yawn, you seem to like a lot of hand waving about 'Statistics dont do that' without much understanding.
There is nothing special here, statistics work just as well here as they do in fusion and fission reactions, in effect very similar things (although there you are trying to create a runway system..). Neutron energy, absorbtion cross-sections, etc, etc.
You still need a bunch of requirements for a 'runaway reaction', and there is not NEARLY enough space junk to cause one, just as all the uranium in the earths crush does not just happen to spontaineously fission..
You should stop treating math as a magical field open to interpretation, and learn that it has real applications.
Yes, you can get a runaway reaction in space junk, no there is not even close to enough up there yet (or likely in the even medium future) for a runaway reaction, and by not even close I mean ORDERS of magnitude too little.