A radioactive nuclear waste storage system includes a cask comprising a hermetically sealed internal cavity configured for holding the waste such as spent nuclear fuel submerged in an inventory of water. One or more pressure surge capacitors disposed inside the cask include a vacuum cavity evacuated to sub-atmospheric conditions prior to storage of fuel in the cask. At least one rupture disk seals a vacuum chamber inside each capacitor. Each rupture disk is designed and constructed to burst at a predetermined burst pressure level occurring inside the cask external to the capacitor. This allows excess cask pressure occurring during a high pressure excursion resulting from abnormal operating conditions to bleed into capacitor, thereby returning the pressure inside the cask to acceptable levels. In one embodiment, the capacitors are located in peripheral regions of the cask cavity adjacent to the circumferential wall of the cask body.