Some Studies on Magnox A-12 Subject to Restricted Deformation Caused by Uranium

Abstract

The sheath and core material which constitutes fuel element through bonding generally functions under different conditions of temperature and atmosphere. When the rigidity of the sheath material is poorer than that of the core material, the sheath is subject to deformation to match the core under service. Hence, it seems very important to clarify such relative deformation considering the thermal fatigue behavior from the view point of the design, safety and usage of the fuel element.In our research “the universal thermal fatigue testing machine” was used which was originally designed by the authors in order to investigate the relative deformation and other various thermal fatigue phenomena in general.The experimental procedures were carried out so as to put Uranium and Magnox A-12 in separated positions. The difference in thermal strain which occurred from the difference in thermal cycling temperature ranges was accurately transmitted to Magnox A-12, and its induced stress behavior, microstructure and failure were observed.The results were as follows.(1) The restricted deformation of Magnox A-12 caused by Uranium.When Magnox A-12 is restricted by Uranium it occasions compressive stress in Magnox at higher temperature range, and the stress in its case is very small in spite of the sudden increase in elongation which is brought about by heating Uranium above its transformation temperature. This fact shows that the deformation of Magnox advances by creep.The increase of the cycling period from 0.5 to 4hr. per cycle promotes tensile creep and stress relaxation at lower temperature range.The maximum stress induced in Magnox under the restricted relative deformation caused by thermal cycling of 190°↔575°C for Uranium and 150°↔450°C for Magnox is higher even after 320 cycles than that of the original state.These results show that Magnox is easily subject to the deformation of Uranium and that the Magnox load on Uranium is very small.(2) Themal fatigue tests on Magnox A-12.The difference in stress and strain values which occur between tension and compression side during the thermal fatigue test, i.e., the non-symmetrical character of stress-strain relationship, increases with the lowering of the thermal cycling temperature, and it is distinguished especially for the thermal fatigue tests of temperature range with the lower temperature limit than 225°C.At higher temperature limit, for instance 525°C, the decrease of the fatigue life is very remarkable and its crack occurs at grain boundary.(3) Observation of microstructure in Magnox A-12Visible basal slips and twins have occured already after one cycle, and the position of deformation has moved toward grain boundary from grain interior with the increase of both cyclic number and cyclic temperature range. In the case of higher thermal cycling temperature range, such as 275↔475°C, the grain growth and the grain boundary flow especially becomes remarkable, resulting in remarkable wrinkles on the surface. The occurrence of wrinkles breeds had influence upon the heat transfer from the core.During the whole experiments, cavitation which was discovered in ordinary creep tests could not be found. This fact seems to depend upon the difference of loading direction between the creep tests and these experiments.Some peculiarities of the above mentioned behaviors and some problems in Magnox A-12 used for sheath material were discussed also from the crystallographic anisotropic point of view.