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CONVENTIONAL NUCLEAR FISSION

Uranium ore, the mineral fuel used in conventional nuclear power plants is a nonrenewable resource present in limited amounts in the Earth's crust.

Uranium deposits arc usually located in sedimentary rocks, but how they got there is not well under stood. It is possible that they accumulated along with the sediments that eventually became sedimentary rock, much as coal deposits accumulated Another possibility is that groundwater containing dissolved uranium seeped through the sediments which gradually became infiltrated with uranium Substantial deposits of uranium are found in North America (43.8 percent of the world's re serves}," Africa {30.2 percent), and Australia (15.6 percent). Uranium ore contains three isotopes U-238 (99.28 percent), U-235 (0.71 percent), and U-234 (less than 0.01 percent). Because U-235, the isotope that is utilized in conventional fission reactions, is such a minor part (0.71 percent) of uranium ore, uranium ore must be refined after mining to increase the concentration of'U-235 to about 3 percent; this refining process is known as enrichment.

The uranium fuel used in a nuclear reactor is processed into small pellets of uranium dioxide, each of which contains the energy equivalent of a ton of coal. The pellets are placed in fuel rods, 12-foot-long closed pipes. The fuel rods are then grouped into square fuel assemblies, generally of 200 rods each. A typical nuclear reactor contains about 250 fuel assemblies.

In nuclear fission U-235 is bombarded with neutrons. When the nucleus of an atom of U-235 is struck by and absorbs a neutron, it becomes unstable and splits into two smaller atoms, each approximately half the she of the original uranium atom. In the fission process, two or three neutrons are also ejected from the uranium atom. They collide with other U-235 atoms, creating a chain

Reaction as those atoms are split and more neutrons are released to collide with additional U-235 atoms.

The fission of U-235 released an enormous amount of heat, which is used in a nuclear power plant to transform water into stream, which is, in turn, used to generate electricity. Production of electricity is possible because the fission reaction results in a nuclear explosion. Fission reactions in the reactor of a nuclear power plant can be started or stopped, increased or decreased, thus allowing the desired amount of beat energy to be produced.

How Electricity Is Produced from Nuclear Power

A typical nuclear power plant has four main parts: (1) the reactor core, where fission occurs; (2) the steam generator, where the heat produced by nuclear fission is used to produce steam from liquid water; (3) the turbine, which uses the steam to generate electricity; and (4) the condenser, which cools the steam, converting it back to a liquid.

Fission takes place in the reactor core, which contains the fuel assemblies. Above each fuel assembly is a control rod made of a special metal alloy hat is capable of absorbing neutrons. The plant operator signals the control rod to move either up out of or down into the fuel assembly. If the control rod is out of the kid assembly, free neutrons collie with the fuel rods and fission of uranium takes place. If the control rod is completely lowered into the fuel assembly, it absorbs the free neutrons and fission of uranium no longer occurs, by exactly 227

Controlling the placement of the control rod, the plant operator can produce the exact amount of fission required.

A typical nuclear power plant has three water circuits. The primary water circuit heats water, thing the energy produced by the fission reaction. This circuit is a closed system that circulates water under high pressure through the reactor core, where it is heated to about 293°C (560°F). Because it is under such high pressure, this superheated water cannot expand to become steam and so remains in a liquid state.

From the reactor core, the very hot water circulates to the steam generator, where it boils water held in a secondary water circuit, converting the water to steam- The pressurized steam goes to and turns the turbine, which in turn spins a generator to produce electricity. After it has turned the turbine, the depleted steam in the secondary water circuit goes to a condenser, where it is converted to a liquid again.

A tertiary water circuit provides cool water to the condenser, which cools the spent steam in the secondary water circuit. As the water in the tertiary water circuit is heated, it moves from the condenser to a cooling tower, where it is cooled before circulating hack to the condenser.