Since “catastrophe” embraces everything from a plumbing break to the Black Death plague, the word is reasonable. But the second paragraph is not good journalism:
Extensive analysis predicts all kinds of bad things. But by assertion of the specific of nuclear weapons use, Reuters leaps out in front of most of the models.
Ample material exists in open source, for example, (CNN) The U.S. vs. North Korea: Inside a Pentagon war game. Reuters owes it to the public to do more work. In this case, there is every reason to use identified sources.
Usually, I criticize CNN for this kind of laxity. Now Reuters joins the club. Do the work. Plenty of information is to be had for a little digging.