1.1.BACKGROUND
The innovative technology of fast reactors offers several advantages, such as higher utilization of the natural uranium, reduction of highly radioactive minor actinides and the ability to operate in a closed fuel cycle. The fast neutron spectrum allows fast reactors to increase the energy yield from natural uranium by a factor of sixty to seventy compared to the thermal spectrum reactors. This increased yield would allow for a sustainable fuel supply for nuclear power programmes that would last for thousands of years based on proven uranium resources as well as for a significant improvement of nuclear waste management. Since the beginning of the nuclear era, sodium has been used in nuclear reactors due to its attractive physical properties, such as high thermal conductivity and relatively high boiling temperature, and also because sodium is a very weak moderator and weak absorber of neutrons. Sodium technology has reached its maturity and has more than 500 reactor-years of operation experience. However, sodium reacts aggressively with water and air oxygen, and thus requires special measures and designs of the heat removal systems to avoid sodium contact with water and prevent sodium fires. To overcome these challenges, other coolants are considered as alternatives. Fast reactors cooled by heavy liquid metals (HLM), such as lead or lead-bismuth eutectic (LBE) alloy also offers improved safety and reliability, as the coolant does not react chemically with water and air, and has a high boiling temperature and other attractive thermo-physical properties. This allows smaller dimensions of the reactor vessel, excludes the need for an intermediate circuit, and reduces the size of the reactor unit, which has a significant impact on economics of the construction. The deployment of the HLM cooled fast reactors could be an attractive benefit for the growing energy sector. The main structural materials used today in fast reactors are mostly stainless steels and various steel alloys. The heavy liquid metal coolants, such as lead and LBE have the ability to dissolve oxygen. With this in mind, by controlling the oxygen concentration in the liquid metal, a protective oxide film can be formed on the steel surface. These oxide films serve as a protection from the dissolution of the steel. State-of-the-art research focuses on new materials such as oxide dispersion strengthened (ODS) steels, refractory alloys, SiC ceramic matrix composites, MAX phase materials and coated materials. Various phenomena affect the properties and behaviour of steels during exposure to heavy liquid metals, including the dissolution of steel alloy elements, liquid metal embrittlement, non-passivating oxidation and others. Corrosion and degradation of standard steels have two major consequences on the operation of lead and lead-bismuth cooled fast reactors (LFRs): the loss of material owing to corrosion affects the integrity of structures and components, and the release of corrosion products in the coolant and formation of solid oxides that could lead to the blockage of the coolant circulation.
One of the main goals of the scientific community focused on LFRs is to explore and identify mitigation strategies that can reduce the degradation of materials in operation in order to ensure the safety and efficiency of the nuclear system. The mitigation measures should be based on a deep understanding of the physical phenomena and processes causing changes in the materials’
properties. The major mitigation measures include HLM environmental control (e.g. operating at low temperature, active control of oxygen concentration or employment of corrosion inhibitors), the adoption of surface engineering solutions (such as protective coatings), and the development of innovative materials capable of withstanding harsh HLM environments.
Operation under controlled oxygen concentration and at low temperatures (below 450–480°C) has proven to be efficient against corrosion issues. However, experiments performed at higher temperatures have shown that oxygen control is less effective, and strong corrosion(depending on the steel type and experimental conditions) can occur as the temperature increases.
Several researchers consider using coatings on standard steels as the most viable solution in the short term having as advantage the use of materials with known properties and qualified for neutron irradiation.
The development of innovative materials (corrosion resistant to HLM) is a promising option to combat degradation phenomena in the long term. Alumina-forming austenitic (AFA) steels have shown very good performance in HLM environment compared to the standard steels or high-entropy alloys, as they have some unique compositions, microstructures and engineering properties. However, physics-based experimental validation is required to develop modelling tools able to simulate and analyse the long-term behaviour of materials in HLM when exposed to corrosive environments and/or irradiation.
In addition, fast-evolving HLM technologies require the establishment of a standardized procedures to perform corrosion and mechanical tests, as well as to assess the performance of manufactured materials in various shapes. The assessment of mechanical properties of materials in liquid metal and coated steels under irradiation is also a high priority; these properties should be measured in the relevant ranges of temperature, neutron fluence, stresses and HLM flow velocity for the different components.
Taking recent developments into consideration, and in order to identify gaps and needs, the IAEA Technical Working Group on Fast Reactors (TWG-FR) recommended a Technical Meeting on this topic. The technical meeting on Structural Materials for Heavy Liquid Metal Cooled Fast Reactors was held in Vienna in October 2019, addressing the IAEA Member States’ expressed need for information exchange on projects and programs in the field, as well as for the identification of priorities based on the analysis of technology gaps to be covered through research and development (R&D) activities to be carried out at the international level under the IAEA’s aegis. This document provides the summary of the technical sessions as well as full papers submitted and presented at the meeting. The papers have been peer reviewed.
1.2.OBJECTIVE
The purpose of the meeting was to discuss experiences, innovations, and technological challenges related to structural materials proposed for advanced reactors cooled by lead and lead-bismuth eutectic. The meeting provided a forum to promote and facilitate the exchange of information on structural materials for HLM cooled reactors at the national and international levels and to present and discuss the current status of R&D in this field. The objective of this document is to provide to the Member States a reference, summarizing the work presented, including the full contributions, by the participants.
1.3.SCOPE
This TECDOC presents the Proceedings of the Technical Meeting on Structural Materials for Heavy Liquid Metal Cooled Fast Reactors, which was held in October 2019 in Vienna. The document compiles summaries of the technical sessions, group discussions and includes the full peer-reviewed papers that were submitted to and presented at the meeting.
1.4.STRUCTURE
Section 1 provide the introduction to the document. Section 2 summarizes the technical/meeting session and discussions from the technical meeting. Section 3 provides a summary of group discussion and Section 4 highlights main summary and conclusions from the meeting. Full papers submitted to the meeting are also included in this document, categorized by technical sessions.