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

Abstract

For more than 50آ years, a thin (3–20آ خ¼m) graphite coating has played an important role in limiting the stress corrosion cracking (SCC) of Zircaloy4 fuel sheathing in CANDUآ® nuclear reactors. Siloxane coatings, which were examined alongside graphite coatings in the early 1970s, demonstrated even better tolerance against powerrampinduced 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 inservice. 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 presentday chemistry, wear resistance, and performance in hot, stressful, and corrosive environments. After subjecting slotted Zircaloy4 rings to iodine concentrations exceeding the estimated inreactor 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 50day exposure to thermal neutron flux ((2.5آ±0.1)أ—1011  n/cm2 s) in the SLOWPOKE2 nuclear reactor at the Royal Military College of Canada.