Dr. Alan Robock has been one of the leading authorities on Nuclear Winter theory for many years. He maintains an excellent web site on this subject here: http://climate.envsci.rutgers.edu/nuclear/
He and Brain Toon (one of the original authors of the TTAPS study*) prepared a new review of Nuclear Winter for the Bulletin of the Atomic Scientists here (PDF) which demonstrates the danger of Nuclear Winter is still present, that the science behind the theory still stands up and that threshold of triggering climatic change from using nuclear weapons is still extremely low.
*Title: Nuclear winter: global consequences of multiple nuclear explosions, Author(s):T.P. Ackerman , J.B. Pollack , Carl Sagan , O.B. Toon and R.P. Turco, Source:Science. 222 (Dec. 23, 1983): p1283.