Several lines of reactor design exist, differentiated by the type of fuel employed, the fluid used to transport the heat (the thermal carrier), and the type of moderating fluid used (to slow down the neutrons).
All current French reactors are pressurised water reactors, (PWR, or REP in France), of American design (Westinghouse). In this type of reactor, water is used as the heat transport fluid and the moderator fluid. This technology (we speak of a line of reactors) represents 63% of world production capacity of nuclear electricity (80% in Europe).
Another type of reactor was developed earlier in France, the graphite-gas reactor (UNGG), using natural uranium, graphite as the moderator fluid (to slow down the neutrons) and carbon monoxide gas as the cooling agent. These are known as first generation reactors.
Other lines exist, using slightly different technologies:
- The boiling water reactor: the thermal transport fluid is water as in the PWR (but the water boils, because it is not under pressure other than atmospheric pressure), the fuel is enriched uranium (the BWR line, or RBMK in the ex-Soviet block);
- The rapid neutron reactor (RNR), also called a super generator: the neutrons are not slowed down and so there is no moderator fluid. The fast-moving neutrons combine with uranium 238, a non-fissile product, and are transformed into plutonium 239, a fissile product. The cooling fluid is sodium. The French Superphenix reactor uses this technology;
- The heavy water reactor (this is the type used in Canada): heavy water (deuterium oxide) is used as the moderator to slow down the neutrons. These reactors work with natural uranium;
Note: reactors in the countries of the ex-Soviet Union are of two types: the oldest, known as RBMK, are boiling water reactors using graphite as the liquid moderator. These reactors do not have a secondary circuit and are not equipped with a containment structure. That is why the accident at Chernobyl
had such damaging effects on people and the environment.
A new technology was subsequently developed: pressurised water reactors (WER), using a concept very similar to the American PWR reactors.
Today, in the 21st century, the PWR’s are being perfected and are becoming even safer, with a reactor of Franco-German conception, the EPR (European Pressure Reactor). The first is being built in Finland. The second is planned for France, on the site of Flamanville.
Research is continuing into reactors of the fourth generation, which could be built around 2020 to 2030.
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