Nitrogen or Air Versus Helium for Nuclear Closed Cycle Gas Turbines

An Atomic Show listener who heard the recent show about atomic gas turbines provided a link to an article posted on Energy Pulse titled The Potential for Air-cooled High-Temperature Nuclear Reactors. That article provided me an opportunity to review just why I continue to be intrigued by the use of nitrogen (N2) or air as the working fluid/coolant for closed cycle, nuclear heated gas turbines designed for widespread, economical power production.

Harry Valentine, the author of the Energy Pulse article, provides a similar derivation of the applicable engineering computations that led me to begin to understand – in about 1994 – that N2 would be a useful alternative to helium in that particular application. The motive for my investigation then was a rather dismissive encounter with a combustion gas turbine designer who chuckled when I told him that I had just formed a company that wanted to build nuclear gas turbines by adapting air breathing compressors and turbines for use with helium. He told me that we would have to start from scratch in machinery design and be prepared to spend several billion dollars in the process of refining that design and building the appropriate tooling before we would reach the point where the machinery could actually be manufactured.

As the sole member of a start-up organization with a rather low capital base, that prospect almost stopped AAE dead in its tracks. The only ray of hope offered was when that designer asked if I had ever considered using a different inert gas like N2. (I wish I could remember that guy’s name, I do remember that he was teaching at the University of South Florida in Tampa and that he had several decades of industrial turbomachinery design experience before becoming a professor.)

The Energy Pulse article has attracted a few interesting comments so far, but it is not yet widely read. Here is my contribution to the discussion:

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.

If you are a bit on the geeky side – and how many Atomic Insights readers do not fit that description – you might be interested in digging into A REVIEW OF HELIUM GAS TURBINE TECHNOLOGY FOR HIGH-TEMPERATURE GAS-COOLED REACTORS.

That paper, published in 2007, offers an up to date view of the efforts to overcome the difficult hurdle imposed by attempting to use helium as the working fluid for closed cycle gas turbines, an effort that has occupied a certain subset of reactor designers for careers long enough to cause grey hair and enable the development of adult grandchildren. If you read closely between the lines, you might better understand PBMR’s recent decision to focus its limited capital budget on the process heat market rather than the continued development of closed cycle helium turbo machinery for electrical power production.