The Steam Generating Heavy Water Reactor (SGHWR) was a United Kingdom design for commercial nuclear reactors. It uses heavy water as the neutron moderator and normal "light" water as the coolant. The coolant boils in the reactor, like a boiling water reactor, and drives the power-extraction steam turbines. A single prototype of the design, the 100 MWe "Winfrith Reactor", was connected to the grid in 1967 and ran until 1990. A larger commercial design with a 650 MWe power rating was selected in 1974 as the basis for future reactor builds in the UK, but declining electricity use led to this decision being reversed in 1976 and no production models were ever built. SGHWR was among a number of similar designs, which include the CANDU-derived Gentilly Nuclear Generating Station in Quebec, the Fugen Advanced Test Reactor in Japan, and the never-commissioned CIRENE reactor in Italy. These designs differ with the baseline CANDU design, which uses heavy water as the coolant as well as the moderator. History SGHWR was a departure from previous UK designs, which had used graphite as the moderator and carbon dioxide gas as the coolant. The original Magnox was designed to run on natural uranium but the subsequent Advanced Gas-cooled Reactor (AGR) abandoned this for a variety of reasons, using low-enriched uranium instead. Although Magnox was technically successful it was expensive. For future orders, several alternative reactor designs concepts were studied during the early 1960s. As part of this program, a 100 megawatt electrical (MWe) prototype of the SGHWR was built at Winfrith in the 1960s and was connected to the grid in 1967. It is often known simply as the "Winfrith Reactor". The other designs produced similar sub-scale prototypes of the High Temperature Reactor also at Winfrith, the Magnox-derived AGR at Windscale, and the Prototype Fast Reactor at Dounreay. This contest ultimately selected the AGR design, and several AGRs began construction in the late 1960s. These quickly ran into problems, and by the early 1970s the design was considered a failure. In 1974, a larger version of the SGHWR with a design power of 650 MWe was selected for future power plant builds. In 1976 this decision was reversed due to the combination of a predicted sharp drop in electricity demand, higher than expected costs, and the lack of obvious export potential in a shrinking nuclear market. Given the limited number of new reactors expected in