A reactor using heavy water as the moderator. A prominent example is the Canadian deuterium uranium (CANDU) type reactor, which is moderated and cooled by heavy water and is fuelled with natural uranium. The fuel bundles, located in horizontal pressure tubes, consist of Zircaloy tubes filled with uranium oxide pellets. CANDU reactors are refuelled on-load and, for safeguards purposes, are treated as item facilities. There are also HWRs that operate with a pressure vessel (similar to LWRs) and those where light water is used as coolant.