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  1. It looks to me like the PB-FHR concept is based on a faulty concept on the power-conversion end.  The designers put far too much importance on load-following and not nearly enough on efficiency.  It’s trivial to reduce output power on a nuclear generator; either steam or electricity can always be dumped.  But reliance on fossil fuels is a huge negative.

    I seriously doubt the thermal efficiency claims made for the open-cycle Brayton turbine.  The GE LMS100, which operates at a pressure ratio of 42:1 and has an intercooled compressor system and 1380°C firing temperature, only reaches 46% efficiency at maximum power.  It surpasses belief that a system with an 18:1 pressure ratio and 670°C turbine inlet temperature could hit 45%, especially given that the gas-fired heat addition is not at the inlet to the high-pressure turbine.  The figures for base-load operation are 236 MW(th), 100 MW(e) or 42.4% efficiency in combined-cycle operation.  I am skeptical.

    GE has no gas turbine designated F7B, but it does have a 231 MW(e) product designated 7F.05.  The GE 7F.05 turbine in fossil-fired service (and more than 600 degrees higher TIT) has a heat rate of 9116 kJ/kWh, or 39.5% efficient; the GT efficiency at 670°C TIT would be much lower.  It would be better to have the PB-FHR drive an independent heat engine and use LMS100’s for peaking.

    If the goal is to eliminate the use of cooling water, the PB-FHR would be better designed with a supercritical steam cycle and dry condensers in natural-draft cooling towers.  It would take a level of effort far beyond a comment to do the analysis, but I’m certain that thermal efficiency in excess of 40% could be achieved.  I could be tempted to do that analysis; we’ll see how much time I’ve got after I finish my other reading.

    1. This gas/nuclear hybrid concept is actually pretty popular these days, and UC Berkeley is not the only place with people looking into it. There is also a group at MIT exploring the concept.

      I don’t work on this stuff myself, so I can’t explain the details, but I did chair a session at a conference last year where these ideas were presented, and from what I understood from what I heard at that conference, I don’t think that you understand the concept.

      The purpose of the nuclear heat source is to improve the efficiency of the natural gas part of the plant. It sounds to me like you have the concept backwards, which is why you have so many questions. The people working on this idea sound very confident in their concept and what it can (theoretically) achieve.

      While it’s always wise to maintain a good deal of skepticism, if I were you, I’d hold off on critiquing the details until you have more information than what can be provided in a blog article about a road trip (which is a very interesting article, by the way). Perhaps you could look up some of the technical papers that have been published on it?

        1. Is there a reasonably short answer to why FLIBE and not another salt? This is not a thorium breeder. FLIBE has the issues with beryllium toxicity and the need for Li enrichment. There some reasons given for why FLIBE is ‘good’ but I didn’t see a comparison to any alternatives … perhaps in the student thesis mentioned?

          1. I think FLIBE is one of the best for low neutron absorption. For salts with higher neutron absorption, if a hotspot develops and a void forms (boiling), the loss of neutron absorption outweighs the loss of moderation, so reactivity goes up and it gets even hotter (“positive void coef”), which is a bad thing.

            For reactors with the fuel dissolved in the salt (e.g. LFTR, DMSR, or IMSR) it is easier to guarantee negative void coef, so more salt choices are possible.

      1. This gas/nuclear hybrid concept is actually pretty popular these days

        I’ve noticed this.  But in a carbon-constrained world where emissions need to drop 80% over present-day levels just to stop the annual increase, building 50-year assets reliant on natural gas is a non-starter.

        What I suspect is that these concepts have been financed by natural gas interests or pushed by their lobby.  They’d love to have their fingers in both the renewable and nuclear pies, and it would help deny oxygen to efforts that eliminate carbon emissions (and their revenue stream).

        The purpose of the nuclear heat source is to improve the efficiency of the natural gas part of the plant. It sounds to me like you have the concept backwards

        I’m no expert in thermodynamics but I can calculate compression and expansion work in gases and also use steam tables to get the textbook problems correct.  I also know a little bit about existing technologies like CCGTs.  I believe I understand the concept just fine, I just think the premise is badly flawed.  Also, 45% efficiency is very low for a CCGT; the best top 60% (LHV).  Why accept such poor performance, when the stakes are so high?  I would follow the money to get insight into that one.

        Not that I don’t think generating heat at a temperature sufficient to keep a modern, high-efficiency gas turbine running at idle power isn’t a great advance, but having the gas-free output be such a small fraction of peak suggests that gas dependency was a design criterion.

        I’d hold off on critiquing the details until you have more information than what can be provided in a blog article about a road trip

        Next Big Future was on this concept some time ago, and the PDF linked by Rod hasn’t cleared up any of the issues that bothered me when it first came to my attention.

        1. Then sorry, I can’t help you. You’re not very convincing, especially since you’re now spouting stuff that sounds like conspiracy theories.

          If look at the first page of the PDF that Rod linked to, you’ll see that this research was funded as a NEUP project. As someone who is listed as PI on a couple of these projects (both approved and pending), I can assure you that the funding for this research is not decided by a bunch of oil barons.

          1. You’re not very convincing, especially since you’re now spouting stuff that sounds like conspiracy theories.


            you’ll see that this research was funded as a NEUP project.

            Yes, I did note that.  If you know anything about Congress, the steering of grants to favored clients comes up fairly regularly (usually in the context of a scandal).  Picking a gas-dependent nuclear concept over others would be so esoteric, it would be unlikely to make the news.  All it would take is a few words from a congressman on the subcommittee which writes their budget.

            I can assure you that the funding for this research is not decided by a bunch of oil barons.

            We have certainty that oil barons are controlling the major environmental organizations through the foundations which give them the majority of their funding.  This would be just a bit more of the same.

            I’m going to dig into the details of an ultrasupercritical steam cycle today, I think; this is just too salient to leave alone.

            1. @Engineer-Poet

              I’m going to dig into the details of an ultrasupercritical steam cycle today, I think; this is just too salient to leave alone.

              The design report indicates that power conversion cycle is covered in more detail in the following two references:

              C. Andreades, R.O. Scarlat, L. Dempsey, and P.F. Peterson, “Reheat Air-Brayton Combined Cycle (RACC) Power Conversion Design and Performance Under Nominal Ambient Conditions,” ASME Journal of Engineering for Gas Turbines and Power, vol. 136, No. 6, doi:10.1115/1.4026506 (2014).

              C. Andreades, L. Dempsey, and P.F. Peterson, “Reheat Air-Brayton Combined Cycle (RACC) Power Conversion Off-Nominal and Transient Performance,” ASME Journal of Engineering for Gas Turbines and Power, vol. 136, No. 7, doi:10.1115/1.4026612 (2014).

              At least two of the authors of those reports have commented in this thread, so if you come up with specific questions, perhaps you can ask them.

          2. Heh … have fun with that. You’re just making yourself look silly. But if you want to learn more you might want to start here:


            You clearly don’t understand how NEUP works. It appears to me that you don’t understand how the economics of the hybrid concept works. Thus, it’s pointless for me to waste my time trying to explain anything further.

            Perhaps one of the people working on this design can answer your “questions,” but since you already have access to their 156-page design report, I’m not sure that even they can help you either.

          3. Brian, I can see what the claims for NEUP are.  That’s the problem:  it’s way, way too reliant on natural gas.

            I’ve finally gotten back to analysis (had time to review the overview PDF for the Hempstead 600 MW “ultrasupercritical” powerplant) and one of the things I note is that the heat rate of the plant is 7246.6 BTU/kWh or a thermal efficiency of 47%.  That is at a maximum steam temperature of 608°C.  I haven’t had time to re-calculate what the efficiency would be if the steam temperature could be increased to 670°C, and I want to take a whack at calculating the losses if the exhaust steam temperature was increased to ~100°C to accomodate dry cooling towers with none of the piping under vacuum.

            I realize that the economic case uses natural gas for rapid load-following.  This looks like a means to accomodate the political insistence on large amounts of unreliables, specifically wind and solar.  However, 66% efficiency is not nearly high enough to cut carbon emissions as far as is required; even a 120% effective efficiency only cuts net emissions to ~250 g/kWh, 5 times the 50 g/kWh the IPCC gives as the maximum.  A superior system design would load-follow using energy from thermal storage, not fossil fuel.  A reactor producing heat at 700°C would have no trouble warming up a heat-storage system using solar salt at 525°C.  Find a way to take stored heat and produce steam at 500°C and 35 MPa and you’re talking.

          4. Well, it’s not my concept, so it’s not really necessary or appropriate for me to defend it. I’m just sharing my impressions.

            But, if you do want to discuss these technical issues in detail, I see that registration for ICAPP-15 is now open. I’m sure that there will be a paper or two presented at this conference on the nuclear-gas-hybrid concept, as there were at last year’s conference. You could take up these issues in person with the folks who are actually working on this idea.

            Plus, this year’s conference is being held in May in the French Riviera — in case you need additional incentive to attend. 😉

          5. Travel to France is way beyond my means unless people want to send me there as a press agent or something.

    2. Regarding the surprising efficiency of the PB-FHR, remember that the Berkeley concept is a combined cycle system, which is to say that the waste heat from the gas turbine (which comes out at 400C for nuclear-only operation) is used to power a boiler (HRSG) and steam turbine which likely produces about half of the total shaft output power.

      I had hoped that someone would find an economical way to couple an FHR with thermal energy storage (based on solar-salt) to provide economical load following in a renewable-rich grid. But as the pumped-hydro companies in Germany are finding out, it’s hard to make a profit by putting storage on the grid; fast-ramping fossil fuel powered plants are simply much more economical. The more renewables we deploy, the more we’ll need fossil fuel.

      The nice things about the PB-FHR’s use of fossil fuel are that it is very efficient (66% for the co-fired gas), plus the ramping is very fast and the minimum gas-fired power is zero (i.e. for spinning reserves).

  2. should of let me know you were coming out here! I was at the colloquium talk for Kirk last month! Could of taken you around the Bay!


  3. Rod, It was great to have you visit and give a colloquium. On the comments on the Nuclear Air Combined Cycle, you are correct that it is better to read the design report to avoid making completely incorrect statements. All electrical power systems need peaking capability to match generation to load, and having power conversion systems that can ramp rapidly and convert chemical fuels (natural gas, bio gas, hydrogen) into peak electricity with a thermal efficiency of 66% generates substantially higher revenues than a conventional base-load-only plant does.

  4. One thing to keep in mind when talking about cycle efficiencies is not to mix up simple cycle (the gas turbine all on its own) with combined cycle efficiencies.

    Having said that, the NACC (proposed power conversion) uses a modified GE 7FB in combined cycle and has two operating modes, namely baseload nuclear heat and natural gas injection for peaking power. In pure nuclear mode, the NACC has a turbine inlet temperature of 670C, producing 100MWe with an overall efficiency of approximately 42%. In cofired mode, the turbine inlet temperature is increased to 1070C by burning NG and has an overall cycle efficiency of 54% and a power output of 242MWe. NG itself is converted into electricity with an efficiency of 66%. As a reference point, the GE 7FB in conventional combined cycle configuration produces 280MWe with an efficiency of approximately 57% and turbine inlet temperature in the 1300C range. As stated, state of the art combined cycles reach an efficiency of 60%.
    In terms of CO2 emissions, what this entails is that the NACC produces nearly half the emissions on a per MW basis compared to the conventional GE 7FB CC configuration.

    As far as the GE LMS100 is concerned, it is an aeroderivative GT, not a heavy industrial frame GT, used for large scale electricity production, hence its much higher PR, intercooling, multiple shaft design. The 46% efficiency is a simple cycle efficiency. The reason for all these design choices is that jet engines need to be lightweight and extremely efficient to reduce fuel load on the plane – and these were carried over when the machine was used for land applications. Large industrial frame type turbines sacrifice performance for ruggedness, therefore the “simpler” design (single shaft, lower PR, lower TIT, and lower overall efficiency, etc).

  5. I don’t know, but I keep coming back to the thought that building nuclear systems that rely in any way on natural gas seems to defeat one of the major arguments we all have been pushing in one form or another to advance the nuclear agenda. Poster Engineer-Poet puts it nicely above:

    “But in a carbon-constrained world where emissions need to drop 80% over present-day levels just to stop the annual increase, building 50-year assets reliant on natural gas is a non-starter.”

    I can’t help but agree 100% with this argument. I really don’t see any advantage in hooking our wagons to the natural gas star. We’ve just seen a case (Vermont Yankee) where a perfectly good piece of generating infrastructure with many more decades of useful life was thrown away because of a combination of politics beefed up by appeal to cheap and supposedly abundant natural gas as a “replacement” for the nuclear generator. Never mind the terrible effects of methane on the atmosphere, or the millions of tons more carbon dumped into the biosphere. Throwing in our lot with those who are running us out of business reminds me an awful lot of the tale of the farmer and the viper.

    1. @Wayne SW:
      It isn’t that simple. Electrical power peaking is an absolute requirement. (You know that). Standalone nuclear is capital intensive and economic (today) essentially only in baseload. (For suitably lax definition of “essential”. You know that as well.)

      An open-cycle GT used for peaking has a thermal efficiency of about 40%. I don’t know how a CCGT behaves in peaking mode, perhaps Drs Andreades or Peterson can help out. But if NG is usually limited to about 40% in peaking mode, and in the NACC lashup “NG itself is converted into electricity with an efficiency of 66%,” this is a huge improvement.

      Its the economics that count: How do we build low-carbon dispatchable energy cheaper than coal? We need baseload power. And we need peaking power. Here we have a machine that combines both, saving on common plant material, personnel. and equipment, and passing on the cooling towers which, though effective, are not cheap.

      Let me explain — with apologies to Prof Peterson — what I see to be the significance. A while back, in a fit of pique I did a back-of-the-envelope estimate of what the capital and emissions costs would be if, say, in NREL’s Renewable Electricity Futures 2012, one were to merely swap out coal in their baseline scenarios and replace it with conventional LWR nuclear, all else being equal.

      Save the wind. Save the solar. Pass on the golden opportunity to obtain 15% of the country’s electric power from carbon-neutral biomass co-fired by coal. Just jack out the coal plants and plug in AP1000-type nukes.

      Spoiler alert: At the 80 – 90% reduction levels considered by NREL, one gets modestly better emissions reduction at substantially lower capital cost over NREL’s optimized “renewable” solution just by replacing coal with conventional nukes.


      That substantially lower nuclear capital cost is still substantially more than the cost of just replacing the old conventional coal as it expires with new PC or combined cycle natural gas. I assumed the new nuclear would run at the same 85% capacity factor as the coal it replaced.

      Not good enough. (1) We need clean energy cheaper than coal. Clean energy more costly than coal is a tough sell. (2) As you observed, 85% emissions reduction isn’t enough either. Not from the electric power sector. There we’ll need, by the end of the century, more like 95% or 99% reduction.

      But that last 15 to 20% of electric power emissions, between 80 or 85% and 99%, comes from peaking gas. And you aren’t going to replace that with conventional nuclear, not at any reasonable cost. But if this UCB effort cuts peaking gas emissions by 40 or 50%, then combined with baseload nuclear either from NACC or in combination with standalone nukes (LWR, MSR, SFR, whatever) and we’re a bit over 90% emissions reduction. Add a bit of wind and we’re probably close to the ballpark.

      By that time Prof Peterson’s grad students’ grad students’ grad students will have figured out how to combine nuclear peaking electric power generation with intermittently required process heat e.g. hydrogen production, desal, or whatever, and they’ll have the emissions problem solved.

      All the rest of us gotta do is let Congress know we actually want this stuff to happen.

      If I read this right,

      1. I guess it comes down to what the driver is for obtaining the last 15-20% peaking requirement. You are assuming cost is the major driver, and I can see how that would lead to your conclusion. I was looking more at Engineer-Poet’s viewpoint, wherein more emphasis is placed on the carbon-constrained world. We have BWRs now that can load-follow with fairly rapid response down to 85% FP on recirc flow rate alone. Not as economic, I know, as running full out, but if your overarching goal is eliminating carbon and GHG gases like methane it would be a way of maintaining the 15-20% peaking capacity without using a carbon-based fuel. So imagine a system with a modest overbuild of BWR capacity which would allow a 15% reserve margin for peaking when needed. You’d still be running those plants at near full power a lot of the time, with some throttling back now and then to allow for lower demand. It would affect the long-term economics to sink those capital costs and recover them over a longer period, but if we also make changes on the regulatory and market side to allow for 60-80 year plant lifetime without a lot of regulatory hassle, you’d probably get that back and more, and avoid the downsides of burning natural gas.

        1. Thanks Wayne. I knew BWR’s were good; didn’t know quite that good. Problem is, that last 16% of peaking generation represents the other 80% of required capacity. Not kidding. My 85% emissions reduction estimate figured as follows:

          Generation Mix (%) Generation Capacity (GW)
          Nuclear: 64% 357 GW
          NG: 16% 395 GW
          Hydro: 8% 79 GW
          Wind: 6% 83 GW

          … the remainder being taken up by geothermal, biomass, and other rubbish. Total capacity is 975 GW and average demand is 447 GW, for net overall generation capacity factor of 46%. Nuclear Cf is (0.64*447)/357 = 80%, gas Cf is (0.16*447)/395 = 18%. Replacing the gas generation AND capacity with nuclear would run the nuclear at Cf = (0.80*447)/(357+395) = 48%. You don’t wanna do this.

          But you are right: that last of the gas will eventually have to go. Demand shifting and storage it is then. (A modest increase in wind and solar generation might also help, but I haven’t run the numbers.)

          Ref: See Figure 6 in 10.5.5 The United States: Renewable Electricity Futures Study 2012, and commentary that follows.

          1. I did some crude number crunching on Cal Abel’s steam-compression heat pump concept, assuming an AP1000 as the steam source and compression to 5000 psia to feed a solar-salt heat battery.  Assuming a diversion of 20% of the steam supply to storage, my numbers came out 712 MW(e) net in full storage mode and 1441 MW(e) net in full extraction mode, a ratio of more than 2:1.  Assuming 6 hours of storage that’s fully filled and emptied every day, the average output would fall from 1114 MW to about 1095 MW, ignoring heat losses.

            The nuclear steam supply would be unaffected by this, so it might be relatively simple to certify.  It would allow the average nuclear capacity to be increased to far more than minimum load levels without having to reduce the capacity factor by much.

          2. I don’t doubt your numbers. I should clarify that not everyone considers load-following nuclear units as falling into the “peaker” class, as they are assumed to be running already and don’t have the quick-start from shutdown feature of a true peaker. So I guess they’re an in-between class (Wiki calls them “mid-merit”, which is as good a name as any). The French-design PWRs can also load-follow fairly quickly since they’ve designed in the “grey” control rods for that purpose. I don’t like those as much as BWRs because any kind of control rod perturbation means flux tilt, which complicates fuel management. I’d much rather go with some kind of global control like BWRs can do in a load-follow situation. If we can extend the low end of the load-follow range without having to mess around with the reheater problem below 85% FP then nukes can really do it all. I understand that there is an economic penalty for not running the nuke at full capacity, but whatever monies must be recovered for that surely must be less than what a gas-fired peaker commands on a per MWh basis.

    1. @Rob Brixey

      So you think that a 1950s vintage essay should dissuade people from believing that they can do better than the rather disappointing market acceptance for non-paper reactors during the past 40 years?

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