Course description
When two nuclei come close together, or else a nucleus of an atom and a subatomic particle (such as a proton, neutron, or high energy electron) from outside the atom, collide, a nuclear reaction can occur to produce one or more nuclides that are different from the nuclide(s) that began the process. Therefore, nuclear reaction is a process in which the structure and energy content of an atomic nucleus are changed by interaction with another nucleus or particle (Figs 12 a, b, c).
In principle, a reaction can involve more than two particles colliding, but because the probability of three or more nuclei to meet at the same time at the same place is much less than for two nuclei, such an event is exceptionally rare.
Many nuclear reactions actually involve two separate stages. In the first, an incident particle strikes a target nucleus and the two combine to form a new nucleus, called a compound nucleus.
A given compound nucleus may be formed in a variety of ways. To illustrate this, Fig. 12 (d) shows six reactions whose product is the compound nucleus (asterisk signifies an excited state). Compound nuclei have lifetimes on the order of 10-16 s or so.
A given compound nucleus may decay in one or more ways, depending on its excitation energy. Thus with an excitation energy of, say, 12 MeV can decay in any of the four ways shown in Fig. 12 (d).
can also simply emit one or more gamma rays whose energies total 12 MeV. Usually a particular decay mode is favored by a compound nucleus in a specific excited state.
"Nuclear reaction" is a term implying an induced change in a nuclide, and thus it does not apply to any type of radioactive decay (which by definition is a spontaneous process).
Natural nuclear reactions occur in the interaction between cosmic rays and matter, and nuclear reactions can be employed artificially to obtain nuclear energy, at an adjustable rate, on demand.
Perhaps the most notable nuclear reactions are the nuclear chain reactions in fissionable materials that produces induced nuclear fission, and the various nuclear fusion reactions of light elements that power the energy production of the Sun and stars. Both of these types of reactions are employed in nuclear weapons.
If a nucleus interacts with another nucleus or particle and they then separate without changing the nature of any nuclide, the process is simply referred to as a type of nuclear scattering, rather than a nuclear reaction.
Nuclear Fission:
In nuclear physics and nuclear chemistry, nuclear fission is either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei) (Fig. 13 a).
The fission process often produces free neutrons and photons (in the form of gamma rays), and releases a very large amount of energy even by the energetic standards of radioactive decay (Fig. 13 b). Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom. The two nuclei produced are most often of comparable but slightly different sizes.
However, the process of breaking up of the nucleus of a heavy atom into two or more less equal nuclei with the release of an enormous amount of energy is known as fission. The new nuclei that result from fission are called fission fragments.
The process actually involves two separate stages (Fig. 13 b). When uranium is bombarded with neutrons, first the uranium nucleus captures a slow neutron to form a new nucleus, called a compound nucleus, whose atomic and mass numbers are respectively the sum of the atomic numbers of the original particles and the sum of their mass numbers. Compound nuclei have lifetime of 10-6 s or so. The compound nucleus then splits into two nearly equal parts. Because heavy nuclei have a greater neutron/proton ratio than lighter ones, the fragments contain an excess of neutron. To reduce this excess, two or three neutrons are emitted by the fragments as soon as they are formed.
In order for fission to produce energy, the total binding energy of the resulting elements must be less negative (higher energy) than that of the starting element.
The schematic equation for the fission process is
92U235 + 0n1 92U236* X + Y + neutrons
92U236* is a highly unstable isotope, and X and Y are the fission fragments. The fragments are not uniquely determined because various combinations of fragments along with emission of various numbers of neutrons are possible. Typical fission reactions are (Figs 13 c, d)
92U235 + 0n1 92U236* 56Ba141 + 36Kr92 + 30n1 + Q
92U235 + 0n1 92U236* 54Xe140 + 38Sr94 + 20n1 + Q
where Q is the energy released in the reaction.
Nuclear chain reaction:
A nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series of these reactions (Fig. 14 a). The specific nuclear reaction may be the fission of heavy isotopes (e.g. 235U). The nuclear chain reaction releases several million times more energy per reaction than any chemical reaction.
A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats (Fig. 14 b). The process may be controlled (nuclear power) or uncontrolled (nuclear weapons). If each neutron releases two more neutrons, then the number of fissions doubles each generation. In that case, in 10 generations there are 1,024 fissions and in 80 generations about 6 × 〖10〗^23 (a mole) fissions. Therefore, the self-sustaining fission process is called chain reaction.
Nuclear Fusion:
In nuclear physics, nuclear fusion is a nuclear reaction in which two or more atomic nuclei collide at a very high speed and join to form a new type of atomic nucleus. During this process, matter is not conserved because some of the matter of the fusing nuclei is converted to photons (energy), i.e., the decrease in mass comes off in the form of energy according to the Einstein relationship E = Δmc2.
Therefore, the process releases excess binding energy from the reaction, based upon the binding energies of the atoms involved in the process.
The power that fuels the sun and the stars is nuclear fusion (Fig. 15 a). In a hydrogen bomb, two isotopes of hydrogen, deuterium and tritium are fused to form a nucleus of helium and a neutron (Fig. 15 b). This fusion releases 17.6 MeV of energy (Fig. 61 b). Unlike nuclear fission, there is no limit on the amount of the fusion that can occur
Idea about nuclear power reactor: A nuclear reactor is a device used to initiate and control a sustained nuclear chain reaction. A reactor is a very efficient source of energy: the fission of 1 gm of a suitable fissionable material per day evolves energy at the rate of about 1 MW. For producing energy at the same rate combustion of 2.6 tons of coals per day will be needed. The energy liberated in a nuclear reactor becoes
becomes heat in its interior, and this heat is removed by circulating a liquid or gas coolant. The hot coolant is then used to boil water, and the resulting steam is fed to a turbine that can power an electric generator to produce electricity.
Main components
Ø The core of the reactor contains all of the nuclear fuel and generates all of the heat. It contains low-enriched uranium (< 5% U-235), control systems, and structural materials. The core can contain hundreds of thousands of individual fuel pins.
Ø The coolant is the material that passes through the core, transferring the heat from the fuel to a turbine. It could be water, heavy-water, liquid sodium, helium, or something else.
Ø The turbine transfers the heat from the coolant to electricity, just like in a fossil-fuel plant.
Ø The containment is the structure that separates the reactor from the environment. These are usually dome-shaped, made of high-density, steel-reinforced concrete.
Ø Cooling towers are needed by some plants to dump the excess heat that cannot be converted to energy due to the laws of thermodynamics. These are the hyperbolic icons of nuclear energy. They emit only clean water vapor.
Fig. 16 (a) shows a nuclear reactor heating up water and spinning a generator to produce electricity. The water coming into the condenser and then going right back out would be water from a river, lake, or ocean. This water does not go near the radioactivity, which is in the reactor vessel.
Fuel, made up of heavy atoms that split when they absorb neutrons, is placed into the reactor vessel (basically a large tank) along with a small neutron source. The neutrons start a chain reaction where each atom that splits releases more neutrons that cause other atoms to split (Fig. 16 b). Each time an atom splits, it releases large amounts of energy in the form of heat. The heat is carried out of the reactor by coolant, which is most commonly just plain water. The coolant heats up and goes off to a turbine to spin a generator or drive shaft. Nuclear reactors are just exotic heat sources. Fig. 16 (c) shows a type of reactor.
Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. Some are run only for research. Today there are about 450 nuclear power reactors that are used to generate electricity in about 30 countries around the world (Fig. 16 d).
Radioactivity and Nuclear Physics
Q1. What are the reasons to occur radioactivity?
Q2. What is meant by radiation?
Q3. Define the SI unit of radioactivity.
Q4. Derive an expression for the law governing radioactive decay. Draw the graph of decay.
Q5. Define the half-life of a radioactive element.
Q6. Show that the half-life of a radioactive element is inversely proportional to its decay constant.
Q7. What is radioactive equilibrium?
Q8. When a daughter product is said to be in secular equilibrium with the parent?
Q9. When a daughter product is said to be in transient equilibrium with the parent?
Q10. Explain the mass defect and binding energy of a nucleus. Give example.
Q11. How does nuclear reaction occur?
Q12. Explain the separate stages involved in nuclear reactions?
Q13. What are nuclear fission and fusion? Give example.
Q14. Show that the nuclear chain reaction is a self-sustaining fission process.
Q15. Explain the working principle of a nuclear reactor.
