VELLA WORLD

MATERIALS

 

The development and qualification of structural materials for core and primary components has been recognized as one of the main key issues on the path to future nuclear systems, as Generation IV reactors, critical and sub-critical transmutation systems, and also fusion devices, to ensure their safe and reliable operation.
Therefore, numerous and well targeted research activities are required both to complete the qualification of the existing materials under the extreme conditions typical of the abovementioned systems and to develop and qualify new materials and coatings, for longer perspectives.
For short-term deployment, ferritic/martensitic and austenitic stainless steels appear a viable solution as structural materials for lead-cooled systems. Therefore, studies are going on worldwide to characterize these materials, especially in terms of resistance to corrosion from lead or LBE, swelling, high-temperature creep strength and fabricability.
In parallel, special effort is dedicate to investigate new improved materials and corrosion protection barriers.
A possible alternative presently on the horizon could be represented by the ODS steels, actually in an early stage of development but object of well targeted research activities dedicated to qualify their behaviour in the LFR environment. These kind of steel in the first tests have shown remarkable high-temperature creep properties but need further improvement. Ceramics, refractory metals, or coated refractories may be necessary for significantly higher temperature (800°C) applications.
Finally, coating and surface modification technology is an important component of the material development program and is needed to be evaluated, particularly for the higher operating temperature concepts.

Corrosion and structure protection
Several experimental campaigns dedicated to characterize the corrosion behaviour of the reference steels already available (namely F/M and austenitic stainless steels) in lead and LBE are going on.
In this framework, a large effort is dedicated to corrosion experiments, both in stagnant and flowing Pb/LBE, in the temperature range between 300 °C to 700 °C, with oxygen concentrations from 10-12 to the saturation.
Different kinds of F/M (from the first generation steels as HT9, EM12 to the fourth generation steels as NF12 and SAVE12, passing through HCM12, NF616, HM12A etc) and austenitic steels (as, i.e., D9, 1.4970, 304L, 1.4984, etc. ), with particular attention dedicated to T91 and AISI 316 are under investigation.
Presently, while the short/medium term corrosion behaviour has been quite extensively investigated, the long term corrosion behaviour of the reference materials still needs to be characterized. In particular, long term tests is flowing conditions are needed, especially in pure lead environment, where the data available are less than for LBE environment.
The knowledge gap on these material long term corrosion behaviour, therefore, mainly in flowing lead, is expected to be filled in the framework of the next R&D programmes.

In parallel to the material development programmes, attention is paid to the realization of always more efficient corrosion barriers.
Coatings realized by means of the surface layer composition modification by diffusion (i.e. surface enrichment of aluminium, chromium and/or silicium), are studied in parallel to coatings realized depositing a metallic or ceramic layer with improved oxidation resistance on the steel surface.
Special attention is dedicated to the GESA treatment, both for Surface layer composition modification by diffusion and for the deposition of metallic/ ceramic layers.

Mechanical behaviour
Regarding the environmental effects on the material mechanical behaviour, several studies on fracture, fatigue, creep, creep-fatigue, stress corrosion cracking and liquid metal embrittlement in Pb-Bi and, to some extent, Pb environment have been carried out in the framework of the past FPs and still are going on. However, data are still missing on the mechanical behaviour of the reference materials after long term exposure in Pb.
T91 and AISI 316, considered as reference materials, have been widely characterized both in lead and LBE in terms of embrittlement and fatigue, while poor information on the creep and fracture behaviour of both T91 and 316L in contact with lead or LBE are available.
Fracture mechanics, low cycle fatigue, fatigue crack initiation, short crack behaviour studies, therefore, are going on in LBE in the present R&D programmes.
Analyses of the creep crack growth are carried out in LBE and similar studies are foreseen in Pb in the next years.
The problem of liquid metal embrittlement for the system lead/martensitic (or austenitic) steel has not been properly studied up to now: the knowledge of the mechanisms involved is limited and the eventual combined effect of neutron irradiation has not been sufficiently investigated. Only few and incomplete data are available , from which the necessity to carry out adequate experimental campaigns, envisaged in different present and future projects.

Irradiation Effects
The material behaviour during irradiation is an important issue to be addressed. The change of material properties because of irradiated with protons and/or neutrons has to be fully understood.
Up to now, some experiments under thermal neutron spectrum have been carried out.
An experimental campaign of irradiation, moreover, under fast neutron spectrum in pure lead is foreseen