A Simple Kinetic Model of Zircaloy Zr(Fe,Cr){sub 2} Precipitate Amorphization During Neutron Irradiation
At neutron flux levels typical for Zircaloy fuel cladding in commercial power reactors, there is insufficient thermal energy below about 600 K to maintain long-range order in hexagonal close packed (hcp) Zr(Fe,Cr){sub 2} precipitates, and these Laves-phase intermetallics gradually become amorphous. The transformation is homogeneous with no change in composition at low temperatures, but above 500 K an amorphous zone containing only 10 at% Fe grows inward from the periphery as Fe moves outward to the adjacent alloy matrix. The shrinking central cores of Zr(Fe,Cr){sub 2} precipitates in Zircaloy-4 remain crystalline, while in Zircaloy-2 these precipitates quickly undergo partial transformation and the low-Fe amorphous front advances into a random mixture of amorphous and crystalline regions, each with the original composition. Above 600 K, the Zr(Fe,Cr){sub 2} precipitates tend to retain both their hcp structure and original chemical composition. These observations suggest that a dynamic competition between kinetic excitation to an amorphous state and thermal recrystallization makes some fraction of the Fe atoms available for flux-assisted diffusion to the alloy matrix by displacing them from hcp lattice positions into metastable, probably interstitial, sites. With one set of kinetic constants, a simple analytic representation of these processes accurately predicts precipitate amorphization as a function of neutron flux, temperature, and time for either Zircaloy-2 or -4. By implication, over the composition range of interest, hcp Zr(Fe,Cr){sub 2} is most stable thermodynamically with about 33 at% Fe, typical of Zircaloy-2, but amorphous Zr(Fe,Cr){sub 2} has the smallest activation energy for recrystallization with the slightly higher Fe content typical of Zircaloy-4.
- Research Organization:
- Knolls Atomic Power Lab., Niskayuna, NY (US)
- Sponsoring Organization:
- US Department of Energy (US)
- DOE Contract Number:
- AC12-76SN00052
- OSTI ID:
- 754944
- Report Number(s):
- KAPL-P-000319; K99115; TRN: US0003549
- Resource Relation:
- Conference: 9th Environmental Degradation Conference, Newport Beach, CA (US), 08/01/1999--08/05/1999; Other Information: PBD: 1 Jul 1999
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
22 GENERAL STUDIES OF NUCLEAR REACTORS
PHYSICAL RADIATION EFFECTS
FUEL CANS
ZIRCALOY 2
ZIRCALOY 4
MATHEMATICAL MODELS
MICROSTRUCTURE
NUCLEAR POWER PLANTS
CRYSTAL-PHASE TRANSFORMATIONS
RECRYSTALLIZATION
AMORPHOUS STATE
LAVES PHASES
ZIRCALOY
LAVES PHASE
NEUTRON FLUX
AMORPHOUS ZONE
IRON DEPLETION