There have been non-combat losses of nuclear-powered warships during sea trials and peacetime patrol missions. Nuclear contamination is spreading from some of these sinking sites. It is also conceivable that combat losses of nuclear-powered warships could cause contamination of civilians, civilian objects and the natural environment. If such combat losses occur at sea, both belligerent and neutral States will have to deal with a difficult question: to what extent and by who can harm resulting from such contamination be compensated for payment of damages. This article attempts to dissect whether nuclear contamination incidentally caused to civilians, civilian objects and the natural environment during international armed conflict can be properly categorized as collateral damage as envisaged by the laws of armed conflict and neutrality at sea, the lawfulness of which is assessed following the principle of proportionality.