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I wonder if anyone's ever heard from SNAP-10. It was the first fission reactor in orbit (1965). It had, uh, problems and was shut down after 43 days. In late November 1979 it had a 'anomalous event', and it had 6 more in the next 6 years... releasing '50 trackable pieces'.

[0] https://web.archive.org/web/20170503083646/https://ston.jsc....

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Great document. One of many eye-openers:

"The West German government sponsors a meeting called Safety Aspects of Nuclear Reactors in Space, in Cologne. Nietrich Rex predicts that Soviet space nuclear reactors will undergo 2-3 on-orbit collisions in the next 300 years. Each will result in world-wide reentry of radioactive debris."

That's a prediction from November 1989.

I can't find a source after a cursory search, but IIRC the Soviet RORSATs are still the second or third biggest source of space junk today (#1 being the 2007 Chinese test of an antisatellite weapon). A few of them had nuclear reactors fail in dramatic ways, although a lot of their debris is just from leaking coolant.

Per this document it appears that (frequent) Delta second stage explosions were one of the most significant contributors. By 1981 27% of all tracked debris (of orbital period under 225 minutes) were from Delta second stage explosions. (I'm guessing these are considered high velocity debris). It's quite interesting that NASA was surprised by this (asking McDonnell-Douglas to find out why in May of 1981) considering how long they had been launching the Delta rockets and tracking debris. From the document, NASA officials continually approved work to track orbital debris, but often didn't give the reports administrative attention.

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