Investigations of the fuel rod behavior in the low pressure phase of a LOCA and of the interaction between ballooning zircaloy claddings and emergency core cooling (REBEKA-programme)

For measuring the temperature distributions in the rods the photoscanning method was improved by determining correction isotherms. Photoscannings were completed for the heater rods of thirteen full length fuel rod simulators. For the capillary pressure measuring system a relation was obtained which allows the actual pressure development at the point of the expected balloon to be calculated by taking into account the temperature distribution over the capillary. A number of experiments carried out at different temperature levels showed good agreement with the theoretical results. Comparative cladding tube burst experiments were performed on shortened fuel rod simulators in air under adiabatic and non-adiabatic experimental conditions, the shroud tube being directly heated only in the case of adiabatic experimental conditions. The adiabatic experiment resulted in slightly higher burst temperatures, slightly lower burst pressures, but considerably higher circumferential burst strains due to much smaller azimuthal differential temperatures. Three bundle heating experiments with subsequent flooding were performed. They served for commissioning and checking the loop for bundle tests and for verification of the improved service program for the display and on-line representation. (orig./RW)