Kolenkova O. Features of structure formation and evaluation of process plasticity using acoustic emission in making nuclear fuel cladding tubes of Zr1Nb alloy

The thesis objective is development of a resource-saving technology for manufacturing nuclear fuel cladding tubes of a new zirconium-niobium alloy Zr1Nb of Ukrainian production. Features of structure formation in ingots obtained by electron-beam, vacuum-arc and skull-furnace melting with electromagnetic stirring have been investigated at the manufacturing stage. It has been ascertained by electron diffractions method that the layer between zirconium platelets in Zr1Nb alloy is niobium with a body-centered cubic lattice. Scheme of structure formation during decomposition of solid niobium solution in zirconium in Zr1Nb alloy ingots has been proposed. Regularities of structure formation and creation of processing properties in hot-extruded tubes of zirconium-niobium alloy depending on the initial cast structure, oxygen and niobium content and extrusion temperature-and-deformation parameters have been established. Schedules of hardening anneal and heat treatment ensuring formation of highest processing plasticity in hot-extruded tubes have been developed. Interconnection between structure, chemical composition (oxygen and niobium contents) and plasticity and damageability of hot-extruded tubes and parameters of acoustic emission (AE) has been established. Using AE method, it has been established that hot-extruded tubes having martensite structure feature a higher level of processing plasticity in comparison with the bainite-type structure. A procedure of assessment of plasticity level of Zr1Nb alloy tubes on the basis of AE data has been worked out. Parameters of making nuclear fuel cladding tubes of Zr1Nb alloy at the step of hot working have been refined. The new parameters ensure formation of the highest plasticity in hot-extruded trex-tubes. Using AE data, deformation ratios for the first cold cold-rolling cycle have been chosen to ensure a complete use of Zr1Nb alloy plasticity resource. The technology of making nuclear fuel cladding tubes of Zr1Nb alloy has been corrected to reduce the number of cold-rolling cycles from five to four cycles. An experimental lot of nuclear fuel cladding tubes has been produced in the plant conditions. The tubes meet requirements of specification ТУ У 27.4-05757883-206:2008.