Investigation On The Fracture Performances Of N18 Zircaloy Thin-walled Tubes

Zircaloy is widely used in nuclear reactors because of the small thermal neutron absorption cross section,good corrosion resistance and excellent mechanical properties.However,the high temperature,high pressure and neutron irradiation environment in the reactor will change its mechanical properties.In order to meet the requirements of reactor development,a new type of zircaloy represented by N18 zircaloy is used in reactor components such as fuel assemblies and fuel cladding.N18 Zircaloy thin-walled tube is a typical reactor fuel cladding structure.Its fracture behavior is of great significance for ensuring long-term safe and stable operation of nuclear reactors.In this thesis,the quasi-static fracture and creep crack growth behaviors of N18Zircaloy thin-walled tube were systematically studied,and the following work was completed:(1)A type of DEAT specimen and the test set-ups for the measurement of the behavior of quasi-static fracture and creep crack growth was designed,and the test systems of quasi-static fracture and creep crack growth based on the DEAT specimens were established.Based on the energy equivalence principle,the semi-analytical expression of stress intensity factor K of the DEAT specimens was derived.A method for monitoring crack length of the DEAT specimens based on flexibility principle,and a real-time crack length determination method based on load separation theory were established.The J-integral and C~*integral formulae for the DEAT specimens were established.A real-time crack length monitoring method for the DEAT specimens in the creep crack growth tests based on DC potential method was established.(2)The quasi-static fracture tests of N18 zircaloy thin-walled tube at room temperature were completed by using the DEAT specimens,and the crack tip constraint effect of N18zircaloy DEAT specimens was analyzed based on J-Q-M theory.The results showed that the effect of crack size effect on J-?a resistance curves or crack initiation toughness was caused by the difference of crack tip constraint levels.The J-Q-M theory can better describe the crack tip stress field of the DEAT specimens,and the parameter Q can be used to characterize the crack tip constraint levels of the DEAT specimens caused by the difference of crack size.J-?a resistance curves of N18 zircaloy DEAT specimens with different crack tip constraint levels can be expressed by the constraint modified resistance curves based on the parameter Q.(3)The creep crack growth tests of N18 zircaloy thin-walled tube at different temperatures and load levels were completed using the DEAT specimens,and the creep crack growth curves were obtained.The results showed that the temperature and load had a significant effect on the creep crack growth of N18 zircaloy thin-walled tube,and the creep crack growth could be divided into two stages:the state of steady crack growth and rapid crack growth.The relation between the creep crack growth rate da/dt and C~*integral of N18zircaloy thin-walled tube under different temperatures and load levels satisfies a good linear relationship under double logarithmic coordinates.