Light enrichment ordinary water reactor

LEO Low Enrichment Ordinary Water Reactor (or Light Water Reactor)... 

A nuclear fission reactor is a device that permits a controlled chain reaction of nuclear fissions. In power plants, a nuclear reactor is used to produce heat, which in turn is used to produce steam to drive an electric generator. A nuclear reactor consists of fuel rods alternating with control rods contained within a vessel. The fuel rods are the cylinders that contain fissionable material. In the light-water (ordinary water) reactors commonly used in the United States (see Figure 21.19), these fuel rods contain uranium dioxide pellets in a zirconium alloy tube. Natural uranium contains only 0.72% uranium-235, which is the isotope that undergoes fission. The uranium used for fuel in these reactors is enriched so that it contains about 3% of the uranium-235 isotope. Control rods are cylinders composed of substances... 

One of the problems of ordinary water, or light water, as a coolant is that it absorbs neutrons. This means that this type of reactor needs to use enriched uranium as a fuel, with obvious cost impUcations. There is a second hazard, which is that if the system suffers a sudden loss in pressurisation, then the hot water will immediately turn into steam. The steam, which is much less dense, will absorb fewer neutrons and so the reactor will become more active at a time when it should become less active. This was a factor in the Chernobyl accident. 

LIGHT-WATER REACTOR (LWR). A nuclear reactor that uses ordinary water (as opposed to heavy water) as the moderator and coolant. Low-enriched uranium (LEU) is commonly used as the fuel in such reactors. 

---