Hydrogen Solubility and Microstructural Changes in Zircaloy-4 Due to Neutron Irradiation

Abstract

The present work deals with two essential degradation mechanisms of zirconium alloys: Hydrogen behavior and microstructural transformations due to neutron irradiation. The studies were carried out by using samples taken from Zircaloy-4 components, which remained for 14 years in the Atucha I pressurized heavy water reactor, accumulating neutron fluences of up to 1022 neutrons/cm2. A hydrogen/deuterium solubility increase was observed in both dissolution and precipitation processes. With the aim to follow this evolution with radiation damage recovery, the irradiated samples were annealed for different periods at 350 and 400°C in a differential scanning calorimeter (DSC). These temperatures were chosen to reproduce those reached by spent fuels in the early stages of the dry storage (DS). Between intervals, the terminal solid solubility temperatures in dissolution (TTSSd) and in precipitation (TTSSp) were determined. They were also determined after additional annealing at 500 and 600°C, trying to simulate long periods at DS. The hydride morphology and the metallurgical state of the Zr(Fe, Cr)2 second-phase particles (SPPs) were followed between annealing by optical microscopy, analytical electron microscopy, synchrotron light X-ray diffraction4 (SXRD), and DSC. As a brief summary, we found from the first to the last run of the annealing sequence an increase of 55°C in TTSSd and 40°C in TTSSp on average. The optical and transmission electron microscopies (TEM) indicate that in the material in the irradiated condition, exists a hydride size distribution of four orders of magnitude, from 10 nm to 100 μm, but biased to sizes smaller than 5 μm. A complete amorphisation of the SPPs was observed in high fluence samples by TEM and SXRD. Similar TEM observations made in irradiated samples annealed above 400°C show a re-precipitation process of nano-SPPs forming clusters. The re-precipitation is affected by kinetics: The SPPs crystallization temperature, Tc, rises from 450 to 500°C as the heating rate increases from 5 to 20°C/min. The measured heat of crystallization was 27.3±2.1 kJ/mol (Fe + Cr).

Experimental Results and Theoretical Interpretations of Solvus Relationships in the Zr–H System

“…As shown in Fig. 8.12, this type of behavior was, in fact, observed by Vizcaíno et al [49,50]. These authors used the DSC technique to determine solvus relationships on neutron irradiated Zircaloy-4 coolant channel material removed from the Atucha 1 PHW reactor which had very high levels of hydrogen content ranging from *150 to 250 wppm and neutron fluences up to 10 26 n/m 2 .…”

“…(from Pan and Puls [29]; with permission from AECL) these lines has recently also been provided by Vizcaíno et al [50], as further discussed in Sect. 8.4.3.…”

“…A recovery peak superimposed to the initial curve is observed that may be related to damage recovery. (from Vizcaíno et al [50]) Considerably greater differences between unirradiated and irradiated Zircaloy-4 coolant tube material were recently documented by Vizcaíno et al [47][48][49][50]. The authors point out that the more irregular features of the DSC signals in irradiated materials made it more difficult to identify the exact location of the terminal stage of hydride dissolution based on the maximum slope and completion temperature locations of the DSC signal.…”

“…These authors used the DSC technique to determine solvus relationships on neutron irradiated Zircaloy-4 coolant channel material removed from the Atucha 1 PHW reactor which had very high levels of hydrogen content ranging from *150 to 250 wppm and neutron fluences up to 10 26 n/m 2 . It was argued by Vizcaíno et al [50] that-based on the results of observations from optical and transmission electron microscopy-the less distinct hydride transformation signals in these irradiated materials are a consequence of the existence of a wide spread in hydride sizes ranging over four orders of magnitude from 10 nm to 100 lm. These observations by these authors are in accord with those proposed by this writer when account is taken of the preferential formation of hydride precipitates in the cores of irradiation-produced dislocations.…”

“…Of course, when applied to the data for the same material this approach provides no extra benefit, since it still does require the hydrogen concentration in the material to be known in order to calculate the reference solvus enthalpy, D " H H d!a in these irradiated material. Vizcaíno et al [47][48][49][50] did not discard the results of their first heating runs, as is the usual custom, because these authors felt that some insight concerning the effect of irradiation on the dissolution solvus could be gleaned from the results of this run. The solvus enthalpies calculated from Eq.…”