Innovative high temperature resistant steels (IS)
The increase of efficiency of energy production systems often implies an increase of the operational temperature. Accordingly, Gen IV nuclear reactors will also operate at higher temperature than Gen II and III reactors, producing steam at higher temperatures and thus requiring higher coolant temperature for a higher overall plant thermal efficiency. As an example the sodium outlet temperature in a sodium fast reactor, SFR, ~550 °C, is substantially higher than the outlet water temperature in a LWR, ~300 °C. Materials used for primary components and fuel cladding in current LWR reactor do not have enough strength, among other mechanical and physical properties, at the higher temperatures foreseen for GenIV systems, thus other materials have to be used in such reactors. Austenitic stainless steels are good candidates for cladding but their use is limited to low burn-up due to their poor resistance to radiation-induced swelling. On the other hand, ferritic/martensitic (F/M) steels are more resistant to swelling allowing higher burn-up, but their uses are limited by their lower creep resistance at 600-700 °C. Improvements on the high temperature strength of F/M steel can be achieved, while maintaining their high swelling resistance, by the addition of oxide particles of by thermo-mechanical treatments. The use of oxide dispersion strengthened (ODS) alloys is currently the option for long term cladding material to achieve high burn-up values in the European fast reactor programmes.