Demands for improved sustainability, efficiency and safety require innovations in the field of nuclear technology and thus push forward the Generation IV (Gen IV) reactor systems. In general, Gen IV reactors use fuel more efficiently than current Light Water Reactors (LWR) due to a closed fuel cycle and/or higher fuel burn-up. Most systems also have enhanced safety features compared to present day reactors. One of the Gen IV type reactor concepts is called Supercritical Water Reactor (SCWR) that uses water as a coolant medium. For components in SCWR designs, general corrosion is one of the most severe degradation modes. Numerous papers in the literature have considered the general corrosion of SCWR candidate materials. Unfortunately, tests have been conducted under various conditions. This work through Publications I - VI aims at identifying and decreasing knowledge gaps in the open literature especially at higher temperatures up to 700°C with different material compositions by using different characterization methods. The most important variables affecting corrosion behaviour of the candidate materials in Supercritical Water (SCW) are alloy class, temperature and treatment of the sample surface. Based on this work, it can be said that in the case of fuel cladding, zirconium alloys and steels with bulk Cr content < 16 - 20 wt-% have generally not sufficient corrosion resistance except possibly with special surface treatment. This observation rules out most of the available nuclear-grade materials in typical metallographic conditions. In terms of oxidation resistance, high-performance alloys like high-chromium steels (> 20 wt-% Cr), Ni-based alloys and Oxide Dispersion Strengthened (ODS) steels are applicable. However, there are some limitations regarding the adverse effect of high Cr and Ni contents in the bulk alloy or the material's joining possibilities. Thus, a study on cold worked Type 316L was performed by exposing specimens up to 3 000 h in SCW. Very promising general corrosion results were achieved at least in laboratory conditions. However, the effect of Stress Corrosion Cracking (SCC) susceptibility of cold-worked material and longer term behaviour (about 1 year or time period of equivalent to the fuel cycle) still need to be studied. In order to estimate the long-term corrosion penetration rate, the Mixed Conduction Model (MCM) was developed and applied to predict the behaviour of selected materials in SCW. The most promising results were found on the 20 wt-% Cr ODS steel. Based on the MCM calculations, the 20 wt-% Cr ODS steel showed slower corrosion penetration rate (mm/a) than the other alloys studied. By contrast, 316NG showed 2 times higher corrosion penetration rate compared to 20 wt-% Cr ODS steel. Modelling results gave new insight into the possible oxidation behaviour of structural materials in SCW and how it can be interpreted through modelling.
|Translated title of the contribution||Rakennemateriaalien korroosiotestaus ja mallintaminen ylikriittisessä vedessä|
|Publication status||Published - 2020|
|MoE publication type||G5 Doctoral dissertation (article)|
- supercritical water