Development of a gaseous iodine testing system to determine the I-SCC properties of zirconium alloys

摘要

Zirconium alloy fuel cladding is susceptible to iodinestress corrosion cracking (I-SCC) degradation. Atpresent, reactor operating parameters are restricted toensure that I-SCC is unlikely. For example, powerramping restrictions limit reactor load followingcapabilities and have an economic penalty. An increasedunderstanding of critical parameters controlling I-SCCinitiation and growth could improve reactor performance.A gaseous iodine testing system has been designed andcommissioned. The system is capable of dynamic loadingand has independent control over several key variables,including iodine partial pressure and specimentemperature. Both material property (compact tensionspecimens) and component (cladding C-ring specimens)testing can be performed. Direct current potential drop(DCPD) is used to measure crack growth in-situ andallow application of constant K loading to Zircaloy-4compact tension specimens. Specimens in the mostsusceptible orientation for I-SCC were tested and thencharacterized using a variety of techniques, includingSEM of fracture surfaces and TEM of crack-tip FIBliftouts. Valid fracture mechanics testing with constant Kloading in a gaseous iodine environment allowed forrelatively slow cracking rates to be detected and forfactors such as KISCC to be accurately determined. Resultsshow that KISCC is lower than 3 Mpa√m in the mostsusceptible cracking plane. Characterization resultsestablish that iodine is present on the fracture surfacesand that basal plane pseudo-cleavage occurs when c-axesare normal to the cracking plane, leading to low KISCCand high cracking rates. These results demonstrate theextensive capabilities of the gaseous iodine testing systemand explain how texture affects I-SCC susceptibility,which can inform fuel cladding design.