Deformation Behaviour of a Transversely Loaded Garter Spring

Abstract

The CANDU power generation system is based on a natural uranium fuelled reactor with a heavy water moderator. A unique feature of the CANDU reactor is the horizontal fuel channel that allows on-line re-fuelling and fuel management. Pressure tubes containing the fuel bundles and pressurized heavy water coolant are the in-core component of the fuel channel assemblies. Calandria tubes span the length of the reactor core and provide passageways for the pressure tubes through the reactor core. The calandria and pressure tubes are each approximately 6 meters long. The calandria tube separates the heavy water moderator (∼80°C) from the pressure tube (∼300°C). Both tubes are subjected to gravity loads but the pressure tube carries the additional load of the fuel bundles as well as experiencing high temperature and irradiation induced material effects. The pressure tube deflects under the combined loading and areas of the pressure tube could come into contact with the calandria tube. This contact would limit the operating efficiency and lifetime of the fuel channel. To maintain a gap between the pressure and calandria tubes, helical springs manufactured from rectangular cross-section wire are placed over the pressure tube. These helical springs are known as garter springs and four such springs are spaced along the pressure tube. Initially, there is no contact between the springs and the calandria tube, but as gravity forces and creep effects begin to act, the pressure tube sags and garter spring/calandria tube contact occurs. As the pressure tube continues to deform, a portion of the pressure tube weight, fuel and coolant is transmitted through the garter spring onto the calandria tube. The calandria tube, in turn, begins to deflect under the applied stresses. This creep deformation of the fuel channel takes place over many thousands of operating hours. Eventually, creep induces a permanent vertical deformation (sag) in the fuel channel. The sag of a fuel channel is an important factor in the operation of the structure and many methods are used to determine the general response of the pressure tube/calandria tube/garter spring system. These methods assume the garter spring is a rigid component. This paper specifically examines the garter spring behaviour with respect to the non-linear material and contact response between the pressure tube/garter spring/calandria tube components. A three dimensional (3-D) finite element solid model of the garter spring is used to determine the non-linear response of the helical garter spring to the transverse forces applied from 3-D shell finite element models of the pressure and calandria tubes. Comparison with experimental, crushing tests on garter springs illustrate the analytical model is well behaved. Applying the operating load to the 3-D model shows that the garter spring’s transverse deformation is small and that assuming the garter spring is a rigid component is valid.