Simulation of Second Phase Particle Evolution in Zircaloys

Abstract

Zircaloy-2 and -4 are a group of zirconium alloys used in water-cooled reactors, primarily as nuclear fuel cladding. The size and distribution of second phase particles (SPPs) in these alloys, which is set by the thermomechanical processing history, is known to play a critical role in their corrosion performance during reactor operation. Both alloys contain the Zr(Fe,Cr)2 Laves phase which is known to undergo amorphization and dissolution, the rate of which has a positive dependence on neutron flux and fluence and is inversely proportional to temperature. These processes are accompanied by a release of solute to the surrounding matrix. The Zr2(Fe,Ni) Zintl phase is only found in significant quantities in Zircaloy-2. In the temperature range typically associated with the reactor core, amorphization of this phase is not observed but solute is redistributed to the matrix through dissolution. The structural and chemical changes associated with both SPP types under irradiation has been qualitatively correlated to a reduced corrosion resistance. A model capable of tracking SPP evolution through material processing and in-reactor operation is therefore a valuable tool for predicting and extending the lifetime of components. Characterization of SPPs in quenched and annealed Zircaloy-4 has been carried out using electron microscopy, generating a statistically reliable data set across a range of conditions. Slower quenches resulted in an increased average SPP size with a reduced number density. The volume fraction ratio of SPPs found at grain boundaries relative to the grain interior was found to increase with a decreasing quench rate, which is related to the level of solute segregation occurring during the beta-Zr to alpha-Zr phase transformation. The data generated from this work was utilised in the development and calibration of a Kampmann-Wagner Numerical (KWN) precipitation model. The KWN model was segmented to account for the variable solute concentration across a grain during quenching, with the segregation process accounted for through a modified version of the Scheil equation. The model predictions and experimental observations both show that, although the quench rate is critical in determining the initial SPP size distribution, the effect of this diminishes with increased post-quench annealing. A sample of Zircaloy-2 neutron irradiated to 27 dpa was compared with an in-situ heavy ion (Ar2+) irradiation of the same material. The temperature in both experiments was 320 °C, with two doses of 13 and 24 dpa achieved through Ar2+ irradiation. It is thought that complete amorphization of the Zr(Fe,Cr)2 phase occurred under all irradiation conditions. Dissolution of this phase under neutron irradiation was evidenced through irregularities at the precipitate/matrix interface and an increased Cr concentration in the surrounding matrix, co-segregating with Fe along the basal trace. Contrastingly, under Ar2+ irradiation, Fe released during the amorphization process was observed to preferentially diffuse in the < 0001 > direction, whilst no change in precipitate morphology was seen at either dose. Following this, the KWN precipitation model was further developed to incorporate an existing amorphization model and a newly derived empirical model of dissolution. This has been calibrated for Zr(Fe,Cr)2 evolution in Zircaloy-4 under neutron irradiation, producing quantitatively meaningful results.

Date of Award	31 Dec 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Joseph Robson (Supervisor) & Christopher Race (Supervisor)