The SUBATECH laboratory is pursuing research on a new type of "4th generation" nuclear reactor, the "molten salt" reactor (MSR), which could offer several advantages: increased safety, better management of radioactive waste, etc. SUBATECH is participating in various projects dedicated to these MSRs.
The nuclear reactor of tomorrow?
Which technology will be the most capable of contributing to the country's energy supply, while offering maximum guarantees in terms of both safety and waste management? At a time when the revival of the nuclear industry is on the agenda, these questions are becoming increasingly relevant.
One model in particular seems to be taking the lead: the "molten salt" reactor (MSR). Since 2015, Lydie Giot, an associate professor at the SUBATECH laboratory (1), which is under the triple supervision of the CNRS, Nantes University and IMT Atlantique, has been working on this technology. Lydie Giot is a specialist in nuclear safety issues related to residual power, and an expert for the IAEA (International Atomic Energy Agency) and the NEA (the OECD Nuclear Energy Agency).
This MSR belongs to the "4th generation" of nuclear reactors. This generation should replace the graphite/gas models (1st generation, currently being dismantled), the pressurized or boiling water reactors (2nd generation, the most widespread in the world), and the EPRs (3rd generation - in reality, an evolution of the former).
"This '4th generation' concept, which marks a real technological breakthrough, is not new," says Lydie Giot. "It was first discovered in American labs in the 1960s, but wasn’t adopted due to a lack of funding." The main characteristic of this type of reactor is that the fuel is dissolved in molten salts. Different variants are being studied, depending on the type of salt (fluoride, chloride), the use of a moderating element such as graphite ("thermal neutron" model) or not ("fast neutron" reactor), or the fuel used (235U, plutonium, 233U or 241Am etc.).
Possibility of draining the reactor
Compared to other types of reactor, the MSR has some serious advantages. Firstly, its 'fuel' (the fissile material), mixed with salts, is liquid and therefore homogeneous: this simplifies its preparation. And above all, in the event of an accident, it would be possible to empty the reactor core into a tank, which would avoid the risk of dissemination. Finally, the MSR can use a wide range of fuels - including those produced in other types of reactors (plutonium, neptunium, americium, etc.). This may (partly) solve the issue of very long-lived waste.
With fewer risks of explosion and proliferation, better use of resources, and easier management of waste from the current nuclear fleet, the FSR has a lot going for it. But this type of reactor is far from ready for industrial deployment. To date, its existence is purely theoretical: its safety has not been proven, and the risk cannot be accurately measured - even though a great deal of work is currently being done on this subject, with different scenarios. A major technological challenge lies in the resistance of (new) materials to irradiation.
In other words, it will take a long time before the first FRL is tested. "It will probably take at least fifteen years," says Lydie Giot. "It will all depend on the financial resources available". The next step will be to develop an industrial sector. In short, there are many challenges still to be met. In the meantime, molten salt reactors are already raising enough hopes that several dozen start-ups around the world (in the United States, Canada, China, etc.) are interested in them. In France, the company Naarea even plans to present a first prototype of a mini-molten salt reactor in 2030, with a capacity of 15 MW.
A European project on MSR safety
SUBATECH is also involved in several initiatives dedicated to MSRs. For example, SAMOSAFER, a European project launched in 2019, aims to develop new simulation and assessment tools for the safety of these reactors. Led by the University of Delft in the Netherlands, SAMOSAFER brings together some fifteen partners (including CNRS, IRSN (2), Framatome, CEA (3) and EDF) from seven countries. "The aim is to develop new risk assessment and simulation tools," explains Lydie Giot, who works in close collaboration with the Laboratoire de Physique Subatomique et de Cosmologie (LPSC) in Grenoble. The SUBATECH team is responsible for evaluating the 'source term' (which would be released in the event of an accident) and coordinating the calculations carried out by the various partners. The final results are expected in June.
Another European project, MIMOSA, aims to develop an optimized strategy for the multi-recycling of plutonium. MSR can use as fuel the nuclear materials that are not currently considered recyclable, in particular reprocessed plutonium. This will reduce both the lifespan and the quantity of spent fuel produced in power plants. Led by Orano, MIMOSA brings together 14 partners from several countries, including SUBATECH.
ISAC (Innovative System for Actinide Conversion) is a new project launched as part of the France Relance 2030 plan. Coordinated by the CEA, it brings together many players in the French nuclear industry - including Orano, Framatome, EDF and the CNRS with IJClab, the LPSC and SUBATECH - to work on the molten salt reactor. The aim is to study how minor actinides (i.e. radioactive waste), such as americium produced in pressurized water reactors, can be recycled in the MSR. This solution would ultimately reduce waste to be stored. SUBATECH is leading studies on residual power, ... "This is an original approach, with several design options, which could offer interesting prospects," says Lydie Giot, who believes that "a demonstrator or a first industrial project could see the light of day within 15 or 20 years."
(1) Laboratoire de Physique SUBAtomique et TECHnologies associées.
(2) Institut de radioprotection et de sûreté nucléaire.
(3) Commissariat à l’énergie atomique.
by Pierre-Hervé VAILLANT