Azimuthal Deformation Particulars of Single-Crystal Cladding of High-Temperature Fuel Elements

Abstract

The particulars of the azimuthal deformation and the reasons for the formation of faceting on single-crystal molybdenum cladding with the <111> axis oriented longitudinally in tests for structural creep at 1650°C under internal argon gas pressure are investigated. It is shown that the periodic local nonuniformity of the azimuthal deformation of <111> cladding and the appearance of faceting are associated with the difference of the maximum shear stress in the active slip planes for single crystals with a bcc lattice in the range of orientations of the stretch axis <110>-<112>. Cladding sections located at the center of the interval <110>-<112> possess weak resistance to high-temperature creep and sites of deformation localization.Heat-resistant single-crystal alloys based on molybdenum and tungsten are the most promising material for cladding of high-temperature fuel elements for nuclear thermionic converters and nuclear power-propulsion plants for use in space [1][2][3]. The long service life at high temperatures under stress and the stringent requirements for preserving geometric dimensions of the cladding require materials with high resistance to creep and an adequate long-time strength limit. Heat-resistant single-crystal materials best satisfy the complex and contradictory set of requirements for the cladding material of fuel elements for nuclear facilities of this kind.A characteristic feature of single-crystal materials is that their mechanical properties depend on the crystallographic direction of the acting load. For uniform azimuthal deformation of single-crystal cladding under the internal pressure of the nuclear fuel and gaseous fission products, a crystallographic orientation of the longitudinal axis of the cladding with maximum possible symmetry in the transverse section is preferred.Of the three main crystallographic directions of the longitudinal axis of the cladding for a bcc lattice <100>, <110>, and <111>, the latter possesses the highest symmetry and therefore highest isotropy of mechanical properties. In the transverse section, the crystallographic directions <110> and <112> appear six times on the cylindrical surface, <123> 12 times, and <110>-<112> segments 12 times (Fig. 1). For single crystals of molybdenum, tungsten and their alloys, the short-time mechanical properties and creep have been studied in detail in a wide range of temperatures in uniaxial tension in the <111> and <110> directions and it has been shown that the <110> direction is least resistant to creep [4,5]. The experimental creep of samples with stretching axis <110> is usually used to determine the maximum deformation of the cladding when validating its service life. However, it is not obvious that this orientation during high-temperature creep is weakest in the segment <110>-<112> of the stretching axis. As one can see in Fig. 1, in cladding with longitudinal axis <111> the possible directions of the axis of stretching occupy an angle range of 30°in the segment <110>-<112>. High-temperature creep and longand short-term st...