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
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
A given compound nucleus may decay in one or
more ways, depending on its excitation energy. Thus
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
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.