Episode:
43
SNAP-10A
Country:
USA
Years of Operation:
1965-1965
Category:
Research & Experimental
Reactor Type:
SFR
Coolant:
NaK (Sodium-Potassium)
Fuel Type:
Highly Enriched Uranium
Moderator:
ZrH (Zirconium Hydride)
Thermal Power (MWth):
0.04
Electrical Power (MWe):
0.04
Status:
Research & Experimental
timeline
First Criticality Year
1965
Commercial Op Year
Shutdown Year
1965
Lessons Learned
✅ Compact reactors can be extraordinarily reliable
✅ Passive safety actually works
✅ Nuclear in space failed politically — not technically
✅ The only U.S. reactor to fly worked better than most PowerPoint reactor concepts today
SNAP-10A didn’t fail.
It was retired by government budget committees and public fear — the most powerful reactivity insertions of all!
sources
ARTICLE
When the Nuclear Reactor Was Meant to Go to Space (Forgotten Reactors Series)
Before “nuclear-powered spacecraft” became a punchline in sci-fi movies, it was a very real engineering program quietly taking shape in Southern California. Meet the SNAP reactors — Systems for Nuclear Auxiliary Power — America’s early attempt at putting fission reactors where solar panels feared to tread.
- Yes, this involved actual nuclear reactors.
- Yes, some of them ran in California.
- And yes… one of them did fly.
The SNAP program kicked off in the late 1950s amid Cold War optimism, when space missions were expected to go farther, last longer, and carry heavier payloads than chemical batteries or early solar arrays could support.
If you wanted reliable power beyond Earth orbit — especially in shadowed or distant environments — nuclear fission was the obvious answer.
Most SNAP development and ground testing occurred at the Santa Susana Field Laboratory in California’s Simi Hills, overseen by Atomics International (a Rockwell division). Multiple reactor concepts were built and operated there, but the star of the show was SNAP-10A — the first and only U.S. nuclear reactor ever launched into space.
The Hardware
SNAP-10A was a compact, zirconium-hydride–moderated reactor cooled by liquid sodium-potassium (NaK). It produced about 30 kW thermal and roughly 500 watts of electrical power via thermoelectric converters.
No moving parts.
No pumps.
No crew.
Just clean, quiet fission doing its thing.
Timeline
Program conceived: Late 1950s
Ground testing: Early 1960s (Santa Susana)
Launched: April 3,1965, aboard an Atlas-Agena rocket
Operational in space: 43 days
Program canceled: Late 1960s
What Went Right
In orbit, SNAP-10A achieved criticality, stabilized, and delivered power exactly as designed. The reactor itself performed FLAWLESSLY. No runaway reactions. No thermal surprises. No glowing green asteroids (sorry, Hollywood).
What Went Wrong
After 43 days, an unrelated spacecraft voltage regulator failed — not the reactor. The electrical system shut down. The reactor passively shut itself down forever, exactly as designed.
The satellite remains in orbit to this day, silent, stable, and still radioactive — a long-term debris lesson before anyone used that phrase.
On the ground, Santa Susana later became infamous for chemical and sodium reactor cleanup issues — mostly tied to non-SNAP activities — but public perception doesn’t bother with footnotes.
Why It Ended
Politics, budgets, and the rapid improvement of solar panels killed SNAP. Nuclear worked — but “worked quietly” isn’t enough when cheaper options exist and public tolerance for nuclear power in space drops to zero.
SLIDE DECK
