Once proposed SP-100 reactor.

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

The report — 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 – notes that there are a number of types of reentry events that can potentially occur with missions containing fission reactors.

The report is an upshot from a Nuclear Power and Propulsion Technical Discipline Team that was directed to consider possible improvements to the launch approval process as it relates to fission reactors.

The paper presents the next step in that examination, which is to accurately describe and frame the problem and suggest safety criteria that might apply to inadvertent reentry. The report includes a discussion of the issues associated with different types of inadvertent reentry, the possible consequences of those events, a review of previous work in the area, security and nonproliferation issues, and options for safety requirements that might be considered.

NASA’s Kilopower project: The power level would be suitable to access, extract, and process lunar ice in permanently shadowed craters and demonstrate propellant production.
Credit: NASA

Postulated scenarios

“Each type of reentry event can produce a variety of possible adverse environments for the fission reactor,” the report notes.

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).”

Former Soviet Union’s Cosmos 954 satellite in an artist’s rendition with labels showing key parts. Spacecraft reentered in January 1978, coming down across northwestern Canada, Major pieces of the nuclear-powered satellite remained intact and impacted the ground, scattering radioactive debris far and wide. Credit: US Department of Energy

Past guidance

The report’s conclusion observes that the general theme is that the likelihood of inadvertent reentry should be kept as low as possible. Further, if reentry is to occur, either burnup or intact reentry is preferred over scattered reentry.

“A significant departure from past guidance is the notion that reentry into the ocean may be considered a success state, whether or not criticality occurs. It is anticipated that the guidance in this report may be modified following the issuance of further policy guidance from the Office of Science and Technology Policy (OSTP).”

Future discussion

In particular, issues that may warrant future discussion, the report says, include:

 Definition of a “hot” reactor

 Whether or not to consider criticality for ocean impacts

 Suggested general design criteria

 Suggested risk criteria

 Application of criteria, i.e., parsing of numbers

 Mission Implications

This study was chartered by NASA’s Nuclear Power & Propulsion Technical Discipline Team (TDT) led by Lee Mason and Mike Houts. The Nuclear Power & Propulsion TDT governance resides under the NASA Office of Chief Engineer with oversight by the NASA Engineering Safety Center (NESC) Power Technical Fellow (Chris Iannello) and Propulsion Technical Fellow (Daniel Dorney).

For more information, go to: 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 at:


Note: Special thanks to the Federation of American Scientists (FAS) for flagging this report.

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