to go into this and I don’t know where it is going to come from.

    1. @David Walters

      I’m a lazy cheapskate. Besides, AAE, Inc. had very limited funds.

      We looked at the cost of developing helium turbomachinery and the time that it would take to complete the task and determined that we needed a different path. That is when we realized that nitrogen would work almost as well as a reactor coolant and far better as a working fluid in conventional, already developed turbomachines of many sizes, makes and models.

      The only moderately sized stumbling blocks are the production of C-14 from neutron irradiation of N2 and the slightly higher enrichments [or shorter core lifetime] required to overcome the negative reactivity of N2 gas compared to helium.

      We (and under current conditions, this is a “royal we”) believe that those obstacles are solvable with a far lower investment than needed for helium turbomachines.

  3. This seems like a good thread to ask a question that’s been on my mind lately – whatever happened to the NGNP – Next Generation Nuclear Plant, which was a project with multiple large stakeholders, to create a high temp gas cooled reactor?