New reactors and fuels
Increasing the sustainability of nuclear energy
Uranium is the nuclear fuel of a reactor and is extracted naturally. In its natural state, uranium consists for 0.7 percent of fissile uranium-235 and for 99.3 percent of non-fissile uranium-238. To increase the content of fissile atoms, uranium is enriched prior to being used in nuclear reactors. The nuclear fuel serves for about four years, after which it is replaced by new nuclear fuel. And yet, 95% of the spent nuclear fuel could still be used after these four years in a nuclear reactor. That is the reason why the sector continues to innovate and look for alternatives to use this natural resource in a more sustainable way. One of the alternatives are reactors of the fourth generation, the so-called ‘fast reactors’. These reactors can also split uranium-238 and re-use spent nuclear fuel. In this way, the volume and radiotoxicity of radioactive waste can be reduced.
Lead-bismuth as cooling agent
Fast reactors operate with fast neutrons. To maintain the speed of these neutrons, the reactor core cannot be cooled with water as it would slow down the neutrons. Gases and liquid metals such as sodium and lead-bismuth do qualify as cooling agent. SCK CEN is building MYRRHA, a research reactor using lead-bismuth as cooling agent. The choice of cooling agent determines the further configuration of the design. Which nuclear fuels will be used in the core? From which structural materials will the reactor vessel and cooling circuits, amongst other things, be made? How does liquid metal behave as cooling agent? Every aspect is meticulously studied to ensure the safe operation of MYRRHA. Thanks to fast neutrons, MYRRHA is able to carry out all planned activities, mainly the demonstration of transmutation. Transmutation converts long-lived radioactive materials into less toxic materials with a shorter lifespan. MYRRHA is at the forefront of research into this very promising and sustainable technique. This technique can reduce the footprint of geological disposal, even though geological disposal remains necessary.
To test every aspect in real conditions, the research centre has disposal of high-tech laboratories and state-of-the-art calculation codes.
Contrary to the current water-cooled reactors, fast reactors operate at a higher temperature. Fast neutrons also have another impact on the structural materials. SCK CEN simulates these conditions in its BR2 research reactor and analyses the damage to these materials. This gives a realistic picture of how the structural materials will behave in the future reactor. To study the reactor physics behind fast reactors, SCK CEN reconverted the VENUS reactor into a ‘mini-MYRRHA’: a scale model of Accelerator Driven Systems (ADS).
Validating lead-bismuth as cooling agent
In the technology hall, various experiments for the future innovative research infrastructure MYRRHA have been set up. With these experiments, the research centre wants to validate lead-bismuth as cooling agent. On the one hand, researchers will test if the cooling of the reactor core remains sufficient, even if lead-bismuth is not actively circulated across the core by pumps. On the other hand, they test different structural materials as lead-bismuth is corrosive to most steel grades to a higher or lesser extent.
MYRRHA (Multi-purpose HYbrid Research Reactor for High-tech Applications) is a multifunctional research facility but above all a unique one. It is the world's first research reactor driven by a particle accelerator and using liquid metal lead-bismuth as cooling agent. MYRRHA paves the way for promising technologies and applications, for instance for optimising the management of nuclear waste, producing new medical radio-isotopes and performing materials research.
Accident-tolerant fuels: game-changer in the nuclear industry
How does the current design of nuclear fuels look like? The uranium fuel has been compressed into ceramic pellets: cylindrical pellets of about 1 cm high are stacked in long tubes. The encasing of these tubes was originally made from stainless steel but in the 1960’s, this material was replaced by a zirconium alloy. Why? Zirconium is chemically resistant, transparent to neutrons and resistant to the extreme conditions in the reactor core. However, zirconium also has a downside: when overheated, it reacts with oxygen. If during a nuclear disaster the core cannot be cooled and the emergency cooling system cannot be started, this accelerated oxidation will make the situation worse. Therefore, researchers study the development of new materials that offer all the benefits of zirconium and are better protected against overheating. SCK CEN contributes actively to the design of these so-called ‘accident-tolerant fuels’. Within the scope of its Il Trovatore Project, the research centre studies, together with thirty partners, concepts that are still in an early stage of development. Various materials and coatings are developed, studied in laboratories and tested for their radiation resistance in the BR2 research reactor.