Contact Author:
Dr John Paul Foster
fosterjp@westinghouse.com
P:803-647-3632
F:
Westinghouse Electric Company
5801 Bluff Road
PO Drawer R
Columbia, SC 29250
USA

In-Reactor Creep Behavior of Zircaloy-2

J.P. Foster (Westinghouse-USA), M.A. McGrath (Halden Reactor Project-Norway)

In-Reactor Creep Behaviour of Zircaloy-2 John Paul Foster Westinghouse Electric Company, 5801 Bluff Road, PO Drawer R, Columbia, SC, USA fosterjp@westinghouse.com Margaret Ann McGrath OECD Halden Reactor Project, P.O. Box 173, N-1751 Halden, Norway maggs@hrp.no ABSTRACT A BWR pre-irradiated Zircaloy-2 tube was creep tested in the Halden Reactor in a dedicated creep rig connected to a light water supply and surrounded by high enrichment fuel rods. The Zr-2 tube sample was connected to a variable pressure gas supply allowing the magnitude and direction of the applied hoop stress to be changed during the test. A three-point contact diameter gauge, utilizing an LVDT, was used for making in-pile outer diameter (OD) measurements on the sample, which was tested for 14,410 hours. A total of eight hoop stress changes were applied in series to the sample during different irradiation periods. The data and two different evaluations of the data were previously reported(1,2). The purpose of this paper is to update the data and the evaluation of the data taking into account new oxide thickness measurements and observations of the morphology of the oxide on the sample and on the diameter gauge calibration steps (machined into the sample end-caps). Since the difference in oxide thickness on the sample and on the end-caps must be subtracted from the OD measurements in order to calculate the irradiation creep strain, the development of the oxide layers must be calculated as a function of irradiation time (or fluence). Oxide thickness equations were therefore developed for the sample and its end-caps. The sample creep strains were re-calculated, correcting for the time dependent oxide layer thickness increases. The correction for the sample oxide thickness depends on the initial oxide thickness after the BWR pre-irradiation which is not precisely known. Therefore, the creep strain calculations were performed parametrically for the cases of no oxide correction, nominal oxide correction and +35% oxide correction in order to evaluate the sensitivity of the creep strains and strain rates to the sample oxide thickness at the beginning of the Halden irradiation. These calculations will be discussed in detail. The sample exhibited both transient and approximate steady state creep components. The transient component was observed to re-occur after each applied stress change. Note that this is the first experimental measurement of the re-occurrence of the transient component at relatively high fluence levels. The effect was observed for both tensile and compressive hoop stress changes. The magnitude of the saturated transient component of strain was approximately linearly proportional to the preceding hoop stress change regardless of a fast fluence accumula-tion in the sample from 6 to 7.7 x1021 n/cm2 (E>1 MeV) during the Halden irradiation. Re-evaluation of the data now indicates that there is a threshold stress change for this effect in tension. In addition, the data appear to be consistent with there being the same threshold behaviour in compression; however, additional measurements would be necessary to confirm this. In the case of the steady state creep component, the newly oxide-corrected data are assessed with respect to having reached true steady state creep, although the rates are considered to be “approximate” due to various operational influences. However, several results are apparent from the approximate steady state creep rates. The stress dependence of secondary creep rate was approximately linear and the creep rate in tension appeared to be higher than that in compression for a given stress. The results will be compared with other irradiation creep tests performed on fully RXA Zircaloy-4. References 1. M.A. McGrath, “In-Reactor Creep Behaviour of Zircaloy Cladding,” International Topical Meeting on Light Water Reactor Fuel Performance, Park City, Utah, April 10-13, 2000, American Nuclear Society. 2. J. P. Foster, “Critical Review of In-Reactor Zircaloy Creep Test Data from the Halden Reactor Project,” International Topical Meeting on Light Water Reactor Fuel Performance, Park City, Utah, April 10-13, 2000, American Nuclear Society.