Powering the moon, mars, and beyond.

Most reactor engineers are familiar with Admiral Hyman Rickover’s famous words on academic vs practical reactors in 1953. Unfortunately very few heed his advice, despite the fact that Rickover has by far made the greatest historical contribution to special purpose reactors (i.e. reactors not providing electricity to the grid – space reactors, test reactors, naval reactors, small power stations, etc.).

“An academic reactor or reactor plant almost always has the following basic characteristics:

(1) It is simple. (2) It is small. (3) It is cheap (4) It is light. (5) It can be built very quickly. (6) It is very flexible in purpose (’omnibus reactor’). (7) Very little development is required. It will use mostly off-the-shelf components. (8) The reactor is in the study phase. It is not being built now.

 

The academic-reactor designer is a dilettante. He has not had to assume any real responsibility in connection with his projects. He is free to luxuriate in elegant ideas, the practical shortcomings of which can be relegated to the category of ‘mere technical details.' 

 

Unfortunately for those who must make far reaching decisions without the benefit of an intimate knowledge of reactor technology and unfortunately for the interested public, it is much easier to get the academic side of an issue than the practical side. For a large part those involved with the academic reactors have more inclination and time to present their ideas in reports and orally to those who will listen. Since they are innocently unaware of the real but hidden difficulties of their plans, they speak with great facility and confidence.”

The above paragraph perfectly describes what has happened during the past 50 years of special purpose reactors in the US; but even after 50 years of failure, decision makers are still wooed by the promises of paper reactor designers. Every special purpose reactor project since Rickover’s retirement has not only failed miserably, but has not even reached the point of an actual, “real” reactor test – until KRUSTY in 2018.

The KRUSTY vacuum chamber (left) containing the core (inside lower left) and PCS (inside upper left) is lifted to be installed on COMET (right), which holds the large SS304 shield (upper right) and the lower shielding on the platen (lower right).

The Kilopower Reactor Using Stirling Technology (KRUSTY) test in 2018 was the culmination of our founder’s efforts to return the US to developing practical/real reactors versus the continued blind pursuit of academic/paper reactors. Those efforts begin with the Demonstration Using Flattop Fissions (DUFF) experiment. In 2012, our founders questioned if it was possible to do any “real” reactor development in today’s environment, and they envisioned and executed DUFF to prove to NASA, DOE, and themselves that was indeed still possible. The primary goal of DUFF was simply to create electricity (to power a light) by carrying power from a fission reactor to Stirling converters via a water heat pipe. It was not prototypic of a useful power system, rather DUFF’s value was to show that if things are kept simple, then real reactor development might be possible.
 

Following the success of DUFF, our founders proposed KRUSTY to NASA as a prototypic space reactor test. KRUSTY was approved in 2014, and successfully completed in 3.5 years for ~$17M, with funding from both NASA and the NSSA.

KRUSTY core assembly. Left photo shows heat pipes fit within the slots in the HEU UMo fuel. Right photo shows core after the installation of the Haynes 230 rings that clamp the heat pipes to the fuel (via interference fit). White BeO axial reflectors seen on top and bottom. Some parts in the photos are part of the temporary assembly fixture, which is later removed.

The KRUSTY nuclear system test showed that the system operated as predicted, and that the reactor is highly tolerant of possible failure conditions and transients. The key feature demonstrated was the ability of the reactor to load-follow the demand of the power conversion system. The thermal power of the test ranged from 1.5 to 5.0 kWt, with a fuel temperature up to 880°C. Each 80-We-rated Stirling engine produced ~90 We at a component efficiency of ~35% and an overall system efficiency of ~25%. The results of KRUSTY testing are documented in several conference and journal papers, including a special issue of the ANS journal Nuclear Technology.

An engineer completes KRUSTY thermocouple connections on the right. The array of 8 Stirling converters and simulators are shown on top. The heat pipes connect to the Stirlings in a conical condenser, from which the sodium travels back down to the core.

The artwork above shows astronauts completing the installation of four 10-kWe SpaceNukes reactors on the Mars surface. In this architecture, the reactors are buried in 1.5m deep holes, which reduces the mass of the landed system by a nearly a factor of 2 (due to less shielding and power cabling). The buried configuration allows the outpost to be positioned closer to the power station based on astronaut dose requirements; although other logistical reasons, such as damage from landing debris, might require ~1-km of separation regardless. The buried configuration would also allow astronauts easier and more frequent/lengthy access to the power site for planned or unplanned maintenance (because of the lower dose rates). It is anticipated that initial reactor missions will accept the mass penalty of leaving the reactor on the lander, until a surface architecture is established that could bury the reactors.

 

The artwork below shoes the power station in greater detail. The The circular grey structures are the deployable radiators atop each SpaceNukes reactor power system, and the Stirling converters are in the radiator shadows. A yellow cable exits each reactor to carry electricity to a  junction box which manages and distributes the power. In this architecture, two power cable exit the junction box. One cable carries power to a human outpost on the upper right, and another cable exits downward towards a station for in-situ resource processing (to create propellant and/or oxygen for a return rocket and other uses). The edge of the lander that carried the power systems to Mars can be seen on the far left.

The initial unit power size for surface applications is expected between 10 and 30 kWe, to match the expected power needs and logistics capability of an early human settlement. These types of power systems are rather simple and have direct heritage to the KRUSTY reactor test, which provides a high level of confidence in system performance, and low uncertainty in development, manufacturing, and cost. The evolution to higher power SpaceNukes reactors, from 100 kWe to >1 MWe is kept simple by retaining the same reactor physics and technology, while changing the basic core geometry and power conversion to accommodate higher power draw. Note that there will be very little difference between systems deployed on the Moon or Mars (or other planetary bodies), while solar power systems would be substantially different for 14 days of darkness versus diminished sunlight and dust storms.

In addition to surface power, SpaceNukes reactors are intended to provide power in space, wherever sufficient sunlight is not available; e.g. the outer solar system and beyond. KRUSTY was prototypic of a 1-kWe space power system that could be used for high power instruments and/or high data transmission rates – perhaps a communications satellite in orbit around Mars or a high fidelity probe of Enceladus. Higher power systems, perhaps 10 to 30 kWe could power ion thrusters to allow missions that orbit multiple moons, or missions to orbit Neptune, Pluto, or further out Kuiper objects, while also allowing powerful diagnostics and high data transmission rates. No matter what the power level, or whether on surface or in space, the basic technology and operation of the system is almost identical and tied directly to the KRUSTY test. The biggest system difference between surface and space reactors will be in radiation shielding and heat rejection.

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