Nuclar bombs as any other weapons don´t kill people, the people that seize control of such weapons are the ones that determine if they kill someone.
So the desire to make a bomb to explode is determined by the human tendency of war, which can´t be calculated or programed.
So the capacity for exploding exists, and their inhability to do so correlates to the persons that holds the trigger.
The world could have gone to hell during the cold war decades ago fortunately for us who where born later those in charge of the decisions took the right ones.
So it was never an oax. It is imposible to predict the explosion of such bombs as it is imposible to predict the reactions of human nature.