Oxide surface peeling (OSP) of advanced zirconium cladding irradiated in PWRs

Oxide surface peeling (OSP) is a relatively new term that has been recognized and proposed by Westinghouse in 2013 to describe the phenomenon of “de-lamination of surface layers of the thin oxide, specifically for high corrosion resistance cladding with thin overall oxide”. Since the first in-depth analysis in the 17th ASTM Symposium on Zirconium in the Nuclear Industry at Hyderabad India, recognizing it as cosmetic effect, OSP has continued to be closely monitored and extensively studied. Although first proposed based on observations in highly corrosion resistant Westinghouse cladding alloys, this phenomenon has been observed in corrosion resistant zirconium cladding alloys from other sources well before the OSP term was described in 2013. The recent observations and evaluations have all confirmed the key characteristics of OSP and the negligible impact to fuel rod performance. There is neither hydride localization nor ductility reduction associated with OSP indications, as opposed to other oxide anomalies such as oxide spalling observed on Zircaloy-4 where a major fraction of the thick oxide delaminates. In the case of OSP, the overall oxide thickness is low, meaning that even if almost the entire thickness of the oxide layer is locally peeled off, the resulting thermal gradient in the underlying cladding remains insignificant. The limited size of cold spots as well as relatively low hydrogen content in the metal are also contributing factors for OSP's benign characteristics. In addition to hotcell examinations focusing on OSP impact on fuel rod performance, potential dose effects of activated niobium released in oxide particles from OSP to reactor coolant systems have been evaluated to be minimal. The estimate is based on conservative calculations of the weight loss of zirconium oxide via OSP, in combination with an inventory and release calculation of Nb-94 for various cladding materials. This report will cover the latest observations and evaluations conducted on OSP. Further, an insight on the possible mechanism and dominating factors will be proposed.