PNNL-13501 - Pacific Northwest National Laboratory

erbium coatings were applied to zircaloy-4 sheet. The
erbium coatings were applied in the pure state and also
with Zr overcoats, and a variety of thicknesses and
coating parameters were investigated.
Coated coupons (both Er-only and Zr-overcoated Er)
were paired with other coated and bare zircaloy-4
coupons, and seal-welded to produce feedstock for hot
roll bonding. Prior to welding, the surfaces to be bonded
were sanded with SiC paper and swabbed with B-etch for
several minutes to remove any oxide film. After
preparation, the coupons were rapidly transported to the
weld chamber to prevent oxide formation. Coupons were
hot roll bonded at 800°C at overall thickness reductions of
10% to 20%. The best results were obtained with either
erbium or Zr-overcoated erbium coatings bonded with
bare zircaloy-4 coupons.
Coupons seal-welded in a He atmosphere were prevented
from completely bonding during hot rolling by the He.
Three alternate coupon welding techniques were
investigated. These included: 1) electron beam welding
in vacuum, 2) TIG welding in He with a brazed Cu tube
serving as a vacuum port, and 3) TIG welding in He with
a brazed Cu tube serving as a gas bladder to accommodate
the He during hot rolling. The TIG welding techniques
were unsuccessful because the braze failed during heating
to 800°C prior to hot rolling. Although electron beam
welding is not suitable for large, manfactured
components, it was used for the remainder of the
development work due to resource limitations. The TIG
welding approach would likely work if Zr or zircaloy
tubes were used for the vacuum ports or gas bladders.
Initial efforts to hot roll bond the coupons fabricated by
electron beam welding were unsuccessful. The time
between surface preparation (sanding and B-etch) and
insertion into the vacuum chamber appeared to be
unacceptably long, allowing an oxide layer to form. Later
efforts were successful when the samples were stored in a
dessicator and evacuated to 10 -6 torr within 10 to 15
minutes after sample preparation. These samples
displayed complete bonding across the width of the
coupons, with no evidence of failures.
The final objective of the development effort involved a
demonstration of the formability of the laminates in a
prototypical width (approximately 20 cm). The most
common method for producing boiling water reactor fuel
channels involves forming the zircaloy-4 sheet into a Uchannel,
and butt-welding two U-channels to form a
continuous square duct. Significant effort was expended
to develop a suitable thermomechanical processing
schedule to produce laminated U-channels of prototypic
364 FY 2000 Laboratory Directed Research and Development Annual Report
width. Difficulties associated with embrittlement of the
zircaloy-4 by oxidation during hot rolling and annealing
in air occured. To alleviate these problems, the surface
oxide picked up during hot rolling was removed prior to
further processing, and annealing was done in vacuum or
inert gas.
After e-beam welding, the samples were milled on both
faces to produce a prototypic thickness of 0.25 cm. They
were then hot-rolled. The hot rolling was done in air.
The oxide film was mechanically removed prior to
annealing. The samples were then annealed and formed
into U-channels using a mechanical press.
Experimental work on this study is in its final stages. A
prototypic-dimension fuel channel section will be
produced from two U-channels produced using the
process described above. Some of the U-channels will be
hot-rolled and/or annealed at 750°C to evaluate the effect
of a lower working temperature. Ideally, a final
thermomechanical processing schedule will be established
as the starting point for manufacturing scale-up activities
that would be conducted as part of a follow-on program.
Summary and Conclusions
To date, the study has demonstrated the feasibility of
producing Zry/Er/Zry laminate structures consistently and
with reasonably reproducible properties. Laminates and
U-channels have been produced in prototypic thicknesses
and widths for boiling water reactor fuel channel
applications. Significant development work remains to
transform the laboratory-scale process to one more
suitable for high-throughput production. However, most
of the methods used for commercial zircaloy production
appear to be applicable to fabricating square ducts
fabricated from Zry/Er/Zry laminates.
Bibliography
American Society for Testing and Materials. 1979.
Standard specification for zirconium and zirconium alloy
sheet, strip, and plate for nuclear application, ASTM B
352-79. American Society for Testing and Materials,
West Conshohocken, Pennsylvania.
Savitskii EM, VF Terekhova, IV Burov, IA Markova, and
OP Maumkin. 1962. “Rare earth alloys.” USSR
Academy of Sciences, p. 58. Moscow.
Love B and C Kirkpatrick. 1961. “Mechanical properties
of rare earth metals” in Rare Earth Research, Ed. E.V.
Kleber, The Macmillan Company, New York.