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Atomic Insights

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

Atomic Insights Nov 1995

ML-1 Mobile Power System: Reactor in a Box

February 1, 2019 By Rod Adams 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.

Filed Under: Army Nuclear Program, Atomic Insights Nov 1995, Small Nuclear Power Plants, Technical History Stories

In the news: November 1995

November 1, 1995 By Rod Adams

Russian Delegation Visits Cuba

(October 10, 1995) Russian scientists and businessmen visited Cuba to discuss the future of the VVER-440 pressurized water reactor at Juragua. According to an official at Cuba’s National Center for Nuclear Security, construction was halted when the reactor was 90 percent complete. Cuba’s government is investigating the possibility of completing the reactor to help relieve the severe energy shortage caused by loss of subsidized Russian oil shipments.

In contrast to the reports from disgruntled former employees who have defected to the United States, International Atomic Energy Agency Director Hans Blix toured Juragua in January and was impressed by the effectiveness of the efforts to preserve what has been already constructed. According to an IAEA spokesman, “It is in good condition, given that it is still in a formative stage of development.”

San Onofre Wins in Court

(October 12, 1995) After two months of testimony and three days worth of deliberation, a jury in San Diego Federal District Court ruled that occupational radiation exposure did not cause Glen James’ chronic myelogenous leukemia. Mr. James, a former contract worker at San Onofre Nuclear Generating Station filed the suit against Southern California Edison, the plant’s owner, claiming that he had been injured by excessive radiation exposure.

According to SCE’s defense attorney, “Federal regulations provide the standard of care necessary to protect employees and the communities surrounding nuclear facilities. That standard was upheld in this case.”

Westinghouse Settles Philippine Dispute

(October 14, 1995) Westinghouse Electric Company will take a third-quarter 1995 charge of $45 million to settle a long standing dispute over the Bataan Nuclear Power Plant. The company completed the plant in 1985, but the Philippine government decided not to operate it, citing concerns over the plant’s quality and safety.

Westinghouse cited a desire to move forward with new business as part of the reason for the settlement. Because of the dispute, the Philippine government had banned Westinghouse from selling any equipment in the country.

Filed Under: Atomic Insights Nov 1995

Prefab Reactors For Off-Grid Users

November 1, 1995 By Rod Adams

The Army Nuclear Power Program was established as part of the Atomic Energy Commission in 1954. It was charged with the responsibility of developing reactors suitable for providing electrical power to remote bases and outposts. Some of the people associated with the Army Nuclear Power Program saw essentially unlimited potential for small, transportable reactors. Most […]

Filed Under: Army Nuclear Program, Atomic Insights Nov 1995, Small Nuclear Power Plants

Army Nuclear Power Plants

November 1, 1995 By Rod Adams

Designation Description of Reactors SM-1 This stationary military reactor was the Army’s prototype and training facility. It began operation in April 1957 at Fort Belvoir, VA, several months before the Shippingport reactor. SM-1 has the distinction of having been the first nuclear power plant to be hooked to an electrical grid. 2,000 kw. SM-1A Built […]

Filed Under: Army Nuclear Program, Atomic Insights Nov 1995, Small Nuclear Power Plants

Letter from the Editor: Portable Nuclear Reactors

November 1, 1995 By Rod Adams

The United States Army ran an innovative nuclear power program for more than 20 years. The men involved operated a series of small, nuclear heated generating plants in some of the world’s least hospitable environments. The story of what those diligent heros did has been all but lost. Though the Army was the lead service […]

Filed Under: Army Nuclear Program, Atomic Insights Nov 1995, Small Nuclear Power Plants

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