The Nuclear Reactors "Generations"

It comes handy to classify on a historical basis the commercial nuclear reactors in 4 "generations":

- The first generation gathers the reactors of the pioneer era, which started generating power between 1955 and 1970. It encompasses a wide variety of different models, many of which were prototypes. This period was characterized by a steep escalation in unit sizes, from 60 to 800 MWe, with many different vendors and an almost total lack of standardization (west of the Iron Curtain).

- The second generation, which started operation between 1970 and 2005, supplies today 16% of the world electricity. Unit sizes range from 450 to 1450 MWe with a large cluster around 1000 MWe. Most of the reactors belong to one of the two types which use ordinary water both to slow the neutrons down and to cool the core, pressurized water reactors PWR and boiling water reactors BWR. Vendors are fewer and they begin offering standardized models, series built.

- The third generation is being commercialized just now. Their design, posterior to the Chernobyl accident, provides for increased safety and enhanced protection against external aggressions. Generation III reactors are almost exclusively "advanced" PWR and BWR types, like the EPR described below.

- Since 1999, several countries have established an international forum to define the main features of reactors fitting the requirements expected to be prevalent around 2040, and to carry out Research & Development programs in order to make their commercialization possible by that date. This is "Generation IV".

EPR, the Evolutionary Power Reactor

Issued from the best of French and German nuclear technologies, the EPR is the archetype of Generation III reactors. Advanced PWR rating 1600 MWe, EPR takes full advantage of the experience accumulated if the two originating countries by keeping a large technological continuity with the recent models N4 and Konvoi (Hence the "evolutionary").

However, EPR creates a revolution in terms of safety: on the one hand, it minimizes even more the risk of a severe accident leading to core meltdown by the multiple redundancy of its emergency and safeguard systems (4 distinct systems located in 4 separate buildings, only one of which is enough to prevent the accident), on the other hand, it guarantees that if such accident were to happen nevertheless – there is no such thing as zero risk in nature – the molten core would be caught and solidified within the containment building, passively, without need for operator action. Radioactivity would thus remain contained within this double walled building. It would be a severe internal accident, but without external consequences.

Furthermore, the reactor architecture was reinforced after the 9/11 terrorists attacks to ascertain that a similar attack with commercial jets would not result in a severe accident.

On January 1st 2008, EPR plants were under construction in Finland and France, ordered in China, and under serious consideration by utilities in the UK and USA. Among the competitors to EPR one may mention General Electric ABWR and ESBWR, Toshiba-Westinghouse AP 1000 and the Russian AES 92.

Generation IV

The "Generation IV International Forum", GIF, started by specifying the qualities expected from these future models. Most important was this consideration: if the current nuclear "renaissance" leads to a rapid growth of the world fleet, the uranium market will tighten around 2040, because the amount needed to fuel the then operating fleet over its lifetime will be close to the ultimate conventional uranium resources. It shall therefore become necessary to introduce Fast neutron Breeders which can use uranium much more efficiently. These same breeders have also the physical capability to transform some long-lived waste (the “minor actinides”) in short-lived waste, which would ease the constraints on the disposal site.

Gen IV reactors are also expected to be sturdy, competitive and even more resilient against aggressions or attempts to misuse for military purposes. They should also be able to do more than only generate electricity: seawater desalination, process heat, hydrogen production for synthetic fuels, etc.

After having devoted two years to review old and new designs (more than one hundred!) against the criteria quoted above, the GIF experts agreed to define six "identikits" of reactors fitting to be commercialized by 2040. Phase 2, R&D, begins now.

Here are the six proposed designs:

- 3 Fastr neutron breeders, respectively cooled by liquid sodium (following Phénix and Superphénix in France), by liquid lead or by high pressure gas,

- 1 Very high temperature gas-cooled reactor, mostly aimed at producing process heat and hydrogen,

- 1 Supercritical Water cooled reactor which might be a breeder,

- 1 Molten Salt reactor, whose fuel is liquid, and optimized to use thorium.

Controlled Fusion

It is possible to release part of the binding energy of very heavy nuclei by fission, but it is also possible to release part of the binding energy of very light nuclei by fusing together two of them. Such nuclear fusion is the source of the energy radiated by the Sun and the other stars. Our sun is a huge nuclear reactor, with enough fuel to heat us for the next 4 billion years or so: solar energy is just one form of nuclear energy.

The knack is to fuse together two nuclei which both have a positive electrical charge and therefore repel each other quite vigorously. Within the stars, the gravity due to their enormous masses is able to overcome this electrostatic repulsion. On Earth, there cannot be the same mechanism nor quite identical reactions.

The only fusion reaction we can reasonably achieve is the one implemented in H bombs⁽¹⁾, involving two hydrogen isotopes, deuterium D the nucleus of which is constituted by one proton and one neutron, and tritium T the nucleus of which possesses one proton and two neutrons. Deuterium is found in large amounts but in very low concentration in ordinary water, as “heavy” water D2O. Extracting deuterium is not too difficult but it requires a lot of energy. As for tritium, it is a short-lived radioactive element (its half-life is close to 12 years), which, therefore, must be produced through the capture of a neutron by a nucleus of lithium, an element relatively well spread in the earth crust. The reactions of interest are as follow:

The fusion reaction produces one stable helium nucleus and one neutron, the latter carrying most of the energy released. This neutron is necessary to breed another tritium atom.

⁽¹⁾In a thermonuclear or H bomb, one uses part of the huge amount of energy released during the explosion of an A-bomb to violently compress the mixture and trigger the fusion during its implosion. One tries to reproduce less violently the process by focussing many laser rays on a tiny marble containing the D-T mixture but any power production using this "inertial containment" method is even farther away than power production using the "magnetic containment" described hereafter.

For lack of enough gravity, one must force the deuterium and tritium nuclei to meet by moving them at very high speed by thermal agitation, i.e. by heating the whole plasma⁽²⁾ to 100 millions degrees! Such a hot plasma must have no physical contact whatsoever with any wall: it must be contained, enclosed within a kind of virtual bottle through a combination of electrical and magnetic fields. A Russian team led by Lev Artsimovitch invented, as soon as 1954, the Tokomak device which realizes this containment.

It is a big tokomak called ITER which will probably demonstrate the physical feasibility of controlling nuclear fusion.

⁽²⁾At very high temperature, it is no longer a gas with atoms or molecules, but a kind of very low density broth mixing bare nuclei (ions) and single electrons.

 

From ITER to commercial fusion ?

On June 28th 2005, all the nations involved in fusion research (European Union, Japan, Russian Federation, United States, China, South Korea and India) embarked upon building together at Cadarache (Bouches du Rhône) the biggest tokomak in the world, christened ITER (the road, in Latin language) standing for International Tokomak Experimental Reactor. This facility should begin its experiments by 2016 and carry them out over the next fifteen years at least. ITER is expected to bring the physical demonstration of controlled fusion by magnetic containment MCF, by generating, during several minutes, more energy than what will have been necessary to heat up the plasma. At that date, ITER shall be, as far as fusion is concerned, the equivalent of what was, for fission, the first chain reaction in the CP1 reactor, December 2nd, 1942.

Before designing the first actual demonstration reactor, a complete program of R&D will have to be performed, in addition to the ITER program, to develop the materials required for this demanding technology, notably those of the wall facing the plasma: those materials must be able to withstand both very high temperatures and very high doses of high energy neutron radiation. This “Demo” plant could be built around 2030 and be followed by a prototype plant around 2050. Only then will it be possible to evaluate with some credibility the economic potential of this very promising energy source.