Analysis of Lattice Change in Aged Zr-2.5%Nb CANDU Pressure Tube Material Using a Neutron Diffraction

Zr-2.5%Nb pressure tube material is known to cause in-reactor deformation in the CANDU reactor operating environment. This is because the pressure tube contains nuclear fuel and is exposed to fast neutron irradiation due to nuclear fission. However, it is not yet understood why the deformation occurs by fast neutron irradiation. This study investigated why the length change occurs in an out-reactor environment without neutron irradiation by examining the behavior of lattice change by thermal treatment. In the experiment, double melt and quadruple melt pressure tube materials were aged at 300, 350, and 400 degrees C for up to 20,000 h, respectively, and the change in average lattice distance was investigated using a neutron diffraction device. At 400 degrees C-20,000 h aging treatment, the double melted materials expanded by 0.05 and 0.09% in (1010) and (0002) planes, respectively. On the other hand, the quadruple melt material expanded 0.02% and 0.08% in the (1010) and (0002) planes, respectively. These results show that lattice expansion can occur only by thermal treatment. It is judged that the in-reactor deformation causes additional lattice expansion to the out-reactor deformation. The lattice expansion behavior by thermal treatment and neutron irradiation was interpreted mechanistically as a concept of increase in entropy by applying an elliptic atom model rather than a sphere.