Zircaloy-2 plate with neutron exposure more than 3 × 10²⁶ n/m² (E > 1 MeV) has been tested to study the effects of ultra-high neutron fluence on the mechanical and physical behavior of Zircaloy-2.

The Zircaloy-2 plate used in this study is a section of a 6.4-mm-thick control blade follower that resided in a pressurized water reactor (PWR) for 17 one-year cycles at an irradiation temperature of 563 K (290°C). The irradiation growth of the plate was 2.2%. The hydrogen content of the plate is in the range of 85 to 110 ppm. The second-phase particle (SPP) size is 0.29 μm for the unirradiated archive material and 0.23 (μm after irradiation. The archive material was characterized using optical metallography, crystallographic texture measurements, microstructural analysis using STEM, and microhardness measurements. In addition to the optical and electron microscopy for microstructural analysis of the irradiated material, post-irradiation mechanical properties have been assessed using Knoop microhardness testing at room temperature, tensile testing at 561 K (288°C), Charpy impact testing in the temperature range from 293 to 648 K (20 to 375°C), and ductile fracture toughness testing at 573 K (300°C). Delayed hydride cracking velocity (DHCV) measurements were made in the temperature range of 473 to 573 K (200 to 300°C).

Knoop microhardness and tensile test results reveal that the material hardened, strengthened, and became less ductile as a result of neutron irradiation. Charpy impact testing and fracture toughness measurements show that the material has considerable ductility at all test temperatures. Comparisons are made to smaller SPP materials tested after lower fluence [1].

The DHCV measurements showed that delayed hydride cracking occurred at temperatures equal to or less than 533 K (260°C). No DHC was observed at higher temperatures.

The implications of the observed microstructural and mechanical property changes, and the temperature dependence of delayed hydride cracking, are discussed.