A water jacket is a water-filled void surrounding a device, typically a metal sheath having intake and outlet vents to allow water to be pumped through the void. It is commonly used around a nuclear power core to contain radiation. The flow of water to an external heating or cooling device allows precise temperature control of the device. Look up Void in Wiktionary, the free dictionary Void can refer to: Aether as the source of all elements, the quintessence. ... The neutrality of this article is disputed. ... Radiation has a variety of different meanings. ...