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  1. Nice article. Karios seem like a BS free enterprise. My only concern is the FLIBE. Would that not need lithium enrichment? Beryllium is a bit nasty. Does this produce tritium when irradiated?

      1. Getting on and doing the job methodically/realistically without the hype/unproven claims.

        I think Dr Charles Forsberg said new reactor designs was a liars club.

      2. BS free, as in free of BS

        Not BS free-enterprise.

        So yes, definitely a compliment.

        Clear English not my strong point. Apologies.

    1. Your question about lithium enrichment is a good one, but was sidestepped.
      The issue has become more serious, with recent political changes in US-China trade relations.

      Similarly, the internal structure of FHR pebbles is different.
      To my knowledge, neither the fabrication nor long term endurance testing have been done, for this novel design (….or have Kairos abandoned that too, reverting to standard fully-tested pebbles?)


  2. “The FHR idea is to combine some of the best features of reactors typically cooled by a gas like helium or nitrogen with some of the best features of molten salt fluid fueled reactors.”

    This is not true. The best feature of a Molten Salt Reactor is the liquid fuel-form. It has infinite negative coolant void coefficient of reactivity, because the fuel and the coolant are the same. Gas cooled reactor also has high negative void coefficient of reactivity and it is safe in loss of coolant. However the FHR has positive coolant void coefficient. FHR power density is high and it is not safe in loss of coolant. Pebbles float in molten salt so it is not possible to design a pool type FHR to prevent loss of coolant.

    Another disadvantage is that FHR uses solid fuel which makes both the ends of FHR nuclear fuel cycle capital intensive. Capital heavy solid-fuel fabrication facility again creates another technology lock-in and delays the development of more efficient and hotter fluid-fuel reactor like vapor-core-reactor. A liquid-fuel molten-salt reactor will not create another technology lock-in that prevents the development of vapor-core reactors. The MSR fuel cycle is less capital intensive and it is more similar to other fluid-fuel reactors, so there wont be another technology lock-in.

    1. the FHR has positive coolant void coefficient.

      Does it?  Is FLiBe not a moderator?  The “fireball reactor” used BeO as its moderator.  Loss of Be should lead to reduced moderation.  Then there’s thermal expansion of the pebbles themselves.  So long as under-moderation and neutron loss shut the reactor down before fuel can be damaged, you’re okay.

      Pebbles float in molten salt so it is not possible to design a pool type FHR to prevent loss of coolant.

      Sure you can.  You just arrange the pebble pool upside down, with pebbles introduced at the bottom and upflow cooling.

      Not that I don’t think Moltex is pretty neat too, but I don’t think you’ve made your case here.

      1. My understanding is that the FHR has positive coolant void coefficient if the coolant salt has lithium that is less than 99.995% Li7 (Li6 being a strong absorber of neutrons).

        By contrast, when the fuel & moderator are mixed, this issue does not exist.

      2. Many analysis tell it is very difficult to achieve zero or negative coolant void coefficient in a FHR.
        “it appears that achieving a zero or negative void coefficient is possible if high-purity 7Li is used (>99.99%) in the Flibe salt and if burnable poisons are present in the core.”
        (link: https://info.ornl.gov/sites/publications/Files/Pub57278.pdf)

        In pool type entire primary loop is in a pool of coolant and gravity will make sure that fuel and coolant are not separated. In case of FHR gravity works the opposite way. How a upside down pool works?

        “I don’t think you’ve made your case here.”

        This is not a “you” versus “me” debate. It’s about MSR versus FHR. There’s shortage of really talented people, R&D funding and testing reactors in nuclear sector. Neat liquid-fuel reactors should get priority for scarce resource like talented people and material testing.

      3. How a upside down pool works?

        It’s not the pool, it’s the pellet container within the pool.  You introduce pellets and coolant at the bottom and extract pellets and hot coolant at the top.  Screens allow coolant to flow while blocking the travel of pellets.

        This allows a novel shutdown method.  If you open the top of the pellet container, the pellets float out and distribute themselves across the top of the coolant pool in a sub-critical configuration.

    2. If you’re spending your time worrying about boiling your salt (and thus what your void coefficient is), then you don’t understand much at all about molten salt reactors.

      1. Heh.  You remind me of the story someone told about the mathematician put in charge of calculating hydrodynamic figures for a submarine who panicked when he found a singularity in the equations for seawater.

        At the speed of sound in seawater, to be specific.

        I had a bit of a laugh at myself there.

      2. Freezing creates voids. Frozen salt is more dense and occupy less volume.

        Also there is loss of coolant scenario, if any pipe or vessel breaks.

        Decay heat in station blackout may be enough to boil the salt locally.

        Molten salts have very low vapor pressure at normal operating temperature and there won’t be any cavitation or boiling.

      3. @ brian mays

        MSR guys like you remind me of the condescending comic book guy from The Simpsons. What you wrote implies amateur MSR fanboys know better than the nuclear professionals, on this subject.

      4. @Scaryjello I’m not nuclear professional. MSR documentation is on the internet and anyone who is interested can read it.

        I have seen nuclear engineering curriculum of many universities in USA. Except 1(University Wisconsin) none teach fluid-fuel reactors. Nuclear professionals who come out of schools learn nothing about liquid fuel reactors. USA once had many Aqueous Homogeneous Reactors in universities, but all got replaced by solid fuel reactors.

      5. Achal – If you are freezing your salt or losing your coolant, you have a lot more to worry about than a positive void coefficient. That is why your comment was stupid. You choose back it up by citing a report that is a decade and a half old! This is why I have no confidence that you know what you’re talking about.

        If you want to avoid freezing your salt then design your decay heat removal systems properly so that they don’t overcool. If you want to avoid losing your coolant then don’t use pipes and put a second tank around your primary pool. If your knowledge of the technical literature was more up-to-date than 2004, you’d know this stuff.

        Scaryjello – And your comment reminds me of … well … of the comment of every anonymous fool who ever bothered to comment on a blog to showcase his/her ignorance.

        “Achal” is the amateur MSR guy. I am the nuclear professional with a PhD and 15 years of experience of working on advanced nuclear reactor designs. What “nuclear professional” are you?

        1. @Brian Mays and Scaryjello

          Please return to your corners.

          I know and respect both of you. Both of you accurately claim the title of nuclear professional. Both of you hold positions of responsibility and have a decade or more of relevant experience. You’re both like many of the nukes I know; educated, knowledgeable and opinionated.

          Please shake hands and cooperate instead of bickering on my blog.

      6. @Achal

        Part of the problem with MSR guys that ‘read stuff on the internet’ is that they always use the wrong ‘state of being’ or possession for MSRs. They write that MSRs ARE this way or MSRs HAVE this benefit or make comments like Brian’s about how “[we who discuss feedback mechanisms] don’t understand much at all about molten salt reactors.”

        See, the thing is that there IS NOT ONE MSR functioning today so, these statements are misleading. Am I being told that I don’t understand much about a reactor that doesn’t exist? How can MSR HAVE anything or BE in any state if there are no examples of them?

        MSR guys need to start phrasing things in the following manner: “If it were built, it would have the following attribute.”

        1. @scaryjello

          Agreed. I’d suggest a slight modification of your suggested phrase.

          Instead of

          “If it were built, it would have the following attribute.”

          A more accurate one might be:

          “If it were built, i think (hope)it might have the following attribute.”

      7. If positive void coefficient is not a concern then FHR can use non Lithium salt and save money.

        If the goal is to make nuclear power better than fossil fuels on every possible way. Then the cost and complexity of the reactor should reduce. My point was liquid fuel molten salt reactor was better than FHR in this respect. The fuel cycle of liquid-fuel molten salt reactor is simple and the same level of safety can be (or might be) achieved in less complex way. This reduces cost.

        TRISO+Graphite waste is hardest to manage. Volume is high because of graphite, and more storage space is needed which dives cost of casks and repositories. Also, TRISO fuel with multiple coatings is the hardest to reprocess and reuse.

    3. China’s TMSR-SF is a very similar pebble-fueled/salt-cooled reactor, and a liquid-fueled variant was announced simultaneously and is expected to follow.

      Worries about void aside, the fuel-cycle of a liquid-fueled reactor can be dramatically simplified and much more efficient. Few see the pebble-fueled variants as anything more than a first step, which is easier to experiment with and may be more palatable to regulators. Any development in pumps, materials, and heat exchangers is directly relevant for liquid-fueled reactors. It appears that LFTR is the goal, but there is little sense in holding up development of other molten salt systems, as a Th-fueled thermal breeder has missing pieces.

      Work on NF3 fluorination has only recently started, and HD-Li isn’t yet available. A rapid scaling of nuclear will also need a great amount of fissile, and expanding mining and enrichment is not desirable. China is already sitting on massive Th stockpiles from rare-earth mine tailings. LFTR variants for producing U233 can also be fueled by actinides recovered from spent fuel. The combination of essentially free fuel with no need for mining and enrichment is very attractive, and others may even foot the bill for the service of recycling spent fuel.

    1. You do have to wonder where this smidgen of honesty came from, at long last.  Did their financiers relent, or did they change patrons?

      If the UCS was truly honest, they would have to admit that the health threat from radiation release from nuclear plants is far smaller than the pollution from fossil fuels and the hazards of building and maintaining most forms of “renewables”.

      1. If the UCS was really honest, they would have to beg forgiveness for the years of damage they’ve done to the environment, the millions who have lung disease who would not if nuclear building had continued in the 70s/80s, the tens of millions who internalized lies about the relative risks of nuclear electricity generation and now have blatantly wrong, anti-nuclear fervor.

        All so a couple of hippies could avoid real work back in the 70s.

        When you lie to people, you steal their freedom. Propaganda, fraud, and lying is just as much a means of oppression as attaching shackles.

        1. I’d hardly classify Daniel Ford and Henry Kendall as “hippies.” They were the two individuals that repurposed UCS name and credibility built during successful drive to halt atmospheric weapons testing.

          In early 1970s, when they decided to use the UCS name to initiate a very public challenge of nuclear energy safety, they were essentially the only active members of the organization.

          Ford was a lawyer. Kendall was the only “Concerned Scientist.” He had lots of resources at his disposal; his family owned the company that made Curity brand bandages and other medical products. He was also a tenured professor at MIT.

      2. If the UCS was really honest, they would have to beg forgiveness for the years of damage they’ve done to the environment, the millions who have lung disease who would not if nuclear building had continued in the 70s/80s, the tens of millions who internalized lies about the relative risks of nuclear electricity generation and now have blatantly wrong, anti-nuclear fervor.

        Baby steps, Jeff.  Baby steps.

      3. You do have to wonder where this smidgen of honesty came from, …

        I searched the obituaries for Edwin Lyman, but came up empty. So your guess is as good as mine.

      4. David Lochbaum is one of the authors. Don’t know the background of the others.

        Are they finally recognizing Germany’s renewable energy/nuclear phase-out has led to increased CO2 emissions?

        Perhaps UCS received a donation from a sane/knowledgable greenie. The Sierra Club changed its longstanding immigration restrictionist policy after a $200 million donation from an open borders liberal.

  3. I wish them luck, but it is a near certainty that Kairos will go the way of the B&W mPower reactor; that is to say it will be canceled after much effort and expense.

    There is no other possible outcome for this overly complicated system that combines two problematic ‘advanced’ (not really advanced) reactor designs into one single unworkable cluster. In reality, besides the hype, there is not a big push to build pebble beds or MSRs – they could have been built yesterday for the last 50-years.

    I have friends at Kairos, but time has passed and they are now more of acquaintances; they all took the job in the past 12-months. I try to reason why they took the job there with the obviously certain future of eventual cancellation… At this time in my career, I can’t afford to lose 4 years working on another science project that gets canceled.

    Of all the advanced reactor design concepts out there (there really aren’t many), Per Peterson’s design gets significant funding.

    Private companies can’t do innovation in Nuclear – too many constraints.

    1. They can do innovation. Unfortunately (in fission) it is limited to art work and computer modeling studies. No hardware.

      Someday that may change if Russia or China demonstrate workable concepts and create a kind of Sputnik moment.

      There are private companies doing innovative fusion work using actual test hardware such as TAE. I’d rather they recieve money being diverted to ITER.

      1. Yes, gist of the ‘private’ nuclear comment was that this particular energy source belongs to the US government… The fuel actually belongs to the US government.

        Ya know, if a PWR runs well (doesn’t trip during cycle) and there is no significant injection/boration, the 10B fraction in the coolant drops from 20% to 16% over 18 months. The residual 6Li in the Kairos RX coolant will deplete if there is no significant make-up. Now, my gut tells me that a positive void coefficient isn’t going to make a pebble bed reactor blow up…. so we’re just hung up on a paradigm that is appropriate for LWR. I recently saw some testing results for the ‘Krusty Kilopower’ NASA Mars RX (on the internet). It looked like they started it up instantaneously – maybe not prompt – but they let the thermal feedback ‘turn it’ so that it established whatever power level. That was a metallic reactor and not really germaine. However, I think it is reasonable to conjecture, with much emphasis on my own ignorance, that a positive void coefficient can accomodated in graphite moderated reactors.

        If you look at PRISM, the EBR2 derivative, it has fuel assemblies replaced by a component known as the Gas Expansion Module (GEM) to counteract a positive void coefficient. When the RCPs trip, the GEM fills with Na to counteract boiling and positive reactivity from it. So, there isn’t a ton of margin to fuel damage in an EBR2-type metallic fueled RX in a pump trip. Maybe that is one of the reasons why the Russians use MOX in BN800. I wonder how Terrapower will address this; maybe it doesn’t matter with the recent China Nuclear Export limitations. I heard that caught Terrapower completely off guard.

      2. @scaryjello: please correct me if I’m mistaken, but wasn’t an RCP trip precisely the sort of failure that EBR2 was designed to accommodate and allow operating staff to walk away from, and operationally tested as well?

        1. @Edward Leaver

          I believe you are correct. The EBRII design features that provided this passive safety protection included both the metal alloy fuel and a large pool of sodium coolant. As far as I can tell from the technical details provided on the GEH Prism web site, the Prism design uses a similar pool type concept with metal alloy fuel. The materials claim the design achieves the passive safety in the case of a loss of coolant flow that EBRII demonstrated.

      3. It’s hard to find the video of the EBR-II loss of cooling test but I uncovered it:


        You’ll find papers under EBR-II loss-of-flow test but no videos.

        One of the things that EBR-II shares with NuScale is that it had no need for power for emergency cooling.  Therefore, it could safely operate without any backup power supply… meaning it was eminently suitable for “black start” of an un-powered electric grid.

        Yes, PRISM shares the same integral shutdown feature as EBR-II.

      4. Of all the shutdown heat removal tests in EBR-2, the most significant one is SHRT-45R. What is the duration of this test? How long this test was conducted before they switched on the cooling and inserted the control rods?

      5. @archal

        R.e. SHRT-45R “unprotected loss of flow” test on EBR-2.

        The test ran long enough to reach asymptotic thermal equilibrium and confirm accuracy of system modelling. An exact time interval is not given, but Table 7.2 (“Plentiful Energy” page 149) plots acquired data out to 8 minutes, which confirmed stable asymptotic equilibrium. Reactor power, initially 100%, had dropped to less than 10% after 3 minutes, continuing to drop to whatever low level was needed to maintain equilibrium core temperature.

        “Following the unprotected loss-of-flow test in the morning, the reactor was
        immediately restarted and the unprotected loss-of-heat-sink test was conducted in the afternoon of the same day. The unprotected loss-of-heat-sink was initiated by the shutdown of the intermediate pump, isolating the primary system from any heat sink. The primary pump continued to function, transferring the heat from the core to the bulk sodium of the pool.”

        The thermal transient for this second test was much slower than for the first, and test data is plotted out to 40 minutes, again confirming inherent safety of this design. These metal-fuel IFRs have substantial negative thermal reactivity above the design power point. In addition, the high thermal conductivity of the metal fuel keeps internal temperature low, and Doppler reactivity to a minimum compared to oxide fuels.

        See Plentiful Energy: the Story of the Integral Fast Reactor, Charles Till, Yoon Chang, available for download from a Science Council near you.

    2. What legal changes are necessary to allow private companies to innovate in Nuclear power? Are there dozens or will one or two changes make the difference?

      1. Thanks Eino,

        I would disagree with your phrase just a bit. “it didn’t strike me as an industry that liked to change.” It seems to me, from your evidence, that it is an industry that government regulators want to suppress change. I think the evidence that they like change comes from the numbers of new designs in the past 10 years.

    1. That’s a pretty impressive margin.  A landslide endorsement of nukes.

      The anti-nuclear propaganda must be failing.  I wonder how many Taiwanese are readers of The Hiroshima Syndrome blog?  How many read Environmental Progress?  Is it home-grown sensibility from the blackouts?

      1. My impression is that it is largely home grown. Taiwan uses 53% of electricity for industry. The industries cannot compete with only on-again, off-again power.

  4. Meanwhile, in France President Macron has announced that a dozen reactors will close by 2035, to make way for 3x more wind power and 5x more solar, funded at 7-8 billion Euros a year. No movement on new EPR reactors, for now, and no mention of third generation designs. The first two reactors to be closed, at Fessennheim, are approaching forty years old, and had two billion Euros spent on upgrades only five years ago. Nearly all the similar-era United States reactors have already been approved for 60 year lifespans, and many have applications for 80 years.

    1. The first two reactors to be closed, at Fessennheim, are approaching forty years old, and had two billion Euros spent on upgrades only five years ago.

      Practically a criminal waste, isn’t it?

      That is not unlike Taiwan’s premature closure of 1272 MW of nuclear capacity at Chinshan, which represents about 5% of electric consumption changed from nuclear to fossil plus some “renewables”.  As an island nation with a massive revanchist neighbor and reliant upon imported energy, it would make a great deal of strategic sense to switch as much energy use as possible from rapidly-consumed coal, oil and LNG over to long-term storable uranium.

  5. When I saw the French news my first thought was that if France wants to move to 50% nuclear, why not keep the excess reactors and designate them for export? It’d be a good income stream.

    1. That would be hard to do given Germany’s frequent spates of exports due to surges in wind and PV generation.  It makes more sense to find other uses for electricity and decarbonize more things.

      The transport sector seems to have a lot of low-hanging fruit in that regard.  Electrify what you can, and use storable electrofuels for some of the rest.  If the electrofuel plants can run well as interruptible loads, they could also eliminate the need for the nuclear plants to do load-following; they could just run at 100% until the next scheduled ramp-down for refueling.

      1. Only having to put in $1100/kW in capital to get maybe 20 GW to balance the accounting for the Gallic ruinables devotees would be quite a deal.

  6. Russia’s recently announced successful annealing of a VVER 1000 reactor pressure vessel indicates that PWRs, even old ones like Fessenheim, can have life extensions of 20-40 years beyond the 60 years contemplated now, by more enlightened regulations. As well, if radiophobic German Greens want to get hysterical, just load new ATF (accident tolerant fuel) into them. Better yet, load Lightbridge fuel into them, and with the excess profits, buy off the German Greens, who have been easily bought by German lignite coal interests, and could conceivably react, Pavlovian style, to a better offer from French nuclear interests.

    1. I do believe that annealing neutron embrittlement was the last remaining obstacle to effectively indefinite lifespans for NPP pressure vessels.  Between that and cavitation peening to eliminate surface cracking by placing it under compression, we should be able to make them new again almost as many times as we wish.

      1. Does anyone have details on the concrete deterioration that is blamed for the winter closure of most of Belgium’s nuclear fleet ? I thought everything in a nuclear plant could be replaced except the pressure vessel and the containment dome. If RPVs can be annealed and peened, what problems can arise with the containment ?

      2. I can’t even imagine the prepwork. Can you imagine heating the vessel to 1100F? That’s the volumetric average fuel temperature at power. Torches?

      3. I can’t even imagine the prepwork.

        Put big fat blocks of insulation around it.  Maybe pump in argon to prevent any oxidation or nitriding during the process.

        Can you imagine heating the vessel to 1100F? That’s the volumetric average fuel temperature at power.

        That and more was done when it was being forged.


        You’re at a site with at least hundreds of megawatts of electric service.  Why would you use torches?

  7. “The VVER-1000 RPV is larger in diameter and has thicker steel structures than the VVER-440 RPV, thus requiring development of a new technology for the annealing of large-capacity RPVs, it said. The metal in the RPV was slowly heated to a temperature of +565 degrees Celsius, after which began the “stationary annealing” process, which lasted 100 hours. The metal was then slowly cooled.”


    I wonder how they do this. Any links to details?

  8. Here’s a description of the Skoda process for annealing the smaller VVER-440 vessels. They are actually annealing specific weld(s), it isn’t like the entire vessel goes into a furnace.

    “The annealing equipment is a ring‐shaped furnace with heating elements on its
    external surface. Annealing equipment basic parameters are a maximum diameter of 4.27 m, a height of 10.6 m and a total weight of 64.8 tons. Installed power output of heating elements is 975 kW, while approximately 200‐400 kW is sufficient for the
    annealing. Heating elements are connected to five adjustable heating sections. The
    equipment also consists of control boxes, a transformer, a power supply cable
    network, and a control system. Power supply is drawn from the main circulation
    pumps feed system. The control system works in a semi‐automatic mode where
    surface temperatures are determined in individual heating sections and these are
    automatically maintained by the control system. The same is applicable for heating
    and cooling rates. Control correction can also be made manually at any time. ”


    1. Great link. Thanks. Lots of prep work – they discuss that the process is executed over a month with the QA taking a year. I have seen videos of Russians using torches to heat RPV during welding on the little PWRs they put in the new floating power station.

  9. Although I accept that their original fossil-gas-augmented air-turbine design is likely the best fit for today’s grid and electricity market (in much of the US), I do like the switch to solar salt and steam for the initial product.

    Such a plant can be easily upgraded to include thermal energy storage (e.g. by providing large tanks for the solar salt coolant). Thus, it will be easy for the public to imagine such a plant as part of a future solar-rich power grid.

    As the Japanese nuclear companies are finding out, it is hard to sell a nuclear plant over-seas if you are not building any at home. California is a technology leader of the US and the world, and California loves solar power. A solar-friendly nuclear plant is probably the only kind California will build.

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