> In the entire history of civil nuclear power "unknown parties stole the radioisotopes" has never happened.

This reads a bit like "why do we need a QA department when we don't have any bugs"?

The reason nobody stole the stuff from reactors is because everyone has, by international law and also nonbinding recommendations, security and armed guards making sure they don't. These are not free.

The Goiânia accident was stupidity, not malice, so you can't predict how many people would die if it was done maliciously from how many were killed. My understanding is what keeps people (relatively) safe from this type of attack at the moment, is the public deployment of radiation sensors since 9/11, which we know about because of people with radioisotopes in them for medical reasons getting caught by them. These are not free.

The Chrenobyl reactors were housed in what’s "best described as a shed" because that was cheap. Same for all of the other design issues with those reactors: it made them cheap.

The rules that make reactors expensive are written in incidents.