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620.178.72 One of the most widely used structural materials in nuclear power is Zr-1%Nb alloy. However, its mechanical properties have not been investigated to a sufficient extent. This applies primarily to the strength and strain characteristics at high strain rates and under high impact loads. This form of loading of the materials and structures of nuclear power plants occurs in emergency situations [1, 2], which usually arise at operating and increased temperatures.The purpose of the present research was to study the mechanical properties of Zr-1% Nb alloy over a wide range of strain rates and temperature.Mechanical tests on the uniaxial stretching of Zr-l%Nb alloy were carried out in the strain-rate range from 2.5-10 -3 to 8-104 see -1 and over a 20-700~ temperature range. The specimens were made from alloy of standard chemical composition, obtained using technology developed in industry, and were not subjected to any additional heat treatment. Because of the need to ensure a quasi-static uniaxial stress state at a strain rate > 103 sec -l [3], in tests with ~ = 2.5.10 -3-6.2.103 sec -I we used specimens with a working section of length l = 5 mm and diameter d 0 = 2.5 mm. Investigations show [4] that these dimensions and shape of the specimen guarantee a uniaxial stress over a working length of 2.8 mm, in which connection, for this rate, fracture occurs in the middle of the working section with a well-developed neck. Tests at higher strain rates of 8-10'* see -l were made using specimens the working length of which was 0.5 mm and the diameter 2.5 mm. A reduction in the length leads to the formation of a nonuniaxial stress state in the specimen. However, the use of the well-developed method described in [5] enabled us to convert characteristics measured in a nonuniaxial stress state to the characteristics of a uniaxial stress state.Tests at a strain rate from 2.5.10 -3 to 0.6 see -I were made on ZDt0/90 and P-1246 machines; I00 sec -1 on electromagnetic equipment [6] and 3000 sec-l and above (high-rate stretching) on magnetic-impulse equipment [7]. We recorded load-time P(t) oscillograms, from which we constructed stress-strain o(t) diagrams. The systematic error in determining the strength characteristics was 4% and the systematic error in determining the plasticity characteristics was 5%.For high-rate stretching of the specimens at an elevated temperature we used the Hopkins composite rod scheme [8], based on magnetic-impulse equipment. In tests using this scheme (Fig. 1) the specimen was placed in a vacuum chamber between two elastic rods and loaded by generating an elastic wave in the transmitting rod, which, having passed through the specimen and deformed it, was recorded by the rod-dynamometer. This method enabled high-rate stretching of the specimens in a vacuum to be carried out, avoiding oxidation of the specimens at an elevated temperature, where the semiconductor strain gauges of the dynamometer were situated at a safe distance from the heated specimen and were protected by additional special co...

Experience in running atomic power plants with thermal reactors has established, by analyzing the reasons for fuelelement failures, that interaction between the fuel and the cladding under unsteady conditions can lead to a breakdown of the hermetic sealing of the fuel elements due to corrosion resulting from stresses when various fission products, particularly iodine, act on the cladding [1].Investigations of iodine corrosion of claddings made of Zr-1% Nb alloy in the last few years have established that corrosion cracking is governed by such factors as the iodine concentration, the stress, temperature, the state of the internal surface, the texture of the cladding and the neutron flux [2][3][4].To predict the behavior of fuel,element claddings under emergency conditions, tests of VVI~R-t000 reactor regular fuel-element claddings made of Zr-1% Nb alloy in the presence of iodine were carried out at elevated temperatures. It was established that iodine facilitates a reduction in the cracking deformation of the claddings and the time taken for the cladding to crack in the 500-750~ range. However, its effect only manifests itself after a certain critical concentration has been exceeded [5][6][7]. According to estimates in [7] at a uranium depletion rate of 36,000 MW.day/t, the iodine content in LWR fuel elements should amount to -2.2 mg/cm 3. This value is obtained on the assumption that it is uniformly distributed over the free volume and that its relative separation is equal to the mean for gaseous fission products. However, redistribution of the fission products together with an additional increase in the separation when the fuel elements split when there is a sudden change in the power and under emergency conditions with a loss of coolant can give rise to a very high local concentration of iodine in the fuel-element claddings. At the same time one must take into account the production of a certain amount due to radiolysis of cesium iodine. The formation of cesium uranates and molybdates must also be borne in mind; this sharply increases the partial pressure of iodine, thereby facilitating the corrosion cracking of the cladding. Moreover, numerous experiments have shown that iodine and cesium separate out independently from the fuel [8].In this paper we present the results of laboratory experiments carried out to establish the temperature dependence of the initial iodine concentration at which its effect on the deformation behavior of fuel-element claddings made of Zr-1% Nb alloy is detected in isothermal tests in the 500-750~ temperature range.Isothermal tests of tubular samples of Zr-1%Nb alloy were carried out at 500-750~ when they were loaded with a constant internal pressure of argon. The samples were 125 mm long, and had an external diameter of 9.15 mm and a wall thickness of 0.72 mm. They were sealed by contact welding, pumped out and supplied with a weighed amount of crystalline iodine. The pressure of the argon f'filing was calculated from the strength of the material of the cladding so as to produce a t...

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