Experimental strategy of fuel performance testing with respect to PCI

Basic metallurgical investigations have revealed that stress corrosion cracking of Zircaloy tubes requires critical stress and iodine concentration for a minimum of time. Based on this observation KWU has structured its experimental strategy on the RSST approach. This means the evaluation of a defect-free power Range below a PCI defect threshold and a defect-free LHGR Step (naturally beyond the threshold), a limited Speed of power increase if both limits are exceeded, and a minimum Time for any mechanism to become effective.KWU has initiated a large ramp test program at HFR, Petten, the results of which are backed by the participation in international ramp test programs at Studsvik. The first target was the determination of the failure threshold as a function of burnup for the different fuel rod designs. Then, the allowable safe speed for passing the defect threshold was investigated. The defect-free range was confirmed by power reactor experiments on a broad statistical basis with rods of original length. In forthcoming experiments in Petten the verification of safe steps shall be a point of main priority. The permissible time above the thresholds may be controlled purely by the crack nucleation. In the Studsvik Demo Ramp II Program this is a point of special consideration.Detailed PIE results show that these performance limits can well be interpreted by observable phenomena like grain growth, fission product redistribution, fission gas release etc.