Publication Laka-library:
A nuclear plant in pictures
| Author | EdF |
| Date | 1980 |
| Classification | 6.01.0.00/71 (GENERAL) |
| Front | 
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From the publication:
INTRODUCTION Basic nuclear theory Nuclear energy is produced by splitting the atomic nucleus of uranium-235, which is achieved by bombarding it with a particle known as a neutron ; in short, the "nuclear fission" of uranium-235. This uranium-235 is one of the isotopes (or species) found in natural uranium, where it represents 0.7% of the total mass, the remainder being composed of uranium-238. We know that the nucleus of an atom is made up of positively-charged particles, or protons, together with neutral particles, or neutrons. The atomic nucleus of uranium-235 and that of uranium-238 both have the same number of protons, i.e. 92 - the atomic number of uranium -, but different numbers of neutrons: 143 for the 235 isotope and 146 for uranium-238. When fission takes place, the uranium-235 structure of protons and neutrons disintegrates to form two new particles, usually radioactive, known as "fission products". Energy is released and three neutrons are lost to the mass. The art of reactor design consists in ensuring that the fissile materials are arranged so that one of these neutrons goes on to split another atom of uranium-235 and the process continues to be repeated, in what is known as a chain reaction. The problem is that most of the reactor's structural components (cladding, frames, etc) absorb the loose neutrons without a fission effect. Similarly, the uranium-238 in the reactor absorbs neutrons and after two radio-active transformations gives a new fissile material: plutonium. When the proportion ofuranium-235 in the reactor fuel is as low as 0.7% (as for natural uranium), it is obviously difficult to maintain the chain reaction. The difficulty is turned by exploiting a property of "slow" neutrons (low-energy particles), i.e. they more easily split uranium-235 atoms than their high-speed counterparts and are less prone to absorption by uranium-238. Their speed of travel is delayed by a series of impacts against what we call a moderator. The nuclear energy produced inside the fissile material is in the form of heat, which then has to be transferred to water, thus generating steam for driving a turbine. This transfer is effected via a cooling fluid selected for its low neutron absorption capacity. When the reactor fuel employed is natural uranium, these considerations imply that graphite must be used as the moderator, and pressurized carbon dioxide as the coolant. Other combinations including heavy water may also be used in this case. The "graphite-gas" family of nuclear plants designed on these lines is represented in France by those at Chinon, Saint-Laurent-des-Eaux, and Bugey.
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