21.7 Nuclear Fission

Our discussion of the energy changes in nuclear reactions (Section 21.6) revealed an important observation: Both the splitting of heavy nuclei (fission) and the union of light nuclei (fusion) are exothermic processes. Commercial nuclear power plants and the most common forms of nuclear weaponry depend on the process of nuclear fission for their operation. The first nuclear fission to be discovered was that of uranium-235. This nucleus, as well as those of uranium-233 and plutonium-239, undergoes fission when struck by a slow-moving neutron. (Other heavy nuclei can be induced to undergo fission. However, these three are the only ones of practical importance.) This induced fission process is illustrated in Figure 21.14. A heavy nucleus can split in many different ways. Two different ways that the uranium-235 nucleus splits are shown in Equations 21.24 and 21.25:




Figure 21.14 Schematic representation of the fission of uranium-235 showing one of its many fission patterns. In this process 3.5 10–11 J of energy is produced per 235U nucleus.

More than 200 different isotopes of 35 different elements have been found among the fission products of uranium-235. Most of them are radioactive.

On the average, 2.4 neutrons are produced by every fission of uranium-235. If one fission produces 2 neutrons, these 2 neutrons can cause two fissions. The 4 neutrons thereby released can produce four fissions, and so forth, as shown in Figure 21.15. The number of fissions and the energy released quickly escalate, and if the process is unchecked, the result is a violent explosion. Reactions that multiply in this fashion are called chain reactions.

Figure 21.15 Chain fission reaction in which each fission produces two neutrons. The process leads to an accelerating rate of fission, with the number of fissions potentially doubling at each stage.

In order for a fission chain reaction to occur, the sample of fissionable material must have a certain minimum mass. Otherwise, neutrons escape from the sample before they have the opportunity to strike another nucleus and cause additional fission. The chain stops if enough neutrons are lost. The amount of fissionable material large enough to maintain the chain reaction with a constant rate of fission is called the critical mass. When a critical mass of material is present, only one neutron from each fission is subsequently effective in producing another fission. The critical mass of uranium-235 is about 1 kg. If more than a critical mass of fissionable material is present, very few neutrons escape. The chain reaction thus multiplies the number of fissions, which can lead to a nuclear explosion. A mass in excess of a critical mass is referred to as a supercritical mass. The effect of mass on a fission reaction is illustrated in Figure 21.16.

Figure 21.16 The chain reaction in a subcritical mass soon stops because neutrons are lost from the mass without causing fission. As the size of the mass increases, fewer neutrons are able to escape. In a supercritical mass, the chain reaction is able to accelerate.

Figure 21.17 shows a schematic diagram of the first atomic bomb used in warfare, the bomb that was dropped on Hiroshima, Japan, on August 6, 1945. To trigger a fission reaction, two subcritical masses of uranium-235 are slammed together using chemical explosives. The combined masses of the uranium form a supercritical mass, which leads to a rapid, uncontrolled chain reaction and, ultimately, a nuclear explosion. The energy released by the bomb dropped on Hiroshima was equivalent to that of 20,000 T of TNT (it therefore is called a 20-kiloton bomb). Unfortunately, the basic design of a fission-based atomic bomb is quite simple. The fissionable materials are potentially available to any nation with a nuclear reactor. This simplicity has resulted in the proliferation of atomic weapons.

Figure 21.17 One design used in atomic bombs. A conventional explosive is used to bring two subcritical masses together to form a supercritical mass.

Nuclear Reactors

Nuclear fission produces the energy generated by nuclear power plants. The "fuel" of the nuclear reactor is a fissionable substance, such as uranium-235. Typically, uranium is enriched to about 3 percent uranium-235 and then used in the form of UO2 pellets. These enriched uranium pellets are encased in zirconium or stainless steel tubes. Rods composed of materials such as cadmium or boron control the fission process by absorbing neutrons. These control rods regulate the flux of neutrons to keep the reaction chain self-sustaining, while preventing the reactor core from overheating. (The reactor core cannot reach supercritical levels and explode with the violence of an atomic bomb because the concentration of uranium-235 is too low. However, if the core overheats, sufficient damage might be done to release radioactive materials into the environment.)

The reactor is started by a neutron-emitting source; it is stopped by inserting the control rods more deeply into the reactor core, the site of the fission (Figure 21.19). The reactor core also contains a moderator, which acts to slow down neutrons so that they can be captured more readily by the fuel. A cooling liquid circulates through the reactor core to carry off the heat generated by the nuclear fission. The cooling liquid can also serve as the neutron moderator.

Figure 21.19 Reactor core showing fuel elements, control rods, and cooling fluid.

The design of a nuclear power plant is basically the same as that of a power plant that burns fossil fuel (except that the burner is replaced by a reactor core). In both instances steam is used to drive a turbine connected to an electrical generator. The steam must be condensed; therefore, additional cooling water, generally obtained from a large source such as a river or lake, is needed. The nuclear power plant design shown in Figure 21.20 is currently the most popular. The primary coolant, which passes through the core, is in a closed system. Other coolants never pass through the reactor core at all. This lessens the chance that radioactive products could escape the core. Additionally, the reactor is surrounded by a concrete shell to shield personnel and nearby residents from radiation.

Figure 21.20 The basic design of a nuclear power plant. Heat produced by the reactor core is carried by a cooling fluid such as water or liquid sodium to a steam generator. The steam so produced is used to drive an electrical generator.

Fission products accumulate as the reactor operates. These products decrease the efficiency of the reactor by capturing neutrons. The reactor must be stopped periodically so that the nuclear fuel can be replaced or reprocessed. When the fuel rods are removed from the reactor, they are initially very radioactive. It was originally intended that they be stored for several months in pools at the reactor site to allow decay of short-lived radioactive nuclei. They were then to be transported in shielded containers to reprocessing plants where the fuel would be separated from the fission products. However, reprocessing plants have been plagued with operational difficulties, and there is intense opposition to the transport of nuclear wastes on the nation's highways. Even if the transportation difficulties could be overcome, the high level of radioactivity of the spent fuel makes reprocessing a hazardous operation. At present, the spent fuel rods are simply being kept in storage at reactor sites.

Storage poses a major problem because the fission products are extremely radioactive. It is estimated that 20 half-lives are required for their radioactivity to reach levels acceptable for biological exposure. Based on the 28.8-yr half-life of strontium-90, one of the longer-lived and most dangerous of the products, the wastes must be stored for 600 yr. If plutonium-239 is not removed, storage must be for longer periods because plutonium-239 has a half-life of 24,000 yr. It is advantageous, however, to remove plutonium-239 because it can be used as a fissionable fuel.

A considerable amount of research is being devoted to disposal of radioactive wastes. At present, the most attractive possibilities appear to be formation of glass, ceramic, or synthetic rock from the wastes, as a means of immobilizing them. These solid materials would then be placed in containers of high corrosion resistance and durability and buried deep underground. The United States is currently investigating Yucca Mountain in Nevada as a possible disposal site. Because the radioactivity will persist for a long time, there must be assurances that the solids and their containers will not crack from the heat generated by nuclear decay, allowing radioactivity to find its way into underground water supplies.