PNNL-13501 - Pacific Northwest National Laboratory

Study Control Number: PN98009/1255
BWR Burnable Absorber Coating
Mikal A. McKinnon, Edward F. Love, David J. Senor
This technology may serve to enhance operations in commercial nuclear facilities and make them more competitive with
fossil fuel plants.
Project Description
Previous projects have completed neutronic analysis of a
boiling water reactor core containing fuel assemblies with
burnable poison in the channels. The results showed
enhanced neutronic performance of the core that resulted
in a lower pin peaking factor and a potential $500K cost
savings per fuel reload (12 to 18 months). In fiscal year
1998, various methods of fabrication were studied
including alloying, coating, and laminating. In fiscal year
1999, the development of the most promising fabrication
technique was pursued using coupon-sized samples. For
fiscal year 2000, several coupons were made and hot
rolled. In addition to the hot rolling of the specimens,
bending of the specimens was demonstrated.
Metallographic analysis was completed to assess the
performance of the coupons. Then the process used in
developing the most promising coupon was used to
demonstrate fabricability of channels at various scales.
Introduction
Three techniques for incorporating burnable absorber
material into a boiling water reactor fuel channel were
explored. One technique involved coating a zircaloy-4
substrate with erbium to form an intermetallic surface
layer, another focused on producing laminates using
erbium foil sandwiched between zircaloy-4 sheets, and a
third explored zirconium-erbium alloys. We discovered
that the foil method for producing laminates was not
particularly promising, and a hybrid technique was
developed in which erbium-coated zircaloy-4 coupons
were used successfully to produce laminates.
Results and Accomplishments
Coating Development
Electrospark deposition was the coating technique
selected as most promising. The technique can be easily
adapted to provide coatings of different compositions or
thickness at different locations on the substrate material.
Two significant modifications to traditional electrospark
deposition coating were developed. The first was
adapting the technique to use square cross-section
electrodes. The second modification was adapting the
technique to produce coatings under an inert cover gas
rather than in air. Erbium coatings produced in air
appeared to oxidize, which led to embrittlement and
cracking upon cooling. With an inert cover gas, erbium
coatings up to 75 µm thick were successfully produced in
a single pass. Metallography of these coatings revealed
excellent coverage with no cracks, good uniformity in
thickness and a relatively smooth surface. Thicker
coatings were produced by producing erbium layers in
successive passes. Coatings up to 150 µm thick were
successfully fabricated.
Laminate Development
The attractive feature of producing a zircaloy-erbiumzircaloy
(Zry/Er/Zry) laminate is that the burnable
absorber is not exposed to the primary loop coolant. By
placing the erbium within the zircaloy structure of the fuel
channel, issues associated with exposure to boiling water
reactor primary coolant are avoided. For economic
reasons, it is desirable to minimize changes to present fuel
channel manufacturing processes. If erbium could be
incorporated into zircaloy feedstock early in the
manufacturing process, fabrication of fuel channels
potentially could proceed with minor additional
considerations due to the presence of erbium (welds and
heat affected zones, nondestructive evaluation for erbium
assay). Therefore, a series of experiments was conducted
to evaluate the potential for roll bonding and subsequent
cold- or hot-rolling of bonded laminates.
A hybrid technique was developed in which electrospark
deposition erbium coatings were sandwiched with
zircaloy-4 sheet to produce laminates. A variety of
Nuclear Science and Engineering 363