The foreseeable effects are in the mind of the reasonable actor. Consequences, the intended and unintended effects (which do not render the act itself as intrinsically evil), remain in the circumstances font. Catholic teaching does not forbid the possession of nuclear weapons for deterrent purposes.
If reasonable and informed prudence allows that nuclear weapons can be used both discriminately and proportionately to good effect in a just war then their use is moral.
Pope Francis has issued an opinion, not a teaching, that nuclear weapons have “devastating, indiscriminate and uncontainable effects, over time and space.” However, the pope is not an expert on nuclear weaponry. If the pope is correct then it follows from jus in bello principles that the use of nuclear weapons is prohibited. However, the experts disagree:
Charles Dunlap, a former Staff Judge Advocate at USSTRATCOM, highlights the ability of nuclear weapons to be used discriminately, noting that “by reducing weapon yield, improving accuracy through delivery system selection, employing multiple small weapons (as opposed to a single, large device), adjusting the height of burst, and offsetting the desired ground zero, collateral damage can be minimized consistent with military objectives.”