There are several reactor (power plant) types including (1) pressurized water reactors (PWRs), (2) boiling water reactors (BWRs), (3) liquid-metal fast breeder reactor (LMFBR), (4) high temperature gas-cooled reactors (HTGRs), and (5) heavy water reactors (CANDU). Gas-cooled reactors (CGRs) include the British CO2 and He cooled reactors and General Atomics HTGR (e.g., Ft. St. Vrain). CGRs are graphite-moderated and have a high thermal efficiency. Liquid-metal, fast-breeder reactors generally use NaK as the coolant in either a loop or pool configuration.
In the U.S. most electric power plants are light water reactors (LWRs), which are the PWRs and BWRs. The LWRs use thermal neutrons (<0.1 eV) to sustain the chain reaction whereas reactors like the LMFBR use high-energy (>1 MeV) neutrons. Most of world's reactors are thermal reactors. Thermal reactors are easier to control than fast reactors and in some designs (e.g., CANDU) can use natural uranium. Fast reactors require a higher power density, but those designs may have the potential for breeding. In a thermal-fission reaction, neutrons are born at fast energy levels, but require the use of a moderator (e.g., water or graphite) to slow the neutrons down, and in most cases, enriched fuel to achieve a sustained chain reaction. The reaction is regulated using control rods typically composed of boron-based neutron absorber material.
Pressurized Water Reactor (PWR) Schematic [Source: Tennessee Valley Authority (TVA)].
Boiling Water Reactor (BWR) Schematic [Source: Tennessee Valley Authority (TVA)].
Pressurized water reactors are the dominant reactor type for electric power plants and are also the basis of naval reactors. The LWR fuel is uranium dioxide (UO2), which is a ceramic fuel pellet that is compatible with the water coolant. The fuel pellets are stacked into a Zircaloy clad fuel rod. Many fuel rods are placed in a square lattice to construct a fuel assembly. A couple hundred fuel assemblies are generally needed to fuel the entire reactor core. This reactor core is housed in a reactor pressure vessel that is composed of steel 8 to 10 inches thick.
Reactor safety basically comes down to making sure that heat is adequately removed from the core in order to avoid release of radioactivity from the plant. This can be ensured by maintaining the fuel temperature below its melting temperature of about 5000°F, and by keeping the cladding temperature below the point at which the zirc-water reaction occurs.
Nuclear waste is classified as either high-level or low-level waste. The high-level waste includes fission products and transuranic isotopes, and is highly radioactive and must be stored for long periods. Low-level waste includes clothing, rags, and tools, which are sealed in a drum for ultimate placement in a dedicated landfill.
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