Pressurised water reactors (PWR) are by far the most common type in use today, making up 100% of France's nuclear reactors, 80% of Europe's installations and 60% of those found in the world. In 1995, they produced altogether a total electrical power of 221.6 Gigawatts.
PWRs belong to the second generation of reactors, which took precedence over early reactors powered by natural uranium fuel. These second-generation reactors are of an American design. They were developed in and after the 1970s, when moderately enriched uranium fuels became available for civilian purposes. In France PWRs were built mainly in the 80’s by Framatome
The fuel moderately enriched (3 to 5%) in fissile uranium-235 can be associated with ordinary water to slow down neutrons. This water also cools the core of the reactor. Kept under high pressure (155 atm) and not allowed to boil, the primary water passes through a heat exchanger where it transfers its heat to a second loop of water at a lower pressure. This secondary water transforms into steam, then sets off giant turbines whose motion produces electricity.
The secondary steam that loses it pressure in the turbine and cools down is not in contact with radioactive elements. It can be recycled after having been condensed. The plume of steam escaping from the tower of a nuclear power plant comes from a third loop of tertiary water that cools the condenser. To be released into the environment, a radioactive product must pass through the zircaloy shell of the fuel rod, from primary water to secondary water, then secondary water to tertiary water. As a result, the PWRs are the cleanest of second-generation reactors.
The safety provided by these multiple barriers comes at the additional cost of the complexity of the two loops heat exchangers.
The pressurized primary water has the dual role of moderator and coolant. This provides the reactor with an intrinsic natural safety. If, for some reason, the water disappears after a burst pipe, the heat is no more evacuated. But fortunately, since water plays the role of a moderator, neutrons are no more slowed down. The chain reaction stops and with it the release of nuclear energy. Stopping the release of energy limits the severity of the accident.
Nuclear engineers design reactors to operate under normal conditions at a carefully-chosen operating point, a point of equilibrium : the reactor is designed in such a way that reactivity decreases if the core temperature increases, and inversely if it decreases.
Safety in the advent of an accident is provided by several barriers separating the radioactive materials from the environment: the zirconium cladding separating the fuel from the primary water, the walls of the reactor vessel and heat exchangers, and the power plant external walls. The worst accident occurred with such a reactor was that of Three Mile Island in the United States in 1979. Because of the existence of these barrier, there were no significant fall-out for the population.
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