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Understanding The Effects Of Zn Injection And OLNC-Treatment On 316 Stainless Steel Oxide Under Simulated BWR Conditions

Austenitic stainless-steel alloys are widely used as structural components in light water reactors (LWR) coolant systems, due to their passivity in high temperature water solutions. After initial passivation, subsequent development and dissolution rates of the protective film are very low. Nevertheless, metal cations and colloidal particles that are generated by superficial corrosion of structural materials, can be activated and generate radioactive isotopes that are responsible for radiation source as they circulate through the reactor core. Specifically, the radioactive 60Co, generated by neutron activation of the inactive 59Co (constituent of the naturally occurring Co), releases high-energy γ rays with a half-life of 5.3 years and is the main radiation source in boiling water reactor plants. Mitigating the incorporation of 60Co into stainless-steel oxide depends on understanding the phenomenon of oxide growth and development as a
function of the water chemistry employed, which involves thermodynamic and kinetic considerations.

Product Number: ED22-17227-SG
Author: Dora Capone, Liberato Volpe, Ben F. Spencer, Tsuyoshi Ito, Jonathan Duff, Tsuyoshi Ito, M. Grace Burke, Yoichi Wada, Fabio Scenini, Makoto Nagase
Publication Date: 2022
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This study investigates the mechanism of incorporation of metal cations (Zn and Co) into the 316 stainless steel (SS) oxide under hydrogenated water chemistry (HWC) and On-Line NobleChemTM (OLNC). Coupons of 316 SS were exposed to simulated boiling water reactor (BWR) conditions (pure water, H:O molar ratio ∼8) at 288 °C and then analysed via surface characterisation techniques. Specifically, coupons were initially exposed under HWC conditions for 500 h, then subjected for 200 h to the OLNC treatment, and then a final exposure under HWC conditions for 500 h, all with simultaneous monitoring of the electrochemical corrosion potential. These exposures were performed both with and without Zn and or Co injection. It was found that Zn decreased the oxide thickness of the inner oxide layer and decreased the size of the crystallites on the outer layer. Addition of 0.2 ppb of Co in the water chemistry containing 10 ppb Zn did not appear to influence the oxide morphology nor its composition. Hard X-Ray XPS analysis showed that Zn did not completely suppress Co incorporation in the outer oxide layer which was still found in concentrations up to 0.6 at. % on the surface of the oxide under Zn addition conditions.

This study investigates the mechanism of incorporation of metal cations (Zn and Co) into the 316 stainless steel (SS) oxide under hydrogenated water chemistry (HWC) and On-Line NobleChemTM (OLNC). Coupons of 316 SS were exposed to simulated boiling water reactor (BWR) conditions (pure water, H:O molar ratio ∼8) at 288 °C and then analysed via surface characterisation techniques. Specifically, coupons were initially exposed under HWC conditions for 500 h, then subjected for 200 h to the OLNC treatment, and then a final exposure under HWC conditions for 500 h, all with simultaneous monitoring of the electrochemical corrosion potential. These exposures were performed both with and without Zn and or Co injection. It was found that Zn decreased the oxide thickness of the inner oxide layer and decreased the size of the crystallites on the outer layer. Addition of 0.2 ppb of Co in the water chemistry containing 10 ppb Zn did not appear to influence the oxide morphology nor its composition. Hard X-Ray XPS analysis showed that Zn did not completely suppress Co incorporation in the outer oxide layer which was still found in concentrations up to 0.6 at. % on the surface of the oxide under Zn addition conditions.