Falling Space Reactors: Assessing the Risk

A new NASA report examines various scenarios in which nuclear reactors that are used to power spacecraft could accidentally reenter the Earth’s atmosphere.

“There are a number of types of reentry events that can potentially occur with missions containing fission reactors. Each type of reentry event can produce a variety of possible adverse environments for the fission reactor,” the report said.

The postulated scenarios include accidental reentry upon launch, reentry from orbit, and reentry during Earth flyby.

“There are three potential outcomes for a fission reactor in a reentry scenario,” the report explains. “First, the fission reactor can burn up in the atmosphere due to the aerothermal loads imparted to it during reentry. Second, it can survive the reentry and impact the Earth’s surface with or without additional spacecraft components. Finally, it can break apart during reentry, but its various components survive reentry and impact the Earth’s surface (a scattered reentry).”

See Fission Reactor Inadvertent Reentry: A Report to the Nuclear Power & Propulsion Technical Discipline Team, by Allen Camp et al, NASA/CR−2019-220397, August 2019.

A conference on “Nuclear Energy in Space: Nonproliferation Risks and Solutions” will be held in Washington DC on October 17 that will focus on the anticipated use of highly enriched uranium in space nuclear reactors, and the feasibility of using low enriched uranium instead. The conference is sponsored by the Nuclear Proliferation Prevention Project (NPPP) at the University of Texas at Austin.

Several previous technical analyses have concluded that use of low enriched uranium in space reactors is in fact feasible, but that it would probably require a reactor of significantly larger mass.

See “White Paper – Use of LEU for a Space Reactor,” August 2017 and “Consideration of Low Enriched Uranium Space Reactors” by David Lee Black, July 2018.

NASA Releases Space Nuclear Power Study

NASA has released a long-awaited Nuclear Power Assessment Study that examines the prospects for the use of nuclear power in civilian space missions over the next 20 years.

The Study concludes that there is a continuing demand for radioisotope power systems, which have been used in deep space exploration for decades, but that there is no imminent requirement for a new fission reactor program.

The 177-page Study, prepared for NASA by Johns Hopkins University Applied Physics Laboratory, had been completed several months ago but was withheld from public release due to unspecified “security concerns,” according to Space News. Those concerns may have involved the discussion of the proposed use of highly enriched uranium as fuel for a space reactor, or the handling of plutonium-238 for radioisotope power sources.

Nuclear power can be enabling for a variety of space missions because it offers high power density in compact, rugged form. Radioisotope power sources (in which the natural heat of decay is converted into electricity) have contributed to some of the U.S. space program’s greatest achievements, including the Voyager I and II probes to the outer solar system and beyond. But development of nuclear reactor technology for use in space has been dogged by a repeated series of false starts in which anticipated mission requirements failed to materialize.

“The United States has spent billions of dollars on space reactor programs, which have resulted in only one flight of an FPS [fission power source],” the new NASA report noted. That was the 1965 launch of the SNAP 10-A reactor on the SNAPSHOT mission. It had an electrical failure after a month’s operation and “it remains in a 1300-km altitude, ‘nuclear-safe’ orbit, although debris-shedding events of some level may have occurred,” the report said.

The development and use of space nuclear power raises potential environmental safety and public health issues. As a result, the NASA report said, “it may be prudent to build in more time in the development schedule for the first launch of a new space reactor. Public interest would likely be large, and it is possible that opposition could be substantial.”

In any case, specific presidential approval is required for the launch of a nuclear power source into space, pursuant to Presidential Directive 25 of 1977.

“For any U.S. space mission involving the use of RPS [radioisotope power sources], radioisotope heating units, nuclear reactors, or a major nuclear source, launch approval must be obtained from the Office of the President,” the report noted.