It's worth mentioning IP law is applied on a per-territory basis (i.e. you have to register your IP in every territory you would like it enforced, and that territory's courts are solely responsible for its enforcement in that territory). Many countries government have laws which restrict IP in national security concerns [0], which nuclear reactors may come under. Furthermore, patents require the innovation used is published publicly, which may not be desirable for sensitive technologies like nuclear reactors. Trade secrets and the like are more likely to be used in this particular case.
[0] - https://www.wipo.int/pct/en/texts/nat_sec.html