Nuclear energy has been used since the mid-twentieth century, mainly for electricity generation and marine propulsion. It has undeniable advantages but also entails high risks, making it controversial.

Among its strengths are its high energy density (a 7 g pellet of nuclear fuel contains as much energy as one tonne of coal) and almost inexhaustible resource potential with certain types of reactors, the absence of greenhouse gas emissions, a low cost and broad independency from geopolitical tensions.

Risks, in turn, are related to the hazards of uranium and radioactivity of the fuel and fission products, which make it mandatory to have draconian security measures in place throughout the fuel cycle and to use long-term waste storage. There is also a risk of proliferation of nuclear weapons with the currently operating reactors that produce plutonium.

There are two main categories of reactors, depending on the level of neutron energy they use: fast neutron reactors and slow neutron reactors:

• "fast neutrons" are produced by fission reactions before they are slowed by a large number of shocks. Their energy is about 0.1 MeV to 2 or 3 MeV. These neutrons are able to split, thus destroying not only the nuclei known as fissile but also the actinides, which are nuclei heavier than uranium accumulating in the reactor fuel. To burn this radioactive waste effectively, fast neutrons are required. It is said that the fast reactor operates as breeders, which means it produces more plutonium-239 (239U) fuel than it consumes: fuel becomes inexhaustible;

• "slow neutrons" or "thermal neutrons" are slowed by a large number of shocks, usually in a medium called a moderator. Their energy is of the order of one electron volt or a fraction of an electron volt, i.e. 6 orders of magnitude smaller than that of fast neutrons. They can only split a small number of nuclei: uranium 235 (235U, the only uranium variety existing in nature), plutonium 239 (239U) and uranium-233 (233U) produced in the reactors. The existing fleet comprises almost only slow neutron reactors.

The notion of the generation of a reactor has been introduced to distinguish the main developments that marked the history of civilian nuclear reactors:

• the first generation corresponds to the first prototypes built mainly in the U.S. until the late 1950s. It encompasses the development of natural uranium graphite gas systems (uranium naturel graphite gaz UNGG in French) in France, with the completion of 9 reactors, whose efficiency was close to 29%;

• the second generation, from 1960 to 1995, marks the first phase of commercialization of nuclear power plants, with three main systems: the light water reactors (BWR or boiling water reactors), pressurized water reactors (PWR, performance about 33%), and heavy water (in Canada). Most operational plants are currently second generation;

• the main objective of the third generation reactors is to increase plant safety, accidents at Three Mile Island in the United States, Chernobyl in Ukraine (formerly in the USSR) and Fukushima in Japan having shown the major risks associated with certain second generation units. The EPR (European pressurized reactor) reactor is a third generation reactor. Its performance is slightly higher than those of second generation: 35%;

• it is primarily the increased efficiencies which justify the current work on the fourth generation (Generation IV) reactors, its great innovation residing in designing the reactor along with the cycle that goes with it and optimizing this system as a whole, so as to properly tackle the issues of sustainability. The efficiency of PWR, which are the most widely used today, are indeed less than 35% in practice, while those of combined cycle gas turbines exceed 55%.

Since the early 1990s, different types of fourth generation reactor projects have emerged, with the objectives of blowing away technological barriers faced by earlier generations and achieving sustainable economic and technical performance, all with a better adaptation to sustainable development criteria.

Some of these projects involve high temperature gas-cooled reactors, called PBMR (pebble bed modular reactor) and GT-MHR (gas turbine modular helium-cooled reactor). Some started in 2001, following the initiative of the United States' Department of Energy (DOE) called Generation IV Nuclear Energy Systems Initiative, which has managed to unite around it a group of nuclear research organizations from 10 countries in a loose cooperation called the GIF (Generation IV International Forum). Its goal is to study systems that could be the fourth generation of reactors, after those in use or planned in the short term (EPR), and deployable by 2030.

After studying in detail a hundred types of reactors, in September 2002 the GIF selected six concepts on which the efforts of its members will focus:

• supercritical water reactor (SCWR);

• high temperature reactors (HTR);

• gas cooled fast neutron reactors (GFR);

• sodium cooled fast neutron reactors (SFR);

• lead cooled fast neutron reactors (LFR);

• molten salt reactors (MSR).

If you want to deepen the study of these reactor types, refer to the following thematic pages:

• Pressurized Water reactors (PWR);

• High-temperature nuclear reactors (HTR).

By analogy with other forms of energy that we have considered so far, we call “nuclear fuel” the uranium producing slow neutrons that are then further slowed down to provide heat at high temperature in the vast majority of reactors existing in the world, i.e. pressurized water or boiling water reactors.

In these reactors, as well as in those that the GIF is studying, the fuel is in the form of enriched uranium aggregates, as natural uranium lacks sufficient amounts of isotope 235.

The fuel cycle (Figure above) is divided into three main phases: upstream, reactor and downstream.

Upstream includes the following main steps:

• mining;

• concentration and refining;

• enrichment;

• preparation of assemblies.

The fuel is then used in the reactor, from which it comes out highly radioactive.

Downstream includes the following main steps:

• reprocessing;

• waste packaging;

• waste storage.

In its natural state, uranium is a metal present in various concentrations in rocks such as pitchblende. Like coal, the ore is mined in underground or open pit mines. Its uranium content being very low (usually in the order of 0.1% to 0.5%), it is necessary to first concentrate it, in an operation realized at the site of extraction by chemical means, which leads to the production of a yellow paste called "yellowcake". Uranium concentration in octaoxyde of triuranium (U3O8) is 75%.

This yellowcake has to be refined to be purified and converted into uranium tetrafluoride (UF4) or hexrafluoride (UF6).

It is then necessary to "enrich" the uranium so that the concentration in the isotope 235 is sufficient for the fission reaction to be maintained, since only this isotope produces slow neutrons. This requires increasing its 235U concentration from a natural 0.7% to 3-5%.

Enrichment is very difficult to achieve because the technological properties of the two isotopes are very close. Their masses being slightly different, separation techniques are based on this property. We call separation work units or SWU the work required to separate a kilogram of uranium into two batches of different isotopic characters. Two methods are mainly used:

• gaseous diffusion, which is a gradual process, requiring many steps (1400) and involving a large amount of energy (2400 kWh / SWU);

• ultracentrifugation, at about 60,000 r / min, which requires much less energy.

Once the enrichment is successful, uranium, in the form of uranium hexafluoride (UF6), is converted to uranium oxide, in the form of black powder, which is compacted into pellets.

These tablets are packaged in assemblies that form the heart of the reactor and whose 3-5% 235U produce slow neutrons that sustain the nuclear reaction. Most of the uranium, 238U, remains stable, with the exception of some nuclei that capture a neutron and become plutonium (239U), also fissile as 235U.

The fuel used in the reactor becomes radioactive and gradually degrades, mainly because of the decrease in the concentration of 235U and production of fission products, plutonium and minor actinides. It is considered worn out when its composition is 95% uranium, 0.9% 235U, 4% fission products, 1% plutonium and 0.1% minor actinides.

It must then be reprocessed, but this is only possible when it has cooled enough, which means leaving it in a "pool" for three years.

Reprocessing serves a double purpose:

• value the uranium and plutonium present in the fuel by recycling them, since they contain a lot of energy;

• extract the non-recyclable waste by reducing and confining it.

Combustible materials are then recycled while waste is packaged for storage. We will not elaborate here on the techniques that are implemented for this, referring the reader to specialized literature.