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  1. One more reason nitrogen is not favored is a potentially dangerous reactivity increase after a pressure drop. N-15 would solve this too.

  2. Great article Rod! Good to hear you’re finally coming around! My powers of persuasion are working, admittedly with a rather long delayed fuse!

    Nitrogen in the air occurs as N2, so not a good form for isotopic separation.

    Probably an attractive means of industrial-scale production would be to co-locate an N-15 production plant with an ammonia production plant. NH3 has only 1 nitrogen atom and the 3 Hs have low atomic weight. Simply sign a contract with the ammonia plant that you sidestream some of their production, enrich it, then send the N15 depleted stream back to the ammonia plant.

    Perhaps a centrifuge train? Mass difference is pretty big compared to U235/U238 and without radioactive and hazardous (hexafluoride is much nasties than ammonia) materials the cost would be much lower than enriching uranium. But even if the cost were the same as enriching uranium in a centrifuge, it would not be outrageous. Not that much mass of N15 is needed for a reactor, compared to the fuel mass…

    What about chemical separation process options?

    1. I thought the same thing, but there’s a fly in the ointment:  N-14DH2 has about the same weight as N-15H3, and IIRC there’s a lot more fraction of D than N-15.  In other words, you’d have to use isotopically separated hydrogen as well.

      There’s only one stable isotope of fluorine, so maybe NF3 would be better-suited to gas centrifuging.  Dangerous to handle, but better-suited.

      1. Yes, there would be the odd H-2 messing up the party, but there’s not so many of them, relative to N-15s. A sufficient enrichment level would be achievable with NH3. Not 100% of course, due to them “darn deuterons”, but practically we’d be happy with a lower enrichment for a real application. Probably 90 or 95% would be a “happy camper” level, and even 80% might scratch by.

        I go by the ol’ “80-20 rule”. Solve 80% of the problem and then revisit the problem later. Go for a 80% N-15 enrichment level and cut down carbon activation by 80%, neutron absorptions by 80%, effect of reactivity by 80% and so on. Once we have an active system operational we can look for further improvements, maybe get another 80% chop-down, but it probably won’t matter much anymore.

        NF3 isn’t that difficult to handle, one of the attractive properties that lends it to a fluorinator application is its relative inertness at low temperatures, but good reactivity as fluorinator at elevated temperatures. F being mono-isotopic is a major factor in its application in enrichment. But, in this case, it hardly seems necessary. F-19 is a lot heavier than H-1 so this doesn’t help compared to ammonia. It also isn’t a common chemical, nowhere near as industrious as ammonia. Ammonia is probably the best bet still.

      1. No, anhydrous ammonia is NH3. NH4+ (ammonium ion) + OH- (hydroxide ion) is formed as ammonia is dissolved in water. NH4 is a neutral ammonium radical and pseudo-alkali metal. It is so unstable that is spontaneously disassociates into NH3 and H (monoatomic hydrogen!). With something dangerously unstable it would probably detonate even if not subjected to any oxygen.

      2. It looks like it ate my reply so I’ll try again (it said duplicate and then did nothing).

        NH4 is an exotic ammonium radical and pseudo-alkalimetal that spontaneously decomposes to ammonia (NH3) and monoatomic hydrogen (BOOM!). Ammonia is NH3, and in water NH4+ (ammonium ion) and OH- (hydroxide ion) is formed.

  3. I agree that N2 needs to be looked at, but comparing helium to nitrogen to heavy nitrogen microscopic cross sections is insufficient arguement to justify heavy nitrogen over natural nitrogen, since N2 is a gas, so has much lower macroscopic neutron absorption cross sections even than water with H microscopic cross section of 0.33b, because water is a luquid instead of a gas. Obviously, natural N2 wouldbe able to work.

    1. Correct. The fact it’s a gas helps with the absorptions. Still, while natural nitrogen would work, the absorptions aren’t trivial. Enough to cause noticeable changes in fuel use, activation issues (recall the tritium in CANDU is significant, despite trivial deuterium absorption x-section) and reactivity coefficients.

      The fact it’s a gas also helps with enrichment – you don’t need so many kg of N-15 as you’d need kg of uranium enrichment… in fact the amounts are rather trivial. Hence the expectation that cost differences are small. It does require an approach where we save the N-15 – a closed loop system, so with a vacuum drawdown and charge-up bleed-in of the drawn out N-15. Not a big deal, very standard tech is involved.

      The effects are more pronounced for pebble bed cores as they have a higher coolant fraction.

      1. Since N-15 costs so much more than a neon-CO2 combo, it may be that my gas-mix scheme might be the better option after all.

      2. Wikipedia claims $240/kg for neon, of which only about half of the Ne/CO2 gas mix would have to be by volume (and less than 1/3 by weight).  Linde offers both compressed and liquid neon in bulk quantities.

        It’s very hard to see how an isotope-separated gas could be as cheap as neon.

      3. CO2 is not suitable for a high-temperature graphite cooled reactor, it reacts with the carbon at these temperatures.

        Neon is suitable, but does have a higher capture cross section than N-15. It also is not compatible with standard Brayton cycles like nitrogen is, it’d require a different turbomachinery design, different heat capacity, viscosity, speed of sound and so on. A big part of the technical issues with helium cooled reactors using a direct Brayton cycle has been the differences in gas properties leading to subtle but complex design and operational issues with the turbo design. Designing a new type of power cycle is rather a big deal.

      4. CO2 is not suitable for a high-temperature graphite cooled reactor, it reacts with the carbon at these temperatures.

        Use an outer SiC coating on the pebbles.

        Neon is suitable, but does have a higher capture cross section than N-15. It also is not compatible with standard Brayton cycles like nitrogen is, it’d require a different turbomachinery design, different heat capacity, viscosity, speed of sound and so on.

        What you really need is to get γ correct, which can be done with a CO2/Ne mix.  The molecular weight is even awfully close.

      5. I like the outer SiC coating on the pebbles. It would also avoid another problem with pebble bed reactors, which is graphite dusting from the pebbles grinding each other. Graphite is used as a lubricant… in fact a bit odd that an outer SiC coating is not done as standard in pebble bed reactors.

        So what is the advantage of CO2/Ne over just pure CO2? Might as well go for SCO2 cycle then? Higher pressure, but the turbo is compact. Reactor vessel could be of prestressed cast iron which the Germans developed years ago.

      6. So what is the advantage of CO2/Ne over just pure CO2?

        It lets you use bog-standard OTS turbomachinery.

        Might as well go for SCO2 cycle then?

        If Allam-cycle plants take off, that may well become an option.  Once someone has the turbomachinery for CO2 in production, it’s no real stretch to use it with a different heat source.

  4. While nitrogen has 40% higher heat capacity/m³ than Helium, pumping/compressor power required is by the mass ratio, 7x higher. So, must also be evaluated.

    Conversely, in the turbine the higher mass is a benefit, higher momentum, smaller turbine, or lower speeds.

  5. Open cycle air turbines are diffeeent designs from closed cycle N2 turbines, so this needs discussion as well. It is not a 1:1 changeover, just add a recooperator. Remember,the reason the ML-1 project failed, was not the reactor (with natural N2),which worked fine, but the PCU failed.

    Please flesh this out more.

    1. ML-1 was designed and built in the very early days of Brayton Cycle machinery commercialization. There were no machines available in the size range used for the project, so purpose-built machines were used.

      There were unintended mismatches between the compressor and turbine that led to redesign and replacement efforts that added costs and did not contribute to project success. But we have had ~60 years worth of Brayton cycle machinery development since that project ended.

  6. C-14 produced via N2 is MUCH orders of magnitude more economic a source as 100% pure C-14 as a gas, than C-14 from graphite me xed with C-12 in a solid.

      1. How so?
        Looking up C-14 I see lots about radiocarbon dating, but aside from the speculative diamond battery I don’t see any uses it could be sold for.

  7. A benefit of heavy nitrogen (hN2) vs natural N2 (nN2) is nN2 has a positive temperature coefficient, increasing tenperature squeezes poison out of the core, increasing reactivity. But, hN2 makes that negligible.

  8. Gas turbines used for backup of solar and wind are jept hot and spinning to be ready when the sun goes behind a cloud or the wind gust decreases. This maintaining heating and spinning requires about 30% gas use, with no load, no revenue to pay for it. If ypu make the turbine part of the decay heat system, vice an independent passive decay heat remivsl to air, the the Turbo-generators & heat exchangers becime safety rated, dramatically increasing PCU system QA costs.

    1. Using decay heat to keep turbine & compressor warm doesn’t mean that is the only method available for keeping core within specified temperature limits under all envisioned conditions.

      It should not change the classification of any components.

    2. Not sure what you’re on about here. Lots of typos and a confused narrative.

      An operational convenience doesn’t mean a safety related item. Any small gas cooled reactor can easily use passive air cooling for safety core and containment heat removal.

      Wind and solar are no good, they need more backup joules of fossil chemical energy than they deliver in useful energy when needed. Typically you need 3 to 6 joules of fossil to keep a single joule of wind and solar on a power grid, depending on solar and wind resources and the amount of industrialization of the grid. This is common knowledge among honest researchers and those who posess a degree of common sense.

  9. Would the reduced size and improved economics of a heavy N2 pebble bed brayton cycle allow the US navy to use nuclear as a power source on ships other than aircarft carriers and subs? Or you fit these systems into destroyers?

    1. It would be very hard to shrink a pebble bed down to a size small enough to be competitive with the PWRs the Navy uses. There is also no real advantage for that use case as far as I know. The HEU PWRs used by the Navy fit all of their needs quite handily and use a known and mature technology. If it ain’t broke, don’t fix it.

      1. Reactor cores are a small portion of the weight and volume of complete power plants.

        There have been gas cooled cores for nuclear rockets with higher energy density than current naval reactor cores.

        IOW – I would not make such categorical statements about technology.

        On the other hand, you have correctly identified the position that NR has taken regarding closed cycle nuclear heated gas turbines. We’ve been talking on and off for more than a quarter of a century.

      2. Although you are probably right about me making too broad of statements here, I also wouldn’t use NTRs as the counter point. Nuclear thermal rockets have incredibly limited core lifetimes (measured in hours at full power) and tend to leak fission products out of the fuel at high power. Neither of those are things I think the Navy (or anyone else) would want out of a power reactor.

        I am excited about gas cooled reactor for stationary power applications, but am doubtful they can fill any of the limited mobile roles (aside from the obvious NTR role already mentioned). Other technologies tend to be a combination of more mature (such as PWRs for ships) or better suited for the task (such as the tiny sodium heat pipe reactors for NASA missions). Although I am quite liable to be proven wrong by some of the gas cooled micro-reactors in development!

      3. Would a molten salt reactor with a heat exchanger to a regular N2 closed brayton cycle (or open brayton cycle) work any better?

    2. No, GA went even smaller than pebble bed, to monolithic sub-dense UC and their reactor is still the size of a school bus, not amenable to compact nuclear for military vehicle applications.

      1. Again, reactor size isn’t necessarily correlated to size of a complete power generation system.

        AFAIK, all of the micro nuclear power plants being developed under Project Pele use some form of gas cooled reactor with a Brayton cycle power conversion system.

        But I’ve been wrong before and might be wrong again.

  10. Personally, I’m generally opposed to isotopic separations, particularly when an isotope is as rare as N-15 is the target, because they are energetically expensive. An exception would be deuterium, since the separation is fairly easily accomplished by the electrolysis of water.

    I love, absolutely love, actinide nitride fuels, because they are so easily recycled.

    The wonderful thing about nitrogen-14 is that it tends to produce carbon-14, which, if one isn’t inordinately terrified of radioactive things, has much to recommend it for carbide fuels, since it has a neutron capture cross section that approximates that of helium, helium having a capture cross section of zero, but being a gas that will become extremely rare and expensive in short order. C-14 will thus be the absolute best low capture cross section isotope available in the future, but regrettably will be something of a moderator, albeit not so much as natural carbon..

    In combined cycle nuclear reactors, with multiple “step down” thermal cycles attached in integrated heat networks, my personal favorite gaseous fuel, after gaseous metals – I’ve convinced myself they are actually quite possible with materials science advances – would be carbon dioxide, in an Allam type “Braytonish” cycle, since carbon dioxide can do chemistry in the supercriticial state. Hot, and compressed, carbon dioxide turns out to be a pretty good oxidant, and of course, as an oxidant is itself reduced. With reduced carbon dioxide, carbon monoxide, or under Boudouard conditions, pure carbon, one can make just about every industrial chemical now produced from dangerous petroleum or dangerous natural gas, and displace coal in metal reductions in steel, aluminum, and FFC reductions..

    In recent years I’ve spent a lot of time thinking about air as a coolant, if only because the gamma, x-ray and UV irradiation of air (isolated from a neutron flux) can address the rising fluoride gas problem which otherwise will be with all future generations.

  11. Why must the air that is going through the turbine also go through the reactor?
    Why can’t you use some other coolant suitable for high temperatures like molten salt, CO2 or whatever and then heat the air for the turbine outside the reactor where you have no neutron flux? Then you can use air or regular nitrogen or whatever you want.

    1. It seems like adding another loop as for air would add more complexity which is a major problem with reactors already.

      1. My philosophy is to keep systems as simple as possible while also using technology that is already in widespread use.

        Best technologies are often a new combination of several well proven technologies. Simple cycle and combined cycle gas turbines qualify. So does coated particle actinide fuel.

      2. Are you really adding much complexity thought? With enriched nitrogen you have to establish a programme to enrich nitrogen before you can build a reactor and that’s a complex thing in and of itself.

        With enriched nitrogen going through the core and through a turbine it seems you’d have a lot more places where it has an opportunity to leak than if you had it in a primary loop with just a heat exchanger. If this enriched nitrogen leaks too quickly somewhere you need to be able to shut down the core and keep a positive pressure with regular nitrogen (so air doesn’t get in to the core; there’s no neutron flux so regular nitrogen is not a problem).

        You also limit yourself to AFAIK only graphite pebble bed /graphite prism type reactors with SiC coated fuel grains. They have low enough power density and high enough heat tolerance to sit there and stew at 1 atm if gas leaks too badly.

        1. @Soylent

          Heavy nitrogen is already available as a specialized science material, so there is a modest production infrastructure already operating. At least one company has plans to build a new production facility and is actively seeking customers. They have recently patented an improved process.

          Making leak tight systems isn’t anything new. Combustion gas turbines need leak prevention to protect operators from combustion gases, for example.

  12. The Russian KLT-40S is a reactor well proven in icebreakers. Here a 150 MWt unit produces 35 MWe (gross) as well as up to 35 MW of heat for desalination or district heating. These are designed to run 3-4 years between refuelling and it is envisaged that they will be operated in pairs to allow for outages, with on-board refuelling capability and used fuel storage. At the end of a 12-year operating cycle the whole plant is taken to a central facility for two-year overhaul and removal of used fuel, before being returned to service.

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