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Research Papers

The Effect of Alkali Metal and Alkaline Earth Metal Impurities on the Iodine-Induced Corrosion of CANDU Fuel Sheathing

[+] 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

David Kerr

Department of Mechanical and
Materials Engineering,
Queen's University,
Kingston, ON K7L 3N6, Canada
e-mail: david.kerr@queensu.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

Evan Veryard

Department of Chemistry and
Chemical Engineering,
Royal Military College of Canada,
Kingston, ON K7K 7B4, Canada
e-mail: evan.veryard@mail.mcgill.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

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

Mark R. Daymond

Department of Mechanical and
Materials Engineering,
Queen's University,
Kingston, ON K7L 3N6, Canada
e-mail: mark.daymond@queensu.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 March 31, 2016; final manuscript received June 15, 2017; published online xx xx, xxxx. Assoc. Editor: Brian Ikeda.

ASME J of Nuclear Rad Sci 3(4), 041004 (Jul 31, 2017) (9 pages) Paper No: NERS-16-1030; doi: 10.1115/1.4037115 History: Received March 31, 2016; Revised June 15, 2017

During normal operation in Canada deuterium uranium (CANDU®) reactors, the stress corrosion cracking (SCC) of fuel sheathing is mitigated effectively, in part, using a thin graphite-based coating known as CANDU lubricant (CANLUB). Mechanisms typically proposed for the demonstrated SCC mitigation offered by CANLUB include lubrication and/or chemical interactions. An additional possibility, that was recently suggested, involves the sequestering of iodine through its interaction with alkali metal and/or alkaline earth metal impurities in the CANLUB coating. This possibility is supported by the systematic analysis and testing in this paper, wherein three prevalent impurities (Na, Ca, and Mg) found in CANLUB were incorporated into SCC slotted ring experiments as metal oxides. When the amount of metal oxide (Na2O, CaO, or MgO) matched or exceeded the amount of iodine (6 mmol = 16 mg/cm3), Na2O and CaO protected the rings from corrosion whereas MgO enhanced their corrosion. When Zircaloy-4 sheathing is subjected to mechanical stress, high temperature, and high concentrations of iodine vapor, it is better protected by siloxane coatings than by graphite-CANLUB coatings. Consequently, since metal impurities (Na, Ca, and Mg) are found more abundantly in siloxane coatings than in graphite-CANLUB coatings, Zircaloy-4 slotted rings were coated with graphite-CANLUB containing Na, Ca, and/or Mg at those more abundant concentrations. Since these concentrations remain below 6 mmol, SCC test results suggest that the siloxane's superior adhesion is an essential first step in preventing corrosion induced by 6 mmol of iodine.

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Figures

Grahic Jump Location
Fig. 1

Ring thickness was calculated using two different algorithms, whereby the thickness is determined by summing up (a) parallel and evenly distributed thickness lines or (b) lines representing the distance from one side to the nearest point on the opposite side. Fewer lines are highlighted in (b) for clarity. Inset: Schematic of a slotted ring highlighting the locations of both the apex and far from apex (FFA) regions.

Grahic Jump Location
Fig. 2

Average deflection of slotted rings exposed to known amounts of CaO, MgO, and Na2O. Deflections were induced by a 95 g mass-load. Error bars = ±1 standard deviation. Since the 6 mmol of iodine is consumed fully at the 6 mmol Na2O level, only the first three Na2O data points were fitted. R2 values for the CaO, MgO, and Na2O best-fit lines are 0.86, 0.78, and 0.89, respectively. As expected, the deflection decreases marginally as the Na2O concentration increases above 6 mmol.

Grahic Jump Location
Fig. 3

Photographs of slotted ring surfaces after the rings were stressed, heated, and exposed to iodine and various metal oxides. Compared with rings exposed to iodine only ((a), top), rings exposed to MgO and iodine formed a consistent rustlike layer on their surfaces ((a), bottom). (b) On the other hand, when Na2O was present, the Na2O accumulated preferentially along the inner surface of the ring, providing a potential chemical barrier. Gray or black oxide films were commonly observed on rings exposed to iodine only, as well as on rings exposed to iodine with the presence of Na2O or (c) CaO. Both the rustlike and gray/black oxide layers were readily removed upon cleaning with water and methanol.

Grahic Jump Location
Fig. 4

Cross sections of rings strained with 12 mm wedges. (a) Ring exposed to I2 only. (b) Ring exposed to I2 and a high concentration of MgO. Although extensive pitting is observed in the I2 environment, the pitting plus numerous transverse and radial striations extending into the cross section were observed in rings exposed to MgO + I2.

Grahic Jump Location
Fig. 5

Ring apex thicknesses measured using two measurement techniques (blue = normal lines and red = nearest distance lines). The dashed line represents the thickness of the apex region, which is averaged over two to three control rings.

Grahic Jump Location
Fig. 6

Micrographs of Zircaloy-4 surfaces after exposure to 6 mmol I2 as well as (a) 0 mmol Na2O, (b) 3.5 mmol Na2O, (c) 6.8 mmol Na2O, (d) 11.1 mmol Na2O, and (e) 13.6 mmol Na2O. As Na2O increased, the Zircaloy-4 surfaces better resembled the control ring (f).

Grahic Jump Location
Fig. 7

Micrographs of Zircaloy-4 surfaces after exposure to 6 mmol I2 as well as (a) 0 mmol, (b) 1.50 mmol, (c) 2.99 mmol, (d) 4.51 mmol, and (e) 5.94 mmol of CaO. Inset: Energy dispersive X-ray analysis of the cross section shows that small amounts of calcium (red), possibly calcium oxide, are detected within the surface oxide (blue) forming above the zirconium substrate (green). Since zirconium is not detected in regions of high calcium, it is more likely that calcium exists as an oxide rather than a zirconate.

Grahic Jump Location
Fig. 8

Oxide growth and stress-induced striations in rings exposed to CaO. (a) At low CaO concentrations (1.5 mmol), transverse and radial striations evolve near the tensile surface. (b) At higher CaO concentrations (6.0 mmol), more of the available iodine vapor is consumed by the CaO, and a sufficient availability of oxygen permits oxide growth on the pitted surface.

Grahic Jump Location
Fig. 9

Micrographs of Zircaloy-4 surfaces after exposure to 6 mmol I2 as well as (a) 0 mmol, (b) 1.4 mmol, (c) 3.0 mmol, (d) 4.5 mmol, and (e) 6.0 mmol of MgO

Grahic Jump Location
Fig. 10

Zoomed-in micrographs of Zircaloy-4 surfaces after exposure to 6 mmol I2 as well as (a) 1.4 mmol, (b) 3.0 mmol, (c) 4.5 mmol, and (d) 6.0 mmol of MgO

Grahic Jump Location
Fig. 11

Images of ring cross sections (light regions) that were previously subjected to plastic stress. Transverse striations accumulate near the surfaces under (a) compression and (b) tension. Radial striations evolve inward from transverse striations near the tensile ring surface. Dashed and solid lines highlight the boundaries of surfaces under compression and tension, respectively.

Grahic Jump Location
Fig. 12

Micrographs of Zircaloy-4 surfaces after exposure to 6 mmol I2 as well as (a) 0.8 kPa, (b) 1.6 kPa, and (c) 7.2 kPa of oxygen. (d) Surface view of a heavily cracked oxide layer (left) in a region far from the apex.

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