The Potential for Air-cooled High-Temperature Nuclear Reactors

Posted on March 31, 2009
Posted By: Harry Valentine
Topic: Nuclear

Nuclear power generation technology has undergone an evolution from fuel rods and heavy water to newer designs of reactors that can be cooled by light water and more recently by gas. There is much research underway in China and the USA involving gas-cooled, high-temperature reactors (HTR) based on the continually evolving pebble bed modular reactor (PBMR) technology. Advanced research into PBMR technology in China is focused on both uranium-fueled and thorium-fueled versions that involve cooling the reactors using pressurized helium circulating in a closed-cycle turbine.

Helium has the advantage of having 5.1708-times the heat capacity or specific heat of air based on equal weight and temperature (BTU/lb- R or KJ/Kg- K). It has a specific heat ratio of 1.66 as compared to 1.4 for air. That higher specific heat ratio allows a power turbine to extract more work from an equivalent mass flow-rate of helium (Kg/sec) as compared to air. It also has a gas constant that is 7.23707-times that of air.

That gas constant offers an advantage in applications like airships as it translates to air having 7.23707-times the density of helium at equal pressure and temperature, causing the entrapped helium inside the airship envelope to produce great buoyancy. When the higher density of air is divided by its lower heat capacity when compared to helium, the result is (7.23707)/(5.1708) = 1.3996 which means that for equal pressure, temperature and volume flow rate, air will have 39.96 percent greater heat capacity than helium with which to remove heat from a high-temperature reactor.

The design evolution of nuclear reactors suggests that gas-cooled PBMR-HTR reactors could eventually be cooled by air, nitrogen or carbon dioxide. Researchers have been seeking methods by which to resolve the flammability problem of pebble bed reactors, an advance that opens the door to cool them using pressurized air. A high-temperature nuclear reactor may be cooled by the equivalent volume of helium, air, nitrogen or even carbon dioxide being pumped through the reactor at the same pressure. The equivalent volume of air will have 7.23707-times the density as helium with up to 39.96 percent greater thermal capacity with which to cool the reactor. Nitrogen can be extracted from atmospheric air and provide comparable performance as air.

Single-stage Closed-Cycle Turbine:

It is possible to compare air (or nitrogen) to helium a theoretical gas-cooled nuclear reactor which operates up to 950 C (1223 K) with the gas being heated to 1170 K with heat been transferred to the gas at 95 percent-effectiveness. For the basis of comparison a single-stage closed-cycle turbine is used with final cooling being done by ocean water with gas cooled to 47 C or 320 K. The single axial-flow compressor and single turbine rotate on a common shaft and both operate at a pressure ratio of seven-to-one with 87 percent-isentropic efficiency.

A single-stage closed-cycle turbine that extracts heat from a gas-cooled, high-temperature reactor (HTR) can operate using air (or nitrogen) at the same temperature, pressure and volume flow rate as helium. The air (or nitrogen) can cool the reactor that can operate a turbine engine with exhaust heat supporting the operation of a thermal desalination facility. The final exhaust of the turbine will be cooled by ocean water prior to the pressurized air or gas being re-compresses through the power producing cycle. Closed-cycle turbine engines can offer a wide range of output at high efficiency by varying system pressure to many times above or below atmospheric between LP turbine exhaust and LP compressor intake.

Multistage Closed-Cycle turbine:

Air or nitrogen-cooled HTR technology could offer higher efficiency using twin-spool turbine engine operation that includes after-cooling between the low-pressure and high-pressure compressors, reheating the gas between the high-pressure and low-pressure turbines and recovering exhaust heat in a recuperative heat exchanger. The system pressure ratio of nine-to-one may operate with 2-stage compression using compounded compressors and turbines with three-to-one pressure ratio and 90 percent isentropic efficiency. The theoretical performance of such a system is provided in the following table.

Helium has an efficiency advantage over air in single stage, simple-cycle turbine operating without a recuperative heat exchanger. The addition of such a heat exchanger increases the efficiency of a closed-cycle turbine that operates on air or nitrogen and that rejects enough exhaust heat to sustain the operation of a thermal desalination plant. Air (or nitrogen) can operate as efficiently as the working medium in closed-cycle multi-stage turbine. Such a turbine would use an after cooler between its compressors; reheat the gas between its turbines and use a recuperative heat exchanger to improve efficiency. The exhaust would still be hot enough to sustain a thermal desalination plant.

Purified and filtered atmospheric air or nitrogen at varying pressures would re-circulate through the closed-cycle turbo-machinery of an air-cooled high-temperature reactor (HTR). A large proportion of the air or nitrogen would remain within the system with a reserve supply stored on-site in large tanks. Using gas from a storage system to cool a reactor reduces the risk of damaging the turbo-machinery or fouling the heat exchange surfaces. Steam turbines in a steam driven nuclear power station can sustain damage and be eroded by from tiny high-speed droplets of saturated steam eroding surface material from the turbine blades.

Future Prospects:

The evolving development of gas-cooled, high-temperature reactors may eventually allow pressurized air or nitrogen to be used as the cooling medium and also as the working fluid that drives the turbines. Air and nitrogen have very similar thermal properties and heat ratios and could be used to cool nuclear reactors once the flammability of the fuel for pebble bed modular reactors is resolved. Such a development would allow the operation of gas-cooled nuclear reactors in countries that have a shortage of helium. The exhaust heat from such power plants can sustain the operation of thermal desalination plants in nations that need additional electric power and that have a shortage of potable water for their populations.

Research into and development into gas-cooled (helium), pebble-bed modular reactors is underway in China with similar research being undertaken in the USA. Advances in that research may eventually lead to the development of reactors that may be cooled by air, nitrogen, or even pressurized carbon dioxide. In all cases, extra gas may be stored in underground caverns and be transferred to and from the reactor as needed. Pebble-bed modular reactor technology can operate on either uranium or thorium with thorium being more plentiful and the spent product less volatile.

Authored By:

Other Posts by: Harry Valentine

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April, 01 2009

Len Gould says

Excellent article, Harry.

April, 01 2009

Alan Belcher says

Question: since air is much more abundant and cheaper than helium, why is it not being proposed more often as a heat transport medium for this radioactive environment? Could it be that it might become radioctive when passing through the reactor?

Just some thoughts -- not an attempt to pour cold water on your concept!

April, 01 2009

Don Giegler says

For a look at high temperature gas-cooled reactor experience with helium as the primary coolant and the evolution to a gas turbine design, try:

Bye the bye, Alan is right. Use of helium minimizes reactor coolant system circulating activity. Lots of interesting activation products occur when air is present in a high temperature reactor coolant loop.

April, 05 2009

Roger Arnold says

You're barking up the wrong tree, Harry. "Focusing on the wrong parameters" to put it formally.

The fact that a given volume of air or nitrogen pumped through a heat exchanger will remove more heat than the same volume of helium is irrelevant. What matters is the energy needed to pump the volume needed to remove a given amount of heat. And there helium, with its lower viscosity, is very much superior to air or nitrogen. (Hydrogen would be even better, but is ruled out due to its effect on metals.)

Since it circulates in a closed system, the cost of its helium load is a one-time capital expense for reactor construction -- and a negligible one at that. I'm sure it would be possible to design a HTGR using nitrogen, but I sure don't see any reason to go there.

April, 05 2009

Brad Fiander says

This article mentions China and the US as centres of pebble bed research, but fails to mention South Africa which is the home of the PBMR (see China is focused on using pebble bed reactors as heat sources to drive Rankine cycle steam plants with a helium primary cooling loop, whereas South Africa has been going the direct helium/Brayton cycle route. But there are rather large problems in designing helium turbines, so China may get a jump on any commercial market.

Using nitrogen would be preferable to using air as a coolant due to the reaction products. I think that wIth nitrogen, 14-Carbon is the predominant nuclide generated. Helium's advantage is that it is not activated by neutrons, but this advantage is eroded somewhat by the difficulty in keeping helium from leaking, and its cost.

April, 05 2009

Brad Fiander says

Roger, the cost of helium will be an ongoing cost. How much, I don't know. But there will always be leakage when trying to contain such a small atom.

April, 05 2009

Malcolm Rawlingson says

Helium is quite a rare gas and as Brad says it will leak out so constant make up will be necessary. Deploying thousands of these reactors may strain helium supply capability. I think the key parameter is the specific heat capacity which is high for helium. Also helium does not become radiactive in the core allowing it to be used in a direct cycle gas turbine without shielding. Using air or nitrogen I suspect will create some gamma radiation from O16 and N19 which are radioactive isotopes of Oxygen and Nitrogen formed in the core by neutron bomardment. It may not be that bad as the air density will be low but both of these are strong gamma emitters. Fire in the core is a concern given that these are graphite (carbon) moderated reactors. It would need to be nitrogen but I think Roger has it...the amount of energy needed to pump N2 around is very much higher that that for He.


April, 06 2009

Michael Keller says

Extremely hot air and the graphite of the gas reactors do not do so well together - causes severe corrosion/oxidation.

Helium (being inert, not prone to absorbing neutrons and with a high heat capacity) is an ideal reactor coolant.

As far as helium leakage is concerned, will occur largely at the turbine/compressor seals. Use of shaft labyrinth seals, brush seals and oil seals should keep it down. Molecular sieve technology should be able recover most of the helium that leaks out into the containment structure surrounding the turbo-compressor.

There is another quite unexpected developing gas reactor technology - see

April, 07 2009

Rod Adams says

Harry - very interesting article. I am happy to have found another person who is interested in using air or nitrogen as the working fluid for gas cooled reactors. That has been one of my major research focus areas since about 1991.

Here are the reasons why I think that N2 (perhaps eventually air) has an advantage over helium in direct, closed cycle gas turbine applications.

1. There are hundreds of commercially proven compressors and turbines that are designed to operate with air as the working fluid. Their blades, cooling systems, seals, and materials have all been carefully chosen and the tooling necessary for their production is already in existence. Adapting these machines to run on N2 is a trivial modification. None of them could use He. As Brad has mentioned, building He specific machines is a costly and long lead time endeavor - there are no helium machines in operation today. By using N2 as the coolant/working fluid, reactor designers gain access to a complete spectrum of low capital cost fluid machinery - the same machinery that enables natural gas power plants to be built with much lower capital costs than power plants requiring steam plants.

2. As some have pointed out, the world's inventory of helium is limited. That gas also has some very special uses in rockets and lighter than air craft. A large building program of helium cooled nuclear plants would add enough demand to the supply-demand balance to drive up the price considerably. Any reactor designed for helium cooling would be a captive customer of a potentially capricious supplier since the reactor would not be able to produce power without a steady replacement supply of helium.

3. There is a vast amount of experience in exposing Nitrogen to neutron flux. We know exactly what activation products get produced (C-14) and how much of those products get produced in any given flow and neutron flux situation. The operational challenge provided is MUCH lower than that already accepted on a regular basis in boiling water reactors. C-14 is an easy material to handle, isolate and store; that necessary activity will be a part of the operational cost of the units, but it is not a major issue compared to the issues that face many other types of power plant designs.

4. As Harry has clearly shown, the "heat capacity advantage" for helium is a chimera. Compressors and turbines operate on gas volume flow, not mass flow and N2 has a higher specific heat capacity per unit volume than helium. The people worried about compressor work need to go back and review their basic thermodynamics and fluid flow text books. If not math inclined, just think about how many air breathing Brayton cycle gas turbines are in use today in power plants, under aircraft wings and on board ships. If compression power was really a big issue. . .

One more thing - the first table is a little misleading. Producing high volume flow compressors and turbines that can create a 7:1 pressure ratio using helium is EXTREMELY difficult due to the nature of the gas. The number of stages required for that pressure ratio is quite high and leads to real machinery challenges due to the overall length of the machines. The math behind that statement is no longer at the tip of my fingers, but I recall that it has a lot to do with sonic velocities, Reynolds numbers, and gas constants.

Rod Adams Founder, Adams Atomic Engines, Inc.

April, 07 2009

Len Gould says

I second Rod's points against helium as a working fluid for a turbine. In addition to his points, I'd point out the problems of implementing sufficiently close blade-end clearances to keep efficiencies anywhere near high enough for commercial operation. Where ARE those helium-cooled PBMR's we were assured were "coming soon" a decade ago?

April, 07 2009

David Walters says

Let me suggest... "slightly" off topic, that the biggest issue in using either N2, CO2 or H2 is actually the turbine itself. No one, to my knowledge, actually produces a closed cycle Brayton cycle turbine of the kind that can drive a turbine/generator set. 99.5% of all R&D into turbines has been open cycle, Ranking aero-derivative or frame units for planes, trains, ships and gas turbine generators.


April, 07 2009

Rod Adams says

David - there is no reason at all why one cannot use a frame type turbine or a turbo expander designed for an open cycle using air cannot work inside a closed cycle system using N2. It is just a matter of setting the pressures properly so that the exhaust is at atmospheric or close to that.

A properly sized cooler is no different from an exhaust stack from a system point of view.

April, 08 2009

Don Giegler says

For those genuinely interested in the demonstration of nuclear-powered helium gas turbines and helium-cooled pebble bed nuclear reactor experience, consider Oberhausen 2 (1975 to 1987), AVR (1966 to 1988) and THTR (1985 to 1988).

At the Fort St. Vrain Nuclear Generating Station, between 1975 and 1989, 4 steam turbine-driven axial flow compressors moved 8900 lbs of helium through a graphite-moderated reactor core past 12 once-through steam generator modules. At rated output, these machines produced a pressure rise of 14 psi in the closed-circuit primary coolant system. Helium storage and purification systems provided any make-up required to maintain 688 psig working pressure in the primary coolant system. These auxiliary systems employed state-of-the-art components in sometimes innovative configurations to deal with helium compression, leakage and, ultimately, scarcity and cost of the systems' working fluid. A fluid whose MASS flow was all-important in achieving performance goals.

April, 09 2009

Michael Keller says

Actually, a number of closed-cycle gas turbines power plants have been built with over 750,000 hours of operation. These units were built in the 1940's and 50's in Germany and several other countries. Interesting book on the subject: "Closed-cycle Gas Turbines - Operating Experience and Future Potential" by Hans Ulrich Frutschi available from the ASME Press.

Gas turbines have been built that can handle all types of gases, largely in conjunction with refinery and chemical processing efforts. Both Germany and Japan have done extensive prototype testing of Helium turbines. The US is apparently doing a fair amount of work with helium turbines as part of the Next Generation Nuclear Plant. However, somewhat difficult to get any specific information.

As Rod pointed out, the nature of helium means that a lot more compressor stages are needed to achieve a set pressure than say using nitrogen. However, the pressure increase required for a closed-cycle helium system is roughly 2.5, thus the machine’s length is much less of a problem.

Len, When you run a financial Pro Forma on the PBMR, the plants small output (~165 mW) and cost cause the investment to be on the somewhat marginal side. Too small and costly to effectively compete with combined-cycle plants and coal plants.

April, 11 2009

Rod Adams says

Don and Michael - you are both correct and do a pretty fair job of helping people to understand just what I am talking about.

Every day there are probably several times 750,000 hours more operational experience with gas turbines operating with a gas that is very similar in thermodynamic characteristics with N2 kept at atmospheric pressure on the compressor inlet and using between 5:1 and 30:1 pressure ratios.

As Don pointed out, the helium circulation system at Ft. St. Vrain "worked", but it was one of the major sources of plant down time and contributed to the early demise of the plant after 14 years with an average capacity factor that would only be competitive with windmills.

Frutschi's book is an excellent technological history that provides many lessons learned on how to take dead end technical paths. They are not dead end because they will never work, but dead end because they were so costly that no one followed through to the next step. Refining the design and manufacturing process for turbo machinery is time consuming and costly. It only makes sense in an enterprise where there is a stream of revenue that can support the effort.

With air breathing machines, that process of refinement has been conducted. N2 machines can build on that base of knowledge and experience. Maybe someday, closed cycle nuclear turbines will be making enough money so that the enterprise operating them can use the cash flow to design and refine something better that uses helium, but that business may also decided that N2 is good enough so that they can deploy their cash resources on other improvement paths.

April, 17 2009

Malcolm Rawlingson says

There is not the slightest chance of any regulatory body approving an air cooled graphite moderated reactor. The risk of a core fire is unacceptable and would not meet any modern licensing criteria. So before we get to the drawing board I suggest consultation with the USNRC would be a good first step. I suspect they will just say no. Nitrogen cooling may be acceptable but gas cooled reactors tend to be large compared to their water cooled equivalents so I expect construction costs would increase. The UK operated Magnox reactors for many years and there still are a few left. They are very large reactors with relatively low output compared to modern water cooled plants. They used carbon dioxide as the coolant with U-metal fuel rods and graphite moderator and a conventional steam system secondary side. Also direct cycle machines are great in theory but practically not a vert smart idea.

While the virtues of various gas compression regimes are discussed at length above the point all have missed is that one single fuel failure will make ALL of this equipment contaminated with fission products and therefore an absolute nightmare to work on. Any material in the reactor that can be moved by the gas flow will potentially add to the contamination of the gas turbine.

Imagine doing gas turbine repairs in a plastic suit? No I don't think direct cycle systems are a practical idea at all. Now I suspect that this fuel has a low failure rate....but it is not zero and it just takes one failure to contaminate everything.

The issue of water droplets in the HP turbine cylinder is a valid one but has essentially been designed out in modern nuclear turbines. Nuclear steam is produced at relatively low temperatures and pressures so after he HP turbine the steam is extracted and passed through moisture separators and reheaters to dry it out and increase temperature before going into the LP turbines. The blading MOST affected by water droplets is not the HP turbine but the large diameter LP turbine blades whose radial velocity is far greater then the HP turbine blades. Striking a water droplet here is where significant turbine damage can occur. Also metallurgical improvements with harder blading steels now available have significantly reduced this erosion problem.

So it is not really a valid reason to select a gas turbine over a steam turbine.

Now there is one approach that could be used and that is to use the pebble bed HTR to increase the inlet steam temperature to that approaching coal temperatures. Passing the steam through a pre-heater heated by N2 or He would make more sense to me and avoid the contamination problem for the turbine.

Any thoughts on that idea.


April, 20 2009

Michael Keller says

Malcolm, Few observations - Fuel failures with boiling water reactors will contaminate the large steam turbines used with these plants. The turbines tend to be slightly contaminated with radioactive "crud" anyway because BWR’s are direct cycle – steam created by reactor goes directly to the steam turbine - Fuel failures with pressurized water reactors (PWR’s) will contaminate the piping, steam generators and reactor coolant pumps. A steam generator tube failure will contaminate the large steam turbine. This has occurred a number of times in the past, but not so much anymore. The “crud” inside the primary loop also does in fact contaminate piping, pumps and various parts of the plant as well.

Having been in a “plastic suit” working on nuclear equipment, I can attest that it is hot and pain-in-the-butt work. I have also been in self contained “plastic suits” in environments that were actually “lethal”, quite unlike nuclear plants. Dealing with radioactive contamination is very much doable and not that big a deal in the broader scheme of things

As far as closed-cycle nuclear plants are concerned, the gas reactor fuel (such as General Atomics’ prismatic core) is exceptionally rugged and highly unlikely to experience fuel failures of any kind. Not so sure the pebble bed reactor fuel is quite as rugged, but the “billiard ball” sized fuel is pretty impressive.

From a technical standpoint, the closed-cycle helium gas reactor has a lot of potential, particularly since you can simply walk away from a plant and the core would not melt. This is quite unlike conventional nuclear plants. However, this capability comes at a cost – the size of the plant is limited to about 600 megawatts of thermal power. This limitation seriously hampers the commercial viability of the technology.

My company has come up with a novel way around the power limitations of the gas reactor. Please visit us at Send us an e-mail and we will happily forward technical papers on the hybrid-nuclear power plant. Difficult to say whether our solution will ultimately win the day, but we are giving it a shot.


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