by Rod Adams » 28 Mar 2009, 12:04
I'll start with a brief word about my background in reactors - I learned my trade on relatively small reactor heated power plants used to push submarines. Though certainly not tiny, these machines operated on a very human scale that made sense to me. They were small enough to be operated and maintained by a small crew (I had less than 30 nukes working for me when I was the Engineer Officer) and also small enough so that I could do a reasonably thorough inspection of the facility in just a few hours. Even with my liberal arts undergraduate education (I was an English major in college), I managed to learn enough about the details of the technology and all of the pieces and parts that made it work so that I was always comfortable that the plant was well under control.
Somehow, I managed to step back a bit from the daily grind of trying to operate this facility under the stringent, human imposed rules and paperwork and realized that it was a very simple machine with a core that was a thing of almost poetic beauty. Simply by adjusting the position of neutron absorbers, we were able to start up from any temperature, establish a steady rate of heating, and then level off the reactor at the desired temperature. After that process was complete, the reactor pretty much took care of itself as we operated the steam plant to produce whatever power level the ship drivers desired. (Because of the way we were trained in the US Navy, I was also a ship driver, not just an engineer.)
In the early 1990s, I had the opportunity to do some library research and realized that most of the world thought that nuclear power inherently meant enormous machines that produced 50-100 times more power than the reactors I learned on. As a guy who has done a little traveling and visited some power plants in smaller towns, I kept wondering why, especially as I learned more about pollution. I kept thinking back to our closed systems on submarines.
I found out about the German AVR project, and about the investigations that had been going on almost since the very beginning of nuclear fission power to try to combine the simple machinery of Brayton cycle gas turbines with reactors that could produce suitable gas temperatures. Those made so much sense to me (several of my Navy buddies have been engineers on gas turbine powered ships and have demonstrated to me that their engines were much simpler than steam plants).
One of the challenges that seemed to be in the way of success of previous attempts to bring high temperature reactors together with gas turbines is that the researchers wanted to take several giant technology steps at one time. They not only wanted to use a closed cycle gas turbine, but they wanted to operate it at high pressure and to use helium as the working fluid. Both of those are problematic in that they require completely new machinery and a full development path for that machinery that had to be completed before they could put it together with the high temperature reactors - which also needed some time and money to complete their development to allow them to operate at an even higher temperature (desired so that the overall system would produce the most thermally efficient nuclear plant ever.) The quest for the ultimate in high efficiency also led the researchers to propose several "improvements" to the simple Brayton cycle including intercooling and recuperation. All of these enhancements worked fine as long as they remained on paper, but practical challenges impeded progress in every project that attempted to develop the ideas using real metal.
I thought back to the simple combustion turbine machines that my buddies had showed me on Navy ships. They have an air intake, a compressor, a burner, a turbine and an exhaust stack. I also thought back to the "heater" on my boat and imagined using the pebble bed technology for a similar heater that would simply stay at a predictable temperature (because of the negative temperature coefficient of reactivity). (My concept for a pebble bed gets rid of the on line refueling system and the notion of moving the pebbles around during operation.)
I learned a bit about the challenge that combustion gas turbine designers have in regards to maintaining temperature and thought that the nuclear version would actually simplify some design considerations. I also thought about the temperatures and pressures used for combustion gas turbines and realized they allowed reasonably compact machinery without requiring thick boundary walls or extraordinary effort to seal between the stages of the compressor and turbine.
The real aha moment came when I tried to work my way past the helium challenges. I tried to explain to a combustion turbine engineer that I wanted to find a shelf model compressor turbine set that would operate reasonably well with helium; the man almost laughed at my ignorance. He explained the differences in Reynolds numbers, specific heat, specific mass, and sonic velocity and told me that I would have to start all over with blade design, interstage sealing, and even rotational bearings. Then I remembered that N2 is a lot like air and that it behaves reasonably well in a neutron flux. I went back to that engineer and asked which existing compressor turbine sets would work with N2 and he gave me a simple answer - all of them.
So, that is a long winded explanation for just one of the possibilities that I see for much smaller reactor power plants. Use a fixed pebble bed reactor as the heat source. In early models, operate that heat source to produce easily achievable and well proven temperatures of 700-900 C. Use a compressor/turbine combination with demonstrated industrial capacity and reliability. Use N2 as the working fluid with compressor inlet pressure at standard atmospheric pressure, a pressure ratio selected using standard Brayton cycle analysis based on the available reactor gas temperature, and then cool the outlet of the turbine with air or water to remove the waste heat and complete the cycle. Control the power output the same way you control the power output of a steam turbine - adjust the gas flow through the turbine.
As one of my non engineer, but watch officer qualified ship driver friends describes it, that system is understandable with the following mantra - suck, bang, blow, go. (In the Navy, we try to teach people simple ways to remember what are occasionally complex concepts. In this case, the Brayton cycle is simple compress gas, heat gas, expand gas, cool gas - repeat.)
Of course, there are enhancements to this system that can be attached AFTER we have built and operated machines, trained a cadre of operators, and built a market in all of those wonderful places in the world where affordable power is not currently available and where cleaner power is a desirable goal that allows us to charge early adopter rates to pay off the development and bureaucratic overhead that has been imposed by people who LIKE selling coal, oil and natural gas.
I personally like the idea of expansion of this knowledge and power to as many people as possible, I am not a big fan of concentrated power or wealth.
Hope this story generates some thought and discussion. (Of course, the Adams Engines also just exist on paper, but we are working hard to change that and seem to be making some progress toward that goal without any physical or technical hurdles in the way. Those bureaucratic hurdles are a challenge, but it is easier to overcome human rules than natural rules.)
Rod Adams
President, Adams Atomic Engines, Inc.
Publisher, Atomic Insights
Host and producer, The Atomic Show Podcast