Seriously, you are concerned small nuclear reactors left behind? The main idea is, that you will be able to load them onto a truck and ship them back to the factory. So the chance of anything left behind is very small.

Why would the risk be small? I've seen pretty expensive machinery been left behind. I destroyed such machinery to take out the copper wires from its transformars to make a net.

What makes you think that this can't happen? It can happen in so many ways, i.e. the owner is criminal and runs away or fucks up and loses everything and the court takes years to decide who gets what from the factories, the new owners put it on sale it takes another 10 years to sell because the repair costs incurred are massive and equipment is getting obsolete therefore you can't find a buyer. People get old, move on and all that decays for 50 years until the land becomes valuable enough for someone to buy it with all that obsolete garbage.

It happens all the time.

You are not going to load a reactor at the end of its lifetime on a truck to ship it back to the factory.

I think you underestimate the amount of mismanagement and human error that happens every day. May I remind you of the Goiânia accident? Additionally, Wikipedia has a seriously long list of “orphan source incidents”.