NUCLEAR FISSION

Nuclear fission is a process in which a heavy nucleus—usually one with a nucleon number of two hundred or more—separates into two nuclei. Usually the division liberates neutrons and electromagnetic radiation and releases a substantial amount of energy. The discovery of nuclear fission is credited to Otto Hahn and Fritz Strassman. In the process of bombarding uranium with neutrons in the late 1930s, they detected several nuclear products of significantly smaller mass than uranium, one of which was identified as 137Ba. The theorectical underpinnings that exist to this day for nuclear fission were proposed by Lise Meitner and Otto Frisch. Shortly after Hahn and Strassman's discovery.

Some heavy nuclei will fission spontaneously. Others can be induced to fission through interaction with a neutron. In both spontaneous nuclear fission and induced nuclear fission the pool of neutrons and protons is conserved. For example, the nucleus ²⁵²Cf (Californium) fissions spontaneously. The 98 protons and 154 neutrons in the nucleus of ²⁵²Cf are reconfigured into other nuclei. Usually a few neutrons are released in the process. Pictorially, a typical spontaneous fission of ²⁵²Cf producing two nuclei and three neutrons is shown in Figure 1.

The pictorial depiction of the spontaneous fission of ²⁵²Cf can be summarized as an equation: The nucleus ²³⁵U (uranium) does not fission spontaneously, but it can be induced to fission through interaction with a neutron. Pictorially, a typical neutron-induced fission of ²³⁵U producing two nuclei and three neutrons is depicted in Figure 2.

The pictorial depiction of the neutron-induced fission of ²³⁵U can be summarized as an equation: Visually, nuclear fission is simply a rearrangement of neutrons and protons, and would seem to be possible for any nucleus. However, energy conservation principles limit significantly the number of possibilities for either spontaneous nuclear fission or induced nuclear fission. Total energy is conserved in a nuclear fission reaction as it is in all nuclear reactions. This means that the total energy before the fission equals the total energy of the fission products. For example, in the induced nuclear fission reaction discussed earlier, the combined energy of the neutron and ²³⁵U nucleus equals the combined energy of the ¹⁴⁴Ba and ⁸⁵Kr nuclei and the three neutrons. The energy accounting includes kinetic energy as well as any potential energy associated with the nuclei and the neutrons. The energy associated with mass through the Einstein relation E = mc² is particularly relevant. The interaction of the neutron and the ²³⁵U nucleus takes place when both are essentially at rest and, therefore, have negligible kinetic energy. Yet the reaction products—¹⁴⁴Ba, ⁸⁵Kr, and three neutrons—have significant kinetic energy. The kinetic energy of the products has its origin in the conversion of potential energy associated with the ²³⁵U nucleus and the interacting neutron. The gain in kinetic energy is accompanied by a loss of mass in the interacting nuclei (neutron and ²³⁵U nucleus). The kinetic energy of the reaction products (¹⁴⁴Ba, ⁸⁵Kr and 3 neutrons) is determined from E = mc², where m is the mass difference between the interacting nuclei and reaction products. The following example illustrates this principle. In atomic mass units the masses of the

nuclei involved in the illustrative nuclear fission reaction are:

Adding the left and the right columns yields 236.052588 u and 235.8665677 u, respectively. The reaction products have 0.1860 u less mass than the interacting nuclei. One atomic mass unit has an energy equivalent of 931.50 million electron volts (MeV). Therefore, 0.1860 u has an energy equivalent of 173.3 MeV. The energy acquired by the reaction products totals 173.3 MeV. This nuclear fission energy release is huge compared to that in a chemical reaction.

A typical chemical reaction such as that in burning a fossil fuel releases about 10 eV of energy. A release of 170 MeV in one nuclear fission reaction liberates the energy equivalent of 17 million chemical reactions in the burning of a fossil fuel.

A nuclear fission reaction will not occur unless the following occur: (1) the total mass of the reaction products is less than the total mass of the interacting nuclei, and (2) the sum of the neutrons and the sum of the protons in the interacting particles equals the sum of the neutrons and the sum of the protons in the products of the fission.

Any combination of reaction products consistent with these conservation principles is possible. For example, in the neutron-induced nuclear fission of ²³⁵U it is possible to produce ¹⁴⁰Xe, ⁹⁴Sr, two neutrons, and 185 MeV of energy. The most likely reaction products are close in atomic number to xenon (Xe) and strontium (Sr), but the possibilities number in the hundreds.

Describing a neutron-induced nuclear fission reaction such as does not convey the likelihood of it actually happening. When a neutron encounters the ²³⁵U nucleus, several nuclear events other than fission may occur. For example, the neutron may bounce off, changing the course of its path much like a marble bouncing off a bowling ball. Or the 235U nucleus may capture the neutron and disintegrate in a way that does not involve nuclear fission. Whatever the event, there is only some chance that the event occurs when the neutron and the nucleus interact, and this probability depends strongly on the energy of the neutron. The fissioning of ²³⁵U by a neutron is likely only for low-energy neutrons. Low in this sense means that the energy is of the order of the energy of a molecule in the air in a room.

A drop of water is a collection of molecules bound together by electric forces. Perturbing the drop in some way often causes the drop to separate into two smaller drops. A heavy nucleus such as 235U has 92 protons and 143 neutrons and has features of a liquid drop that are useful for visualizing the dynamics of nuclear fission. The strong nuclear force that tends to bind the protons and the neutrons together competes with electric forces between protons that tend to cause the nucleus to break apart. Competition between these forces produces collective oscillations that are analogous to oscillations of a liquid drop. The energy added to the nucleus when it captures a neutron enhances the oscillations, and on some occasions the nucleus separates into two parts. The model is useful for visualizing the nuclear fission mechanism as well as for doing quantitative calculations involving the probability aspects of the process.

NUCLEAR FISSION CHAIN REACTION

A chain reaction results when some event triggers a sequence of identical events. A chain reaction on a crowded highway develops when a car runs into the rear of another car, the struck car collides with a car in front of it, and so on. An expanding chain reaction develops if a car hits two cars, each struck car moves on to strike two cars, and so on. A chain reaction is central to the operation of a nuclear fission reactor and a nuclear fission bomb. It begins with a neutron-inducing fission in a nucleus. Neutrons liberated in the fission in turn induce fission in other nuclei, and so on (see Figure 3). Each fission releases energy, and unless there is some purposeful control, the energy release proceeds uncontrolled. Controlled or uncontrolled, the chain reaction does not propagate unless (1) the fuel is properly arranged and (2) there is sufficient likelihood that a neutron induces fission.

This is tantamount to saying that a chain reaction of cars does not propagate unless (1) the cars are arranged properly and (2) there is a sufficient chance that a car will make other collisions.

NUCLEAR FUEL

Commercially, there are only two nuclear species that will function in a self-sustaining nuclear fission chain reaction; 235U and 239Pu (plutonium). Uranium occurs naturally in two predominant forms, ²³⁵U and ²³⁸U. The bulk, 99.3 percent, of natural uranium is ²³⁸U. Even though ²³⁸U differs from ²³⁵U by having only three additional neutrons, the likelihood of neutron-induced fission of 238U is so low that it cannot be used in a self-sustaining chain reaction. Thus the great bulk of natural uranium is not of a form that can be used in self-sustaining chain reactions. 239Pu is not found naturally in appreciable amounts. Rather, ²³⁹Pu is made through nuclear transmutations. The process begins by having 238U capture a neutron to form 239U. 239U is unstable and undergoes radioactive decay, with the end product being ²³⁹Pu.

NUCLEAR FISSION REACTOR PRINCIPLES

The fuel in a nuclear fission reactor is generally 235U atoms arranged appropriately in a reactor vessel. Neutrons instigate fission of nuclei of ²³⁵U atoms and liberate energy. The energy output may be controlled either by regulating the fuel and/or adjusting the neutron supply. Generally, the energy output is controlled by regulating the number of neutrons. Cylindrical control rods made of materials that readily absorb neutrons are moved up or down in strategically arranged spaces in the fuel assembly. Water circulating around the nuclear fuel removes heat produced by nuclear fission reactions and reduces moderates) the speed of neutrons liberated by fission. The main components of a nuclear fission reactor are shown in Figure 4.

Moderator

Neutrons produced in nuclear fission reactions are very energetic and are referred to as fast neutrons. To optimize the chain reaction, the speed of the reaction neutrons must be reduced. Water molecules surrounding the fuel help slow the neutrons through collisions with atoms. A proton is an optimum target for a neutron. Hydrogen atoms in water are a good source of protons because each water molecule has two hydrogen atoms and each atom has a single proton in its nucleus. A neutron produced in a nuclear fission reaction bounces around in a pool of water, losing energy with each collision with a proton. After its energy is reduced sufficiently, it may react with a ²³⁵U nucleus to induce fission. Reducing the energy is referred to as moderation and the material used in the moderation is called a moderator. An object colliding head-on with a stationary object loses the greatest amount of kinetic energy when the masses of the two objects are the same. This is apparent when a billiard ball collides head-on with another billiard ball at rest. A hydrogen atom has roughly the same mass as a neutron. Accordingly, a neutron loses virtually all its kinetic energy in a head-on collision with a hydrogen atom. Because a water molecule (H₂O) has two hydrogen atoms, water is an efficient moderator for neutrons produced in nuclear fission reactions. Water is the moderator in nearly all reactors.

Control Rods

A nuclear chain reaction can be controlled by inserting materials within the fuel that deprive the fuel of neutrons. These materials are usually rod-shaped and are referred to as control rods. Control rods are made of materials that are more likely than the fuel to absorb neutrons. Boron and cadmium are favorite materials for control rods. To decrease the energy output, the control rods are inserted closer to the fuel. To increase the energy output, the control rods are pulled away from the fuel. Boric acid (H₃BO₃), a boron-containing liquid that strongly absorbs neutrons, is often included in the water moderator to provide further control of the energy output of the reactor.

Fuel

Uranium is a metal that is found naturally as a constituent of chemical compounds in minerals such as pitchblende. Uranium ore is mined much like coal: Open pits are used to mine shallow deposits, deeper deposits require shaft mining. Commercial ores yield 3 to 5 lb of uranium compounds per ton of ore. A material called yellow-cake is produced that is rich in the uranium compound U₃O₈. About 7 of every 1,000 uranium atoms in the yellow cake are of the variety ²³⁵U.

Some nuclear fission reactors are designed to use natural uranium having 0.7 percent 235U and 99.3 percent 238U. CANDU reactors, manufactured in Canada, are examples. These reactors do not use ordinary water for the moderator. Most nuclear fission reactors use ordinary water for a moderator which requires that the fuel be about 3 percent ²³⁵U and about 97 percent ²³⁸U. Achieving this enrichment requires that the solid uranium compounds in the yellow cake be converted to gaseous uranium hexafluoride (UF₆). Following enrichment, gaseous UF₆ is converted to solid uranium oxide (UO₂) for fabrication of fuel elements for a nuclear reactor.

Uranium oxide fuel is formed into solid pellets about ? in. in diameter and ½ in. long. Fuel pellets are stacked in tubes about 12 ft long. Neutrons must be able to penetrate the tube walls, called cladding, to interact with the uranium, and the walls must withstand the temperature required for heating water for a steam turbine. Stainless steel or a material called zircaloy is used for the cladding. Some 200 tubes are packed into a fuel bundle. About 175 appropriately arranged fuel bundles form the reactor core, where the energy is produced. A cylindrical steel vessel about 20 ft in diameter and about 40 ft high houses the core. Energy and neutrons are liberated from nuclear fission reactions within the fuel pellets. About 83 percent of the energy is in kinetic energy of the fission fragments. Fission fragments convert their kinetic energy to heat in the process of coming to rest in the fuel pellets. Heat is conducted from the pellets by water circulating around the fuel cladding. Water has two very important roles: conducting heat from the reactor core and moderating neutron speeds.

Energy Production

There are about 100 tons of uranium in the fuel bundles. With the control rods in their lowest position, there is no chain reaction, no energy produced, and no neutrons liberated from nuclear fission reactions. Energy production starts by raising the control rods and inserting a source of neutrons. Nuclear fission reactions begin producing energy and neutrons. Neutrons escape from the fuel elements into the water, where they lose speed and change direction with each collision with a proton. After losing sufficient speed, a neutron may reenter a fuel element and induce a ²³⁵U nucleus to fission to help foster the chain reaction. If on the average one neutron from a nuclear fission reaction goes on to produce another nuclear fission reaction, the chain reaction is self-sustaining, and energy is produced at a steady rate. Raising the control rods allows more nuclear fission reactions and more energy production. Higher, but steady, energy production occurs at the new control rod position. Control of the chain reaction depends on

• the arrangement of the fuel elements
• the quality of the moderator
• the quantity of ²³⁵U
• the neutron energy required for a high probability for inducing nuclear fission
• the material surrounding the reactor core that is used to minimize the escape of neutrons.

Although protons are very efficient neutron moderators, they also efficiently capture neutrons to form bound proton-neutron pairs called deuterons. Reactors using ordinary water for the moderator compensate for neutron capture by using fuel enriched to about 3 percent ²³⁵U.

Usually atoms resulting from nuclear fission are radioactive. There are also radioactive atoms produced from neutron capture by both 235U and 238U. Both types of radioactive atoms remain in the nuclear fuel. It is these radioactive atoms that comprise the nuclear wastes that require disposal in an environmentally acceptable manner.

Boiling-Water Reactors and Pressurized-Water Reactors

Current light-water reactors have two basic designs. One is termed a boiling-water reactor (BWR), the other a pressurized-water reactor (PWR). The principles of the PWR design is illustrated in Figure 5. Water comes to a boil, producing steam inside the containment vessel of a boiling-water reactor. Water is kept under pressure and does not boil inside the containment vessel of a pressurized-water reactor. Steam for the turbine in a nuclear power plant using a pressurized water reactor comes from water brought to a boil in a steam generator garnering heat from water circulating around the reactor core. In a boiling-water reactor, steam is produced in the reactor vessel. In a pressurized-water reactor, steam comes from a system separated from the reactor vessel. Pressure and temperature in a boiling-water reactor are about 1,000 lb per sq in. and 545°F (285°C). These numbers rise to about 2,250 lb per sq in. and 600°F (316°C) in a pressurized-water reactor. Roughly 70 percent of the reactors in the United States are of the PWR type. Essentially, the safety aspects of PWR and BWR reactors are equivalent.

At the beginning of the twenty-first century, 439 nuclear power reactors in 31 countries were generating 352 billion watts of electric power. More than 16 percent of the world's electricity was produced by these electric power plants. On average the power output of one of these plants is roughly 800 megawatts and is comparable to the power output of a large coal-burning electric power plant. Either one of these plants could supply the electricity needs of about a quarter of a million homes. The electric power outputs of solar and wind systems, for example, are usually factors of ten to a hundred times less than that of a large nuclear-fueled or coal-burning electric power plant.

REACTOR CONTROL

The products of nuclear fission reactions are radioactive and disintegrate according to their own time scales. Often disintegration leads to other radioactive products. A few of these secondary products emit neutrons that add to the pool of neutrons produced by nuclear fission. Very importantly, neutrons from nuclear fission occur before those from radioactive decay. The neutrons from nuclear fission are termed prompt. Those from radioactive decay are termed delayed. A nuclear bomb must function on only prompt neutrons and in so doing requires nearly 100 percent pure 235U (or 239Pu) fuel. Although reactor fuel is enriched above its natural 0.7 percent abundance, it is still some thirty times less concentrated than the fuel needed for bombs. A light-water nuclear reactor cannot achieve a self-sustaining condition using only prompt neutrons; delayed neutrons are needed as well. Control rods are slammed into position if a critical excursion from normal operation is sensed. The time involved in delayed neutron emission is crucial for success of the control operation.

A nuclear reactor and its auxiliaries involve a complex assembly of pumps, pipes, and valves for circulating water and steam. Minor malfunctions of the components are inevitable, and routine maintenance is inescapable. Distinct from the routine malfunctions is the possibility of loss of water in the main cooling of the reactor core. This bears the name "loss of cooling accident." Possible causes of such an accident include reactor operator errors, faulty construction, improper maintenance, natural disaster, and sabotage. A loss of cooling terminates the nuclear fission chain reaction because of the need for a moderator. However, radioactivity in the reactor core continues to produce heat at a level of 5 to 7 percent of its rated power. This percentage may seem low, but 5 percent of 3 billion watts of thermal power from normal operation amount to 150 million watts of thermal power that remains after the chain reaction stops. Many of the radioactive products are short-lived, and the power level drops accordingly. Still, there is sufficient heat to begin melting the core in about thirty seconds. Unless checked, this molten radioactive mass could melt its way through the bottom of the containment facilities. Emergency core-cooling systems must operate within this thirty-second time period. Although safety measures make the chance of an accident releasing radioactivity unlikely, it cannot be made zero.

THE NUCLEAR FISSION BREEDER REACTOR

Contemporary nuclear fission reactors are fueled with either ²³⁵U or ²³⁹Pu. In a nuclear fission reactor fueled with ²³⁵U, some ²³⁸U in the fuel is converted to ²³⁹Pu through nuclear transmutations and becomes part of the fuel inventory. However, the reactor is not designed to optimize the production of 239Pu. A nuclear breeder reactor is designed to both produce energy and convert 238U to 239Pu for fuel. The conversion process is termed breeding.

The fission of ²³⁵U or ²³⁹Pu liberates, on average, two to three neutrons. One neutron is required to sustain the nuclear fission chain reaction. In a nuclear breeder reactor, the extra neutrons are used to induce nuclear reactions that lead to the production of ²³⁹Pu. The sequence begins by arranging for a neutron to be captured by a ²³⁸U nucleus to form ²³⁹U. 239U is unstable and disintegrates producing a variety of neptunium, 239Np. 239Np is unstable and decays, forming 239Pu. The breeding principle requires both fissionable fuel (²³⁵U or 239Pu) and a breeding material (²³⁸U). Both resources are depleted as time passes.

The chance of inducing nuclear fission of 235U is optimum for slow neutrons. Neutron capture by ²³⁸U is significant only for energetic (or fast) neutrons. Accordingly, the nuclear fission reactions must be initiated by fast neutrons even though it is more difficult. Water cannot be used as a coolant, as in a light-water nuclear reactor, because of its moderating effect on neutrons. The alternative to water is usually liquid sodium. These reactors are termed liquid metal (for cooling) fast (for energetic neutrons) nuclear breeder reactors.

Nuclear Breeder Reactor Projects

An experimental nuclear breeder reactor has been operating at the Argonne National Laboratory near Idaho Falls, Idaho, since 1963. However, widespread commercial utilization of nuclear breeder reactors has not materialized. A commercial nuclear breeder reactor electric power plant was built in Monroe County, Michigan, in 1963, but was permanently terminated in 1966 after a blockage in a sodium cooling line produced a partial meltdown of its core. Plans were announced in February 1972 for a major nuclear fission breeder electric power plant to be built on the Clinch River in eastern Tennessee and planned for completion in 1980, but the project ultimately was not funded by the U.S. Congress. France and Japan have constructed major nuclear breeder reactor electric power plants, but at the turn of the century all had been closed. Reprocessing nuclear fuel to recover 239Pu is necessity in nuclear fission breeder technology. This is a major stumbling block because of concerns for proliferation of 239Pu for making nuclear bombs.

Joseph Priest

See also: Electric Power, Generation of; Environmental Problems and Energy Use; Explosives and Propellants; Meitner, Lise; Military Energy Use, Historical Aspects of; Molecular Energy; Nuclear Energy; Nuclear Energy, Historical Evolution of the Use of; Nuclear Fission Fuel; Nuclear Fusion; Nuclear Waste.

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Priest, J. (2000). "Nuclear Energy." In Energy: Principles, Problems, Alternatives, 5th ed., ed. J. Priest. Dubuque, IA: Kendall/Hunt.

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