Nuclear power fission reaction

The fact that neutrons can be absorbed by nuclei without overcoming a threshold (1 = 0 or s-wave reactions) makes neutrons extremely effective nuclear reactants. Neutron-induced reactions are the energy source for present-day commercial nuclear power (fission reactors) while charged-particle-induced reactions remain under study as power sources (fusion reactors). In this chapter we will consider the general features of nuclear fission reactors, following by the general features... 

Both fusion and fission reactions can be used in bombs. The fusion re- A nuclear explosion at sea. actions require a very high temperature to get started, so they are initiated by fission reactions. (When controlled at slower rates in nuclear reactors, fission reactions are used to produce power and additional nuclear fuel.)... 

Del y for Dec y. Nuclear power plants generate radioactive xenon and krypton as products of the fission reactions. Although these products ate trapped inside the fuel elements, portions can leak out into the coolant (through fuel cladding defects) and can be released to the atmosphere with other gases through an air ejector at the main condenser. 

Nuclear power reactors cause the transmutation of chemicals (uranium and plutonium) to fission products using neutrons as the catalyst to produce heat. Fossil furnaces use the chemical reaction of carbon and oxygen to produce CO2 and other wastes to produce heat. There is only one reaction and one purpose for nuclear power reactors there is one reaction but many puiposes for fossil-burning furnaces there are myriad chemical processes and purposes. 

The nucleus of an atom consists of protons and neutrons that are bound together by a nuclear force. Neutrons and protons are rearranged in a nuclear reaction in a manner somewhat akin to rearrang ing atoms in a chemical reaction. The nuclear reaction liberating energy in a nuclear power plant is called nuclear fission. The word fission is derived from fissure, which means a crack or a separation. A nucleus is separated (fissioned) into two major parts by bombardment with a neutron. 

A nuclear power plant generates electricity in a manner similar to a fossil fuel plant. The fundamental difference is the source of heat to create the steam that turns the turbine-generator. A fossil plant relies on the combustion of natural resources (coal, oil) to create steam. A nuclear reactor creates steam with the heat produced from a controlled chain reaction of nuclear fission (the splitting of atoms). 

Plutonium-239 is a fissile element, and vvill split into fragments when struck by a neutron in the nuclear reactor. This makes Pu-239 similar to U-235, able to produce heat and sustain a controlled nuclear reaction inside the nuclear reactor. Nuclear power plants derive over one-third of their power output from the fission of Pu-239. Most of the uranium inside nuclear fuel is U-238. Only a small fraction is the fissile U-235. Over the life cycle of the nuclear fuel, the U-238 changes into Pu-239, which continues to provide nuclear energy to generate electricity. 

Schematic view of a nuclear power plant. The energy source is the core, in which a fission reaction occurs. The rest of the plant is designed to transfer the energy released during fission and convert it into electricity.

Notice that the reaction consumes one neutron, but the reaction releases three neutrons. Those three neutrons are then free to initiate additional fission reactions. This type of situation in which there is a multiplier effect is a chain reaction. We can use isotopes that undergo chain reaction in both the production of bombs and in nuclear power plants. U-235 is fissionable, but U-238 is not. There is a certain minimum quantity of fissionable matter needed to support a chain reaction, the critical mass. 

Fissile materials are defined as materials that are fissionable by nentrons with zero kinetic energy. In nuclear engineering, a fissile material is one that is capable of snstaining a chain reaction of nuclear fission Nuclear power reactors are mainly fueled with manium, the heaviest element that occurs in natnre in more than trace qnantities. The principal nuclear energy soiuces are maninm-235, plutonium-239, uranium-233 and thorium. 

Because the isotope uranium-235 is fissionable, meaning that it produces free neutrons that cause other atoms to split, it generates enough free neutrons to make it unstable. When the unstable U-235 reaches a critical mass of a few pounds, it produces a self-sustaining fission chain reaction that results in a rapid explosion with tremendous energy and becomes a nuclear (atomic) bomb. The first nuclear bombs were made of uranium and plutonium. Today, both of these fuels are used in reactors to produce electrical power. Moderators (control rods) in nuclear power reactors absorb some of the neutrons, which prevents the mass... 

The most common use of plutonium is as a fuel in nuclear reactors to produce electricity or as a source for the critical mass required to sustain a fission chain reaction to produce nuclear weapons. Plutonium also is used to convert nonfissionable uranium-238 into the isotope capable of sustaining a controlled nuclear chain reaction in nuclear power plants. It takes only 10 pounds of plutonium-239 to reach a critical mass and cause a nuclear explosion, as compared with about 33 pounds of fissionable, but scarce, uranium-235. 

Modern nuclear power is based on harnessing the energy released in a fission reaction. The development of atomic energy started in the 1930s with the discovery that atoms could be split with neutrons. This discovery laid the foundation for building the first atomic bombs during World War 11. A basic reaction representing the fission of uranium can be represented as ... 

Traditional nuclear power involves using the heat generated in a controlled fission reaction to generate electricity. A schematic of a nuclear reactor is shown in Figure 17.7. The reactor core consists of a heavy-walled reaction vessel several meters thick that contains fuel elements consisting of zirconium rods containing enriched pellets of U-235 in the form of... 

CRITICAL MASS. The amount of concentrated fissionable material that can just support a self-sustaining fission reaction. See also Nuclear Power Technology. 

---