1. Wow – for over a decade I thought the “Integral” in IFR referred to the closed-loop fuel cycle of the design – whether pyroprocessing was local or regional. Part of me suspects some kind of “rebranding” but that may be unkind, and it doesn’t matter to the big picture.

    Since I first heard of the IFR, I have believed it to be the most rational method to reuse the used fuel rods from LWRs – namely to destroy/reduce the TRUs while generating useful amounts of electricity. I envision a relatively small fleet of IFRs that run off the used fuel rods we already have, but not using IFRs for whole scale replacement of coal and natural gas. Disclosure: I am an advocate of all rational fission power plants but lean heavily to SMRs, especially MSR/LFTR, for replacing coal and natural gas.

    Thank you for showing that IFR is not dead, as I had feared!!

  2. Glad to hear John Sackett is still involved in fast reactors. He was a great speaker when he was on a panel for NAYGN many years back and it is good we still have such experienced folks to show the way…

    …if we are smart enough to go forward and not stall over the politics.

  3. I’ve long wondered why sodium-cooled plant designs include steam generators at all. That’s the part I’d like to be rid of, if possible, & as I understand it, the “closed-cycle” Aircraft Nuclear Propulsion Program reactor concepts used sodium or Na-K as an intermediate coolant to transfer heat to the air turbine.
    I’m sure it would mean a loss of thermal efficiency, since sodium-cooled reactors operate at a relatively low temperature for modern gas turbines, but (as you so often point out, Rod) fission heat is cheap, & eliminating the whole steam loop would reduce the capital investment substantially, in addition to gains in safety/reliability/capacity-factor.

    1. I thought Sodium plants weren’t quite high temperature enough for gas-cooled turbine systems? I could be wrong though – I’m a little fuzzy on those sorts of technical details.

      1. A sodium plant can power a nitrogen driven turbine. It’s a lot safer because nitrogen is unreactive, like an inert gas. Yet it is less efficient than steam driven turbines. Chemically, Na + CO2 requires an induction time at 500°C or less, due to CO formation, and the resultant reaction between Na and CO. “At higher temperatures, a fast global reaction occurs”. I believe the expected operating temperature of the IFR is 550°C, and that 650°C or more is normally considered right for supercritical CO2 Brayton cycle. [ Excuse my engineering ignorance but, for me, ultra-safe N2 driven turbines get my vote for the IFR. ]

        1. Mark Pawelek: Well, from a fuel efficiency standpoint, it’s very likely worth the tradeoff, I would think – lower efficiency than steam, perhaps, but the fuel (assuming you’re using SNF left over from light water reactors, maybe be ‘free’ or *even have a negative cost* [I think there’s an argument to be made that since light water reactors have been paying for decades into a waste disposal fund, that fast-breeder reactors ought to be elligible to claim those disposal fees for any waste they take; then they would in turn pay their own waste disposal fee to dispose of the waste they generate and ship out – that way, the DOE isn’t being paid multiple times for the same waste].

          Of course, in order to use that waste, an IFR would have to process the waste, as discussed in this article, so the fuel wouldn’t really be free, but quite possibly very low cost, and more to the point – because of breeding and recycling, there is an absolutely enormous amount of potential energy in that fuel, so the cost per unit energy should approach zero. (That’s just fuel cost – not capital costs for the power plant, recycling center, etc).

          But, I wonder about nitrogen turbines – efficiency aside, how would the cost compare to SuperCrit CO2 turbines? My understanding is that, on the one hand, SC-C02 turbines have to use highly specialized materials in their construction, which would make them more expensive, but they are also supposed to be a LOT smaller, which might make them ultimately cheaper than larger Nitrogen turbines?

          Is it possible that it’s worthwhile going with SC-C02 not for the efficiency, but for the capital costs?

          1. That’s an interesting possibility, is it not?  Reducing the size of the turbine to fit in a pickup bed, while slashing the maximum operating temperature from the 1380°C of high-performance gas turbines to the ~500°C limit of CO2, is bound to cut a bunch of costs in both manufacturing and materials.

            The problem I can see is that the small size requires high speed, so a direct-drive synchronous alternator is no longer feasible for grid power.  Frequency conversion electronics running at hundreds of megawatts will add a bunch of capital cost, and O&M too.  There’s always a mechanical motor-generator but those run into money too.

    2. The European ASTRID project officially switched from steam-Rankine to nitrogen-Brayton a couple of years ago — for safety reasons.


      It seems unlikely that IFR/PRISM are going to make much progress without matching ASTRID in this respect. The sooner the better.

      The other important trend in recent decades, for SFR design, has been a trade-off of fissile breeding ability for safety.

      This is a direct trade-off, because it involves making the core more leaky for neutrons, such that a coolant void incident (sodium boiling in one or more fuel channels) has a negative reactivity effect, rather than a positive one.

      In the ASTRID project, this trade-off results in a reactor that is slightly below iso-breeding.
      It seems unlikely that an IFR/PRISM without similar core design would get licensed.

      TerraPower’s TWR looks to be following similar safety design philosophy — avoiding the top part of the breeding blanket, that characterized EBR-II for example.


      1. Thanks for the response Jaro. I have a followup question:

        I have read in several sources, including “Plentiful Energy”, that the IFR reactor has a negative temperature coefficient of reactivity based on some physics principle (doppler something, if IIRC), that causes the reactor to automatically and passively shutdown if the temperature gets above a certain point, and that certain point is well below the boiling temperature of sodium.

        So, the scenario you mention, sodium boiling, is impossible, isn’t it? Why should a physically impossible contigency be regarded as a licensing problem?

        1. I also read Plentiful Energy ( as well as Tom Blees’ ‘ Prescription for the Planet’ , plus numerous blog entries from Barry Brook, who first convinced me that nuclear was necessary.) They give a very favourable assessment of sodium cooled reactors, for me only called into question by two things-
          1/ The Enrico Fermi unit 1 reactor was, I think, the only one, apart from EBR II and the Fast Flux Test Facility, to use metal fuel, and that was switched to oxide fuel after its coolant blockage and partial meltdown. The Europeans, the Russians, the Japanese, the Chinese, and the Indians are all sticking to oxide fuel. Do they know something I don’t? ( I think the South Korean KALIMERA design follows most of the EBR II recipe, but it’s only a design, not a reactor.)
          2/ The Russians, who have by far the most practical experience of running sodium cooled reactors, with the BN600 and BN800 both putting watts into the grid, and the BN350 having run for thirty years, seem to have called a pause on their proposed follow-up BN1200, and are continuing to work on nitrate fuels, and on the supposedly much more corrosion-prone lead-cooled reactor.

        2. Right. That passive shutdown mechanism refers to reactivity increase due to withdrawal of an absorber rod. That causes a rapid rise in fuel temperature, which has the negative reactivity effect you describe.
          The whole transient is very quick — a power pulse with a high peak but narrow half-width, which means very low total energy release ( = peak power x half-width).
          Certainly not enough to boil the coolant.
          But it is presumed that coolant boiling can occur in other ways.
          The question then becomes which reactivity effect “wins” — the positive reactivity effect due to voiding, or the negative reactivity effect due to Doppler.
          The answer may depend on details of the design.
          Evidently the ASTRID and TWR design groups are taking a conservative safety approach in making their reactor core leaky, with a negative void reactivity coefficient (but reduced breeding ratio).
          The IFR/PRISM group have traditionally been much less forthcoming in their disclosure of design details, so we may never find out — until some regulatory agency reviews the design and publishes their findings.
          In the case of the UK proposal for a plutonium burner, this should not be an issue, since the reactor is supposed to be a burner in any event.
          It may become an issue for anyone wanting a fissile breeder however.

      2. These slides have a nice diagram comparing the efficiencies of N2/CO2/steam on page 9: slides. Here’s a paper with the same comparison: pdf download (2014). As expected N2 Brayton loses a lot of efficiency over steam of CO2, but it is so many orders of magnitude safer; being so unreactive.

  4. Rod: Question about your idea of regional recycling facilities – I can see how, for most types of material streams, regional facilities makes sense – economy of scale – one large plant instead of dozens of small plants.

    However, isn’t there a problem specifically related to the fuel coming out of a nuclear reactor – that it is HIGHLY radioactive for months or years? I thought the advantage of on-site recycling was that you didn’t have to transport highly radioactive material, which brings it’s own set of costs (particularly, political costs as you will have antis banging the drum to the public of just how radioactive the partly used fuel is, and that it’s traveling through their town on rail or truck).

    Would it be safe to transport such highly radioactive material? Or, would you just let the material cool off for a decade onsite before transporting it to the regional recycling center?

    With onsite processing, you just use robots to move the fuel out of the reactor, through a tunnel, and into the recycling center, yeah?

    I thought with IFR, the real idea to get economy of scale is that you build several reactors at a single site (like maybe 5 or 6), and that way, you can share a single recycling center between them?

    1. To me, one of the beauties of the IFR concept is the simplicity of the reprocessing step. This simplicity makes local reprocessing possible.

      The metallic fuel elements are put into a bucket, and the contents of the bucket electroplated into a molten salt bath. The current is reversed, and the actinides (fuel) are plated onto a rod to be fabricated back into metallic fuel elements.

      The trickiest part seems to be the distillation of the molten salts to remove fission products.

      1. @donb

        You neglected to mention the hot cell handling requirements. Ever seen remote manipulators, observed the challenges of training operators, or thought about the life cycle costs of such facilities?

  5. The article gives me the impression that the piping problems with liquid Sodium have been dealt with. It made me curious as to how much easier it would be to pump liquid Sodium than molten salt.

    I guess you can use magnetic pumps for Sodium.


    I would suppose these pumps would have less wear and tear than those with large impellers. Seems like this may be a big advantage over a molten salt reactor. Is this wrong?

  6. As clarification, sodium is a strong reducing agent and very benign in a reactor environment.

    Sodium reacts with spectacular vigor with water, but not so much with air. Sodium spontaneously ignites in presence of air, but it burns with a (relatively) cool, smoky flame, not at all as it does with water.

    Lead-cooled reactors have their own trade-offs. Pure lead melts at 325 C, PWR’s run at about 305 C max. To obtain something more workable, lead is usually cut with bismuth to form a lead-bismuth eutectic (LBE) which melts at 124 C. But molten lead is corrosive, which presents a design consideration possibly affecting lifetime of plant.

    There is another trade-off with neutron activation products. Sodium absorbs neutrons to give activated sodium-24, a strong gamma emitter with a half-life of 15 hours — short enough not to be a disposal hazard, as it will all go away within a week of reactor shut-down.

    In contrast, Bismuth-209 absorbs a neutron to produce (after beta decay) Polonium-210, a potent alpha emitter with half-life – 138 days – long enough to be a disposal concern. Not an insurmountable concern — alpha is not penetrating radiation — but still an additional something to take into account.

    1. Polonium-210 has both useful (makes a great RTG heat source for moderate life batteries) and sinister applications (Litveninko).

  7. Would N2 going through a heat exchanger with activated sodium make any Carbon 14?

    1. @John ONeill

      No. The activation of N2 to C-14 requires neutrons. Those are only available inside the primary shield that stops reactor generated neutrons.

  8. @John ONeill

    I believe that other nations are sticking to oxide fuel rods due to being conservative and sticking to what is familiar. Metal fuel rods are better at containing fission products, especially gaseous FPs, without cracking or releasing. They also expand with temperature more, which IIRC reduces reactivity which is safer. And the heat transfer is somewhat better, so the internal peak temperature will be a bit lower – I think an annular fuel rod design would help either type of fuel form. Of course the down side is that the metal fuel rods melt at much lower temperatures.

  9. Well if IFR is going to be built. it is either with no revisions or with some revisions.

    I would advocate to move away from Na to Pb-alloy since it does not burn and “freezing” issue can be dealt with.

    1. @JM

      There are many measures of effectiveness associated with coolant choices. I’m not a fan of using molten lead

      • it is hazardous to humans even if it isn’t burning
      • There is less experience with using it compared to sodium
      • Its activation products are longer lived than those associated with sodium
      • Its melting point is much higher, complicating maintenance and start-up processes
      • Processes that have been worked out to deal with the visibility issues for sodium compared to water don’t translate to lead
      • Experience shows that the fire hazards of sodium can be handled with relatively straightforward procedures and materials
      1. “There is less experience with using it compared to sodium”

        I must admit one point that Sodium run reactors have longer running time.
        But Running time of Lead- Bsimut cool reactors stem from 80 year of operating time in Alfa-class. Which is not insignificant. Only issue was freezing issue aka those reactors could not be restarted. But now they have soled this issue in planed reactors. Of 1GW size as well as SMR where freezing is bonus since you transport whole reactors frozen..

        There would have to be Cost/ benefit analysis. But with Lead /Pb you can drop those double walled steam generators and fire hazard. Leaks of Na are not uncommon. French Phoenix suffered the same issue. On disadvantage side you have to solve metallurgy of vessel. Yes but if you solve it you solve for good. It is solved = done
        With Na you pray that those fancy steam generators do not breach again once they are punctured somehow, the Inert gas cover stays there, there is no leak with water around… . Well I am big fun of Black Swan book. And with Na it is just black swan nest.

        To reprocessing on site. Prime rational was ….drum roll… PROLIFERATION.
        Yes it is stupid to have small expensive chemical plant next to every IFR site, but somehow Pu must stay there was great benefit.

        “Its activation products are longer lived than those associated with sodium”

        With U/Pu cycle you would still have some waste. At worst is could still be good 2nd loop coolant assuming that it will not get activated too.

        IFR did not get licence so still time for improvement. If I have to go fast I would go for Dual Fluid Reactor 😀 Breeding ration of 1,6 and doubling time of 4 years is tempting as COMBO for DMSR for not trusted parties running mox fuel and DFR for Big boys.

        1. @JM

          But Running time of Lead- Bsimut cool reactors stem from 80 year of operating time in Alfa-class.

          Perhaps. But the operational and technical details associated with those machines is less available to western countries that the operational and technical details of US navy nuclear plants.

          In other words, we know they operated. That’s about it.

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