Types of Nuclear Reactor

A nuclear reactor is a device for controlling nuclear reactions so that the energy released by the reaction can be converted to a useful form at a constant rate. The nuclear reactors presently in use are fission and breeder reactors.

Fission Reactors

Fission Reactors produce energy from the splitting of U-235. While these reactors differ in design, there are certain components common to all. These are fuel, moderators, control rods, coolants, and shields.

Fission Reactor

Pressurized Water Reactor

Fuel

Uranium-235 is the only naturally occurring isotope of uranium that undergoes fission. However, only 0.7% of U-235 is fissionable and 99.3% is not fissionable. Therefore, methods of separating, converting, and enriching the isotopes are done to increase the fissional U-235 to 2% or 4%. They are prepared as pellets of uranium (IV) oxide, UO₂ and are packed in stainless tubes called fuel rods.

Moderator

The neutrons produced from the fission reactions move at high velocities. These are slowed down by moderators so that they can be captured by U-235 nuclei. Some moderators are used to absorb more of the energy without absorbing the electrons. In modern reactors, the moderator also acts as a coolant. Light-water reactors use water as moderators. In heavy-water reactors, D₂O is used.

Control Rods

Control rods such as cadmium and boron regulate the rate of chain reaction by absorbing neutrons. The movable control rods are placed side by side with the fuel rods. By inserting and withdrawing these rods, the supply of neutrons are regulated. In cases of emergency they are shut off completely.

Coolants

The fission reaction produces a large amount of heat. As the coolant flows around the fuel rods, pumps circulate it through coils that transfer its heat to the water reservoir. The steam in the reservoir turns the turbine that runs the generator which in turn produces electric power.

Accidents at nuclear plants cause negative public reactions toward their use to generate power. In 1979, malfunctions of the cooling system at the Three-Mile Island Facility in Pennsylvania led to melting of some fuel. This caused serious DAMAGE to the reactor core and release of radioactive gases to the atmosphere. In 1986, the failure of a cooling system at the Chernobyl plant in Ukraine caused greater melting of fuel and caused the building to explode and release radioactive materials. Many believe that several thousands of people near the accident have developed o will soon develop cancer. Radiation spread across Scandinavia, Northern Europe, and Great Britain. In 1999, another facility in Tokaimura, Japan was temporarily closed due to a leak within the reactor.

Despite the accidents,several countries like France, Canada, The United States, Japan, and almost all European countries continue to use nuclear power for their electric power source.

Breeder Reactors

            Nuclear reactors that are designed to produce fissionable fuel are called breeder reactors. These reactors convert the non-fissionable U-238 to Pu-239 which is a fissionable isotope. This is made possible by the transmutation of U-238 to Pu-239 when neutrons strike U-238 nuclei.

²³⁸₉₂U  +  ¹₀n  →  ²³⁹₉₃Pu  +  2⁰_-1e

            Thus, while the reactor is using U-235 for its fuel, it is also producing Pu-239 which can be used as a fuel in a fission reactor.

²³⁹₉₄Pu  +  ¹₀n  →  ¹⁴⁷₅₆Ba  +  ⁹⁰₃₈Sn  +  3¹₀n

Fusion Reactors

            Harnessing power from nuclear fusion is a continuous challenge for research scientists. Its fuel, deuterium, is inexpensive and abundant in seawater, and it is a “clean” process. The isotopes produced are stable unlike the radioisotopes produced in fission reactions.

            The major disadvantage is the extremely high activation energy that requires 10⁹ degrees Celsius. No conventional material could withstand such high temperatures. The most promising efforts to date involve the enclosure of the plasma within a magnetic field called the tokamak. The tokamak design has a doughnut-shaped container in which the helical magnetic field confines the plasma and prevents it from getting in touch with the walls.

Radioactive Wastes

            One of the serious objections against the nuclear industry is the radioactive waste. Radioisotopes produced by nuclear power plants such as ⁹⁰₃₈Sr and ¹³⁷₅₅Cs have half-lives of 29 and 30 years, respectively. Radioisotopes with medium and long half-lives are sealed in special containers and stored in underground chambers or in isolated areas. Low-level radioactivity used in medicine can be released into the environment as long as it does not pose health risks.