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For the past five decades—from the Apollo-era lunar science experiments to the Mars Curiosity and the New Horizons missions—Pu-238 Radioisotope Thermal Generators (RTG) have served as a power source. While some of the NASA’s forays will continue to rely on these RTGs, others will require larger power sources to enable human space and planetary exploration and establish reliable high bandwidth deep-space communications. Solar power cannot handle this goal. A larger nuclear-based power source is required.

In a recent Washington Post article, Jeff Bezos, founder of amazon.com and creator of Blue Origin space project said, “I think NASA should work on a space-rated nuclear reactor. If you had a nuclear reactor in space—especially if you want to go anywhere beyond Mars­—you really need nuclear power. Solar power just gets progressively difficult as you get further way from the sun. And that’s a completely doable thing to have a safe, space-qualified nuclear reactor.”

Calls for space nuclear power are not new. In fact, numerous reactor concepts have been proposed in the past. Their development is often dampened by the perception that nuclear is too hard, takes too long and costs too much.

Los Alamos National Laboratory, in partnership with NASA Research Centers and other DOE National Labs, is developing and rapidly maturing a suite of very small fission power sources to meet power needs that range from hundreds of Watts-electric (We) to 100 kWe. These designs, commonly referred to as kiloPower reactors, are based on well-established physics that simultaneously simplifies reactor controls necessary to operate the plant and incorporates inherent safety features that guard against consequences of launch accidents and operational transients. This concept, which received an R&D 100 Award in 2013, is illustrated in Figure 1. Equally important, designers have taken a fundamentally different approach for rapidly maturing the concept from design to full-scale demonstration. Feasibility of the design was demonstrated in 2012 and since then designers have focused on successfully overcoming the remaining R&D challenges driving towards a full-scale demonstration in 2017.

Full-scale system being readied for engineering demonstration. In this case, core is electrically heated to demonstrate overall system – including Stirling engines – performance. Radial reflectors and Stirling engines are missing from the picture. Heat pipes, core, steel rings used to attach the heat pipes and instrumentation wires are visible. This system was subjected to numerous thermal cycles over prolonged periods of time to examine its thermos-mechanical performance. Source: Los Alamos National Laboratory

Inherently safe design

During steady state, a reactor operates with a neutron multiplication factor of ‘1.000’; that is, the number of neutrons in the core remains unchanged from one generation to the next generation. Almost every perturbation in a reactor’s operation ultimately translates into either a positive or a negative reactivity insertion incident, defined as the state in which the core neutron multiplication factor deviates from its steady state value. Sudden and significant positive reactivity insertion can lead to runaway reactor kinetics, wherein temperatures can exceed thermal limits very rapidly. Past development approaches relied on sophisticated control systems to reduce or eliminate such a likelihood. Luckily, reactors also have an inherent ability to self-correct via negative temperature reactivity feedback; reactor power automatically decreases as core temperature increases, and vice versa.

It has been known that strongly reflected small compact fast reactors, such as kiloPower, can be designed to maximize these mechanisms to a point of being totally self-regulating. Our objective is to design-in self-regulation as the front-line feature in order to minimize technical and programmatic risk and to demonstrate via testing that self-regulation is both reliable and repeatable. To that end, multi-scale and multi-physics simulations are relied upon to perform high fidelity design studies that explicitly examined (a) how choices related to fabrication, alloying and bonding techniques would affect the internal crystalline structure of each nuclear component and in turn (b) how that morphology affects that components thermal, mechanical and nuclear performance at conditions of interest. Figure 2 provides expected temperature trace of kiloPower reactor subjected to sudden loss of 25% of Stirling engines, a rather large power mismatch between the core and the power conversion system. Nevertheless, reactor recovers from this perturbation and regains steady state, assuring us that there is no need for advanced autonomous control system.

Rapid prototyping and engineering demonstration

A key objective of the affordable strategy is that the nuclear components can be fabricated to the exacting tolerances demanded by the designers. This includes not only the physical dimensions, but also density and crystalline phase of the alloys. The materials’ characteristics determine thermal and mechanical performance of the core, which in turn affects its nuclear performance. After several joint efforts, an exact replica of the kiloPower core was fabricated at Y-12 with depleted uranium. This provided needed experience and data on casting, machining and material characteristics of the reactor core.

The second phase involved engineering demonstrations where the DU core is assembled together with the rest of the system (including the heat pipes and Stirling engines) in the configuration needed for a flight space reactor. Finely controlled resistance heaters were used to closely mimic the nuclear heat profile that is expected in the nuclear core during regular operation. These tests were performed in a vacuum chamber to simulate the environment in outer space.  

Data collected during these tests confirmed the predictions of computer simulations of the reactor. The data showed a well-characterized thermal response of the system including demonstrating that the Stirling engines could meet the required electrical output.  Other data, like the thermal expansion of the reactor core, were measured as input to computer simulations of the nuclear kinetics and system dynamics. These data were then used to help complete the design for the nuclear demonstration experiment that is planned for later in 2017.

Figure 1: KiloPower Reactor is constructed of solid metallic U-Mo core, brazed with eight heat pipes and Stirling engines that convert core thermal power into electricity. Not shown here is the BeO neutron reflector that surrounds the core. Source: Los Alamos National Laboratory

Full-scale nuclear test

The nuclear demonstration test will occur in late summer or early fall of 2017. The test will be conducted at the Device Assembly Facility at the Nevada National Security Site (NNSS). It will be comprised of a ~32 kilogram enriched uranium reactor core (about the size of a circular oatmeal box) made from uranium metal going critical, and generating heat that will be transported by sodium heat pipes to Stirling engines that will produce electricity. 

The test will include connecting heat pipes and Stirling engines enclosed in a vacuum chamber siting on the top of a critical experiment stand. The critical experiment stand has a lower plate than can be raised and lowered. On this plate will be stacked rings of Beryllium Oxide (BeO) that form the neutron reflector in the reactor concept. A critical mass is achieved by raising the BeO reflector to generate fission in the reactor core. Once fission has begun, the BeO reflector will be slowly raised to increase the temperature in the system to 800 degrees Centigrade. The heat pipes will deliver heat from the core to the Stirling engines and allow the system to make ~250 watts of electricity. For the purpose of testing only, two of the eight Stirling engines will make electricity,the others will only discard heat.

The data gained will inform the engineers regarding startup and shutdown of the reactor, how the reactor performs at steady state, how the reactor load follows when Stirling engines are turned on and off and how the system behaves when all cooling is removed. This data will be essential to moving forward with a final design concept.

Potential for missions to Mars

Once the nuclear demonstration testing has been completed, the path to putting a nuclear reactor on a NASA mission to deep space or the Mars surface is still several years away. A finalized design must be completed along with rigorous testing of the system for reliability and safety.

The most recent NASA studies have focused on the use of KiloPower for potential Mars human exploration. NASA has examined the need for power on Mars and determined that approximately 40 kilowatts would be needed. Five 10-kilowatt KiloPower reactors (four main reactors plus one spare) could solve this power requirement. 

The 40 kilowatts would initially be used to make oxygen and possibly propellant needed by the Mars Ascent Vehicle to send astronauts back into Martian orbit. After making oxygen or fuel, the power would then be available to run the Martian habitat or provided power to Martian rovers all needed by the astronauts during their stay on Mars.  Nuclear power has the advantage of being able to run full time day or night, as well as being able to operate closer to the Martian poles where it is believed water exists in substantial quantities.

Lessons learned

Lessons learned from the kiloPower development program are being leveraged to develop a Mega Watt class of reactors termed MegaPower reactors. These concepts all contain intrinsic safety features similar to those in kiloPower, including reactor self-regulation, low reactor core power density and the use of heat pipes for reactor core heat removal. The use of these higher power reactors is for terrestrial applications, such as power in remote locations, or to power larger human planetary colonies. The MegaPower reactor concept produces approximately two megawatts of electric power. The reactor would be attached to an open air Brayton cycle power conversion system. A Brayton power cycle uses air as the working fluid and as the means of ultimate heat removal. 

MegaPower design and development process will rely on advanced manufacturing technology to fabricate the reactor core, reactor fuels and other structural elements. Research has also devised methods for fabricating and characterizing high temperature moderators that could enhance fuel utilization and thus reduce fuel enrichment levels.

This story was written by: Dasari V. Rao,  director of the Office of Civilian Nuclear Programs, Patrick McClure, System Design and Analysis, of  Los Alamos National Laboratory 

Figure 2: Simulations, as confirmed by feasibility test, show that sudden loss of 2 of the eight Stirling engines can be easily handled by self-regulation – an inherent design feature. Full-scale nuclear experiments scheduled for 2017 will validate the design.
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