Development of materials for LMFBR fuel elements of high burnup

The potential of the currently used austenitic stainless steel 1.4970 as cladding and wrapper material for the SNR may be limited to fluences of the order of 100 dpasub(NRT). The increase of this potential to 150 dpasub(NRT) or more would be an important step towards the economic operation of commercial fast reactors. The paper reports on the development of new austenitic alloys of the type Fe/25Ni/10Cr and on the optimization of tempered martensitic steels with 12% Cr. The new austenitic alloys showed a superior swelling resistance in simulation irradiations at 575, 625 and 675 deg. C. Regarding inner corrosion, simulation experiments (UO2 doped with fission products) revealed a behaviour of the matrix-strengthened variants similar to that of the reference material 1.4970. These positive results need confirmation by high-dose neutron experiments. The embrittlement of these alloys after pre-irradiation is very similar to that for the reference steel and there was no indication of an additional ductility loss due to segregation and precipitation phenomena. Tempered martensitic steels with 9-12% Cr are highly resistant to swelling and high-temperature embrittlement. Their in-pile creep behaviour is different from that of austenitic alloys and may be described as irradiation-affected thermal creep. Additional creep strength and dynamic toughness data for high neutron doses are required before a final judgement of the full potential of these very promising materials can be made. (author)