As portrayed in the film (and who knows in real life), he only knew there was a flaw in the reactor design; he didn't know – with certainty – that the flaw would lead to an explosion. Under those circumstances, with that kind of uncertainty as well as the classified nature of his knowledge, I consider his answer is even more impressively measured. Anyone savvy about the KGB and the Soviet nuclear power industry would see the red flags go up and drop the line of questioning immediately.