Microstructure evolution of Zircaloy-2 irradiated at low temperature

Abstract

The CANDU®¹ reactor, which is based on natural uranium fuel and heavy water moderator, has 380 or 480 horizontal fuel channels in the core. A fuel channel consists of two concentric tubes, each approximately 6 meters long. The inner tube, known as the pressure tube, contains the uranium fuel bundles and the pressurized primary coolant. The outer tube, known as the calandria tube, separates the hot pressure tube (∼300 °C) from the cool heavy water moderator (∼70 °C) and provides support to the pressure tube through the garter spring spacers. The calandria tubes used in CANDU reactors are made from Zircaloy-2. The fast neutron flux (E > 1 MeV) in the reactor produces irradiation damage in zirconium alloys, through nucleation and growth of dislocation loops. X-Ray diffraction (XRD) and transmission electron microscopy (TEM) analyses have been performed on calandria tube materials that were irradiated in CANDU reactors to characterize microstructural changes produced by neutron irradiation. Under these fluences and temperatures applicable to the calandria tube service life, the predominant form of irradiation damage observed is a-type dislocation loops. The a-type dislocation density evolves rapidly at low fluences and approaches saturation after a fast neutron fluence of about 7 × 10²⁵ n/m². This saturation fluence is significantly higher than that observed from Zr-2.5Nb pressure tubes operating at a temperature of about 300 °C. The trend in the a-type dislocations, as a function of fast neutron fluence, can be correlated with the observed calandria tube deformation behaviour in the reactor. Furthermore, it aligns with the changes in mechanical properties of irradiated Zircaloy-2 materials documented in the open literature.