BWR環境下鋅、鈷離子對304不銹鋼表面氧化膜之影響研究

Effects of Added Zinc and Cobalt on The Oxide Compositions upon Type 304 SS in the Simulated BWR Conditions

沸水式核能反應器（Boiling water reactor, BWR）在正規水化學（Normal water chemistry, NWC）環境下，反應器爐水為氧化性含溶氧約200-300 ppb。此環境下，過去運轉經驗中曾發生多次的爐內組件沿晶應力腐蝕龜裂（Intergranular stress corrosion cracking, IGSCC）。加氫水化學環境（Hydrogen water chemistry, HWC），此環境下其溶氧＜10 ppb是抑制IGSCC發生最有效的運轉方式，但它會使BWR停機時爐心外管線表面的Co-60輻射劑量率上升，有效之抑低方法是加氫與加鋅同時進行。本研究係於模擬BWR水環境中，探討鋅、鈷離子對304不銹鋼表面生成的氧化膜組成之影響。研究中將以掃瞄電子顯微鏡（Scanning electron microscope, SEM）、X射線能量解析光譜（Energy dispersive spectroscope, EDS）、二次離子質譜（Secondary ion mass spectroscopy, SIMS）及化學分析光電子光譜（(Electron spectroscopy for chemical analysis, ESCA）等固態表面分析儀解析氧化膜之組成變化與重組情形，以及對BWR水環境的影響。研究結果發現Zn2+對氧化膜中金屬離子的取代能力為Fe3+, Co2+ >>Ni2+。另一方面，CO2+取代氧化膜中金屬離子的能力為Ni2+>Zn2+>>Fe。此研究結果對國內BWR電廠未來的加鋅水化學，深具重要價值。

The dissolved oxygen in the reactor water of a boiling water reactor (BWR) operated under normal water chemistry (NWC) condition lies in the range of 200 to 300 ppb in general. Several events of intergranular stress corrosion cracking (IGSCC) on in-reactor components accordingly happened in the past due to this relatively oxidizing environment. However, hydrogen water chemistry (HWC) condition of a BWR could mitigate such failure owing to its relatively reducing environment of the dissolved oxygen decreasing below 10 ppb. There was one main side effect under such HWC operation condition, i.e. shutdown dose rate buildup on the surface of piping out of core due to Co-60 deposit, based on the plant operation experiences. It is known that zinc addition under HWC condition could reduce this shutdown dose rate buildup and recover to the level as that under NWC condition. Thereby, the experimental conditions of test specimen followed the sequence: as received (step1) → NWC (step 2) → HWC (step 3) → HWC/Zn (test 1) or HWC/Co (test 2) → HWC/Zn/Co (test 1) or HWC/Co/Zn (test 2) for the purpose of investigating the mechanism of zinc and cobalt ion competing in penetration into the outer layer of oxide film formed on the surface of Type 304 stainless steel in the simulated BWR conditions. In the paper, SEM was used to observe the morphology and particulate size and EDS was used to analyze the local composition of surface oxide. And, SIMS and ESCA were used for the determination of elemental compositions in the depth profile. We found that the tendency of zinc ion in replace of the metal ions in the spinel oxide formed in the simulated BWR conditions was Fe3+, Co2+ >>Ni2+. On the contrary, the tendency of Co2+ replacement was Ni2+>Zn2+>>Fe3+. And, the capability of Zn2+ of reactor water replacing Co2+ in the spinel oxide was higher than the reverse. The detailed description and explanation of mechanisms appeared in this text.