It is well known that in stress corrosion cracking the stage in which cracks slowly develop may amount to 75-90% of the overall time until the tube fractures [1][2][3]. This stage obviously includes both the initiation of the crack and its increase in size, for which the further development of the crack is described by the relations of fracture mechanics. The need to investigate the process by which the tube becomes damaged is due primarily to the possibility that surface cracks may form on its inner surface under transient conditions, which do not lead to depressurizing of the fuel-rod tube. The ability of the fuel-rod tube to operate will be limited by the tendency of the cracks formed to increase further. Hence, a determination of the manner in which the initial stage of tube cracking proceeds in an iodine environment should enable predictions to be made on the behavior of the fuel-rod container in water-cooled reactors during operation, including when power is being ramped.The purpose of the present paper is to give an experimental estimate of the parameters governing the initial stage of iodine corrosion: deformation, leading to the initiation of cracks, and the time required for a crack of a def'mite size to form.The Sample Investigated and the Experimental Method, To carry out laboratory tests under stress corrosion cracking conditions we used VVER-1000 fuel-rod tubes with an external diameter of 9.15 mm and a wall thickness of 0.7 ram, made from Zr --1%Nb alloy. The tubes were in the delivered state after annealing for three hours at 580~ Two types of tubes were investigated: standard and plated, The latter had a plastic zirconium plating about 100/~m thick on the inner surface. The mechanical properties of the materials of the tubes are described in [3, 4], Metallographic investigations showed that the mean grain size in the specimens tested is 3.93 :h 0.12 t~m for Zr --l%Nb alloy and 12.3 + 0.9/~m for a plating layer of pure zirconium. The estimate of the grain size was made both by the ASTM method and by the traditional secant method [5]. The sample-simulators of the fuel-rod containers were loaded with an internal pressure of argon. This method of investigation has been highly recommended, since a large number of microcracks have been observed in containers tested under similar conditions. The construction of the simulator is shown in Fig. 1.The technology used to manufacture the samples was as follows. 1. The damper 1 and the transition 3 were connected to the container of Zr-l%Nb alloy 2 using electron-beam welding (welded joints No. 1 and 2).2. The sample was pumped out to a residual pressure of 0.26 Pa (2 • 10 -3 mm Hg), maintained at that pressure for not less than 10 sec and then filled with argon to an excess pressure of 1.03 • 105 Pa.3. Crystalline iodine was loaded into the inner cavity of the sample in a vertical position in an amount required to produce a specified concentration of corrosion reagent.4. After the simulator was filled with gas to an excess pressure of 13 MPa it was sealed by ...
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 =...
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