Investigation of the growth rate of cracks in shells made of the alloy Zr-1%Nb under conditions of iodine corrosion cracking under stress

Abstract

During the operation of fuel elements under conditions when the power per unit length varies, there is danger that the cladding will rupture by the mechanism of corrosion cracking under stress [ 1, 2]. To take account of this phenomenon in the prediction of the working capacity of fuel elements in transient states, it is necessary to know the laws governing the fracture of the fuel element cladding.It is known that the fundamental characteristic determining the resistance of a material to crack propagation in a corrosive medium is the threshold stress-intensity factor KISCC, characterizing the resistance of the material to simultaneous action of a corrosive medium and stress [3][4][5]. For a specific structure and stress state KISCC is determined by the size of the admissible defects, which do not develop for values of Klscc below the threshold value. Therefore KlSCC can serve as a quantitative criterion for comparing cladding materials with respect to resistance to cracking or depth of admissible initial technological defects.In this paper we report a method and the results of investigations of the alloy Zr-1% Nb concerning the threshold stress-intensity factor Kxscc and the rate of growth of cracks under the conditions of an iodine medium.Experimental Procedure. The investigations showed that the values obtained for KIscc can be different even for the same material. The reason for this could be that different methods are used. For example, for one method the threshold of stability is found from the conditions that a fatigue crack produced beforehand does not propagate, and in a different approach it is determined from the conditions that a corrosion crack stops as a resuk of a decrease in the instantaneous stress-intensity factor [6]. Other reasons could be that the structure of the metal in standard samples of plates does not correspond to the samples of fuel element cladding; the geometry of the initial crack, obtained either by the fatigue method or a mechanical cut, could also have an effect [6][7][8].Analysis of methods for determining the rate of growth of a crack and estimating Klscc taking account of the effect of the material texture and character of fracturing of fuel element claddings on corrosion cracking in iodine accompanying the interaction of fuel with the cladding made it possible to choose a method based on tests of fuel element claddings loaded with internal gas pressure (helium, argon) in the presence of iodine under the cladding [8]. A fatigue crack, which does not penetrate through the cladding, is created beforehand on the inner surface of such claddings. The construction of the sample is displayed in Fig. I. The range of stresses required to perform the tests was determined so as to ensure that the deformation is planar. For fracture under a planar deformation, the requirement [3]where try is the yield point and h is the thickness of the cladding, must be satisfied.It follows from this inequality that the region of admissible stress-intensity factors must satisfy the condition (for try =...