Pebble Bed Reactors

Addressing Recommendations of MIT Future of Nuclear Energy In a Carbon Constrained World

The Massachusetts Institute of Technology (MIT) is a world renowned institution that has produced thousands of highly educated engineers and scientists. It is generously supported by foundations, corporations and governments. In 2003, the MIT Energy Initiative, began publishing a series of reports on the future of various energy sources.

The Future of Nuclear Energy in a Carbon Constrained World, the ninth and most recent report in the series, identifies the major issue facing nuclear energy advocates and developers. Unless nuclear energy costs can be controlled, there isn’t much room for significant growth.

The report offers three recommendations that would help nuclear energy developers establish better control over system costs. The fourth recommendation is more about increasing costs for competitors than about lowering nuclear costs.

1. An increased focus on using proven project/ construction management practices to increase the probability of success in the execution and delivery of new nuclear power plants.
2. A shift away from primarily field construction of cumbersome, highly site-dependent plants to more serial manufacturing of standardized plants.
3. A shift toward reactor designs that incorporate inherent and passive safety features.
4. Decarbonization policies should create a level playing field that allows all low-carbon generation technologies to compete on their merits.
   Aside: The title of the report indicates that the study group leaders believe this fourth recommendation is the most likely to be implemented successfully. The first three recommendations are rather conventional wisdom that has yet to be implemented successfully in “The West.” There is hope if governments can agree to implement programs that penalize energy sources that produce carbon dioxide or that reward nuclear for its emission free power generation, or that both penalize emissions and subsidize nuclear. End Aside

Though it would be good for nuclear and may be good for all of us on several different measures if the playing field was leveled, it’s not a good idea to build an industry on hopes and promises that depend on election cycles and political pressure alone.

Though this article is mostly a revision of an article published in August 2017, it is an overview of a short cut way of addressing recommendations 1-3. It isn’t a radical new reactor, but instead is a power production system that combines well-developed systems. The end result is a new system with well established supply chains, an experienced labor force with refined skills, and a functional maintenance model.

The description is purposely hazy on such details as size and power output because the intent is to be able to take full advantage of a large infrastructure that has already produced and operated millions of power-production machines covering a broad spectrum of sizes and power levels.

I cannot resist the temptation to point out that the committee-produced report from the MIT study group, despite its high powered institutional support and broad range of contributors, didn’t mention nuclear gas turbines. Instead, this article was written by an individual who has been refining this concept since 1991. It’s possible that the path described here is too straight and has too few remaining challenges to capture institutional attention.

Heat engines, not reactors

It is a modern day truism that natural gas power plants are cheap while nuclear power plants are expensive. Natural gas plants can also be built in a variety of sizes by a larger number of suppliers who have a pretty good record of completing projects in a relatively short and predictable amount of time.

Another truth is that nuclear fuel can be very cheap on a per unit heat basis, even when compared to what is often called “cheap natural gas.” For the past three decades, the total cost of nuclear fuel has been consistently close to 60 cents per MMBTU; the price of natural gas has been somewhere between $1.80 and $4.50 for the past ten years.

Mobile low power number 1 (ML-1). Closed cycle nuclear gas turbine circa 1962.
Credit: U.S. Army

When natural gas prices are near their low point, efficient, affordable, and reliably constructed natural gas combined cycle plants give that fuel the ability to dominate electricity production and sales. That is even more true in a market where gaseous waste product disposal is generously provided for free.

That’s our current situation. Nuclear plant construction costs are out of sight and schedules cannot be predicted. Fuel costs remain consistently low and emissions are virtually non existent, but gas is dominating.

It’s time to change the game by adapting the well-proven, flexible and reliable combined cycle to be able to use nuclear fuel. That will match the best available heat conversion system with the superior fuel that is more abundant, needs no pipelines, produces manageable quantities of solid waste, and can operate inside sealed buildings.

If you can’t beat them, copy them.

Simple Cycle Gas Turbines (Brayton Cycle Machines)

Modern natural gas plants include Brayton Cycle gas turbines. The cheapest ones per kilowatt of generating capacity are classified as simple cycle machines. They have no additional components designed to improve plant thermal efficiency. They consist of a compressor, a heat source, and a turbine with some inlet air ducting and filters, a fuel supply system and a stack for exhausting waste gases.

They are so cheap that they can be economically viable even if run a few hundred hours per year to meet peaks in demand. Their fuel costs are high due to low thermal efficiency. They are so simple that they can be remotely controlled and often don’t need on site operating staff. They can spin up from a cold condition to full power in 15-20 minutes.

They can burn either natural gas or distillate fuel; the main requirement is that the fuel needs to be zero or very low ash and it cannot contain sulfur or other contaminants that can attack turbine blades.

When run on natural gas, these peaking plants are dependent on “just in time” fuel delivery systems. In some regions, especially New England, natural gas delivery can be interrupted when the turbines are needed the most. That problem can be solved by installing dual fuel capability and a distillate fuel oil storage tank. Duel fuel machines with on-site storage can then burn distillate fuel oil in an emergency situation.

Unfortunately, distillate fuel often costs 4-10 times as much as natural gas on a heat content basis. This choice is made affordable by running the turbines on premium fuel only when the need is high enough to spike electricity prices.

Brayton cycle machines made their way into the electrical power business because they filled an important customer need for reliable sources of power to meet demand peaks. Since they were not expected to operate steadily, their construction costs needed to be low enough so that they could sit idle without driving the accountants crazy.

Advantages Of Brayton Cycle Machines Over Steam Turbines

ML-1 shown being off-loaded from a 1960s vintage transport plane

These machines, derived mainly from experience in producing powerful, compact, lightweight, reliable, flexible, and cost effective engines for aircraft and ship propulsion, have numerous advantages over the venerable steam turbine power plants that have been in use on ships and in electrical power production since the First World War.

Unlike steam plants, gas turbines depend on a working fluid that doesn’t change phase over the full range of pressures and temperatures found in the operating cycle. It is a gas at the beginning, middle and end of the cycle. This attribute shrinks and simplifies the system by eliminating the need for valves, piping, and heat exchangers to separate and recover condensation. Water droplets can destroy rapidly spinning turbine blades and must be eliminated before using the high pressure, high temperature water vapor to turn a turbine.

The gases used as the working fluid also undergo a much reduced change in volume per unit mass over the range from the highest system pressure and temperature to the lowest. This cycle attribute leads to compact turbines where the size difference between stages is far less than a comparable steam turbine. Depending on the gas used, the number of stages in the turbine may also be significantly reduced compared to a steam turbine.

Gas turbine power plants do not require large tanks of tightly controlled pure water to provide system make-up to compensate for inevitable leakage and purposeful use of working fluid to clean the surfaces of heat exchanger tubes.

Simple cycle gas turbines also save space and weight by using fuels that produce purely gaseous waste products with little, if any ash particles. By using only zero or very low ash content fuels, simple Brayton cycles eliminate expensive, space-consuming heat exchangers (boilers or steam generators) that steam plants use to transfer the heat of combustion into boiling water. Instead, they spin turbines by directly expanding heated, compressed gas through the turbine’s acceleration nozzles and blade stages.

As a result of the above features, complete Brayton cycle gas turbine-based power plants are often less that 1/4th the size and mass of a comparably capable steam turbine power plant. They thus require less construction material and fewer labor hours to assemble. They are simpler to harden against seismic stresses. They need fewer, smaller buildings for the plant and also for supporting the smaller staff of people compared to steam plants.

Turning Peakers Into Longer Running Facilities

Gradually, Brayton cycle peaking plants proved their value and demonstrated their easy reliability. Customers showed they were interested in spending more for improved performance, especially in terms of specific fuel consumption.

Engineers suggested that the most dramatic efficiency improvement could come from combining Brayton cycle machines with Rankine cycles in a cascading series of heat conversions. The exhaust gases depart Brayton cycle turbines at a high enough temperature to function as the heat source for water boilers known as heat recovery steam generators (HRSG).

Maximum thermal efficiency in a simple Brayton cycle tops out at less than 40%; in a combined cycle, it is possible to achieve efficiency closer to 60%.

Of course, combining Brayton cycles with Rankine cycles reintroduces some of the cost, size and complexity of pure Rankine steam systems, but a 1000 MWe combined cycle plant includes a steam plant that is less than 1/3 the size of a 1000 MWe steam plant since about 2/3 of its output comes from the Brayton cycle turbines. It is also a more flexible power system that usually includes at least three turbines, each of which may be able to operate without the other turbines being in operation. (Many HRSG’s have the ability to be directly fed from an auxiliary boiler instead of from waste gases from the Brayton Cycle machines.)

Why Does Steam Still Dominate In Nuclear Projects?

One might logically ask if Brayton cycle machines are so obviously cheaper and simpler than Rankine cycle steam turbines, why are any steam turbines being built? Why haven’t at least a few nuclear projects using Brayton Cycle machines been commercially deployed?

Even though a few of the advanced systems being designed have Brayton cycle heat conversion systems, they are still in the minority. Even those that do are not as simple as they could be, despite the fact that capital cost reduction should be at the top of a design criteria list for any plant designer who wants to succeed in the market.

It is unfortunate, but many people trained as nuclear engineers have only a dim understanding of the massive structures, systems and components required outside of the highly refined reactor cores that are their main area of responsibility. They are experts at modeling the interactions of neutrons, a wide range of isotopes, varying coolant properties and widely differing support structures, but they don’t appear to spend much time thinking about the challenges of building, operating and maintaining enormous steam plants, pure water systems, cooling water systems, valves, pumps and some of the world’s largest saturated steam turbines.

They spend even less time thinking about the costs and schedules associated with building the massive foundations, buildings and supporting systems that those large steam plants require.

Addressing the costs of systems and structures related to converting nuclear fission heat into useful power is more important that most nuclear engineers realize. Though it’s difficult to find published studies with this information, a statement often heard in gatherings of nuclear project experts is that ~ 80% of the construction budget for fission heated steam plants is consumed outside of the “nuclear island.”

Heat Conversion Requirements Drive Nuclear Technology Selection

If a plant designer starts with the goal of adapting gas turbines to effectively operate with fission heat, it becomes almost immediately obvious that conventional light water reactor plants and their associated fuels cannot do the job.

The maximum temperatures available from such a system can’t drive a Brayton cycle machine with anything close to the desired efficiency.

Sodium cooled reactors come a bit closer, but the boiling point of sodium at atmospheric pressure is still too low for reasonable Brayton cycle efficiency. Many engineers have good reason to be wary of sodium coolant at low pressure; they would have even more cause for concern if asked to consider high pressure sodium as a primary coolant.

Even molten salt reactors, which were initially developed with the idea of using them as the heat source for Brayton cycle jet engines or turbo-props, have heat exchanger material limitations that make it unlikely they can produce gas turbine inlet temperatures above about 800 ℃. A complex Brayton cycle that includes large, expensive components like recuperators and intercoolers can effectively use that temperature to produce reasonable cycle efficiency, but there isn’t much headroom for improvements without a material breakthrough.

The added complexity of the system negates some of the previously described Brayton cycle advantages.

High temperature gas cooled reactors have been under almost continuous development and refinement since the 1940s. They have demonstrated the capability to deliver gas at temperatures at temperatures as high as 1200 ℃ using technologies available in the mid 1960s. Though most high temperature reactor designs today aim for a more modest introductory temperature of 750-800 ℃, there is headroom for future improvements.

Triso Inside™

One of the keys that may unlock high temperature gas reactors and free them for wide use in an almost unlimited number of applications is the Triso coated fuel particle. This innovation, developed almost 50 years ago, coats a tiny particle of fission fuel in multiple layers of material that combine to seal fission products inside the particle.

As long as the coatings are properly applied and remain intact, radioactive materials remain securely contained. Even if there are minor releases from improperly coated or damaged particles, the public is protected by several additional barriers. Through a lengthy, well-controlled, well-designed and consistently managed development program, the U.S. DOE and its partners have developed credible, repeatable processes that result in reliable fuel capable of long core exposures.

As long as high temperature reactors have “Triso Inside™” and are designed to ensure that their fuel temperatures remain within a broad band, currently reaching as high as 1800 ℃, they should be able obtain construction and operating permission on the basis of providing adequate protection.

Fuels with Triso Inside will be more expensive than conventional nuclear fuels when they are first introduced, but there is a well trodden path towards mass production cost improvements. Triso fuel suppliers should act like traditional fuel suppliers to encourage widespread use of their product, including creating identity advertising, assisting with licensing, training system designers, and potentially offering financing to developers who are creating innovative ways to use their product.

Perhaps some well capitalized fuel suppliers with exceptional project management and marketing skills could leverage their chemical engineering expertise into a lucrative new line of work.

Cooled By Nitro™

The other decision that can throw the door wide open on developing nuclear heated Brayton cycles for almost any conceivable energy application is selecting a working fluid that can take advantage of existing Brayton cycle machinery.

A small niche of nuclear plant designers has been interested in using hot gases passed directly though high temperature reactors to spin gas turbine machines since the earliest days of nuclear power development. Unfortunately, nearly all of them assume that helium is the only gas that can be used to directly cool a nuclear reactor.

That assumption – which is false – leads in two possible direction. One is the path that General Atomics and PBMR took on paper, which was to conceive of the use of a helium turbo machine. That path turned into an effective dead end in the real world; the challenges associated with developing helium turbo machinery are far greater than imagined. PBMR ran out of money before approaching a useable machine; GA took the safer path of not even trying to exit development studies on their own dime.

The other alternative is to use helium cooling for the reactor, but to add a gas to gas heat exchanger to move the heat into air or nitrogen so that conventional turbo machinery designed for combustion turbines can be used. This path adds a large, costly component and reduces system efficiency because of the inevitable drop in turbine inlet temperature caused by the heat transfer process. Adding the heat exchanger is particularly problematic when looking into the future; there are well identified paths for increasing the temperature capability of Triso particle-based fuels, but material breakthroughs will be required to enable cost effective heat exchangers at temperatures higher than 800-850 ℃.

There have been three nuclear heated Brayton cycle machines operated in the US. Two – HTRE-1 and HTRE-2 – used atmospheric air and put the turbine gases directly into the desert air in the location where they were tested. The other was the ML-1, which was designed to produce 300 KWe and to be mounted on a trailer that could be pulled to a remote communications site.

The ML-1 was “cooled by Nitro™” which allowed it to use conventional turbo machinery. After all, nitrogen gas makes up 80% of atmospheric air and has almost identical thermodynamic characteristics.

Unlike helium, nitrogen is available in almost unlimited quantities in an unlimited number of geographic locations. It also works well in the same machines that have been developed and refined to use atmospheric air and combustion products. Using nitrogen as the reactor coolant and the working fluid for the turbo machines eliminates the need for a separate heat exchanger.

High temperature nitrogen cooled reactors will be large, low power density components, but the “reactor” performs the functions of heating the turbine gas, storing the fuel, and providing effective decay heat mitigation. When comparing sizes to other potential power sources, the important metric is the size, cost and schedule for complete systems instead of focusing on certain components.

During early stages of development and deployment, system designers should keep it simple and not worry much about thermal efficiency. They can build roadmaps for future improvements, however, that build on the advances that have already been proven to work well in combustion Brayton cycles, including developing combined cycles, co-generation for process heat applications, co-generation for district heating, and inter cooling and regeneration.

One improvement path that can be carefully developed is uniquely available to closed Brayton cycle systems. Unlike air breathing systems, it is possible to increase the pressure in closed Brayton cycles so that the same volumetric gas flow produces greater amounts of heat transfer and improved power output. Designers should be cautioned to start slowly in this area, there are safety and simplicity benefits associated with using system pressures that are the same as those used in combustion gas turbines.

Though all of this may seem logical and almost obvious when laid out in this manner, there are some valid reasons why this path has not yet been taken. Diving into those reasons is beyond the scope of this document; suffice it to say that most of the barriers have been overcome except the one at the starting line. There are impressive returns available to those that recognize that it’s time to open the gate and let the contestants begin their race.

Disclosure: Rod Adams founded the now defunct Adams Atomic Engines, Inc. His company developed nuclear gas turbines from 1993-2010.