鍍著氮化鉻之鋯四合金護套材料在高溫水氣環境中的氧化行為

The Oxidation Behavior of CrN-coated Zircaloy-4 Cladding Material under High-temperature Steam Environment

鋯合金因為在水化學環境下擁有好的抗腐蝕能力，較低的熱中子捕獲截面積還有良好的機械性質，被廣泛地使用在核能電廠的護套材料上。但是當意外發生時，特別是在冷卻水喪失的意外中，爐心的溫度會不斷上升，此時鋯合金燃料護套將會喪失其抗氧化能力並且與環境中的水蒸氣劇烈反應生成二氧化鋯並且放出氫氣，除了護套本身毀壞會導致核燃料外洩，釋放出的氫氣也可能產生氫爆，因此改善鋯合金護套材料在高溫下的抗氧化能力是相當重要的議題。在本研究中，我們透過非平衡磁控濺鍍系統在鋯四合金上鍍著氮化鉻薄膜，並研究鍍膜的鋯四合金在400℃的高溫水氣環境中2，4及6天的抗氧化能力。實驗結果，在400℃的高溫蒸汽環境中，通過在鋯四合金上進行CrN鍍膜，鋯四合金的氧化增重可減少近90%。此外，具有CrN鍍膜的樣品在實驗後仍保持其完整的薄膜結構，而裸材樣品在表面形成了幾微米的二氧化鋯層，二氧化鋯層中也發現了幾處裂縫。實驗結果證明氮化鉻塗層可以提高鋯四合金的高溫抗氧化性。

Zirconium alloys are widely used as sheath materials in nuclear power plants because of their good corrosion resistance in water chemistry environment, low thermal capture cross-section of neutron, and good mechanical properties. However, when an accident occurs, especially in the event of loss of cooling water, the temperature of the reactor core will continue to rise. Zr-based alloys will lose their corrosion resistance and react violently with the steam to produce zirconia and hydrogen. In addition to the damage of the cladding itself, which will lead to the leakage of the fission product, the released hydrogen can also cause hydrogen explosion. Therefore, it is a very important issue to improve the corrosion resistance of zirconium alloy cladding materials at high temperatures. In this study, we used an unbalanced magnetron sputtering system to deposit a chromium nitride film on Zircaloy-4 substrate, and studied the oxidation resistance of the coated Zircaloy-4 in a high-temperature steam environment of 400 ℃ for 2, 4, and 6 days. The results show that the weight gain of the Zircaloy-4 can be reduced by nearly 90% using CrN coatings in a high-temperature steam environment at 400 ℃. Moreover, the CrN-coated samples still maintain similar microstructure after oxidation tests, while the uncoated samples form a zirconium dioxide layer on the surface with several microns in thickness, and several cracks are also found in the zirconium dioxide layer. This study suggests that the chromium nitride coating can improve the high temperature oxidation resistance of Zircaloy-4.