The JET Vacuum vessel (VV) and Neutral Injector Boxes (NIBs) contain complex coolant systems (demineralised water and freon) for plasma facing and beam power handling components and for the in-vessel divertor coils. There are also in-vacuo cryopumps with liquid (LHe) or supercritical (ScHe) helium and liquid nitrogen (LN2) shields. VV temperature during operations is 320oC. Good VV and NIB vacuum conditions require continuous operation of all these services. The close proximity of water channels to masses of several tons at elevated temperatures (<= 593K in the case of the VV and in-vessel divertor target structure) or cryogenic temperatures (<= 77K in the case of the cryopumps and their supply lines) entails risk to component safety from boiling, freeze up or thermal stress if a Loss of Flow Accident (LOFA) occurs. In addition, there are more severe Loss of Coolant Accidents (LOCA) which might result in a VV breach (from the pressure surge of evaporating liquids); or potential Loss of Vacuum Accidents (LOVA) which, by raising the heat transfer within the vacuum envelope, enhance the boiling and freezing risks.