The water-cooled environment in LWR is a complex corrosive medium. The materials designed for the LOCA -type accidental conditions in LWR must withstand both the normal operating conditions for the whole life-time of the nuclear fuel spanning several years, and in addition the harsh accidental conditions for as long as possible. It is thought that even some hours of extra time to set up emergency cooling for the nuclear fuel could prevent the catastrophic consequences of a full-scale nuclear fuel core meltdown.
First, we tested a larger set of sintered oxide bulk materials to screen the best candidates to be produced as pulsed laser deposited thin films. The screening was done under normal operating conditions in water at 360 °C and 18.6 MPa and characterized after 3, 10 and 24 days of exposure. The overall best performing material, Chromium oxide (Cr2O3), was selected to be studied further in the project. The PLD process for amorphous Cr2O3 was then optimized to produce dense and flaw free microstructure (Figure 1).
We measured an average corrosion rate of + 1.21 ± 0.82 µm/year for the optimized PLD Cr2O3 coatings on steel. In practice, the coating gained mass during corrosion in 360 °C water up to 24 days, most likely due to healing of oxygen vacancies in the deposited non-stoichiometric material. The substrate material was fully covered with the chromia PLD coating, and therefore the mass gain is connected purely to the chromia coating. This signals that the material could be used a protective layer for the primary cladding under nominal operating conditions. In the second phase, we performed accidental condition tests for the PLD Cr2O3 on steel in steam (water)/Argon mixture at 1200 °C for 60 minutes and we found that the material corroded severely exposing the substrate material for further corrosion. The results regarding the Cr2O3 coatings are disseminated at the 2020 TMS Annual Meeting & Exhibition 26.2.2020 with the title “Ceramic Oxide Coatings for Water Reactors: Corrosion Protection in High Temperature Pressurized Water”.
In PLASTICERA, the feasibility of the oxide coatings as new type of ATF coating was tested successfully. The nominal operating conditions can be sustained by the coating, however, the selected material, amorphous chromium oxide (Cr2O3), underperformed in the accidental condition tests. Therefore, it is not certain whether the possible plasticity of these coatings show any practical use under LOCA conditions as the dissolution of the coating is very fast.
