Will heavy nitrogen become a widely used fission reactor coolant?
Heavy nitrogen has the potential to become as important to the future of atomic fission power system development as heavy water has been up until now. That’s a bold statement, so let me explain why I believe it’s true.
Are any nitrogen cooled reactors being used today?
One nuclear fission power system – the US Army’s “reactor in a box” the ML-1 – is known to have used nitrogen as its primary coolant and working fluid.
Nitrogen has features that make it an intriguing coolant option in a closed, Brayton cycle fission system; its thermodynamic qualities are virtually identical to atmospheric air. The overwhelming majority of Brayton cycle machines in operation and production today have been designed to use air.
With few, if any, modifications, a conventional Brayton cycle compressor can push nitrogen through a high temperature nuclear reactor. The resulting hot, high pressure nitrogen can then turn a conventional gas turbine machine. The turbine can discharge its gas into a large, low pressure cooler to return the gas to the initial compressor inlet conditions.
Like helium and CO2, nitrogen is compatible with the graphite that is used as a moderator and structural material in gas cooled reactors. Unlike oxygen, it does not react with carbon (graphite).
All this has been well understood for decades and is the basis for choosing nitrogen for the Adams Engine.
Why have almost all other nuclear system designers dismissed the nitrogen alternative?
The main objection to using nitrogen as a reactor coolant is that nitrogen has a feature that makes it seem less useful than helium. That is the coolant gas selected by the overwhelming majority of those who believe gases have utility as reactor coolants in reactors capable of achieving gas temperatures higher than 700 ℃. (There are many nuclear reactor designers who dismiss all gas coolant options. Reasons for their dismissal are beyond the scope of this article.)
Unlike helium, nitrogen absorbs neutrons. In fact, its broad spectrum neutron absorption coefficient – a measure of probability for the reaction to occur – is fairly high at 1.83 ± 0.03 barns. That number is high enough that nitrogen is used as a secondary means of shutting down the British CO2-cooled Advanced Gas Reactors (AGR). (See pg. 21)
Nuclear reactor engineers work diligently to eliminate materials that absorb neutrons from their designs; neutrons absorbed by any material that is not fuel are wasted and reduce fuel efficiency. All else being equal, a reactor that contains more neutron absorbing materials will need more fissile material to be loaded to achieve the same operating cycle length.
Effects on fissile material efficiency are not the only reason to worry about putting neutron absorbers into fission reactors. When an atom absorbs a neutron it becomes a different isotope; that new isotope can create its own problems.
When atmospheric nitrogen is exposed to a neutron flux, it undergoes an N-P reaction. (That the shorthand for a reaction where an atomic nucleus absorbs a neutron and promptly emits a proton.) In the case of N-14, the most common nitrogen isotope, the N-P reaction creates carbon-14 (C-14).
C-14 decays with a low energy beta (nuclear electron) emission to become N-14. Essentially, one of the neutrons in radioactive carbon becomes a proton, producing stable nitrogen. That decay event is a lot slower than the N-P reaction that created the C-14 – after 5,730 years, only half of a mass of C-14 will have turned back into N-14.
C-14 is part of our earthly environment because it is constantly being created in the upper atmosphere where nitrogen is exposed to cosmic radiation. However, elevated quantities of C-14 are perceived to pose a risk to living organisms. Other nuclear reactors produce C-14, but releases of C-14 are tightly controlled. Production is avoided if at all possible.
Aside: There are valuable uses for C-14 today and more that are being developed. It is possible to turn the disadvantage of constantly producing C-14 into a revenue source that might even become a profit center, but that path isn’t within the scope of this article. End Aside.
One other difference between helium and nitrogen that sometimes enters the discussion about coolant alternatives for gas cooled reactors is the fact that helium has an attractively high specific heat transfer coefficient.
Per unit of mass, helium will transfer 5 times as much heat as nitrogen. But, helium is a light, monatomic gas. Its molecular weight is 4 atomic units. In comparison, nitrogen is a stable diatomic gas with a molecular weight of 28. Since all gases have the same molar volume, at the same temperature and pressure, nitrogen is 7 times as heavy as helium.
A volume of nitrogen has about 40% more capacity for moving heat as the same volume of helium. Compressors and turbines move volumes, not masses.
Partly out of habit and partly because of the challenges associated with managing C-14, virtually all high temperature gas cooled reactor designers have stuck with helium as their choice of coolant. Even though many reactor-decades worth of operational experience has been accumulated with CO2 as a coolant, that gas breaks down at the temperatures envisioned for HTGRs.
Choosing helium has forced gas cooled nuclear power system designers to deal with the considerable challenge of designing and fabricating special purpose helium machinery. Reactors heat sources tend to work well with helium as their cooling medium. It’s a much more difficult gas to move with compressors or circulators and to use to spin turbines.
But the engineers who design reactors are usually not well versed in heat engine design and manufacturing processes. They choose the gas that seems best for their part of the power system. They are often in charge in nuclear power plant design organizations.
Enter heavy nitrogen
Atmospheric nitrogen consists of a predictable ratio of two stable isotopes. 99.67% of them are N-14, an atom that contains 7 neutrons and 7 protons. But 0.36% (36 atoms out of 10000) are N-15, an atom that contains 7 protons and 8 neutrons.
That extra neutron makes the atom extremely reluctant to allow another neutron into the nucleus. The broad spectrum cross-section for neutron absorption for N-15 is roughly 8,000 times lower than it is for N-14. N-15’s absorption cross section is even lower than helium.
Aside: I need to credit Atomic Insights participants for teaching me that heavy nitrogen might be a good solution to a difficult problem. Cyril R. first introduced N15 into a discussion about the NGNP project in March 2013. As you might notice if you review the comment thread, I resisted the idea at the time. John ONeill reintroduced the idea in an Atomic Insights comment posted on Nov. 6, 2018. Those discussions have been running around inside my mind for years, with periodic efforts to learn more. I admit it. I’m slow. End Aside.
Closed Brayton cycle machines using a reasonably pure form of N-15 as the fluid for both turning turbines and transferring heat from a high temperature gas reactor should overcome two obstacles that have stopped nuclear gas turbines from being developed.
They would be using a gas whose thermodynamic properties are virtually identical to atmospheric air. That allows the use of a broad spectrum of refined compressors and turbines that are in production today. Those machines have fully established supply chains for various components. The machines are accompanied by blueprints, maintenance manuals, operating manual and experienced technicians.
There will be some refinements required in bearings and ductwork, but those are largely external to the main parts of a turbo machine.
A small portion of N-14 will remain in an inventory of gas that is vastly enriched in N-15. It will still require some management. But reactor designers and operators inevitably must deal with impurities.
An interesting aspect of making this design choice is the fact that the reactor will be most reactive when its coolant is pure. Any event that results in a reduction in coolant purity will tend to make the reactor less reactive and may even result in halting the fission reaction.
If there is a major loss of coolant event, there will be provisions for refilling the system with available gases, likely either conventional nitrogen or atmospheric air. A major loss of coolant would likely be accompanied by shutting down the reactor for repairs, so there will not be a significant neutron flux and the replacement gas will not accumulate a substantial quantity of C-14 during the repair period before the system is again filled with an inventory of N-15.
There are current uses for N-15 on a laboratory scale. It’s a useful isotope for tracing biological processes like fertilizer uptake. According to current suppliers, the world market for high purity N-15 is less than $1 M annually. And that is for a gas where one supplier’s catalog lists a 5 L bottle as being available for $2,190.00.
There are existing production facilities and several different available processes that can separate N-15 from atmospheric nitrogen. A patent for one of the processes was granted to Taiyo Nippon Sanso Corporation in 2010 (US Patent Number 7,828,939 B2).
Aside: Soon after the original version of this post was published and shared, @Syndroma pointed out that there is serious interest in using N15 for nitride fuels for fast reactors. Nuclear Engineering International published an article titled Russia looking at isotope-modified nitride nuclear fuel about that application for heavy nitrogen. End Aside.
It seems reasonable to believe that production processes could be scaled to meet any substantial demand for the product. It’s also worth noting that this is not a material that will be consumed. It will be continuously cycled through closed loop systems. Any leaks from those systems will return the gas back into the atmosphere.
Opportunities, not predictions or guarantees
Closed Brayton turbo machinery using a fission heat source has been an elusive goal for a small number of people since the earliest days of atomic energy development. Nothing in this post is new information, so it’s entirely possible that its publication will not make any difference.
But the potential for addressing some of the world’s energy needs with a power system that combines an emission-free, abundant, affordable and reliable heat source with refined Brayton cycle heat engines is too attractive to ignore completely.
It is the Brayton cycle that makes natural gas power plants so quick and easy to erect. It is the Brayton cycle that makes them responsive and thermally efficient.
I’ll close with one final thought. When natural gas fired gas turbines are shut down because there isn’t enough demand for electricity or other power, they cool down quickly. Operators don’t purposely keep them warm because that consumes fuel.
A fission-heated Brayton cycle machine will stay warm for many hours as a result of radioactive decay heat generation. That might be a feature that makes the system even more attractive in applications where flexibility has a market value.
One more reason nitrogen is not favored is a potentially dangerous reactivity increase after a pressure drop. N-15 would solve this too.
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?
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.
D (H2) is 0.0115% of naturally occurring hydrogen.
N15 is 0.36% of naturally occurring nitrogen.
IOW, N15 is more abundant. (Significantly)
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.
NH4 – Anhydrous ammonia, not NH3. NH3 is formed as NH3 OH in water.
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.
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.
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.
Rough calculations using 1.22g/cc homogenized graphite/HALEU with N2 at 10 bar and ~1000C, loosely based on the PBMR fuel (9gU + 200gC per pebble) shows about +$0.60 when the N2 is absent.
Did that for Rod in 11/2018 after a comment from handle RRMeyer called attention to the fact in 2018 article (https://atomicinsights.com/turning-nuclear-into-a-fuel-dominated-business/)
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.
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.
Have you received a quote for bulk quantities of N-15? How do you know how much the gas will cost?
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.
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.
Use an outer SiC coating on the pebbles.
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.
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.
It lets you use bog-standard OTS turbomachinery.
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.
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.
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.
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.
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.
Agree. C-14 production and sale is a potential source of revenue.
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.
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.
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.
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.
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.
Speaking of common sense….
https://www.climatecentral.org/news/solar-study-sees-ecological-risks-19568
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?
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.
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.
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!
Would a molten salt reactor with a heat exchanger to a regular N2 closed brayton cycle (or open brayton cycle) work any better?
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.
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.
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.
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.
It seems like adding another loop as for air would add more complexity which is a major problem with reactors already.
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.
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.
@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.
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.