The removal of fuel assemblies from a production reactor is timed to maximize the amount of plutonium in the spent fuel (Figure 2). Production reactors produce less energy and thus consume less fuel than power reactors. Power reactors and those used to produce plutonium for weapons operate in different ways to achieve their goals. Approximately one third of the energy produced by a thermal power reactor comes from fission of this plutonium. It is important to realize that while the U-235 in the fuel assembly of a thermal reactor is undergoing fission, some of the fertile U-238 present in the assembly is also absorbing neutrons to produce fissile Pu-239. Cooling towers are also employed with coal and natural gas plants. Finally, the most obvious components of many nuclear power plants are the cooling towers, the external components, which provide cool water for condensing the steam to water for recycling into the containment structure. The containment structure, composed of thick concrete and steel, inhibits the release of radiation in case of an accident and also secures components of the reactor from potential intruders. The pressure vessel, made of heavy-duty steel, holds the reactor core containing the fuel assembly, control rods, moderator, and coolant. Reactors employing gaseous coolants (CO2 or He) use graphite as the moderator. In reactors using either light water or heavy water, the coolant also serves as the moderator. The coolant is a liquid or gas that removes the heat from the core and produces steam to drive the turbines. The position of these control rods in the reactor core determines the rate of the fission chain reaction. Rods composed of neutron-absorbing material such as cadmium or boron are inserted into the fuel assembly. Since slow neutron reactors are highly efficient in producing fission in uranium-235, they use fuel assemblies containing either natural uranium (0.7% U-235) or slightly enriched uranium (0.9 to 2.0% U-235) fuel. Light water (H2O), heavy water (D2O), and carbon in the form of graphite are the most common moderators. Thermal reactors operate on the principle that uranium-235 undergoes fission more readily with slow neutrons than with fast ones. Fast neutron reactors, also known as fast breeder reactors (FBR), use high speed, unmoderated neutrons to sustain the chain reaction. These reactors require a moderator to reduce the speed of neutrons produced by fission. Thermal reactors use slow neutrons to maintain the reaction. The speed of the neutrons in the chain reaction determines the reactor type (Figure 1). The components common to all nuclear reactors include a fuel assembly, control rods, a coolant, a pressure vessel, a containment structure, and an external cooling facility. In this respect they are similar to plants fueled by coal and natural gas. Power reactors use the heat from fission to produce steam, which turns turbines to generate electricity. While most reactors generate electric power, some can also produce plutonium for weapons and reactor fuel. Nuclear reactors produce energy through a controlled fission chain reaction. In January 2009, 43 reactors were under construction in 11 countries, with several hundred more projected to come on line globally by 2030.Ī Nuclear Power Plant. In 2008, 435 nuclear reactors in 30 countries provided 16% of the world’s electricity. This expansion is driven by concerns about global warming, growth in energy demand, and relative costs of alternative energy sources. Today many nations are considering an expanded role for nuclear power in their energy portfolios.
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