High-Temperature Precipitation Kinetics of Metal Oxide Corrosion Products

在核电站中使用金属和合金通常会暴露在腐蚀性、含水、高温和高压的环境中。在CANDU反应堆中，碳钢用于给料器（输送~300摄氏度和10 MPa重水冷却剂的管道），因为它具有良好的耐腐蚀性，因为它形成了保护性的双氧化物磁铁矿层。一次回路中热交换器或蒸汽发生器的温度梯度导致冷却剂铁浓度相对于磁铁矿的温度依赖平衡溶解度过饱和，因此，在沉淀中超过了耐腐蚀所需的薄层。这导致了不利的结果，例如传热减少，以及放射性物质的积累，这些物质在整个初级侧纳入可用的氧化物沉积物。准确了解材料的降解机制和建筑材料的主要腐蚀产物的行为对于最小化辐射场以确保核工人的安全至关重要。这些机制被整合到综合计算模拟包中，允许对腐蚀缓解策略和污垢清除事件进行预测建模。关于磁铁矿动力学的知识是有限的。一般认为沉淀遵循一阶动力学，文献中没有关于沉淀常数的高温数据。估算是根据站点数据得出的，但混合金属氧化物的存在使得仅提取磁铁矿的基本动力学信息具有挑战性。机械预测和台站观测之间缺乏一致性常常导致模式开发者不重视水化学。相反，尽管有证据表明需要更复杂的传质和动力学组合，但经常使用基于传质的方法来进行物质传输。该研究计划提出确定与CANDU环境相关的磁铁矿和其他混合金属氧化物的新动力学数据，并将其整合到现有的UNB CANDU代码中，该代码用于预测管道表面的腐蚀速率、传热和表面放射性。大部分短期工作将在高温（260-310摄氏度）流经回路中进行，在足够的温度范围内进行测量，以获得动力学降水常数的Arrhenius参数，同时也适用于站点条件。这项研究支持基础（阿伦尼乌斯参数和整体沉淀动力学）和应用工业工作（通过预测建模继续支持现有核反应堆机组的运行）。拟议活动所产生的技术发展有可能使UNB CANDU守则成为腐蚀和放射性模拟的重要工具。

The use of metals and alloys in nuclear power plants often involves exposure to corrosive, aqueous, high-temperature, and high-pressure environments. In CANDU reactors, carbon steel is used for the feeders (piping that transports the ~300 degree Celsius and 10 MPa heavy-water coolant) as it exhibits good corrosion resistance due to the formation of protective, double-oxide magnetite layers. The temperature gradient in the heat exchangers or steam generators in the primary circuit results in an oversaturation in the coolant iron concentration relative to the temperature-dependent equilibrium solubility of magnetite and, thus, in precipitation that exceeds the thin layers required for corrosion resistance. This results in unfavourable outcomes such as reduced heat transfer, and the accumulation of radioactive species, which incorporate into available oxide deposits throughout the primary side. An accurate understanding of material degradation mechanisms and the behaviour of the key corrosion products of the materials of construction is essential in minimizing radiation fields to ensure nuclear worker safety. These mechanisms are incorporated into comprehensive computational simulation packages, allowing for predictive modelling of corrosion mitigation strategies and fouling removal events. Knowledge on the kinetics of magnetite is limited. Precipitation is generally assumed to follow first-order kinetics, and no high-temperature data on the precipitation constant are available in the literature. Estimates are derived from station data, but the presence of mixed metal oxides makes it challenging to extract fundamental kinetic information solely for magnetite. The lack of agreement between mechanistic predictions and station observations has often led model developers to de-emphasize water chemistry. Instead, a mass transfer-based approach to material transport is often used despite evidence that a more complex combination of mass transfer and kinetics is necessary. This research program proposes to determine novel kinetic data for magnetite and other mixed metal oxides relevant to CANDU environments and to integrate this into an existing UNB CANDU code, which is used for prediction of corrosion rates, heat transfer, and surface radioactivity on pipe surfaces. The bulk of the short-term work will be performed in a high-temperature (260-310 degree C) flow-through loop, with measurements over a sufficient temperature range to obtain Arrhenius parameters for kinetic precipitation constants, while also remaining applicable to station conditions. This research supports both fundamental (Arrhenius parameters and overall precipitation kinetics) and applied industry work (continuing to support the operation of the existing nuclear reactor fleet through predictive modelling). The technology development derived from the proposed activities has the potential to establish the UNB CANDU Code as a substantial tool in corrosion and radioactivity modelling.