Fission is an elegant way to heat a gas

Adams Engine

Adams EngineTM

What if it was possible to combine the low capital cost, reliability, and responsive operations of simple cycle combustion gas turbines with the low fuel cost and zero-emission capability of an actinide (uranium, thorium, or plutonium) fuel source? Machines like that could disrupt a few business models while giving the world’s economy a powerful new development tool. That would be especially true if the fuel source was in a form that could withstand all possible temperature and pressure conditions without being damaged.

I’ve been inspired by and chasing that goal with varying levels of intensity since 1991. The pursuit has been rather disruptive to my personal life but with the help of a supportive family and a lot of exceptional friends, the disruptions have combined in a mostly positive way. (I’ll admit — and my wife and children will testify — that it has not always been easy be me or to live with me.)

I cannot claim originality in the idea of a nuclear fission-heated gas turbine. It is a concept that has been discussed and evaluated since the days immediately following World War II.

Both of the necessary parts of the system, however, were quite primitive at that time. Brayton Cycle gas turbines were just starting to enter into useful service and were not yet commercially available. It had only been four years since the very first atomic pile had proven that it was possible to control and sustain a heat-producing reaction using an actinide fuel source in a carefully configured array of selected materials.

One of the primary conceptual leaps required to understand how such a system can work is to recognize that a Brayton Cycle does not have to discharge directly to the atmosphere; all the basic cycle needs is a way to return the gas temperature and pressure to the starting point of the cycle – normally atmospheric air at available temperature and pressure.

In combustion gas turbines like jet engines and simple cycle gas turbines, the standard practice is to discharge the heated combustion products and use a continuous supply of fresh air, but in a system that does not use the air as part of the heat producing chemical reaction, it is better to simply direct the discharge of the turbine through a cooler and then return the cooled, depressurized gas back to the compressor inlet.

A closed Brayton Cycle allows designers to choose among a larger number of working fluid options, keeps any stray fission products or gas activation products inside a controlled system, and allows the possibility of using higher system pressures to shrink component and piping sizes.

Most of the gas cooled reactor designs that have ever been proposed, dating all the way back to the Daniels Pile, have been based on using helium as the working fluid. (See page 63 of the document linked with “Daniels Pile.”) The primary exception has been the CO2 cooled Magnox and Advanced Gas Reactor (AGR) in the UK and the several French gas-graphite reactors built before the decision to focus on developing a series of reactors based on a Westinghouse-designed pressurized water reactor.

Neither of those gases, however, enables the use of conventional gas turbine machinery. They both require specialized compressors and turbines, so there is little synergy between the sixty years of design refinements, manufacturing infrastructure, proven operational performance, or trained operators and maintenance personnel that enable combustion gas turbines to be such an obvious choice for power plant purchasers.

Most of the proposed nuclear-heated, closed-cycle gas turbine projects have also envisioned systems that operate at high pressures and strive for maximum thermal efficiency using several stages of preheat and recuperation. Those design choices add to the complexity of the system and move away from the initial goal of combining the low capital cost, reliability, and responsive operations of a simple cycle gas turbine with the low cost and emission free nature of an actinide fuel source.

Combustion gas turbines use atmospheric air, a readily available gas composed mainly of 80% nitrogen, 19% oxygen, 1% argon, and a complex mixture of various trace contaminants. That choice leads to some engineering complications, but it works and provides a key component of combustion, the heat-producing chemical reaction using hydrocarbon fuel sources. Since a nuclear heated system will use a closed-cycle, it is worthwhile to more carefully control the working fluid to remove the argon, which would be activated as it flows through a fission reactor, and to remove the trace contaminants.

It might also be worthwhile to more carefully control the ratio between N2 and O2 for optimal compatibility with the system materials. Because combustion turbine machinery is designed for operation over a relatively narrow range of input pressures, it is best for the early models of closed cycle gas turbines to keep the temperatures and pressures in a similar range.

Since nuclear fuel costs on a per unit of heat basis are substantially lower than the cost of the clean hydrocarbon fuels suitable for use in a combustion turbine, thermal efficiency is not as important as engineers normally assume, so may be beneficial — especially for early generation machines — to avoid any system complications in order to keep capital costs low.

Though not in widespread use, gas cooled reactor technology has made some significant advances since the Daniels Pile conceptual design. There is a prototype operating in China called the HTR-10, and a demonstration project called the HTR-PM under construction that will use two pebble bed helium-cooled reactors to produce steam that will be combined into a common header to supply a single steam turbine.

The fuel form and materials chosen for that reactor are compatible with an N2/O2 gas mixture. As the existence of the HTR-10 demonstrates, the pebble-bed fuel form enables a variety of reactor power levels to use exactly the same fuel element. Size changes do not require altering the length of fuel assemblies, they simply require changing the internal volume of the reactor to allow more or fewer pebbles.

There is nothing magic about the 6 cm diameter pebble that seems to have always been the chosen fuel element of pebble bed reactor designs – smaller fuel elements would provide some advantages. Nuclear rocket engine designers have proposed fuel elements as small as a grain of sand, but there are disadvantages of making the elements too small. A fuel element the size of a shooter marble might be optimal, but early generation units would probably stick with the 6 cm pebble.

Pebble-beds also allow designers to capture some of the advantages of mobile, liquid fuel without having to deal with the disadvantages of fission products that are not tightly contained in a solid fuel matrix. Most of the pebble bed designs that have been produced have a system that enables continuous refueling. Some have included features allowing portions of the core to be “dumped” to a holding tank under the reactor that is designed to ensure safe retention of hot fuel and a guaranteed sub-critical configuration. In their simplest form, however, pebble-beds can be stationary and not include the complexities of a continuous refueling system.

The basic Adams EngineTM includes a variable speed air (N2/O2) compressor, a stationary pebble-bed reactor, a throttle-controlled turbo-expander, and a gas cooler that uses either an available water source or air as the cooling medium. The highest internal pressure in the system will be roughly 1 MPa – ten times atmospheric pressure. That pressure offers interesting options for the reactor pressure vessel construction and piping manufacturing.

As long as the reactor thermal power is less than about 300 MW, the reactor can be designed so there is no need for any decay heat removal systems. The fuel can withstand any possible temperature without being damaged.

There is no fundamental reason why a designer cannot add a heat recovery steam generator to make an Adams Engine into a combined cycle, but it is not obvious that the improved power output and thermal efficiency would be worth the additional capital cost and system complexity.

One of the greatest compliments an engineer can receive is acknowledgement from his peers that he has created an elegant solution to a difficult problem. Antoine de Saint-Exupéry made the classic statement of engineering elegance:

A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away.

I hope at least some of you will agree that an Adams Engine could be an elegant solution to the challenge of combining low capital costs, reliability, responsiveness, low fuel cost, and zero emissions without having to make too many sacrifices.

There was once a patent on the power control system, but that expired about ten years ago when I decided not to pay the maintenance fee.

The above is a simplified description. There is more detail available if you follow the links for the various mentions of Adams Engines. Questions are always welcome.

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