Out of this world

On Earth, as the debate continues about the merits of nuclear power, testing is underway on a compact, 1kWe reactor, which if scaled up, could support human missions to the moon, Mars, and beyond.

The Kilopower project was launched in 2015 as a joint venture between the US National Aeronautics and Space Administration (NASA) and the US Department of Energy (DOE). The project team is led by NASA’s Glenn Research Center, in partnership with NASA’s Marshall Space Flight Center, the DOE National Nuclear Security Administration and several DOE laboratories: Los Alamos National Laboratory, Y-12 National Security Complex, and the Nevada National Security Site (NSSS). The $20 million, three-year research project aims to demonstrate that nuclear fission is able to provide a safe, abundant and reliable power source for human missions to space.

Power is typically generated in space by solar arrays or by radioisotope thermoelectric generators (RTGs) that convert the heat from naturally decaying plutonium238 into electricity. But during future missions on Mars, sunlight may be unavailable due to dust storms, and power needs are likely to exceed the capabilities of RTGs. On the moon, the cold, dark lunar night lingers for 14 days.

“What we are striving to do is give space missions an option beyond RTGs, which provide a couple of hundred watts or so,” says Lee Mason, power and propulsion technologist at NASA’s Glenn Research Center. “The big difference between all the great things we’ve done on Mars, and what we would need to do for a human mission to that planet, is power. This new technology could provide kilowatts and can eventually be evolved to provide hundreds of kilowatts or even megawatts of power.”

Study into the potential for reactors in space has been under way for more than forty years. Many NASA/DOE space reactor programmes were attempted between the 1970s and 2010, but with limited success, according to Patrick McClure, project lead for Kilopower at Los Alamos National Laboratory. The programmes proved too complicated and costly, relied too heavily on new materials and processes, and had long, often prohibitive development times. In 2010, the Planetary Science Decadal Survey carried out by the National Research Council’s Space Studies Board (SSB) recommended a technology assessment to determine whether fission reactors could provide an alternative to RTG systems. A proof of concept test took place in 2012 at the DOE’s Nevada National Security Site Device Assembly Facility. The experiment, which produced 24MW of power, demonstrated the first use of a heat pipe to cool a small nuclear reactor and power a Stirling engine. This was achieved in less than six months with an investment of under $1 million.

Kilopower design

The Kilopower reactor has a highly-enriched uranium core built by the National Nuclear Security Administration, heat pipes supplied by Advanced Cooling Technologies through a NASA Small Business Innovation Research contract, and Stirling generators provided by Sunpower, Inc.

The core – a solid block of uranium alloy approximately 6in in diameter – is surrounded by a beryllium oxide reflector. A single boron-carbide (B4C) rod is used to start up the reactor. The self-regulated fission reactions make complicated control systems unnecessary. Sodium heat pipes surrounding the reactor are used to transfer heat to Stirling generators. The Stirling convertors use heat and pressure to move a piston, which is coupled to an alternator to produce electricity. Each generator will produce about 100W of power and is cooled by a radiator. 

The reactor has been designed with flight-like components and integrated into a flight-like power system. It will also be tested under flight-like conditions (vacuum environment, thermal power) with consideration given to ground safety issues, transport and assembly requirements.

The team at the Nevada National Security Site began tests on the Kilopower reactor in November 2017. Dave Poston, chief reactor designer for the project, said there will be four main test phases.

• Component criticals where the reactor core, neutron reflector, and startup rod are tested alone to measure reactivity.
• Cold criticals with heat pipes and power conversion added, in which reactivity is gradually added until the system is critical but no heat is produced.
• Warm criticals where reactivity is increased until full reactor power (4kWt) is achieved at moderate temperatures of about 400°C.
• Full power run, simulating a notional mission profile including reactor start up, ramp up to full power, steady state operation at about 800°C, several operational transients, and shut down.

The experiments should conclude with a full-power test lasting approximately 28 hours in late March. As well as confirming reactor performance, this test must demonstrate Kilopower’s load-following ability and accident tolerance. The reactor power will be increased and decreased by removing Stirling engines, with no reactor control action. A heat pipe or engine failure will be demonstrated by halting power removal from a Stirling simulator, again with no reactor control action. The final test, to be completed at the end of the 24-hour test-run, will demonstrate the ability of the reactor to remain operational after an acute failure of all active heat removal.

Poston stresses that the operation and dynamics are similar to any proposed Kilopower flight reactor (from 500We to 10kWe). “Even if the technologies and materials change, the coupled thermal neutronic behaviour is essentially the same,” Poston says.

Safety is a priority for NASA and DOE. The experiments meet all federal and DOE regulations for safe operations, including 10CFR830 for nuclear safety management.

All work will also comply with the National Environmental Policy Act (NEPA), assessing possible adverse environmental impacts and allowing for public engagement. A NEPA review would be completed before NASA went ahead with a flight mission and any operational hazards would be analysed by an independent inter-agency nuclear safety review panel, according to a presentation by Patrick McClure, project lead on Kilopower at Los Alamos.

Measures have been taken to protect the launch and operational safety of fission power systems. These include: limiting the radiological hazard to naturally occurring levels in the uranium reactor core (<5 curies), ensuring the reactor does not operate until it has left Earth’s trajectory or reached the surface of a plant, and providing radiation shielding to protect crew members and prevent damage to equipment.

Applications in space

Four Kilopower systems could provide the 40kWe of around-the-clock power needed to establish a human outpost on Mars or the moon. “A reliable and efficient power system will be essential for day-to-day necessities, such as lighting, water and oxygen, and for mission objectives, like running experiments and producing fuel for the long journey home,” NASA says.

The power source must handle extreme environments. The surface of Mars has a carbon dioxide atmosphere, three-eighths of the gravity and a third of the solar flux of Earth. It has a >12-hour night, large seasonal and geographical variations in solar flux, long dust storms and high winds.

“Kilopower opens up the full surface of Mars, including the northern latitudes where water may reside. On the moon, Kilopower could be deployed to help search for resources in permanently shadowed craters,” Mason says.

Characteristics of fission power that make it beneficial for space outposts and deep space robotics also apply to space missions. Eventually, nuclear fission reactors could be scaled up and used to power nuclear electric propulsion vehicles to transport heavy cargo beyond Mars or to shorten the transit time to Mars and beyond.