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

Abstract

The formation of elongated zirconium hydride platelets during corrosion of nuclear fuel clad is linked to its premature failure due to embrittlement and delayed hydride cracking. Despite their importance, however, most existing models of hydride nucleation and growth in Zr alloys are phenomenological and lack sufficient physical detail to become predictive under the variety of conditions found in nuclear reactors during operation. Moreover, most models ignore the dynamic nature of clad oxidation, which requires that hydrogen transport and precipitation be considered in a scenario where the oxide layer is continuously growing at the expense of the metal substrate. In this paper, we perform simulations of hydride formation in Zr clads with a moving oxide/metal boundary using a stochastic kinetic diffusion/reaction model parameterized with state-of-the-art defect and solute energetics. Our model uses the solutions of the hydrogen diffusion problem across an increasingly-coarse oxide layer to define boundary conditions for the kinetic simulations of hydrogen penetration, precipitation, and dissolution in the metal clad. Our method captures the spatial dependence of the problem by discretizing all spatial derivatives using a stochastic finite difference scheme. Our results include hydride number densities and size distributions along the radial coordinate of the clad for the first 1.6 h of evolution, providing a quantitative picture of hydride incipient nucleation and growth under clad service conditions.

Perspectives on multiscale modelling and experiments to accelerate materials development for fusion

“…66 . The γ -phase has been observed to form in rapidly loaded getters in ex-reactor settings [338,339] .…”

Long-Term Corrosion Testing of Zy-4 in a LiOH Solution under High Pressure and Temperature Conditions

“…When the solubility limit for picked-up hydrogen (about 100 mg/kg at 330 • C) is exceeded in the Zr alloys, the excess hydrogen precipitates as Zr-hydride. Precipitated hydrides in Zr alloys are predominantly in the delta phase (ZrH x , x~1.66) [35,36]. The formation of elongated zirconium hydride platelets during corrosion of nuclear fuel cladding is linked to its premature failure due to embrittlement and delayed hydride cracking.…”

Corrosion Behavior of Chromium Coated Zy-4 Cladding under CANDU Primary Circuit Conditions

“…Zr clad is susceptible to corrosion from the coolant and fuel sides, due to the oxygen and hydrogen penetration. The oxidation and hydrogenation of zirconium in LWR are taking place according to reactions ( 1) and ( 2) as was evidenced in literature [37][38][39][40][41]. Coating of Zr with chromium introduced the third reaction (Equation ( 3)), representing Cr oxidation process which will complete.…”

“…The fraction of δ and γ hydrides is related to the H concentration and cooling rate of Zr. A higher concentration of H and/or a slower cooling rate will lead to more δ hydrides, while the opposite will lead to more γ hydrides [40]. The relatively slow cooling rates during the normal operation of nuclear fuel rod claddings lead to δ hydrides formation, which causes most of DHC (Delayed Hydride Cracking) under actual PHWR's operating conditions.…”