Crack initiation and crack growth during thermal shock tests in the reactor pressure vessel of the HDR under corrosive medium conditions

A survey is presented of the status of comprehensive thermal shock investigations performed at a reactor pressure vessel (RPV). In a separate group of experiments it was first studied which impacts are exerted on the undamaged RPV wall by fluid transients resembling emergency cooling conditions. Furthermore, crack formation and crack growth were enforced in selected wall zones by providing appropriate boundary conditions. It is proposed to load in a final step the wall zones heavily damaged by quasi-reality crack fields under conditions similar to emergency cooling. The investigations are performed to verify the capability of non-destructive testing methods and of computations (of temperatures, stresses, crack behaviour) to provide relevant information. The computations have yielded clear overestimations as well as underestimations. For instance, the investigations performed at the plant proper have produced the result that the thermal shocks acting on the RPV wall surface are substantially reduced by thermal mixing processes in water. In the subsequent computations of the RPV wall structure, rotation symmetric computer codes are found to fail completely under conditions of intensive local cooling. Finally - although the crack growth computations for the special cyclic thermal shocks under corrosive medium conditions were extremely conservative - it has been demonstrated, by use of acoustic emission analysis and other techniques that the present crack growth computations do not adequately take into account crucial parameters influencing the material behaviour. This means that the results obtained with laboratory-scale specimens cannot be safely transferred to the components under the testing conditions prevailing for the time being. (author)