Nuclear: could fast neutron reactors take over from EPRs?

They were announced by President Emmanuel Macron in February 2022: between 6 and 14 EPR (European Pressurized Reactor Where Evolutionary Power Reactor) will be built by 2040, in order to maintain the nuclear as the main source of electricity in France. These reactors are based on the same principle as those currently used: from heat is created in the tank by nuclear reactionsthen extracted by water maintained liquid under high pressure. This water circulates in a closed loop in the primary circuit, itself in indirect contact with a secondary circuit. It cools on contact with it, by transmitting its heat to the water circulating there. The latter then vaporizes and turns a turbine which triggers a alternator : Electricity is generated!

Uranium resources for less than 100 years

This reactor technology has been favored by France for decades. Since then, many feedbacks have been available, allowing in particular to improve the plant safety. So why change technology, when you know how to use the existing one perfectly?

Many reasons. The wastefirst of all: just like every industrial exploitation, the nuclear center generate waste. Except that a large part of them are radioactive, some of which have a duration of radiotoxicity for hundreds or even thousands of years. But also, the cost related to the price of materials first, because the resources in uranium are limited. This element, the heaviest in its natural state on Earth, is 10,000 times rarer than iron. According the International Atomic Energy Agency (IAEA), uranium resources allow at least meet demand by 2040. But what about the distant future? Futura questioned a CEA expert on another reactor technology: fast neutron reactors.

This concept is as old as pressurized water reactors (REP). Called RNR for Fast Neutron Reactorsthese reactors use a different fuel and cooling technique from PWRs, in particular, “ they make better use of atoms fissile and fertile fuel than pressurized water reactors, because fast neutrons cause more nuclei to fission,” explains Eric Abonneau, program assistant 4^e generation of the CEA. Indeed, in a pressurized water reactor, the neutrons are slowed down by the liquid water circulating in the tank. This allows them a greater probability (also called cross section) of fission than with a fast neutron, when they encounter a uranium 235 or plutonium 239 atom, isotopes fissile uranium and plutonium.

But, contrary to the slowed down neutrons, known as “thermal”, the fast neutrons have the ability to fission many more isotopes other than those called fissile, because they have greater energy. Thus, in a fast neutron reactor, on the contrary, “ fast neutron reactors do not require a moderator because the neutrons must not be slowed down,” continues E.Abonneau. They therefore only require a coolant to transport heat.

” The core inlet temperature is around 390°C, and the outlet temperature is 550°C. The heat is transmitted in a secondary circuit, which is also in sodium. Then, the secondary circuit exchanges its heat with a tertiary circuit, either by a sodium/water exchanger, or by a sodium/gas exchanger. » In addition, this possibility of fissioning many more nuclei would alleviate the problem of uranium resources: “We could reuse the plutonium that comes from PWRs, in addition to usingdepleted uranium which is currently set aside in France: we would obtain a much better sustainability », explains E.Abonneau.

Undeniable advantages for fast but insufficiently competitive neutron reactors

This technology sparked great interest in the early days of civilian nuclear. ” Very early on, scientists understood the interest of the fast spectrum for neutrons. From the 1960s, we began to use fast neutron reactors: first with Rhapsodie, then Phénix and Superphénix”. explains E.Abonneau. Reactors remained in operation for a few decades in total, but today all stopped. ” For SuperPhénix, we had started an instruction in the 1990s for its restart, but a political decision led to the shutdown of the reactor. »

However, although the experience feedback is minimal compared to that of the PWRs, the RNR have undeniable advantages, both in terms of operation and design: “RNRs operate at atmospheric pressureunlike PWRs where the pressure imposes thick hulls”. Advantages which, for the moment, do not weigh against PWRs, but could in the near future, in particular for the question of raw material resources: “With the scarcity of uranium, PWRs will come up against a lack of supply. It may remain between 50 and 100 years, but no more. FNRs could last several thousand years, because they can be isogenerators: they produce as much fissile material as they consume”.

Despite everything, there remains the question of the coolant: sodium in most technologies, very reagent with water and air. In other RNRs, leadlead-bismuth, or even molten salts are used, as E. Abonneau explains: “Recently, we have also focused on another technology: molten salt reactors. The fuel is there in liquid form, which has presumed safety advantages but also brings a certain number of locks technologies to raise (corrosionmaterials…) “.

Finally, the question could well arise once the PWRs lose their competitiveness. At this moment, “even if the construction of an RNR costs more than that of a PWR, it will be much more advantageous once in operation in terms of economy, sovereignty and services rendered…”, concludes E.Abonneau.