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  1. “… a strong emphasis on building indigenous capacity to build HTR without outside assistance.”

    Except that their fuel technology came straight from Germany. They literally bought and shipped the equipment.

    Why are they bragging about their pressure vessel? This must be some sort of misunderstanding by the communist propagandists writing this piece. A Pebble Bed HTGR has a pressure vessel that is roughly equivalent to a BWR — similar size, similar pressures.

    1. Brian – manufacturing without outside assistance doesn’t include doing all of the invention and designing domestically – especially in context of Chinese development.

      They can make the pebbles without importing anything else. I’d be willing to bet that they used the purchased equipment to supply fuel for the HTR-10, but they have copied all they need and added some refinements to produce the fuel pebbles for HTR-PM.

      1. Well, I was more referring to the TRISO fuel. That’s the hard part to get right, since the safety case depends on it.

        Anyhow, I’m sure that they have reverse engineered and replicated the German equipment by now. They’re sticking with UO2, so they’re sticking with what they know works.

        The other thing that I don’t understand is the “super critical … steam turbine.” Modern coal plants use supercritical water, but I haven’t been able to find anything to indicate that the pressures of the HTR-PM steam generator output are anywhere near high enough to produce supercritical water. It probably provides high-quality superheated steam, but that’s not the same thing.

          1. They probably did that to avoid having to reheat the steam.  Eliminate the HP turbine section and you don’t need to.

            Just an aside here:

            I dug up a diagram of an ultrasupercritical steam plant (original is off the web but pdf available here) and the high-pressure boiler puts out steam at 3623.7 PSI (gauge, I assusme) and 1114.3 F (601.3 C) and the reheat yields steam at 748 PSI and 1125.6 F (607.6 C).  These temperatures should be easily achieveable with the X-energy reactor using somewhat better heat exchangers.

            Any of these HTGRs should be able to repower an ultrasupercritical steam plant, albeit it may take ten or so of them.  But then, they’re small aren’t they?

            1. HTR modules are not physically small, but neither are the furnaces and boilers in a large coal plant.

              I’m open to a quantified discussion assuming some can bring in the fact we need to address.

              Good thing about HTR modules is that they should be mass producible. Certainly the fuel ball are.

      1. If you dig through the history of the technology, you’ll notice that the Xe-100 also has many common features with the German HTR-Modul. There seems to be a sweet spot that the reactor designers keep coming back to.

        [Disclaimer: I work for X-Energy, but this comment is mine alone and in no way represents the opinion of my employer.]

        1. As I recall, the basis for the “sweet spot” of about 100 MWe (~250 MWth) is passive safety modeling as verified and validated by physical tests.

          For the usual configuration of core height and diameter, HTRs <300 MWth can maintain all fuel within acceptable conditions without any forced coolant flow for all design basis events. It’s also true for most, if not all, beyond design basis events.

          1. Rod – You are correct. The key factor that determines the power level is the ability to reject the stored thermal energy and decay heat during a DLOFC (depressurized loss of forced convection) accident, otherwise known (mostly by General Atomics people) as a Depressurized Conduction Cooldown.

            The core geometry is an essential part of this process, and this has been realized since the mid-1980’s. The primary heat path is in the radial direction. The core is sized (here I mean power level, but physical dimensions also matter) such that the heat can get out of the vessel before the fuel temperatures become high enough to begin causing fuel failures and the release of radionuclides. TRISO fuel is very forgiving, so these temperatures are quite high.

            As these analyses have been performed over and over, with even more modern tools, the calculations keep coming up with roughly the same answer. The tools may change, but the physics remains the same. The old-timers knew what they were doing.

    1. It is unlikely to change very much if that is the only change. It’s likely that designers would maintain the same system pressure, which is a big driver of vessel wall thickness and mass.

      1. Are these oressure vessels any easier/faster/cheaper to construct than the containment domes of Light Water Reactors? Can they be factory built? Can the rest of the power plant?

        1. @jim

          You’re asking the right questions. As I noted in my “aside” talking about possible reactions to the size and weight of HTR pressure vessels. “there are more factors to be considered in atomic fission power plant economics than the size and weight of the pressure vessel”

          One way to understand why some of those additional factors might significant outweigh any cost increases associated with pressure vessels is by perusing this collection of nuclear power plant wall charts. In many of them, FINDING the pressure vessel is like playing “Where’s Waldo?”


          With the right industrial infrastructure, it’s quite possible to mass produce pressure vessels, especially if they are as simple as possible. The wall thickness is also an important consideration. A thinner wall makes manufacturing and quality control simpler and cheaper even if the vessel diameter and height make the mass larger than that of a shorter, thinner, but thicker vessel.

          In the HTR, the maximum operating system pressure is less than half of a PWR, and there is no potential for the kind of rapid pressure increases that are at least theoretically possible in a PWR.

          1. I know you’ve got a lot of love for pebble fuel, but would it be possible to take the molten salt in fuel pins concept from Moltex’s reactor and stick them in this helium cooled reactor for high temperature heat produciton?

            Of course online refueling would be more complicated with tubes than dropping pebbles in the top of the pressure vessel and extracting them out of the bottom using gravity.

            Are their other drawbacks that are not obvious to a non nuclear engineer such as myself?

            I’m asking because pebble fuel seems to be regarded as relatively expensive and also difficult to reprocess, while molten salt fuel is apparently cheaper and much easier to reprocess.

            1. Are Moltex fuel pins clad with a material that can provide similar resistance to damage at high temperatures?
              Can those fuel pins be replaced continuously while the plant is operating?
              Does Moltex fuel pin technology rest on proven performance in both operating power plants and carefully executed test programs?
              Where did you get your cost estimates?

            2. I’m replying to my own comment as I cannot seem to reply to yours.

              I got the cost estimates from the Illinois Energy Proffessor video:


              Where he states:
              Gas = 2 cents per kW-hr
              Coal = 3 cents per kW-hr
              X-Energy’s Xe-100 = 8.4 cents per kW-hr

              If the gas cooled pebble bed reactor costs more than natural gas and coal power then it won’t replace them without subsidies, a carbon tax, or mandates, all of which are controlled by politicians and lobbyists.

              If I understand it your clever idea is to overcome the higher cost of the pebble fuel by using heavy nitrogen and a directly heated brayton cycle turbine without heat exchangers and a steam turbine system to get the overall cost down by reducing the total cost of the power plant.

              I was spitballing if instead of pebble bed fuel, cheap molten salt in fuel pins could be used in a gas cooled design as an alternate way to get costs down to beat natural gas and coal while keeping the heat exchangers and steam turbine system. Or if you could even combine molten salt in fuel pins with your heavy nitrogen direct cycle brayton turbine approach to eliminate heat exchangers and get costs down even futher? I assume the cheaper the electricity or high quality heat from these plants the faster they will replace natural gas and coal.

              1. @jim

                Your professor uses second hand information without any real context.

                The Triso particles that form the basis for HTR fuels and the pebbles that consolidate 10,000 or more of those particles in each pebble are both items whose economy will change when they are being mass produced in highly automated factories with automated quality assurance inspections.

                At this time, both particles and fuel elements have been almost hand made with very low volumes, even compared to the amount that will be needed to fuel a single Xe-100.

                After I started getting excited about pebble bed reactors in the early 1990s, I had a period when I needed to seek outside employment. I landed a job as the General Manager of a small plastics products manufacturer. I produced hundreds of cost models for our wide range of products and learned in a first hand way how much volume mattered.

                I remember one customer coming in with a nice looking drinking “glass” to be made from colored plastics. I asked him how many he wanted. He said he needed just a handful to show customers. Then he figured he could line up enough orders to make hundreds of thousands. I told him that the first small batch would cost $10,000. Once volume reached 100,000 every month, they would cost about $0.25 each.

  2. IIRC there has been some work done on a pebble bed reactor with fluoride salt coolant. Perhaps just a paper reactor, but a coolant that can run at high temperature & low pressure sounds like an advantage. Stuff that is more common than helium is an an advantage too.

    1. @James R. Baerg

      Kairos Power is developing the KP-FHR, a molten fluoride salt cooled high temperature pebble bed reactor. Like any new technology, it started out on paper, moved to computer modeling and is now steadily being turned into real, extensively tested hardware at the component and subsystem level.

      The company was awarded a “risk reduction” grant from the DOE Advanced Reactor Demonstration Program that will help it build a small scale prototype of its system near the Oak Ridge National Laborator.

      Kairos has been covered here several times. https://atomicinsights.com/?s=Kairos

  3. All I wonder about is if the HTGCRs can be scaled-up and away from the radial conduction “sweet spot” mentioned in this comment thread… Heat pipes in the periphery? Void the inner reflector for draft? Typical armchair expert musings… UK AGRs use boilers and blowers to remove decay heat in off-normal/isolated conditions; many were built and there were no major events/accidents in probably 500+ reactor operating years of experience. They weren’t really a resounding success compared to LWRs (low fuel utilization, teething problems), but the technology could undoubtedly be evolved/improved. Modern HTGCR technology is hung-up on the radial conduction decay heat safety case, which is limiting their output to literally an order of magnitude less than seen in the AGRs, which [it could be argued] limits their value for first-world grid-scale power applications. Additionally, the PB types are giving up a lot of finesse with their cores that are literally hopper-fed heaps of dusty, chipping spheres with estimated/averaged properties/arrangement. I imagine the pebbles are cheaper, and easier to handle than thoughtfully-loaded, but screaming hot, prismatic fuel (Ft. St. Vrain)… in air no-less. Of course the 10%+ enriched PB cores are on the order of 0.05gU/cc, compared to ~2.7gU/cc for LWR, which translates to 25x higher spent fuel volume (25x more dry casks per MWD). Is the fuel expensive to make? https://www.nrc.gov/docs/ML0310/ML031000210.pdf Can they run HTRPM with 50 guys?
    Regardless, one may hope the Chinese will eventually depart from their tight-lipped precedent and allow all of us fans to share in the presumably un-eventful operational experience of these interesting reactors. Perhaps much of what I perceive as unnecessary secrecy is a kind-in-kind reaction to 10CFR810 – after all, the recent Chinese fuel leaker story ‘leaked’ to CNN had genesis in Framatome seeking US Government permission to share information with its own office in Lynchburg, VA.

    Go China! Maybe someday they’ll do something new, and then, let the competition begin!

    1. It sure does seem like they are doing new stuff. They have purchased models of many of the Western reactors. They have the CANDU, the AP1000, the EPR, their own Huonglong One and the molten salt reactor. They are developing this stuff and copying it. They are in the process of developing a small modular reactor for export.

      That country blows my mind. I mean this is red China. They were an agrarian poor country when I was a kid. There were few if any high rise buildings, no high speed trains and enormous dams. My dad had been there in World War 2 and shook his head in sadness when he told me how poor the people were.

      It’s hard to buy manufactured products today that are not made in China.

      They are not a Democracy. They don’t believe in free speech and yet they seem committed to helping their people. I don’t think they have the dog eat dog philosophy in many areas. They also seem to have more patience in developing technologies because they are not tethered to the profit motive in the same way as the West. It’s the Yin and Yang thing.

      Did I mention they’ve gone into space? They’ve even gone to the moon to get rocks.

      Sorry, if I veered off a bit there Rod. I just wanted to make a point. The US used to be the pioneers at a lot of this stuff. Now, you got folks afraid of basic science.

      If things keep going like they have been, I think the competition will be totally one sided. I think investors will put their money on China like they have in virtually everything else.

      Some of you nuclear engineers might want to start brushing up on your Mandarin and Cantonese. 🙂

    2. I guess I understand people admiring the Chinese flexing… Can investors actually invest in China? Did you intend to be humorous with the dog remark?

      Yes, the Chinese can build PWRs and have a MSR experiment in the desert – gravy for a generation of youtube fans. If any stable nation wanted to buy a SMR on the export market, I’m fairly certain the Russians would be happy to unplug the Lomonosov barge and tug it over.

      To be innovative in nuclear power… that is truely elusive! There’s nothing new under the sun. A gen-boomer, who openly takes credit for the GA EM2 core design, said the problem with nuclear power is incrementalism, but that continues to work well for aviation. If the development of a super structural material enabled a new design point, you must imagine that each of the world’s dominant cultures would jump on the opportunity to improve the fission reactor.

      It often seems we are tugged in too many directions by our democracy – we seem paralyzed by situational complexity, differing viewpoints, special interests, fake and un-fake news… but the result is that time passes, we don’t chase geese [usually] and what is a real crisis eventually comes to a head without cheerleading, and censorship or mind games. Everything will work out in the end.

      1. Thanks for responding:

        Can you invest in Red China? You betcha. In fact, according to this article investment has been increasing. Here’s an excerpt:

        “ Despite economic and financial tensions and a plethora of foreign restrictions on the transfer of technology to China, China continues to attract record amounts,…”

        Here’s the link:


        Capital is going to flow where the greatest return is expected. If you sell out Grandma, when it’s done folks will justify it by saying Grandma was a liability on the balance sheet.

        I intended no humor with the dog remark, but I’m mean enough to like the idea. In reality, I kind of understand why a country jam packed with 1.4 billion people eats dogs, cats, rats and Covid carrying bats. Unlike a lot of other people, I could understand their former one child policy.

        I’m one of those You Tube fans of the molten salt reactor. I’m surprised that some big company didn’t hire Gordon McDowell and Kirk Sorensen to pitch cars and refrigerators.

        As far as being innovative, maybe you don’t have to be that innovative. Look at all the reactor types that have been paper reactors over the last 70 or so odd years. Bill Gates is hoping to build one in Wyoming. GE- Hitachi was touting it for some time.

        Why aren’t we doing more with the many ideas in the US? I think Mr. Adams may be on to something when he implies how the oil companies have sort of rigged the game.

        I’ve given some thought about those different directions you mention recently. I’ve come to really wonder about this philosophy about privatization, the free market and small government. This philosophy relies on the “invisible hand” to move the progress of mankind forward. I look at the Chinese and while they have the free market, they aren’t afraid to lend their capitalism a “guiding hand.” It seems to work.

        One example not related to nuclear power. This statement is from the Wikipedia article on Healthcare in China.

        “The Chinese government is working on providing affordable basic healthcare to all residents by 2020.”

        This ain’t supposed to be the thing you read about Red China. It wouldn’t have been in the comic books I read as a kid.

        Yeh – Everything will work out in the end.

        1. I’ve come to really wonder about this philosophy about privatization, the free market and small government. This philosophy relies on the “invisible hand” to move the progress of mankind forward. I look at the Chinese and while they have the free market, they aren’t afraid to lend their capitalism a “guiding hand”…

          Indeed. There are no “free markets”. None. Nor did Adam Smith envision any. Rather, Smith made the modest proposal that economic markets be regulated so as to best serve the interests of society, and suggested that the fewer such regulations, the better. His “invisible hand” followed.

          However, had Adam Smith lived a century later, and witnessed the rapaciousness of US railroad and oil barons, he would likely have rolled his eyes and cheerfully lobbied for FTC and Sherman Anti-Trust.

          Basically, any market here is deemed a “free market” if it serves the interests of those interested in deeming it “free”. Any reforms that threatens those interests, such as allotting nuclear-generated energy some small fraction of the subsidies such free-markets afford to wind, solar, and gas, will (and have) see wind and solar interests cheerfully make common cause with gas interests to stymie such reforms — lest they “break the free market”.

          Books have been written about the highly-unfree nature of “deregulated” U.S. electricity “markets”. Meredith Angwin’s Shorting The Grid is excellent, and a rollicking good read.

    1. Some of it goes into heating up the large mass of mostly graphite in the core. Another portion gets radiated to the core pressure vessel and into surrounding environment. Radiative heat transfer increases as core temperature increases.

      At the same time, decay heat production rate slows quickly after fission stops. Most of the isotopes producing it are quite short lived.

      After a relatively short time – measured in minutes to tens of minutes – heat addition rate equals heat losses. Core reaches a steady state temp that is well below the temp that produces fission product release and fuel damage.

      If simple cylinder core is less than 300 MWth, this process works without any operator action or electrical power.

  4. Two days ago I listened to Evergreen Action’s “Policy + Pints with Sen. Wyden: Why Clean Energy Tax Credits Are Crucial” webinar. Evergreen Action is Jay Inslee’s climate PAC. Panelists were

    Becca Ellison, Evergreen Action, moderator.
    Jason Walsh of Bluegreen Alliance.
    Gilbert Campbell, Volt Energy Utilities — a Solar company involved in policy and lobbying.
    Sen. Ron Wyden, (D OR), Chairman Senate Finance Committee.

    Walsh is a former policy advisor in Obama’s DOE. He works on getting Labor onboard with climate action, and climate justice and job retraining issues. Bluegreen Alliance includes Sierra Club and steel unions.

    Gilbert Campbell is a tax expert, and explained how crucial ITCs are in the requisite effort to expand solar from its present 4% of US power generation to the 45% required by 2035. He speaks very well.

    Sen Wyden explained the far-ranging (or fetched) reforms in the Senate’s version of the $3.5 trillion Infrastructure Package. He explained there are 44 separate energy tax breaks in the federal code. His bill would eliminate all of them. The Clean energy for America Act would eliminate all explicit fossil fuel subsidies as well. Senate Finance’s version would replace all this with three classes of technology-neutral tax credits for Clean Energy, Transportation, and Efficiency.

    These would be technology-neutral and follow a results-driven market mechanism: the more you reduce emissions, the greater your tax break. You’d be able to choose between ITC, PTC, and Direct Pay.

    There is considerable daylight between Senate Finance and House Ways & Means. As some might imagine, the House version does *not* have Finance Committee’s tax break elimination and replacement with results-driven low-carbon credit. This is in reconciliation.

    Senator Wyden stressed that such credits would not be just for wind and solar, and gave an example of how a Republican colleague from “a very conservative district” had explained the importance to his constituents of Geothermal.

    Nuclear was never mentioned. The way Sen Wyden stressed “Geothermal”, it didn’t need to be.

  5. Honestly, you gloss over the reality of the THTR failure, which detracts from the credibility of the article.
    The reality is that the pebble feed jammed at 2 am and the operators, rather than call in help, attempted to clear the jam with a broomstick. through an access hatch.
    They broke more than 3 dozen fuel spheres, releasing some undetermined contamination into the countryside. They also destroyed the credibility of the nuclear industry in Germany, it was one egregious lie too many.

    1. I’d like to learn more. I haven’t found that information in any sources that I’ve read. Admittedly I do not read or speak German, and Google Translate (or similar tools) did not exist in the late 1980s.

      But I can’t imagine that any radiation release from such an event would hurt anyone or cause any environmental damage. I CAN imagine a number of differently motivated groups blowing the incident way out of proportion.

      Don’t forget that coal plant are designed to dump tens of thousands of tons of polluting gases and ash into atmosphere every day – and no one seems to freak out about “leaks” from smokestacks.

      1. I googled ‘THTR reaktor’ to get the German Wikipedia entry on it, then hit the ‘Translate to English’ option. It’s much more detailed than the English version – for example, I’d never heard that they used 93% enriched U235 to enable breeding from thorium, but the U233 produced would have been contaminated by U236 bred as a side reaction from U235 ( not that they did any reprocessing.) Or that helium becomes more viscous when it gets hotter, which interferes with the cooling, and disrupts the modeled flow pattern of the fuel spheres. You probably know all this stuff, but I’d never seen it in English sources. Nothing about broom handles, though – I wouldn’t have thought that advisable for the broom operator !

  6. Rod and Brian make the important point of temperature limitations relating to shutdown heat removal for pebble bed gas cooled reactors. Certainly, a pebble bed has only point contact, and so, is a poor conductor heat sink. Without high pressure gas coolant around (depressurized state, due to a pipe break or other malfunction) this leaves it mainly to thermal radiation for heat transfer, which is only effective at high temperatures, hence high accident temperatures in the fuel.

    This is certainly borne out by some interesting analysis work done by the venerable Syd Ball, where the GT-MHRs prismatic, solid core had lower accident temperatures than the pebble fueled PBMR, despite the GT-MHR being a much bigger reactor.

    That’s unfortunate, since pebble fuel is simpler, cheaper and allows online fueling to improve fuel utilization and plant capacity factor.

    This makes me wonder if we can have it both ways: a pebble bed core with a bunch of solid graphite cooling fins in it. Perhaps a simple radial geometry, where the pebble bed is divided into slices. This could really improve conduction heat transfer through the bed. Not sure how the neutronics would work out – one would have many slices of reactor sections arranged radially.

    Similarly, it could be advantageous to have cooling fins between the core barrel and the reactor vessel, for designs that use a core barrel annular arrangement.

    Further downstream it may also help to add cooling fins to the vessel outside surface, for improved thermal radiation to some passive heat sink system for ultimate heat removal.

    Surely, one could do better than 300 MWt with some simple cooling and heat sink enhancements?

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