This isn't career or educational advice. I just wanted to ask a qestion that I think is very interesting, and then it went on hold. Why?
It's not career or education advice, but it is too broad. Learning about developing software in life-critical applications is something that some people make a career out of. There are a number of practices, standards, processes, methods, and tools that people working in this environment would use. These would also vary by country and organization. These generic questions aren't really a good fit for the Stack Exchange format.
If you'd like, I can change the close reason so it better reflects why the question isn't a good fit.
well, can it be reopened, because I think that there can be great answers? Now it's not off-topic. What do I need to do to be less to-broad, but still on topic? --Josip Ivic
You need to make it so that the question isn't a poll. There is no one answer to this. Siemens may have written code in C, and then C++, and looked at using Java but stuck with C++. The Navy may have mandated that Bechtel used Ada while Westinghouse write it in assembly for a custom microprocessor for which Ada wasn't an option, and who knows about Combustion Engineering or General Election which also made reactors for the US Navy (and that is just one navy - there are other nuclear powered naval vessels run by other countries).
The certification requirements for a civilian reactor in one country may differ from that in another and those may either mandate specific languages, or it is the case that specific languages make it easier to meet the requirements.
This goes to the core of the 'too broad' - there are too many possible answers.
The tangential part to all of this is that unless someone has specific in house knowledge (and that is not restricted by an NDA or other security clearance aspect), you won't get an answer beyond what is specified in the various certification requirements (which you can find with google). Your best course of action is to do the leg work and find out who makes the various reactors and ask their public spokesperson for information - that will get you a better answer than anyone here can give you.