By Andrew Sowder, Ph.D., CHP 

In March 2018, the National Aeronautics & Space Administration (NASA) successfully demonstrated the operation of a kilowatt-scale nuclear reactor suitable for powering space missions to the moon, Mars, and beyond. As the first novel nuclear reactor concept to be completed in the United States in 40 years, the milestone achievement brings to light the opportunity to utilize small reactors for extreme environments and critical missions on Earth.

There are many civilian and military applications in which safety, security, and economic survival depend on the availability of megawatts of reliable, always-on power. In most cases, the responsibility for providing this essential power falls to diesel generation that is reliant on costly and vulnerable fuel delivery. Both physics, and experience, have shown that nuclear energy offers an unmatched alternative for reliable, resilient, and continuous power for environments that range from half-akilometer beneath the ocean’s surface to outer space. And in terms of size, nuclear does “small” particularly well. Smaller scales bring many benefits, including simpler, safer designs and operation, and decreased unit capital costs.

Smaller physical dimensions facilitate factory fabrication, transportation, and modular construction and deployment. Reduced scales and shorter construction durations are expected to translate into all important cost- and risk-savings as compared to conventional nuclear plant projects.

Meanwhile, the energy density of nuclear fuel (over a million times that of fossil fuels) allows for extended operation periods of years to decades without the need for refueling.


Many of the “new” reactor concepts and designs today were demonstrated at some scale in the 1950s and 1960s.

Fission power was initially demonstrated in the form of compact reactor systems for naval propulsion. On land, the U.S. Army Nuclear Power Program deployed and commissioned several very small (1-MWe to 10-MWe) reactors at remote installations from 1957 to 1967, including a 10-MWe bargemounted reactor aboard the USS Sturgis for the Panama Canal Zone. Some five decades later, in the spring of 2018, Russia launched a floating nuclear plant comprising two barge-mounted 35-MWe pressurized water reactors.

Over a decade-long period from 1957 to 1967, the Army deployed several small (1-MWe to 10-MWe) reactors to provide heat and power for remote facilities. Above, a 2-MWe nuclear plant is off-loaded at McMurdo Station, Antarctica, in December 1961. PHOTO COURTESY NATIONAL SCIENCE FOUNDATION, U.S. ANTARCTIC PROGRAM


Remote artic communities in North America face electricity prices that can exceed mainland grid-based electricity rates by a factor of 10 or more. In fact, one study found lower-income rural Alaskan residents spend 47 percent of their income on home heating.

The costs of supplying power to military operations can come at a severely high price in terms of logistical support and human casualties. Closer to home, the extended loss of power to critical civilian and national security infrastructure, as in the cases of Hurricane Katrina and Hurricane Maria, can lead to loss of life that far exceeds the initiating event itself as well as hinder the ability of business and commerce to carry on.

Economic analysis of future market opportunities for advanced nuclear done by the Electric Power Research Institute (EPRI) indicates that technology competitiveness is equally driven by policy intervention, technology value or revenue, and by capital costs. For remote, off-grid communities, industrial sites, defense facilities, and other national security-related installations, power generation options are limited. Diesel generation currently dominates the market in the megawatts electric range and below, despite high fuel costs and the risk of fuel supply interruptions.

Justifying the higher upfront capital costs of nuclear small and very small modular reactors, with their low and stable fuel costs and ability to operate without refueling, could become a reality—displacing diesel generators in critical applications.


The advanced reactor landscape continues to grow globally. In a 2017 assessment, Third Way counted 137 advanced reactor projects (fission and fusion), with North America claiming 40 percent of that total.

Most small modular reactor designs maintain the traditional engineering approach, pairing a nuclear island with a conventional power conversion system. A notable exception is the gas-cooled HolosGen reactor, which adapts a nuclear-heated jet engine design to yield a closed-cycle system for electricity generation. The heat pipe reactor design demonstrated by NASA offers another novel approach. The underlying technology for this new class of solid-state reactors was developed at Los Alamos National Laboratory, N.M.


Modular reactors are classified in three different size classes: very small, small, and large. Each has different requirements as well as applications.

Very Small. The microreactor class falls below 50-MWe in capacity and extends down to kilowatt-scale technologies. These highly transportable reactors are designed for full factory fabrication and rapid deployment, with those in the lower range offering varying degrees of portability. Most terrestrial applications fall in the 1-MWe to 10-MWe range.

Small. Small modular reactors are generally considered to fall under 300-MWe. These reactors rely on a high degree of factory fabrication for modular onsite construction. Smaller commercial designs can go down to 50-MWe.

Large. Commercial nuclear units were originally rated in the hundreds of megawatts and were scaled up to 1,000- MWe and above. Most new advanced light-water reactors fall in this range.


Institutional issues present the primary barriers for economic, near-term deployment of small and very small modular reactors in new land-based applications, most of which depart substantially from the established role of large central power station generation. The paradigm shifts envisioned for transportation, portability, and control of reactors also pose significant challenges. These require buy-in from legal and regulatory authorities to allow rapid deployment, periodic movement, and remote or autonomous operation, especially for situations and locations that may lie outside of the traditional nuclear regulatory envelope. This is in addition to the challenge of moving innovative technologies from research and development to demonstration and deployment.

On the technical side, solutions for many of the small and very small modular reactor challenges come from advances in modeling and simulation; material science; manufacturing methods; instrumentation and control; cooling; and power conversion. The extensive experience of the commercial and military operating fleet should also be fully leveraged for all stages of the reactor lifecycle (EPRI recently published a historical review and analysis of government and industry roles).

However, in an industry plagued by delays and escalating construction costs, the focus on modularity to streamline fabrication, delivery, and onsite assembly is the hook that many proponents for new builds hang their hats on. There are two important distinctions that fall out of modular construction that define two general use cases for small modular reactors and very small modular reactors: transportability and portability.

The ability to deliver major systems and components intact from the manufacturer is seen as a solution to overcome costly delays—reducing construction times from many years to just a few. Transportability is essential for practical, economic, long-term deployment in conventional and remote locations. Portability, meanwhile, imposes additional restrictions on size, weight, safety, operational, and decommissioning attributes. Ultimately, such characteristics help enable more rapid deployment and removal by truck or air.


Requirements for many critical power needs in the defense and national security arena echo those of space missions and naval propulsion: robust, resilient, reliable, and extended operation without refueling and other external intervention.

The Department of Defense could provide nuclear technology developers with what has been missing: a customer with the resources and risk appetite needed for the demonstration and early adoption of new small-scale nuclear reactors.

In August 2018, the U.S. Congress directed the Department of Energy to outline a potential pilot program for deploying microreactors at national security facilities. If the pilot program takes off, the advanced nuclear industry’s wish for a first-mover could soon be realized.

Andrew Sowder, Ph.D., CHP, is Technical Executive, Electric Power Research Institute;

[This article first published in the March-April 2019 issue of The Military Engineer]