An affirmative value which might be seen a lot is utilitarisnism. However, Rawls' Justice counters this theory quote well, and more so. In the beginning of Rawls' Justice he says that it is necessary for him to make his theory because of the inadequacy of utilitarinism. Taking this route, however, you need to make a link between possession and use. Thus you prove that the use of nuclear weapons is wrong as per the Just War Theory and therefore the threat of the use of nuclear weapons is immoral.
Most negative cases take the stance of deterrance.
In showing deterrance to be immoral recognize the principles of it. Niether the U.S. nor the Soviet leadership has the moral right to threaten the lives of the people on the other side in this way. I recommend going to the Seattle Times homepage and searching for 'Hanford'. There are two articles on it that explain the dangers of possession of nuclear weapons. I suggest the value of morality for the affirmative, thus forcing the debate onto the criterion.