I think one paper shows that there were almost 40 near misses, and I think that was put up by the Future of Life Institute, so some people can look up that paper, and I think that in general it seems that experts agree some of the biggest risks from nuclear would be accidental use, rather than deliberate and malicious use between countries.
Possibly you are thinking of the Global Catastrophic Risks Institute, and Baum et al.βs A Model for the Probability of Nuclear War ?