1. Are molten salt reators being funded and developed only in the private commerical market? Why doesn’t DOD/DOE just spend the 2 billion in couch cushion pocket change to reboot the ORNL project to lay the technichal/regulatory foundation for advance nuclear? Thus paveing the way for companies like Tranatomic Power to come in afterwards.

    Colonel Paul E. Roege gave a great talk on the military aplication/importance of such technology.

    1. Closest thing I know of in this arena is FLiBe – circa 2012, their target was a 40 MW reactor targeted for certification by the DoE, rather than the NRC (which would OK it for military use, but not commercial). I don’t know if this is an easier target to hit, but at least the market looks more enthusiastic.

      Anyway, should they succeed, the seed money and operational experience would give them a leg-up for later NRC certification.

  2. I wonder why they are using zirconium hydride for neutron moderation instead of zirconium deuteride. They are right there in Canada — the center of the heavy water universe, after all.
    While protium is a substantially better moderator, the neutron adsorption is so much more than deuterium. It seems that they could include more Thorium in the salt and step over into using U233 fuel faster. This method would be particularly advantageous if the salt were simply sequestered after use to get better neutron economy.

    1. You’re right — using zirconium deuteride as a moderator would improve the neutron economy because deuterium has a smaller neutron capture cross section than hydrogen. It came down to a supply chain issue for us, though. It’s much easier to make ZrH, and the advantages of ZrD just aren’t big enough to justify the difficulty of sourcing it. (We’ve looked at using ZrD in our models, though, and it’s something that potentially could still be on the table if it becomes easier to source.)

      1. If I may attempt a more nuanced response…..

        Aside from the sourcing issue (much less daunting than Li7, in my opinion), it makes no sense to combine pure deuterium, with its amazingly low neutron absorption but high cost, with zirconium, which is a worse neutron sink than ordinary hydrogen.
        At least not in a bulk moderator application, where the quantities of zirconium are unavoidably very large — in contrast to zirconium used as just a structural component material like thin-walled fuel rod tubes or even calandria tubes (Candu), where the quantities are minimized by design.
        In other words, in a bulk moderator application, substituting D for H in zirconium hydride would be a total waste, as one would get all the down-side (high cost) with very little of the benefit (neutron economy, cancelled by the zirconium).
        The big difference in heavy water is that the molecular partner in that particular bulk moderator is oxygen, which has an extremely low neutron absorption, like D.
        It makes sense in that case.

        1. OK, Jaro, I see your point. My very old Handbook of Chemistry and Physics does contradict you on the comparison of ordinary Hydrogen and Zirconium, but I’ve noticed several errors in it over the years. Here is what it shows me:
          H = .332 barns
          D = .51 millibarns
          Zr = .182 barns
          O = .178 millibarns
          Anyway, if the high cost of the D was reduced by using somewhat lower purity (say, 90% D and 10% H), I wonder how much the economics would change. My understanding is that the cost of somewhat less pure D2O is very much less than the cost of the purest “reactor grade” D2O. Another (admittedly minor) economic factor could be the production of some Tritium by using D.

          1. Further down the page, Rod speculates that if laser enrichment becomes very cheap, it could make Lithium 7 much easier to procure. If enrichment was really cheap, it might make it possible to use Zirconium 90 in the moderator. This isotope makes up more than half of natural Zirconium, and has an absorption cross section of only .011 barns, three times better than hydrogen, and about 17 times less than the normal isotopic mix. It’s not the thousand(s) fold improvement you get from using Deuterium or Lithium 7, but it might come in handy down the track.

          2. @ John O’Neill
            Interesting point about Zr 90. Actually, you don’t even need isotope separation for ultrapure Zr 90: if you start with Strontium 90, about half of that will be Zr 90 in about 30 years.
            But that sort of reminds me of the Steven Wright joke:
            “Everywhere is within walking distance — if you have the time.”

          3. @ Bryan Elliott

            N-15 would make the Adams Atomic Engine even more attractive. Not only does N-15 have a much lower thermal neutron capture cross-section than N-14 (about 24 microbarns for N-15), but it also doesn’t turn into Carbon 14.

          4. Thanks for the answer; now I’m curious.

            Googling around (because I wanted to check out the design), it looks like atomicengines.com isn’t working properly – but then, it also looks like the domain had been allowed to expire recently (the cached version of / is a GoDaddy parking page).

            Anyway, can you provide a link to any public information you have published on the AAE design?

          5. Bryan,
            No need to Google. Just go to the bottom of this page and under “Archives” you will see a search field. Type in “Adams Engines” and you’ll get plenty of info.

      2. Unless I’m mistaken, ZrD would throw a different big kink in the design. ZrD would require a significantly larger fuel inventory because of different neutron scattering properties relative to ZrH.

    2. My guess is the cost of deuterium outweighs the advantage of having a less absorbent moderator.

  3. I’d be interested to hear Leslie and Mark’s thoughts on what specific changes in government policies would do the most to further the progress and commercial adoption of advanced designs for nuclear power like those that they working on.

    1. By far the most useful change would be to mandate development of a pathway for advanced reactor licensing in the US — right now, there simply isn’t a way to license any of the advanced designs in this country. And even the rules governing test/demo/research/prototype reactors aren’t fleshed out enough to be effective at this stage. I think that Terrestrial Energy has a very big advantage in that they operate under the Canadian regulatory regime, which is much more performance-based and reasonable.

      Another extremely helpful piece would be to make it easier to build demonstration-scale advanced reactors in the US. Specifically, by clarifying that it’s possible to build and operate demo-scale advanced reactors at national lab sites without requiring an explicit license ahead of time from the NRC. (National labs are, by some interpretations of the law, not under the NRC’s jurisdiction — but it’s murky, and I’ve heard that the powers that be are reluctant to permit this.) But if this were allowed you could, for example, build the demo plant at INL, under the auspices of DOE, and (potentially) require NRC staffers to be stationed at the site, so that they can observe the construction and operation. As they do this, the NRC staffers would be building up the necessary expertise to license commercial-scale plants in the future. It’d still be expensive to pay the staffers, but much cheaper and more feasible and flexible than the current system.

      In an ideal world, this could be funded by some of the $25 billion remaining in the Nuclear Waste Fund. So many of the advanced reactor designs feature high burnups (or, like Transatomic’s, can consume the waste directly) and can therefore reduce the long-lived waste burden. I know that in reality the money in the fund has already been shuffled around for different parts of the budget, and isn’t sitting static in a bank account, but it seems so logical to find a way to use it for this.

      1. As I recall from elsewhere, the US military is not under NRC jurisdiction either.  Perhaps DOD could be the initial sponsor for a project aimed at e.g. powering remote bases or producing synthetic fuels from seawater.

        1. Not to get political, but it isn’t going to happen with this administration. From what I understand, the admin’s coolness to new nuclear has been blocking Flibe from building on a base near Huntsville.

          1. @Casey

            Nothing major is going to happen with new nuclear within the remaining time of the current administration. That is not making any particular political statement, just recognizing that there are fewer than 24 months remaining before the election of 2016.

            This is one of the reasons that I persistently remind all nuclear advocates that we must make friends in both parties. No administration lasts long enough to help nuclear energy recover its 1970s vintage luster, but every administration is in office long enough to do serious harm. The current one has done no favors, despite a few positive words now and again. It is a huge disappointment to me.

      2. In the interview, you mentioned that “demo scale” for fluid fueled reactors is much smaller than you’d like to build, since it wouldn’t give you the kind of data you’d need with respect to the reactor’s hydraulics. What useful data _could_ be obtained from a demo scale reactor that might justify the cost of building a prototype, and do you think a reactor of that scale could be spun into a marketable DoE-licensed ruggedized mobile reactor for military use?

      3. Leslie – why not build a demonstration scale prototype in a location outside of the jurisdiction of US regulators? It seems that US regulations doom you to a more cautious approach than the world needs Transatomic to pursue. Is there a “friendliest” country regulatory regime for advanced reactor development (other than China)?

  4. Thor Con states that there is no commercially viable way to procure highly purified Lithium 7 for the flouride salt. They call it unobtanium. Transatomic must disagree, it would seem. I am also curious as to whether they think if they can directly put the contents of old PWR fuel rods (minus the zirconium cladding) into their molten salt, or whether they require some sort of prior processing. I would presume that if the fuel rods were aged (say, 20 years) the isotopic mix would be more favorable?

    1. @Paul Wick

      As I understand the plans from both Thor Con and Terrestrial Energy, the “unobtainium” status of Li-7 is a temporal thing. Those companies are run by people who have reasons for being more impatient than the founders of Transatomic Power. There is a licensed breakthrough in isotopic separation being seriously considered for commercialization in Wilmington, NC. If GEH’s laser enrichment tools are turned towards Li, the unavailability issue may disappear quicker than some believe.

      With regard to the suitability of reusable fuel from reactors, I am no chemist, but I assume that molten salt chemistry would prefer metals rather than oxides. Anyone can feel free to correct me, but I’m pretty sure the conversion from oxide pellets would be a required process step.

      1. Fluorides will steal from an oxide, but you’re right; they will more preferentially dissolve metals, since there’s no competing oxidizer in the mix. Additionally, the oxygen would eventually find itself a dance partner, and while I don’t know the whole chemistry of the periodic, I can speculate that some of those might precipitate out of solution.

        Transatomic peoples: is this a real concern, and if so, what mechanism in the reactor would be responsible for handling it? Would it be the same method as plating-prevention (that is, via chemical means; e.g., keeping the fuel salt slightly reducing – though I might be remembering that one from LFTR)?

    2. Transatomic must disagree, it would seem.

      I’m not sure that’s true. The whitepaper appears to assume they will be using enriched Li-7, but not fully enriched, as that would exacerbate costs:

      Our $2 billion overnight cost estimate at nth-of-a-kind scale includes lithium-7 material (a key salt component) and custom fission processing equipment. Our reactor’s strong neutronics benefits us in permitting the use of slightly less pure lithium-7 and somewhat less frequent fission processing than has been cost-modeled in other reactors

  5. Transatomic has a well written white paper on their site. I didn’t read it all but I want to thank them for leaving out the triple integrals and the ones with the little circle in the middle. It greatly un-confuses the document. Here’s the link.


    There are some good diagrams within the white paper. It’s worth a look.

    First question – How does one “scrub” tritium? This subject had a lot of discussion on another website? (This is from a non nuke.)

  6. Kirk Sorensen might take issue with the NASA write off comment @ the 54-55 minute mark of the interview. His entire science/nuclear engineering career was literally birthed into existence by the idea of space exploration/colonization.

    “The school I went to, something like 75% of the students said they wanted to be astronauts…” How many of those students ended up in the STEM field? How many of those STEM students end up in the nuclear field?

  7. How does TAP consume (100%?) actinides if its working in the (epi)thermal spectrum? I thought that only fast spectrum reactors like the IFR could actually consume actinides, leaving only fission products as waste?

    1. It’s not an epithermal spectrum reactor. It’s a dual-spectrum reactor, with hard spikes in the fast and thermal specrtums.

      The core has about 50% moderation by ZrH, however, it also has an unmoderated region specifically to destroy transactinides.

      1. The dual spectrum thing is a bit more nuanced. Any hydrogen moderated reactor is “dual spectrum” but that doesn’t mean the fast neutrons are doing much. There are fast neutrons in a thermal reactor because they are born that way. But they do not last very long since they are far more likely to bounce off a nucleus and slow down than to be absorbed. Furthermore, the likelihood that a thermal neutron will be absorbed by a fissile nuclide is hundreds of times greater than the likelihood that a fast neutron will be absorbed by a fissile nuclide, which is why we like thermal reactors since they need less fuel than fast reactors to go critical. But this also means that thermal neutron absorptions and fissions are far more likely to occur than fast absorptions and fissions, so the thermal reactions dominate what happens in a reactor. That said fast fissions do occur, but account for just a few percent of the total fissions in a hydrogen moderated thermal reactor. Also, the unmoderated part of the reactor is dominated by epithermal neutrons since the heavy metal density in salts is low enough so that neutrons are far more likely to bounce off fluorine or lithium and slow down than induce fast fission.

      2. The dual spectrum effects are a bit more nuanced. All thermal reactors have fast neutrons, they are born that way. And hydrogen moderated reactors have a larger fast peak than graphite moderated reactors, as shown in the TAP white paper, so they have more of a dual spectrum, but the fast neutrons in a thermal reactor don’t account for many fissions. The fissile actinide thermal fission cross section is hundreds of times greater than the fast fission cross section. That means thermal fissions dominate what happens in a reactor. In fact fast neutrons account for maybe 5% of the fissions in a thermal reactor. And the fissile metal density is too low in the unmoderated region to sustain a fast spectrum. The fast neutrons are more likely to bounce off fluorine and lithium nuclides than cause fission. That means thermal reactors don’t enjoy the large fission to capture cross section ratios seen with fast fissions, so they burn U-235, Pu-239, and Pu-241, but don’t burn the higher actinides.

        I think a fast reactor would couple nicely with the TAP system though, and one fast reactor could take the discharge salt from a dozen or so TAP reactors and eat up the leftover actinide inventory.

        1. The TAP concept has been promoted to me by a nuclear enthousiast friend of mine as a full solution for the destruction of all higher actinide ‘waste’, similar in that respect to metal fueled and cooled fast breeder reactor concepts like the IFR. It isn’t though, if I understand it correctly.

          1. You are correct, it cannot destroy the higher actinide waste, you will need a fast spectrum for that. The TAP concept can burn some of the Pu-239 and remaining U-235, and some of the Pu-241, but the plots in their what paper seem to show that the Pu-241 decreases more slowly than the natural decay of it so it appears they are actually producing Pu-241 from neutronic interactions. Meanwhile the reactor generally produces higher actinides instead of destroying them. There is a reason that engineers and scientists dating back to Enrico Fermi have pursued fast reactors to close the fuel cycle.

  8. The TAP design has many fast neutrons seen by their dumbbell neutron spectrum. Will fast spectrum fission be important?, about what proportion will it be, and will it be mainly from uranium-238?

    1. I imagine fast neutrons will account for 5-7% of fissions in TAP’s reactor, mostly of Pu-239, Pu-241, U-238, and U-235. The rest would be thermal fission, mostly of Pu-239, U-235, and Pu-241.

  9. There was mention in the interview as to what happens if the cladding fails… it sounded like the moderating effect of zirconium hydride would be lowered? If the zirconium dissolves wouldn’t the moderating effect continue and you’d also lose the ability to retract the zirconium from the core because now it is part of the fuel mixture?

    (Unlike many of the commenters here, I’m not an engineer so if this question is dumb for any reason, I’m happy for anyone here to spell it all out for me.)

    1. @Gordon McDowell

      Zirconium is not a moderator, but hydrogen is. As I understood Mark’s explanation, if the zirconium hydride is dissolved, the hydrogen will percolate out of the system.

        1. Now here’s my dumb question. It seems to me that the big problems in the nuke business have involved hydrogen burns. It was a concern at TMI, it made for the spectacular pictures at Fukushima and I remember it causing some welding issues with spent fuel due to radiolysis. Is the amount of hydrogen (tritium too) that bubbles out an insignificant concern? How long will any of it need to be stored before being vented?

          1. The venting question is a political consideration, not engineering. The EPA seems inclined to get even stricter on tritium release, despite the utter lack of evidence that tritium poses any biological threat whatsoever. It’s a low energy alpha emitter, you could vent it freely and it will simply exit the atmosphere.

          2. They outline in their whitepaper that tritium is routinely removed from the reactor via the off-gas system – my assumption is that they sparge the gas stream with hot O2 to convert the tritium to heavy water prior to the cryogenic process, distilling off the highly tritated water before anything else – and stored in waste containers.

            This is because, like all reactors, tritium is evolved from low-mass fissions of lithium within the reactor. If they don’t swing a supply for pure Li-7, the Li-6 has a large fission cross section, which they estimate puts their reactor’s tritium evolution rate between that of an LWR and a CANDU. With Li-7, it’s less of a concern, and gets their neutronics up – but tritium would still need managed.

            This would of course be a key monitoring point for quick response. If your tritated water bottles suddenly jump up by several grams, you know you’re losing a moderator rods, and should either replace the damaged ones quickly (in the case of a common mode failure, indicated by a _very_ quick rise in off-gas water), or if it’s just one, let it dissolve and replace it when the off-gas water settles down (indicating that the Zr is completely incorporated).

            That begs a question, though. Transatomic: what other sort of monitoring would be able to detect a specific damaged moderator rod?

          3. Just to clarify, tritium is a beta emitter only, doesn’t even emit gammas during decay. Makes it a great nuclear battery. And as stated, it is way over regulated…

  10. Rod’s comment here:

    Like many nuclear engineers, Massie and Dewan have focused most of their early design efforts on the reactor portions of their system. The power production portion has received little attention so far.”

    should be throught about alot by AI readers.

    One of those issues is metallurgy. Highly corrosive salts running at very high temperature commands R&D metallurgy nukes to develop the kind of materials *that do not yet exist* that can be used in such reactors of the near (hopefully) future.

    Second are closed Brayton cycle turbines that use hot gas as their motive force instead of lower temperature steam cycles. This will add raise thermal efficiency from approx 33% to over 50%! But the tasks of developing such turbines for utility scale generators remains a task yet uncompleted.

    We need to make such R&D “sexy” and rewarding.

      1. Bryan Elliot – Thanks for answering my question. On a little inspection, it looks like there are many hydrogen recombiners currently available in the marketplace to handle the bubbled off gas.

        From your link – looks like it might be a few years before the CO2 turbine is ready.

  11. Casey: I shouldn’t belabor this point on hydrogen, but there is also the flammability issue. To my limited perspective the major nuclear problems that have occurred have been due to fires. Chernobyl had a burning graphite moderator, Browns Ferry had some cables burning and the others had some hydrogen burns or potential burns. I just wonder how you go about ‘bubbling ” it out of the molten salt solution and collecting it. I’m sure it can be no big deal if engineered right as a lot of generator stators are hydrogen cooled and have a good track record.

    1. Eino –

      I’m not sure about TAP, but most designs include Helium sparging of the salt. Tritium will decay to Helium-3 with a ~12 year half life. With high-purity Lithium-7 the inventory of tritium is going to be too small to be a significant fire risk, but you’d still need to store it.

      1. I think the tritium handling issue is going to be harder for Li based systems than folks realize, but very manageable with engineered systems. I saw a presentation from MIT work that the FLiBe salt cooled/solid TRISO fuel system with enriched Li-7 will still produce something like 8000 Ci/day of escaping Tritium to an air brayton cycle without some kind of engineered trap or barrier. That’s a decent amount of tritium. But honestly, who cares? It’s not really a threat anyway.

  12. As someone who left nuclear for software development around 15 years ago (for the aforementioned money), and presently regret my impotence to drive innovation in the nuclear space forward, let me say: Leslie, Mark and Rod, I’m glad you exist, and I certainly wish Transatomic the greatest success.

    Something you didn’t point out in the interview (but was present in the whitepaper) is the unmoderated outer area of the core, designed to produce the high-energy “weight” of the neutronic dumbbell. Have you guys worked out a breeding ratio estimate in that region?

    This question is assuming you’ve also worked out a reference reactor size, or that the breeding ratio doesn’t float based on reactor size; if either of these aren’t correct, please let me know, and if you can back-of-the-envelope a breed ratio for your smallest-useful-data reactor and an infinitely-scaled-up reactor, I’d be very appreciative.

    1. With 1,200 MWth and 86MWth/m3, TAP’s volume comes to 14m3 — a cube slightly less than 2.5m on a side.

  13. As mentioned in the podcast, and in some of the comments above, I also would like to emphasize the importance of the design of the steam generator heat exchanger. Replacement of those heat exchangers is a high cost maintenance item for PWRs, and something as seemingly simple as properly controlling the chemistry of the feedwater in the secondary plant becomes vitally important.

  14. In college Nuclear Science was not even on my radar as a possible area of study, but I have developed a deep appreciation for the need for innovation in the nuclear energy field as I have gotten older and wiser over the past 18 months. I want to be directly involved in this field because I feel it is irresponsible for me personally not to get involved. I tried typing the reasons why I feel this way but it read as corny and melodramatic.

    Anyways, recently I have dusted off my college physics textbook and started reading from the beginning. I am looking forward to the section on thermal hydraulics; howerver, in the meantime does anyone have any suggestions on how I can transfer from the IT industry to the nuclear industry?

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