Development of rolled joint detachment system for low enriched uranium based advanced heavy water reactor

Advanced heavy water reactor-low enriched uranium (AHWR-LEU) is designed for high discharge burnup and large scale commercial utilization of thorium with LEU as an external feed in a once through fuel cycle. It is a 300 MWe vertical pressure tube type, thorium based reactor, cooled by boiling light water and moderated by heavy water. The reactor incorporates a number of passive safety features and is associated with a fuel cycle having reduced environmental impact and offers more proliferation resistance compared to contemporary light water reactors (LWRs). Some distinguishing features which make AHWR-LEU unique from other conventional nuclear power reactors are core heat removal by natural circulation, negative void coefficient of reactivity under all event scenarios, easily replaceable pressure tubes, long design life, passive safety features, etc. The long design life of the reactor is possible to be met only when the pressure tube which has a limited life of 15–20 full power years (FPYs) due to a life limiting mechanism called irradiation enhanced creep and growth is easily replaceable. Pressure tube is joined at its ends to SS 403 end fittings through rolled joints. A system has been designed and developed for detachment of pressure tubes from the end fitting and its associated hardware without disturbing the surrounding components or coolant channels so that the end fitting can be reused. The system works on the principle of shock heating and fast cooling of the pressure tube in the rolled joint region and application of axial load. Though rolled joint detachment by induction heating has been used earlier in CANDU reactors for removal of pressure tubes and calandria tubes, the present design incorporates a liquid nitrogen based faster cooling arrangement and an integrated load application and gripping module for detachment and removal of pressure tube from the end fitting in much lesser time. This paper describes the methodology of rolled joint detachment, challenges involved in the design, features of the detachment system, findings of experimental trials and observations.