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Media kits | Nuclear energy | Fast neutron reactor | ASTRID | Nuclear reactors
ASTRID: an option for the fourth generation of nuclear reactors
The ASTRID design studies were launched in 2010. In this feature report, the CEA - the project owner and contracting authority - has outlined the profile of this fourth-generation integrated technology demonstrator.
"A reactor built according to the highest safety standards in the industry, capable of optimising the management of nuclear materials." This is how Nicolas Devictor - head of the research programmes in this field at the CEA - defines a fourth-generation reactor system. "Maximum safety" because it should be at least equivalent to that of the EPR, while taking into account experience feedback from the stress tests performed for all French nuclear power plants in the wake of the Fukushima accident. "Optimal performance" because this innovative technology must be able to extract "one hundred times more energy" from the available resources than the current reactor fleet in France. These specifications explain the main technological choices made by the CEA teams for the ASTRID project in collaboration with their industry partners.
"Once built, ASTRID will be a 600 MWe reactor connected to the grid" explained Nicolas Devictor, "since its purpose is to demonstrate that the technical options chosen for the project can be extrapolated to an electricity-producing reactor technology thanks to progress made in reactor operability". The reactor's power was defined to achieve a sufficient degree of operational flexibility while maintaining a level of representativeness that is consistent with the main industrial issues. When it comes to commercial operation, a fourth-generation reactor must ensure a level of availability wherein it generates power at least 90% of the time. For this reason, the industry needs to be shown that the sodium-cooled fast reactor technology can reach this objective through the progressive development of suitable procedures.
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Choosing the fast reactor technology raises a number of constraints with respect to the type of coolant that can be used in the reactor core. As it must not slow down the neutrons, water cannot be used as the coolant like it is in the PWRs operated by EDF. Other criteria such as the viscosity, corrosiveness and thermal characteristics must also be taken into account. According to Nicolas Devictor, all things considered, "even if gas can be used, the best possible choice is liquid sodium at temperatures between 200 and 550°C."
The drawbacks of using sodium as the coolant must therefore be resolved as early as the design phase. In reactors such as Superphénix, the core's reactivity increased when its sodium was drained. The engineers and researchers at the CEA now want to avoid this issue by developing a core with enhanced safety features. To limit the consequences of the accidental contact between liquid sodium and water, the same teams are looking into the possibility of replacing the steam generator used to produce the electricity with a circuit and turbine operating with pressurised nitrogen.
Lastly, the risks of fire due to a sodium leak in the building will be limited by the progress made in leak detection techniques, as well as by consolidating or inerting the rooms most at risk. High-temperature ultrasonic transducers or robots designed to resist high temperatures must also be employed during in-service inspection operations, which are more complex in a sodium-cooled fast reactor due to the opacity of sodium.
The facts as presented herein does not mean that all the technical options have already been finalised. Innovation must be given top priority, which calls for sustained R&D efforts to support the engineering studies co-funded by the CEA and its industry partners.
Nor does this state of affairs give any indication as to when the first Generation IV power plants will be ready for the market, since the ASTRID project has no commercial purpose.
Its purpose is "to prepare for the future and make sure that a fourth-generation technology reaches a sufficient level of technological maturity by the second half of this century in France". To achieve this, the ASTRID reactor will need to have accumulated more than fifteen years of operation and have performed a series of experiments designed to demonstrate its capabilities: operability tests, material ageing tests, transmutation tests, etc.
Patented in 2010, the core with a low sodium void effect is one of the key components that differentiates ASTRID's design from the previous fast reactors. The objective of the CEA and its partners was to develop a core whose reactivity would drop in the case of a sodium leak, going as far as bringing the nuclear reactions to a stop.
When the sodium flow slows down, it either reflects or captures neutrons. Consequently, its disappearance or local expansion in the core causes the reactivity level to vary, resulting in two antagonistic effects: a positive effect since the neutrons are not slowed down as much and therefore not captured as much, and a negative effect due to the increase in neutron leakage outside the core. As the neutron leak rate drops as the size of the core increases, the sodium voiding effect on the reactivity is largely positive for large high-power cores of a traditional design, which is a complex safety issue to manage for sodium-cooled fast reactors.
To obtain a negative effect - or even a low effect - on reactivity, the CEA and its partners decided to develop a core designed to amplify leakage thanks to the combined effect of several geometric considerations:
These developments have been possible thanks to the results of experimental studies and numerical simulations. The CEA has carried out a number of programmes to manage the specificities of the CFV core and validate the calculations. More specifically, a series of tests was performed in a test reactor in Russia, with comparative analyses conducted separately by the CEA and the US Department of Energy, which both confirmed the performance of this core.
Among the innovations prompted by the ASTRID project, the development of instrumentation capable of visualising objects in liquid sodium is a key issue. The objective is to improve the safety case of this inspection and monitoring tool.
How can we possibly see through hot sodium, an opaque substance that looks much like molten aluminium? Based on feedback from fast reactors having operated across the world, a CEA Nuclear Energy Division team joined forces with the CEA-List institute to develop two non-destructive inspection techniques using ultrasounds. These techniques use electromagnetic and piezoelectric transducers (devices that convert a physical signal into a mechanical signal) immersed in sodium, as explained by Olivier Gastaldi, section head at the CEA: "They emit ultrasounds and record the echoes that bounce back. The data is then analysed by signal processing algorithms to generate an image allowing us to visualise the object or surface encountered in 3D. In this way, we can detect any cracks in a component or even identify a fuel sub-assembly in the core.”
This is an amazing feat considering the restrictive reactor conditions: high temperatures (200 to 600°C depending on the application), irradiation, chemical compatibility of materials with liquid sodium, etc. Once developed, these techniques were first tested in water since it behaves in a similar manner to sodium in terms of ultrasonic wave transfers.
They were then tested in liquid sodium. Scientists are currently investigating the possibility of improving the techniques to allow for the electronic deviation of the ultrasonic beam. For an object to be seen by a single-element transducer, it must fall on the transducer's diffusion axis, which is a limiting factor when trying to produce an image.At the same time, work is also being continued to develop algorithms to better process information so as to produce an optimal image.
CEA is a French government-funded technological research organisation in four main areas: low-carbon energies, defense and security, information technologies and health technologies. A prominent player in the European Research Area, it is involved in setting up collaborative projects with many partners around the world.