Hydride precipitation kinetics in Zircaloy-4 studied using synchrotron X-ray diffraction

Abstract

a b s t r a c tAs a result of in-reactor corrosion during operation in nuclear reactors, hydrogen can enter the zirconium fuel cladding and precipitate as brittle hydride particles, which may reduce cladding ductility. Dissolved hydrogen responds to temperature gradients, resulting in transport and precipitation into cold spots so that the distribution of hydrides in the cladding is inhomogeneous. The hydrogen precipitation kinetics plays a strong role in the spatial distribution of the hydrides in the cladding. The precipitation rate is normally described as proportional to the supersaturation of hydrogen in solid solution. The proportionality constant, a 2 , for hydride precipitation in Zircaloy-4 is measured directly using in situ synchrotron X-Ray diffraction, at different temperatures and with three different initial hydrogen concentrations. The results validate the linear approximation of the phenomenological model and a near constant value of a 2 = 4.5 Â 10 À4 s À1 was determined for the temperature range studied.

In situ synchrotron X-ray diffraction study of hydrides in Zircaloy-4 during thermomechanical cycling

“…The difference between the two values is caused by the precipitation hysteresis normally observed in Zr hydride precipitation and likely caused by the energy required for hydride nucleation [10,[30][31][32].Thus the behavior of the a-Zr under such a thermomechanical treatment is well in accord with current understanding. Figure 4.…”

Multidimensional simulations of hydrides during fuel rod lifecycle

“…The final case (4) adds in finite rate precipitation according to the rates measured by [18] and confirmed by [24]. The strong temperature dependence of this form for the precipitation rate coefficient somewhat reduces the formation of hydrides near the cold boundary.…”

“…Various fits for K p have been inferred from experiments to determine T SS d and T SS p [18,23]. In addition, [24] performed an experiment to more directly determine the rate S and obtained reasonable agreement with the deduced rate dependences reported previously, but did not find a consistent temperature dependence for S. Nevertheless, an Arrhenius rate has been useful to model the effects of hydride rim formation in BISON because it improves results near the cold boundary. We have used the rate from [18] …”

“…One : A : possible explanation is that the heat of transport Q * is :::: could ::: be smaller near the water side edge of the cladding. There are large discrepancies in the measured values of Q * and experiments have not found a clear temperature dependence [18,24,31,32]. Nevertheless, if Q * is reduced near the water side due to dependence on temperature, local hydride concentration, or residual stress in the manufactured cladding, this could explain why a thick, lower concentration rim is often observed in experiments.…”

The effect of cooling rate and grain size on hydride microstructure in Zircaloy-4

“…Within zirconium and Zircaloy-4, δ phase (FCC) hydride tends to precipitate at relatively low cooling rates (< 10 °C/min) [11], whereas at cooling rates of > 10 °C/min the γ (FCT) phase hydride is more predominant [12]. For a cooling rate of 1 °C/min, synchrotron XRD based peak deconvolution also implied that δ phase (FCC) hydride could be present, though in the Courty et al study no other hydride phases were discussed [13]. In Blackmur et al [3] controversy regarding the nucleation route for δ phase is discussed, but again only the δ phase was analysed in their synchrotron XRD based analysis of hydride solubility and phase fraction using a Rietveld refinement with a priori supposition about the presence of α-Zr and δ-hydride.…”