Nuclear fission: Encyclopedia II - Nuclear fission - Physical overview

Nuclear fission - Physical overview

Nuclear fission differs from other forms of radioactive decay in that it can be harnessed and controlled via a chain reaction: free neutrons released by each fission event can trigger yet more events, which in turn release more neutrons and cause more fissions. Chemical isotopes that can sustain a fission chain reaction are called nuclear fuels, and are said to be fissile. The most common nuclear fuels are ²³⁵U (the isotope of uranium with an atomic mass of 235) and ²³⁹Pu (the isotope of plutonium with an atomic mass of 239). These fuels break apart into a range of chemical elements with atomic masses near 100 (fission products). Most nuclear fuels undergo spontaneous fission very slowly, gradually disintegrating over periods of eons. In a nuclear reactor or nuclear weapon, most fission events are induced by bombardment with another particle such as a neutron.

Typical fission events release several hundred MeV of energy for each fuel atom that undergoes fission, which is why nuclear fission is used as an energy source. By contrast, most chemical oxidation reactions (such as burning coal or TNT) release at most a few tens of eV per event, so nuclear fuel contains at least ten million times more usable energy than does chemical fuel. The energy of nuclear fission is released as kinetic energy of the fission products and fragments, and as electromagnetic radiation (gamma rays); in a nuclear reactor, the energy is converted to heat as the particles and gamma rays collide with the atoms that make up the reactor and its working fluid (usually water).

Nuclear fission produces energy because the binding energy of intermediate-mass nuclei (with atomic numbers and atomic masses close to ⁵⁶Fe) is greater than the binding energy of very heavy nuclei, so that energy is released when heavy nuclei are broken apart. The total mass of the fission products from a single reaction is less than the mass of the original fuel nucleus, and the excess is released as energy via Einstein's relation E=mc².

The variation in binding energy is due to the interplay of the two fundamental forces acting on the component nucleons (protons and neutrons) that make up the nucleus. Nuclei are bound by an attractive strong nuclear force between nucleons, which overcomes the intense electrostatic repulsion between protons. However, the strong nuclear force acts only over extremely short ranges (it follows a yukawa potential), so that large nuclei are less tightly bound than small nuclei, and breaking a large nucleus into two or more intermediate-sized nuclei releases energy.

Because of the short range of the strong binding force, large nuclei contain proportionally more neutrons than do light elements, which are most stable with a 1-1 ratio of protons and neutrons. Fission products have, on average, the same ratio of neutrons and protons as their parent nucleus, and are therefore usually very unstable because they have too many neutrons compared to stable isotopes of similar mass. This is the fundamental cause of the problem of radioactive high level waste from nuclear reactors. Fission products tend to be beta emitters, emitting fast-moving electrons to conserve electric charge as neutrons convert to protons inside the nucleus.

The most common nuclear fuels, ²³⁵U and ²³⁹Pu, are not major radiologic hazards by themselves: ²³⁵U has a half-life measured in billions of years, and although ²³⁹Pu has a half-life of only about 25,000 years it is a pure alpha particle emitter and hence not particularly dangerous unless ingested. Once a fuel element has been used, the remaining fuel material is intimately mixed with highly radioactive fission products that emit energetic beta particles and gamma rays. Some fission products have half-lives as short as seconds; others have half-lives of tens of thousands of years, requiring long-term storage in facilities such as Yucca mountain until the fission products decay into non-radioactive stable isotopes.

Nuclear fission - Spontaneous and induced fission; chain reactions

Many heavy elements, such as uranium, thorium, and plutonium, undergo both spontaneous fission (a form of radioactive decay) and induced fission (a form of nuclear reaction). Elemental isotopes that undergo induced fission when struck by a free neutron are called fissionable; isotopes that undergo fission when struck by a thermal (slow moving) neutron are also called fissile. A few particularly fissile and readily obtainable isotopes (notably ²³⁵U and ²³⁹Pu) are called nuclear fuels because they can sustain a chain reaction and can be obtained in large enough quantities to be useful.

All fissionable and fissile isotopes undergo a small amount of spontaneous fission, which releases a few free neutrons into any sample of nuclear fuel. The neutrons typically escape rapidly from the fuel and either decay into protons (with a half-life of about 15 minutes) or impact and are absorbed by other nuclei in the vicinity. However, some neutrons will impact fuel nuclei and induce further fissions, releasing yet more neutrons. If enough nuclear fuel is assembled into one place, and/or if the escaping neutrons are sufficiently contained, then these freshly generated neutrons outnumber the neutrons that escape from the assembly, and a sustained chain reaction will take place.

An assembly that supports a sustained chain reaction is called a critical assembly or, if the assembly is almost entirely made of a nuclear fuel, a critical mass. The word "critical" refers to a cusp in the behavior of the differential equation that governs the number of free neutrons present in the fuel: if less than a critical mass is present, then the amount of neutrons is determined by radioactive decay, but if a critical mass or more is present, then the amount of neutrons is controlled instead by the physics of the chain reaction. The actual mass of a critical mass of nuclear fuel depends strongly on the geometry and surrounding materials.

Not all fissionable isotopes can sustain a chain reaction. For example, ²³⁸U, the most abundant form of uranium, is fissionable but not fissile: it undergoes induced fission when impacted by an energetic neutron with over 1 MeV of kinetic energy. But the neutrons produced by ²³⁸U fission are not, themselves, energetic enough to induce further fissions in ²³⁸U, so no chain reaction is possible with that isotope. Instead, bombarding ²³⁸U with slow neutrons causes it to absorb them (becoming ²³⁹U) and decay by beta emission to ²³⁹Pu.

Nuclear fission - Fission reactors

Critical fission reactors are the most common type of nuclear reactor. In a critical fission reactor, neutrons produced by fission of fuel atoms are used to induce yet more fissions, to sustain a controllable amount of energy release. Devices that produce engineered but non-self-sustaining fission reactions are subcritical fission reactors. Such devices use radioactive decay or particle accelerators to trigger fissions. The home-built fission pile built by David Hahn is an example.

Critical fission reactors are built for three primary purposes, which typically involve different engineering trade-offs to take advantage of either the heat or the neutrons produced by the fission chain reaction:

• power reactors are intended to produce heat for nuclear power, either as part of a generating station or a local power system such as a nuclear submarine.
• research reactors are intended to produce neutrons and/or activate radioactive sources for scientific, medical, engineering, or other research purposes.
• breeder reactors are intended to produce nuclear fuels in bulk from more abundant isotopes. The most common type makes ²³⁹Pu (a nuclear fuel) from the naturally very abundant ²³⁸U (not a nuclear fuel).

While, in principle, all fission reactors can act in all three capacities, in practice the tasks lead to conflicting engineering goals and most reactors have been built with only one of the above tasks in mind. (There are several early counter-examples, such as the Hanford N reactor, now decommissioned). Power reactors generally convert the kinetic energy of fission products into heat, which is used to heat a working fluid and drive a heat engine that generates mechanical or electrical power. The working fluid is usually water with a steam turbine, but some designs use other materials such as gaseous helium. Research reactors produce neutrons that are used in various ways, with the heat of fission being treated as an unavoidable waste product. Breeder reactors are a specialized form of research reactor, with the caveat that the sample being irradiated is usually the fuel itself, a mixture of ²³⁸U and ²³⁵U.

For a more detailed description of the physics and operating principles of critical fission reactors, see nuclear reactor physics. For a description of their social, political, and environmental aspects, see nuclear reactor.

Nuclear fission - Fission bombs

One class of nuclear weapon, a fission bomb, otherwise known as an atomic bomb, is a fission reactor designed to liberate as much energy as possible as rapidly as possible, before the released energy causes the reactor to explode (and the chain reaction to stop). Development of nuclear weapons was the motivation behind early research into nuclear fission: the Manhattan Project of the U.S. military during World War Two carried out most of the early scientific work on fission chain reactions, culminating in the Little Boy and Fat Man bombs that were exploded over Japan in August of 1945.

Fission bombs are thousands of times more explosive than a comparable mass of chemical explosive. For example, Little Boy weighed a total of about four tons (of which 60 kg was nuclear fuel) and yielded an explosion equivalent to 15,000 tons of TNT, destroying a large part of the city of Hiroshima. Modern nuclear weapons are literally thousands of times more energetic, so that a single bomb could destroy an entire city.

While the fundamental physics of the fission chain reaction in a nuclear weapon is similar to the physics of a controlled nuclear reactor, the two types of device must be engineered quite differently (see nuclear reactor physics). It would be extremely difficult to convert a nuclear reactor to cause a true nuclear explosion (though fuel meltdowns and steam explosions have occurred), and similarly difficult to extract useful power from a nuclear explosive (though at least one rocket propulsion system, Project Orion, was intended to work by exploding fission bombs behind a massively padded vehicle!).

The strategic importance of nuclear weapons is a major reason why the technology of nuclear fission is politically sensitive. Viable fission bomb designs are within the capabilities of bright undergraduates (see John Aristotle Phillips), but nuclear fuel to realize the designs is thought to be difficult to obtain (see uranium enrichment and nuclear fuel cycle).

Adapted from the Wikipedia article "Physical overview", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki