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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...