SGHWR fuel performance, safety and reliability

The design principles involved in fuel pins and elements need to take account of the sometimes conflicting requirements of performance, safety and reliability. The principal factors involved in this optimization are discussed and it is shown from fuel irradiation experience in the Winfrith SGHWR (Steam Generating Heavy Water Reactor) that the necessary bias towards safety has not resulted in a reliability level lower than that shown by other successful water reactor designs. Reliability has important economic implications and has to be paid for. By a detailed evaluation of SGHWR fuel defects it is shown that very few defects can be shown to be related to design, rating or burnup. This demonstrates that economic aspects have not over-ridden necessary criteria that must be met to achieve the desirable reliability level. It is possible that large-scale experience with SGHWR fuel may eventually demonstrate that the balance is too much in favour of reliability, and consideration may be given to whether design changes favouring economy could be achieved without compromising safety. The safety criteria applied to SGHWR fuel are designed to avoid any possibility of a temperature runaway in any credible accident situation. The philosophy and supporting experimental work programme are described and the fuel design features which particularly contribute to maximizing safety margins are outlined. Reference is made to the new 60-pin fuel element to be used in the commercial SGHWRs and how it compares in design and performance aspects with the 36-pin element that has been used to date in the Winfrith SGHWR. (author)