KWU's experimental basis for power ramping and cycling performance of LWR fuel

Right from the beginning of commercial operation of nuclear power plants KWU has studied the consequences of load follow operation for the fuel performance. The first power cycling experiments were started in 1970 in KWO and HFR Petten. Altogether four power cycling tests have been performed at various power and burnup levels. The fuel rod performance compares well with fuel rods under steady operating conditions at comparable power levels, demonstrating the excellent load follow capability of Zry/UO2 fuel rods. After recognition of the PCI failure mechanism KWU initiated also an experimental power ramping programme on the basis of pre-irradiating segmented rods in power reactors and subsequent ramp testing in the HFR Petten. These tests were backed by ramping of KWU test rods in many international power ramp projects conducted by Studsvik Energiteknik. Additionally, integral power ramp tests were performed in KWO and KWB-A to verify the applicability of the results gained in the research reactors to the power reactors on a broader statistical scale. The paper provides a synopsis of the power ramp test results and an analysis in the frame of the RSST-approach: Failure thresholds (safe range) were determined for PWR rods up to 45 GWd/t(U) and BWR rods up to 35 GWd/t(U), pellet density and pellet to clad gap were important variables; The safe power step above the threshold from a conditioned power level could be derived for BWR rods up to 20 GWd/t(U); Power ramp speeds for safely passing the threshold were determined with BWR rods up to 30 GWd/t(U); The safe time was determined to be in the range of minutes, a time span well above the duration of reactivity induced transients. Multiple ramp tests were performed to study the de-conditioning during further irradiation. The above experimental data together with the operational experience from power reactors form the basis for defining operating guidelines and control system set points for KWU power plants. (author). 15 refs, 9 figs, 2 tabs