An assessment of delayed hydride cracking in zirconium alloy cladding tubes under stress transients

Abstract

Zirconium alloy cladding tubes used in nuclear fuel rods are susceptible to delayed hydride cracking, which is a time dependent crack growth process resulting from the stress assisted diffusion of hydrogen to the crack tip, followed by the formation of radial hydrides and the subsequent fracture of the hydrides in the crack tip region. This article reviews the current understanding of the delayed hydride cracking behaviour of zirconium alloy cladding tubes for fuel rods, focusing on the degradation mechanisms in high burnup fuel rods and transient loading scenarios, which could potentially lead to substantial changes in the hydride microstructure and cladding failure by delayed hydride cracking following removal from the reactor and during storage and disposal of spent nuclear fuel rods in a waste repository. A brief summary of the general characteristics of delayed hydride cracking in zirconium alloy cladding is presented first. Relevant information on the cladding stresses under various usage conditions is then compiled and categorised into several characteristic stress transients that can be anticipated during reactor operation. Delayed hydride cracking in cladding tubes under stress transients is then examined under various temperatures, cooling rates, burnup levels and loading conditions.

Hydrogen concentration and hydrides in Zircaloy-4 during cyclic thermomechanical loading

“…It is known that the hydrogen diffuses favourably into high tensile stress regions, such as at service-induced flaws or manufacturing weak spots (e.g. welding points) and a degradation process called delayed hydride cracking (DHC) can potentially initiate from the pre-crack [3][4][5]. DHC is a stable, time-dependent crack growth mechanism where hydrides repetitively dissolve and re-precipitate within the zircaloy tubes during thermo-mechanical cyclic loading.…”

Kinetic Model of Incipient Hydride Formation in Zr Clad under Dynamic Oxide Growth Conditions

“…Calculations and experiments point to the large misfit strains between the cubic δ-hydride and the host α-Zr as the reason behind such preferential alignment [35,[38][39][40][41][42], which may also impact the mechanical response of the clad. Indeed, the formation of brittle hydride phases is a principal cause of delayed hydride cracking (a subcritical crack growth mechanism facilitated by precipitation of hydride platelets at the crack tips in Zr clad [19,[43][44][45]).…”