George Paul Sabol

Development of a cladding alloy for high burnup

An advanced Zircaloy cladding containing niobium has been developed and tested extensively both in long-term out-of-pile autoclave exposures and through high burnup irradiation in a pressurized water reactor (PWR) environment. Tubing of zirconium-based binary alloys containing 0.5, 1.0, and 2.5% niobium, and a quaternary composition containing tin, niobium, and iron was fabricated in such a manner that the second phase was fully precipitated, but with minimal particle growth. Autoclave testing in pure water and steam over the temperature range of 589 to 727 K indicates that all of the alloys except the 0.5% Nb have a lower post-transition corrosion rate than does Zircaloy-4, with the relative benefit increasing with temperature. Additional autoclave testing in LiOH solutions indicated a marked sensitivity of the Nb binaries to accelerated corrosion, and in these solutions only the Sn-Nb-Fe alloy was superior to Zircaloy-4. Experimental fuel assemblies with cladding of the advanced alloys were examined after one, three, and four cycles in the BR-3 reactor. Rod average burnups of up to 71 GWD/MTU were obtained with total residence times of up to 66 months. Results of post-irradiation examinations are given only for the Sn-Nb-Fe alloy as compared to Zircaloy-4. These examinations revealed that the Sn-Nb-Fe alloy showed the lowest overall corrosion, up to 50% better than Zircaloy-4 at the highest burnups, and was superior to the Zr-Nb binaries. The Sn-Nb-Fe alloy, called ZIRLO, also displayed lower irradiation growth and creep than the others.

A new strain gage method for measuring the contractile strain ratio of Zircaloy tubing

Abstract An improved strain gage method for determining the contractile strain ratio (CSR) of Zircaloy tubing was developed. The new method consists of a number of load-unload cyclings at approximately 0.2% plastic strain interval. With this method the CSR of Zircaloy-4 tubing could be determined accurately because it was possible to separate the plastic strains from the elastic strain involvement. The CSR values determined by use of the new method were in good agreement with those calculated from conventional post-test manual measurements. The CSR of the tubing was found to decrease with the amount of deformation during testing because of uneven plastic flow in the gage section. A new technique of inscribing gage marks by use of a YAG laser is discussed.

Microstructure and Growth Mechanism of Oxide Layers Formed on Zr Alloys Studied with Micro-Beam Synchrotron Radiation

The structures of oxides formed in water and lithiated water on three Zr-based alloys with varied corrosion behavior were studied with micro-beam synchrotron radiation and optical microscopy. Micro-beam synchrotron radiation (0.2 µm spot) has a unique combination of high elemental sensitivity (ppm level) and fine spatial resolution that allowed the determination of various oxide characteristics such as phase content, texture, grain size, and composition as a function of distance from the oxide-metal interface. Micro-beam X-ray fluorescence shows that the oxides formed in lithiated water have increased levels of Fe absorbed from the autoclave environment indicating greater oxide porosity in these oxides. The phase content, texture, and grain size of oxides were studied in detail using synchrotron radiation micro-beam diffraction for samples corroded in water and lithiated water. A remarkable periodicity was observed in the oxide structures using various techniques including X-ray peak intensities for both monoclinic and tetragonal zirconia, texture, and optical microscopy. The periods were similar to the transition period and were less visible in the oxides that behaved worse in lithiated water. These results are discussed in terms of models of oxide growth and of the differences between alloys.

ZIRLO™ — An Alloy Development Success

Significant data and milestones in the development of the ZIRLO alloy for PWR applications are reviewed. From humble beginnings as an R&D program, an approach to improving the corrosion resistance of zirconium-based alloys was developed. The corrosion resistance of dilute alloys is controlled by the microstructure, and both solid solution and precipitate effects impact corrosion performance. Several transition metals are beneficial for corrosion at low concentrations, but the most favorable alloy system is zirconium-niobium, because it offers flexibility in processing options. Strength of the binary alloys can be improved by additions of tin, which, in combination with low levels of iron, provide corrosion resistance in lithium hydroxide solutions. Numerous experimental ingots, thermo-mechanical processing routes, corrosion tests, and materials characterizations were performed along the path to the final alloy composition. The resulting material, ZIRLO, has proven itself as a successful alloy for PWR fuel cladding and structural applications.

Load relaxation studies of zircaloy-4

Abstract Tensile and load relaxation tests as a function of temperature were performed on zircaloy-4 tubing with applied stress in the direction of the tube axis. The phenomenological model, based on Harts plastic equation of state, was capable of describing a portion of the load relaxation data. The effects of strain aging and deformation twinning are discussed, based on the same data.

Precipitation behavior in Zr/2.5 wt% Nb alloys

Abstract The hardness response and structure of high-purity and commercial purity Zr/2.5 wt% Nb alloys have been studied as a function of aging treatment. In both alloys, the age hardening increment is small but finite, and the aging curves are characterized by two hardness peaks separated by a hardness trough. Correlation of the hardening response with microstructure reveals that the samples aged to the first hardness peak are characterized by a fine spherical precipitate of about 100 A diameter. At aging times corresponding to the second hardness peak, this fine precipitate is replaced by a needle-like precipitate. This sequence of precipitation was valid for both the high-purity and commercial alloys, however, precipitation kinetics were more sluggish in the commercial alloy.