Lee Mason, NASA

Kilopower overview and mission 

applications 

1
Kilopower Project

• Innovation

§ A compact, low cost, fission reactor for exploration and science, scalable 

from 1 kW to 10 kW electric

§ Novel integration of available U-235 fuel form, passive sodium heat pipes, 

and flight-ready Stirling convertors

§ Would provide about 10x more power than the Multi-Mission Radioisotope 

Thermoelectric Generator

• Impact

§ Could be scaled up to provide modular option for human exploration 

missions to the Mars/Lunar surface

§ Potentially enables Decadal Survey Planetary Science missions without 

reliance on limited plutonium dioxide fuel

• Goals

§ Full-scale nuclear system-level test of prototype U-235 reactor core 

coupled to flight-like Stirling convertors at relevant operating conditions

§ Design concepts that show scalability to 10 kWe for Mars surface power

• Leveraging

§ Leverages existing DOE/NNSA nuclear materials, manufacturing 

capabilities, test facilities, nuclear safety expertise, and DOE/NNSA cofunding 2
Potential Applications

• Government Missions

§ Human Mars surface missions

§ Deep Space Gateway: lunar surface operations

§ Planetary orbiters and landers: Europa, Titan, Enceladus, Neptune, Pluto, etc.

§ Planetary nuclear electric propulsion: Small Bodies, Ocean Worlds, Interstellar, etc.

• Commercial Missions

§ Space power utility (pay-for-service)

§ Asteroid/space mining

§ Lunar/Mars settlements

• Terrestrial Adaptations

§ Military Forward Operating Bases

§ Unmanned Underwater Vehicles

• Power uses: drilling, melting, heating, refrigeration, sample collection, material 

processing, manufacturing, video, radar, laser, electric propulsion, telecomm, 

rover recharging,... 3
• Projected human exploration power needs...

– Up to 40 kW day/night continuous power

– Power for In-situ Resource Utilization propellant production (pre-crew arrival)

– Power for landers, habitats, life support, rover recharging (during crew operations)

– Technology options: Nuclear Fission or Solar Photovoltaic & Energy Storage

– Desire compact stowage, robotic deployment, survivable for multiple crew campaigns 

(>10 yrs), lunar extensibility

Mars Surface Power

• No off-the-shelf options exist to power long- term human surface missions on Mars

– Power systems used on previous robotic missions 

(e.g. Spirit/Opportunity, Phoenix, Curiosity) do not 

provide sufficient power: all less than 200 W

4

• Mars surface presents major challenges

– CO2 atmosphere, 3/8th gravity, 1/3rd solar flux 

of Earth orbit, >12 hour night

– Large seasonal and geographical solar flux 

variations, long-term dust storms, high winds
Patrick McClure, Los Alamos

Reactor overview and development 

history 

5
The Road to Kilopower

• 1970-2010: Many NASA/DOE space reactor programs attempted with 

limited success and NO flight missions

§ Too complicated and costly

§ Too dependent on new materials and processes

§ Too long to develop, usually longer than the mission can wait

§ Too much optimism for out-of-the-box system performance

• 2010 Planetary Science Decadal Survey: Technology assessment study to 

determine if fission reactors can provide an alternative to radioisotope systems

• 2012 Proof-of-Concept Test: Demonstration Using Flattop Fissions; 24 watts produced in test that took less than 6 months and cost less than $1M

• 2014 NASA Nuclear Power Assessment Study: Small fission option 

examined for Titan Saturn System Mission and Uranus Orbiter Probe

• 2014 NASA Evolvable Mars Campaign: Small fission power baselined for 

pre-crew propellant production and post-landing crew operations

• 2015 Kilopower Project starts under STMD Game Changing Development 

Program: 3 years and <$20M to design, build, and test a prototype reactor

6
Demonstration Using Flattop Fissions

• Proof-of-Concept Test

§ Los Alamos sponsored test at DOE Nevada National 

Security Site, Device Assembly Facility (DAF)

• Test Configuration

§ Highly enriched uranium core with central hole to 

accommodate a single heat pipe

§ Heat transfer via the water based heat pipe

§ Power generation via two Stirling convertors

• Significance

§ First-ever use of a heat pipe to extract thermal power 

from a fission reactor

§ First-ever use of a Stirling convertor to produce electric 

power with a fission heat source

§ Demonstration of nuclear reactivity feedback and 

dynamics with representative components

Stirling Convertor

Assembly

Notional Flight

Concept

Flattop

Critical

Experiment

7

• Sept 13, 2012: Success! 24 Watts produced

§ Completed in less than 6 months with a total cost <$1M

§ Proof that a nuclear reactor ground test can be 

conducted quickly and affordably
§ The KRUSTY experiment adheres to all Federal and Department of Energy 

regulations for safe operations, including 10CFR830, “DOE’s Nuclear Safety 

Management”

§ This test and any future testing will comply with National Environmental Policy 

Act (NEPA) processes to assess possible adverse environmental impacts and allow 

for public comment & engagement

§ A NEPA review would be completed before NASA decides to proceed to a flight 

mission, and potential launch and operational hazards would be analyzed by an 

independent interagency nuclear safety review panel

§ Several factors inherent in space fission power contribute to launch and 

operational safety, including

• At launch, the radiological hazard would be limited to the naturally occurring radioactivity 

present in the uranium reactor core (<5 curies)

• The reactor would not be operated until reaching the surface of a planet or being placed on 

a trajectory leaving Earth

• During planetary surface operations, the reactor would have sufficient radiation shielding to 

protect crew members and prevent damage to sensitive equipment

• The reactor design includes inherent fault tolerance, so that loss of cooling would result in 

an automatic reduction in fission power that prevents uncontrolled scenarios

Kilopower Safety

Safety is a top priority of NASA and the Department of Energy 

8
Marc Gibson, NASA Glenn

Kilopower system design and overall test 

plan

9
Kilopower System Design 

Titanium/Water Heat Pipe Radiator

Stirling Power Conversion System

Haynes 230/Sodium Heat Pipes

Lithium Hydride/Tungsten Shielding

Beryllium Oxide Neutron Reflector

Uranium Molybdenum Cast Metal Fuel

Possible Mission Configurations

(Artist’s Concepts)
Kilopower Technology Development

11

Electrically Heated 

System Test with 

Depleted Uranium 

Core (NASA GRC)

Reactor Experiment 

with Highly Enriched 

Uranium Core

(DOE NNSS)

Flight Concept Flight-prototypic 

Power Pack
System Test Profile:

Expected Operating Temperatures

1 2 3 4 5 6 7 8 9

1. Heat up and steady state Operation, 

simulators set to match Stirling heat 

input

2. Half power Stirling operation

3. Return to steady state

4. Full thermal power 

5. Return to steady state

6. Turn off Stirling engines and cooling 

(Simulators), adjust core power to 

simulate temperature feedback

7. Add additional cooling to 

Simulators, up core power, engines 

still off 

8. Turn Stirling engines back on, return 

to steady state

9. Cut core power, GN2 backfill to 

chamber

This test profile has been performed numerous times with the surrogate Depleted Uranium core 

using an electrical heater. It will be repeated, with the Highly Enriched Uranium core at NNSS.
Dave Poston, Los Alamos

Nuclear design and benefits of 

completing the test 

13
Kilopower Reactor Using Stirling 

TechnologY = KRUSTY

• Designed with flight-like components

§ Uranium core, neutron reflector, heat pipes, 

Stirling engines

• Integrated into flight-like power system

§ Realistic configuration and interfaces

• Tested at flight-like conditions

§ Vacuum environment

§ Thermal power and operating temperature

§ System dynamics

• Exercising flight-like infrastructure

§ Analytical modeling

§ Nuclear test operations

§ Ground safety issues

§ Transport and assembly issues

§ Interagency cooperation

Key things missing from KRUSTY: radiator, full suite of Stirlings, startup-rod, zero-g, launch 

approval, flight hardware, launch loads, flight qualification, lifetime effects, spacecraft integration.
• Component Criticals: The reactor core, neutron 

reflector, and startup rod are tested alone to 

measure reactivity.

• Cold Criticals: Heat pipes and power conversion 

are added, and reactivity is gradually added until 

the system is critical but no heat is produced.

• Warm Criticals: Reactivity is increased until full 

reactor power (4 kilowatts thermal) is achieved at 

moderate temperatures of about 400 C.

• Full Power Run: A notional mission profile is 

simulated including reactor start up, ramp up to full 

power, steady state operation at about 800 C, 

several operational transients, and shut down.

KRUSTY: Summary of Nuclear 

Experiments

The KRUSTY Test is being conducted in four phases over 3 to 5 months 

beginning November 2017, with fission reaction levels iteratively increased 

during each test phase to validate models and gain confidence
KRUSTY Full-Power Run

• Demonstrate start-up, stability, and steady-state 

performance.

§ Start the same way as warm criticals, but continue to add 

reactivity until an average fuel temperature of 800 C is 

reached.

§ Turn on Stirling engines when temperature reaches 650 C.

• Demonstrate reactor load-following ability.

§ Increase and decrease power removed by Stirling 

engines/simulators, with no reactor control action.

• Demonstrate reactor fault tolerance.

§ Simulate a failed heat-pipe or engine by halting power 

removal from a Stirling simulator, with no reactor control 

action.

• Demonstrate ability of reactor to remain 

operational after acute failure of all active heat 

removal (at end of ~24 hour run).

The nuclear operation and dynamics of KRUSTY are similar to 

any proposed Kilopower flight reactor (from 500 We to 10 kWe) –

even if technologies and materials change, the coupled thermalneutronic behavior is essentially the same.
Walk-off Slide

17
Project Participants

18

DOE Los Alamos National 

Laboratory

NASA MSFC

DOE Nevada National 

Security Site

NASA GRC

Advanced Cooling 

Technologies, Inc.

Sunpower, Inc.

DOE Y-12 National 

Security Site

NASA JSC