Pope Francis is not an innovator in his approach to the topic of nuclear weapons (though he did extend the logic of earlier ethical thought). The consistent teaching of the Roman Catholic Church since Pope John XXIII has been that the use of nuclear weapons in war is immoral because it conflicts with the principles of just war theory. Problematically for the principles, the violence wrought through nuclear war is extensive enough that there can be plausible victor in a nuclear exchange, and no chance to protect noncombatants from becoming involved. In short, the church teaches, the use of nuclear weapons is indiscriminate and always disproportionate to the good that can be hoped to be achieved.
Pope Francis' whole papal agenda was brought into focus by Dr. Love when she summarized his mission as care for the three 'P's — his concern for the poor, for the planet, and for peace. Various elements of this are obvious from his Laudato Si', among other addresses and initiatives, but all three of these come together in Francis' concern to see a world free from the threat of nuclear weapons and potential of nuclear catastrophe.