Experience with Material Combinations for Sliding Applications in PFBR In-Vessel Fuel Handling Machine

Abstract

Prototype Fast breeder Reactor (PFBR) is a pool type reactor of 500 MW(e) capacity using mixed oxides of Uranium and Plutonium as fuel and liquid sodium as coolant, which is currently under advanced stage of construction at Kalpakkam. In-Vessel handling of the core subassemblies (SA) is carried out by an offset arm type machine called Transfer Arm (TA). Different types of material combinations are utilized for various sliding pairs used in the machine for various motions. Criteria for the selection of these material combinations are decided by the compatibility of the respective machine element with the working environment, magnitude of the contact stress, working temperature, linear speed, availability of external lubrication, required life, required tolerance etc. Transfer of a SA is achieved by gripping/ungripping of SA using fingers, raising / lowering the gripper outer tube and rotation of TA. The drive for gripper finger operation is at ambient environment and finger actuation is in liquid sodium. An inner tube links the linear actuator to the finger actuator and is housed and guided inside the gripper outer tube. Relative movement of inner tube with respect to outer tube results in open / closing of gripper fingers. Initially, combination of material pairs at five nos. of guide locations was SS 304 LN for the outer tube and hardchrome plated SS 304 LN for inner tube. During testing in air after 20 cycles, jamming of inner tube with respect to outer tube was observed. This was solved by reducing number of guides to two, by changing the surface contact to line contact and by changing the material combination to SS 304 LN against colmonoy coated SS 304 LN. Similar failure was observed for sliding movement at guide locations between the outer tube and shielding sleeve during hoisting of the gripper. Initial material combination of SS 304LN and hardchrome plated SS 304 LN was changed to colmonoy coated SS 304LN and hardchrome plated SS 304 LN. The selected material combinations were validated by testing on a separate subassembly simulating the geometry & loading before actual implementation on the machine. Guide tube, which is used to guide the gripper is raised / lowered by means of a screw-nut mechanism. Initial material pair used for the screw and nut, which are working at ambient conditions was SS 304LN and SS 410 respectively to provide corrosion & galling resistance. However during initial performance testing, this material combination failed and the nut got jammed. Subsequently the problem was studied and overcome by changing the material of nut to phosphor bronze, which is relatively softer and hence provided uniform contact across the nut surfaces. Appropriate material selection and proper design of the geometry of guiding surfaces are very essential for the smooth operation of machine elements in sliding conditions. With improvements in the material choice and geometry of the guides, qualification testing of transfer arm was successfully completed in air and hot argon. Testing in sodium is under progress and the experience at high temperature has been encouraging.