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Research Papers

Mitigating the Stress Corrosion Cracking of Zircaloy-4 Fuel Sheathing: Siloxane Coatings Revisited

[+] Author and Article Information
Graham A. Ferrier

Department of Chemistry and Chemical Engineering,
Royal Military College of Canada,
Kingston, ON K7K 7B4, Canada
e-mail: Graham.Ferrier@rmc.ca

Mohsen Farahani

Department of Chemistry and Chemical Engineering,
Royal Military College of Canada,
Kingston, ON K7K 7B4, Canada
e-mail: farahani-m@rmc.ca

Joseph Metzler

Department of Chemistry and Chemical Engineering,
Royal Military College of Canada,
Kingston, ON K7K 7B4, Canada
e-mail: Joseph.Metzler@rmc.ca

Paul K. Chan

Department of Chemistry and Chemical Engineering,
Royal Military College of Canada,
Kingston, ON K7K 7B4, Canada
e-mail: Paul.Chan@rmc.ca

Emily C. Corcoran

Department of Chemistry and Chemical Engineering,
Royal Military College of Canada,
Kingston, ON K7K 7B4, Canada
e-mail: Emily.Corcoran@rmc.ca

Manuscript received May 4, 2015; final manuscript received September 16, 2015; published online February 29, 2016. Assoc. Editor: Jovica R. Riznic.

ASME J of Nuclear Rad Sci 2(2), 021004 (Feb 29, 2016) (9 pages) Paper No: NERS-15-1068; doi: 10.1115/1.4031620 History: Received May 04, 2015; Accepted September 17, 2015

For more than 50 years, a thin (3–20 μm) graphite coating has played an important role in limiting the stress corrosion cracking (SCC) of Zircaloy-4 fuel sheathing in CANDU® nuclear reactors. Siloxane coatings, which were examined alongside graphite coatings in the early 1970s, demonstrated even better tolerance against power-ramp-induced SCC and exhibited better wear resistance than graphite coatings. Although siloxane technology developed significantly in the 1980s/1990s, siloxane coatings remain unused in CANDU reactors, because graphite is relatively inexpensive and performs well in-service. However, advanced CANDU designs will accommodate average burnups, exceeding the threshold tolerable by the graphite coating (450  MWh/kgHE). In addition, siloxane coatings may find applicability in pressurized and boiling water reactors, wherein the burnups are inherently larger than those in CANDU reactors. Consequently, a commercially available siloxane coating is evaluated by its present-day chemistry, wear resistance, and performance in hot, stressful, and corrosive environments. After subjecting slotted Zircaloy-4 rings to iodine concentrations exceeding the estimated in-reactor concentration (1  mg/cm3), mechanical deflection tests and scanning electron microscopy (SEM) show that the siloxane coating outperforms the graphite coating in preserving the mechanical integrity of the rings. Furthermore, the baked siloxane coating survived a 50-day exposure to thermal neutron flux ((2.5±0.1)×1011  n/cm2s) in the SLOWPOKE-2 nuclear reactor at the Royal Military College of Canada.

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Fig. 1

As shown via optical microscopy, scratch marks reveal that low mass-loads (50–200 g) penetrate wider and deeper into the graphite-CANLUB coating than into the Pyromark 1200 coating. The mass loads required to breach each coating and expose the underlying Zircaloy-4 surface are 500 g and 1500 g for graphite CANLUB and Pyromark 1200, respectively. Scale bars correspond to 100 μm.

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Fig. 2

Zircaloy-4 rectangular wedges increase the openings of Zircaloy-4 slotted rings from 2.3 to 9.0 mm, thereby exerting a maximum elastic stress of 677 MPa to the inner surfaces of the rings.

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Fig. 3

Sealed iodine vial containing a fragile gooseneck is immersed within the sealed Pyrex ampoule containing stressed, slotted rings. When ready to begin the heating regimen, the fragile gooseneck is broken by gentle agitation.

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Fig. 4

Class-1 lever system measures the deflections of slotted rings caused by mass-load forces.

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Fig. 5

Scanning electron microscope images from (a) the control Zircaloy-4 surface, and (b)–(d) the uncoated, CANLUB-coated, and Pyromark-coated surfaces affected by the thermomechanical stress regimen.

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Fig. 6

Hydrogen ion intensity (cps) versus depth is determined using SIMS. Pyromark-1200 data are taken from various samples and locations (1-1 refers to location 1, sample 1).

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Fig. 7

Scanning electron micrographs illustrate that the Pyromark coating continues to survive in neutron and gamma flux after 25 and 50 days.

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