Atomic Insights

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.