Forderungen an die eigenschaften von werkstoffen für reaktordruckbehälter im hinblick auf betriebs — und störbelastungen — eine übersicht

Abstract

In view of the stringent requirements for safety and operating reliability of nuclear power plants, considerable attention has to be focused to the structural design of the highly stresses components of the reactor system. The design of reactor pressure vessel and primary piping must comply with most exacting standards. The demands, unprecedented in convential applications, require additional knowledge on the behaviour of materials. The primary objective of this article is to critically review and assess the adequacy of the foundations of mechanical design of steel reactor pressure vessels on the basis of recent German research work, pointing out those aspects which are presently considered essential to be further investigated with the goal to achieve a more adequate quantitative evaluation of structural reliability and safety under various loading conditions. (Neutron-induced changes of the mechanical properties of pressure vessel materials, however, are outside the scope of the present considerations.) In the 1st part of this paper the maximum strength of pressure vessels for internal pressure loading is discussed showing that not only the tensile strength σ B of the steel but also the uniform strain ϵ G and the true stress at maximum pressure σ G are of crucial importance for structural safety (figs. 1 to 3). At areas of stress concentrations the weakening effect on bursting strength expressed by the reduction factor ν B in general seems to be covered in a conservative way by the strength reduction factors at the onset of effective yield ν 0.2 as they are presently prescribed in Germany for conventional pressure vessel design (fig. 4). However, experimental programs with larger specimens are required. Expected deformations at maximum internal pressure (fig. 5) and the significance of material ductility (reduction of cross-sectional area at tensile rupture ψ, notch impact toughness a K) are indicated (figs. 6 to 8). The 2nd part considers the problem of low-cycle fatigue. Experimental data are given for fluctuating internal pressure (fig. 9), alternating plastic straining at elevated temperatures (fig. 10), alternating low frequency loading in water environments (fig. 11) and for thermal shock (fig. 12). In the 3rd part the influence of specimen-size on tensile strength of bars with notches (fig. 13) and the influence of multiaxial stress states on the failure characteristics (fig. 14) are discussed.