Atomic Insights

Atomic energy technology, politics, and perceptions from a nuclear energy insider who served as a US nuclear submarine engineer officer

Army Nuclear Program

ML-1 Mobile Power System: Reactor in a Box

By 11 Comments

This is a modest update of an article first published in November 1996. DOD’s recent issuance of an RFI for mobile, modest power output atomic power systems shows that the challenges that were clearly described in 1963 have not been addressed – yet. Now is a good time to start addressing them.

The ML-1 experimental reactor was unique. It was not a pressurized water reactor with a steam energy conversion system. Instead, ML-1 was the first nitrogen cooled, water moderated reactor with a nitrogen turbine energy conversion system. Its major design criteria was compactness.

The below video begins with a brief, illustrated explanation of the problems ML-1 was designed to address and alleviate. It makes a persuasive case that will inevitably beg the question – why was this nascent program halted so early in its development.

ML-1 could be packed into four transport packages – a trailer carrying two connected skids for the reactor and the complete heat conversion system, a shipping box for the control room, and two others for cabling, auxiliary gas storage and handling equipment and miscellaneous tools and critical supplies.

The two major containers weighed 15 tons each while the four additional containers each weighed between three and four tons. The complete system weight was about 38 tons. The systems were designed to fit into any of the Army’s transport systems including C-130 aircraft, standard Army trucks, and rail.

In order to reduce the weight of shielding needing transport, the reactor was designed to be installed with a human exclusion boundary of 500 feet.

Design Challenges

In order to minimize the engine volume and mass, the decision was made to operate the engine with nitrogen pressurized to approximately 9 bar – 9 times normal atmospheric pressure – at the compressor inlet. This decision, though it helped reduce the size of the heat exchangers and turbomachinery somewhat, made the design uniquely difficult.

Essentially every other gas turbine ever built has operated with air at atmospheric pressure as the working fluid. The designers of ML-1 had the difficult challenge of making the machine perform as desired with a high density working fluid. This requires the reduction of critical machine clearances and makes accurate balancing far more critical for long term, reliable operation.

A second design decision that made the engine construction more challenging than required was the decision to add a recuperator to the system. Though recuperators have proven that they can improve gas turbine efficiency by several percent in stationary applications, they are not normally used in mobile engines because the additional heat exchanger adds more weight and space than it is worth.

The reactor heat system also required a stretch of existing technology. In order to minimize the size of the reactor, designers decided to use water inside pressure tubes as the neutron moderator. In order to prevent boiling, the water in the tubes was circulated to maintain the temperature below 250 F.

The water tubes were interspersed throughout the core between the fuel bundles. The nitrogen gas flowed past both the water tubes and the fuel bundles and ranged in temperature from 800 F at the core inlet to 1200 F at the outlet. The physical distance between the inlet and the outlet was less than two feet; the temperature extremes made material selection very important.

Testing Experience

The designers of the ML-1 decided to test two different heat engines that could each be connected to the reactor heat source. Once of the machines had an 11 stage axial flow compressor designed and constructed by Fairchild-Stratos Corporation while the other included a two stage centrifugal flow compressor designed and built by Clark Brothers Company.

Neither heat engine was able to meet its designed power output because neither compressor was able to produce the required flow at the required differential pressure. Rather than achieving a power output of 300 kw the best that the tested system could achieve was less than 200 kw. Engineering evaluations were made indicating that some minor adjustments could be made that would raise the performance of the machine, but it is not apparent from the historical record that this kind of rework was ever completed.

A second problem that surfaced during the testing program was related to the moderator water tubes. The high thermal and temperature stress of the tubes combined with manufacturing flaws to cause cracking in the tube welds. The cracks allowed water to enter into the coolant system and required a lengthy hiatus in the test program to correct the problem.

A final problem that had a major effect on the system was the failure of the internal insulation of the regenerator. This was installed under the assumption that it would reduce heat losses and thus improve performance. The insulation consisted of a blanket of fine particles covered with a metal foil. The foil tore loose because of aerodynamic buffeting during testing, causing the distribution of the fine particles throughout the system. After the dust was removed from the engine, testing continued without the insulation.

Lessons Learned

Though the difficulties experienced by the ML-1 testers were the type that are common with the first of a kind of any complex piece of machinery, they proved to be fatal for the program and helped destroy any budding interest in nuclear gas turbines.

Because of the increasing amount of money needed to fight the Vietnam War, the Army’s research and development budget for non weapons items was severely constrained. There was little support for funding experimental nuclear systems in 1963, particularly experimental systems that seemed to have so many difficulties that needed fixing.

Now, however, after more than 30 years of technological developments, it is worth summarizing the lessons that can be learned for future closed cycle gas turbine development.

  • Pressurizing gas turbine cycles may be a good idea on paper, but there are practical engineering difficulties that must be overcome if it is to be used in a real system.
  • Recuperators are troublesome, particularly if space and weight are constraining factors in system design.
  • Water tube reactors are unnecessarily complex, particularly since there have been excellent results achieved by high temperature, graphite moderated reactors.
  • Any material that can potentially contaminate a closed cycle turbine system should be avoided.
  • If possible, well-proven components should be integrated into a complete system rather than designing each component from scratch.

Postscript – When I wrote this article in 1996, I had been working on a direct cycle, low pressure, nitrogen-cooled, pebble bed heated atomic engine for about five years. I was the founder of a struggling 3-year old company called Adams Atomic Engines, Inc.

I’d started publishing Atomic Insights as a paper newsletter in an attempt to widely share what I knew about nuclear energy.

At that time, natural gas was plentiful and cheap, no one was very concerned about climate change, the established US nuclear industry was competing for the business of destroying the existing power plants as they approached the end of their initial 40 year operating licenses, and no one thought that the US would continue being involved in power-hungry expeditions overseas.

Aside: During a 33 year career serving in the US Navy and Naval Reserves, I had a multitude of personal experiences with the Military Industrial Complex. I’ve also participated actively in the early lives of six grandchildren.

I now know enough to want to do everything in my power to ensure that US overseas activities are focused on enabling prosperity, not on expanding our role as a “superpower.” There are so many better ways to invest most of the money that we currently spend on offensive wars. End Aside.

Can Gas Turbines Using Nuclear Fuel Change The Energy Game?

By 49 Comments

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

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

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.

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.

Treasure trove of documents about the ML-1, the US Army’s trailer-mounted, nitrogen-cooled, atomic fission-heated generator

By 11 Comments

I recently published an article featuring a video from the Army Nuclear Power Program that focused on the Army’s mobile, low power closed cycle nitrogen cooled nuclear reactor designated the ML-1. The article generated a good discussion that indicated a strong desire for more information about the program. My initial searches didn’t turn up a […]

McMurdo Station – the New York of the Deep Freeze South

By

(Note: If you are impatient and do not want to watch cute photos of penguins, skip to 19:06 to learn more about the reasons why the PM-3A, a 1,500 kilowatt nuclear electricity generator and process heat supply system, was such a valuable contributor to sustained Antarctic research.) Nearly all of the images that are used […]

Camp Century – Nuclear plant designed, manufactured, constructed & tested in less than 2 years

By

Under current rules and assumptions, anyone who claims that they can design and build a power-producing nuclear reactor in less than 10-15 years is considered to be naive or hopelessly unrealistic. However, there is no reason to believe that everyone with the technical capacity for completing the task will follow the same rules. Even in […]

McMurdo veterans’ cancers FAR more likely to be caused by cigarettes that by Nukey-poo

By

Note: (This story corrected on March 8, 2011) The world’s ignorance regarding the health effects of radiation is occasionally incredible. I just came across a story from New Zealand titled Health fears around polar nuke leak that attempted to implicate the Naval Facilities Engineering Command’s PM-3A, a small nuclear plant that supplied heat and electricity […]

Small Reactors, Safety Culture, Safe Design, and Lessons Learned From SL-1

By

The United States has been operating small nuclear power plants continuously since the early 1950s. These reactors have been used for research and development, power generation, and ship propulsion. There have been tens of thousands of people associated with the design, development, manufacture and operation of these smaller reactors. The enterprise has accumulated an admirable […]

The Atomic Show #133 – Todd Tucker, Author Atomic America

By

Atomic America has a thought provoking subtitle – “How a Deadly Explosion and a Feared Admiral Changed the Course of Nuclear History”. The unifying story through the book is one that most nuclear trained people know a little about – the accident at the Army’s SL-1 that took place on January 3, 1961. In Atomic […]

The Atomic Show #107 – Wind Monster Neighbors; Installing a Reactor in a Single Season

By

Charlie Porter and Roger Hale both have stories of interest. Charlie lives in the shadow of several monster wind turbines, Roger helped to build the PM-2A at Camp Century. Charlie Porter had a nice little horse farm in northwest Missouri. He now really wants to sell that farm and move away because his neighbors are […]