Abstract
This PhD project has aimed to support current operation and future development of pressurised light water reactor (PWR) designs in the UK. A central aspect to a PWR’s design is the steam generator, which transports heat from the reactor core to the secondary coolant circuit and generator. With the steam generator making up a significant fraction of the PWR primary water circuit, the improvement of its constituent materials are crucial to long-term, safe service. In western PWRs, the steam generator is typically comprised of a large network of tubes made primarily of the Ni-based alloy, Alloy 690. During start-up and continued operation, the hot water coolant creates oxide on the alloy surface, and the behaviour of this oxide has a strong bearing on the steam generator lifetime. Localised stress corrosion cracking (SCC) events have been the historical cause of repair and replacement within steam generators. The spalling of oxide can create activated corrosion product that increases dose to reactor workers. The quality of the passivating oxide is a significant factor to mitigate both of these issues. Two factors which affect this passivation are the surface condition and the water chemistry. Previous research has suggested that the injection of 5-40 ppb range of Zn can provide a more protective passivatingoxide that generates less spalled corrosion product. The surface condition is largely effected by the extrusion process during tube manufacture. Two such processes are cold-drawing and pilgering. The results of this project are focused upon the quality of oxide passivation resulting from the application of Zn water chemistry (ZWC) and surface condition between a cold-drawn and pilgered
tube commonly seen in a PWR design. In this project, Alloy 690 tubes in both the cold drawn and pilgered condition were tested in high temperature and high pressure water autoclaves both with and without Zn injection. The microstructure of the tubes were characterised before and after oxidation using a combination of advanced electron microscopy techniques. The results show that the population and nature of crevices in the pilgered condition shows a significant improvement providing fewer localised sites for grain boundary oxidation through the matrix which is the precursor to an SCC event. Pilgering, whilst a more aggressive extrusion method, can provide a surface with fewer crevices for corrosion product nucleation or redeposition, thus also providing
fewer corrosion products available for release within the primary circuit and providing a uniform, secure passivation less prone to localised corrosion events such as SCC. After exposure to pure, high temperature, high pressure water for durations ranging from 100-1000 hours, the pilgered specimen exhibits a thinner internal, matrix-contacting Cr2O3 layer, with a visible migration of finely dispersed TiN precipitate particles at the metal-oxide boundary, potentially limiting element transport through the matrix, leading to a thinner Cr rich internal oxide layer. Both pilgered and cold-drawn specimens were also exposed to high temperature, high pressure water with 40ppb of zinc added. The application of ZWC displays the most significant alteration to the oxide with no large, low adherence corrosion products nucleating upon the surface, providing a uniformly distributed, densely packed polyhedral type oxide layer with an internal Cr2O3 layer contacting the matrix. The Zn incorporates within the oxide at all layers uniformly, however does not incorporate into the grain boundary oxides formed within the matrix in either tube specimen. Overall, the thesis results suggest an optimal condition for Alloy 690 tube both after pilgering is
used in manufacture, and when Zn injection is used in the water chemistry in operation.
| Date of Award | 17 Jun 2025 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | Edf Energy Nuclear Generation Limited |
| Supervisor | Nicolas O Larrosa (Supervisor) & Tomas L Martin (Supervisor) |