Funny, since it's 99.9% assured any nuclear supercomputers over in Iran will be runnig some free copy of Linux they got from someone like Novell.
What are the chances the found a hole in SUSE Linux because the could read the source code? That can't be the reason why because the OSS crowd has assured me that the more eyes on code ensures that it's secure.
You don't need a supercomputer for nuclear weapons unless you're simulating detonations, and you only need to simulate detonations if you're treaty bound to NOT detonate the real thing.
Iran can develop and target their nuclear weapons using any plain old desktop PC.