The issue of stress state during mechanical tests to assess cladding performance during a reactivity-initiated accident (RIA)

The mechanical test procedures that address fuel cladding failure during a RIA are reviewed with an emphasis on the development of test procedures that determine the deformation and fracture behavior of cladding under conditions similar to those reached in a RIA. An analysis of cladding strain data from experimental research reactor test programs that have simulated the RIA is presented. These data show that the cladding undergoes deformation characterized by hoop extension subject to a range of multiaxial stress states and strain paths comprised between plane-strain (no axial extension of the cladding tube) and equal-biaxial tension (equal strain in both the hoop and the axial orientations). Current mechanical test procedures of cladding material are then reviewed with a focus on their ability to generate the appropriate deformation response and to induce the prototypical multiaxial stress states and failure modes activated during a RIA. Two main groups of tests currently exist. In the first group, the deformation behavior of the cladding is examined by several variations of hoop tensile tests in which an axial contraction of the specimen gage section occurs such that a near-uniaxial tension stress state results; finite element analyses are then usually employed to deduce the deformation response, often under conditions of an assumed coefficient of friction between the specimen and test fixtures. The second group includes test procedures which attempt to reproduce the deformation and failure conditions close to those seen during a RIA such that any stress-state corrections of the failure conditions are comparatively small. The advantages and disadvantages of all of these deformation/fracture tests are discussed with special reference to testing high burnup fuel cladding.