Introduction, types of nuclear reaction, nuclear reactors

2. INTRODUCTION
• Nuclear Chemistry is sub discipline of chemistry. It is
concerned with changes in the nucleus of atom. Nuclear
changes are source of radioactivity & nuclear power.
• It deals with radioactivity, nuclear process and
transformation in the nuclei of atom (transmutation) and
nuclear properties.
• Deals with production and use of radioactive source and
fuels.
• Atom of the element consists of three fundamental particles
proton, electron and neutron which are called sub-atomic
particles.
• These particles are mainly responsible for physical, chemical
and also nuclear behavior of atoms of all the elements.
• Out of them protons and neutrons are jointly called
nucleon.

3.  Nuclear reaction can be brought about by the interaction of
two nuclei or under the impact of a subatomic particle on the
nucleus. Nuclear chemistry deals with the study of nuclear
particles, nuclear forces and nuclear reactions.
• Isotopes - An atom have the same number of protons, but a
different number of neutrons. (same atomic no. but different
mass)
• Isobar- same atomic mass
• Radioisotopes:unstable isotopes which are distinguishable by
radioactive transformation.
E.g-C6
12
Here 12- mass no. –It is the sum of the no. of protons & the no. of
neutrons. denoted by A
6- atomic no.- Atomic No.: It is equal to no. of protons present in the
nucleus of its atom. denoted by Z

6. Nuclear reaction
Nuclear reactions are processes in which one or more nuclides
are produced from the collisions between two atomic nuclei
or substraction one atomic nucleus and a subatomic particle.
Types-
1. Nuclear fusion
2. Nuclear fission
3. Radioactive decay
4. Chain reaction

7. Radioactivity
The process in which an unstable isotope undergoes
changes until a stable state is reached . When the atomic nucleus
undergoes spontaneous transformation, called radioactive decay.
During this various radiation is emitted i.e alpha particles, beta
particles and gamma rays.

8. Important parameters
Physical half-life- it is the period of time required to
reduce the radioactivity level of a substance to exactly
one half its original value due solely to radioactive
decay.
Biological half-life-The time required for a living
organism to eliminate one-half of a radioactive
substance which has been introduced into it.

20. 2. Nuclear fission
 refers to the splitting of an atomic nucleus into two or more
lighter nuclei. This process can occur through a nuclear
reaction or through radioactive decay.
 Nuclear fission reactions often release a large amount of
energy, which is accompanied by the emission of neutrons
and gamma rays (photons holding huge amounts of energy,
enough to knock electrons out of atoms).

21. • Nuclear fission was first discovered by the German chemists Otto Hahn
and Fritz Strassmann in the year 1938. The energy produced from fission
reactions is converted into electricity in nuclear power plants. This is done
by using the heat produced from the nuclear reaction to convert water
into steam. The steam is used to rotate turbines in order to generate
electricity.
Examples
An important example of nuclear fission is the splitting of the uranium-235
nucleus when it is bombarded with neutrons.
• 235U + 1n → 141Ba + 92Kr + 3 1n
• 235U + 1n → 144Xe + 90Sr + 2 1n
• 235U + 1n → 146La + 87Br + 3 1n
• 235U + 1n → 137Te + 97Zr + 2 1n
• 235U + 1n → 137Cs + 96Rb + 3 1n
• Another important example of nuclear fission is the splitting of the
plutonium-239 nucleus.

24. 3. Nuclear fusion
 In nuclear fusion reactions, at least two atomic nuclei
combine/fuse into a single nucleus. Subatomic particles such
as neutrons or protons are also formed as products in these
nuclear reactions.
nuclear fusion reaction between deuterium (2H) and tritium (3H) that yields helium (4He)
and a neutron (1n). The fusion of deuterium and tritium nuclei is accompanied by a loss of
approximately 0.0188 amu of mass (which is completely converted into energy).
Approximately 1.69*109 kilojoules of energy are generated for every mole of helium
formed.

26. • The increases in binding energy per nucleon are much larger for fusion
than for fission reactions, because the graph increases more steeply for
light nuclei.
• So fusion gives out more energy per nucleon involved in the reaction
than fission.
• Fusion has a number of advantages over fission:
 greater power output per kilogram,
 the raw materials are cheap and readily available,
 no radioactive elements are produced directly,
 irradiation by the neutrons leads to radioactivity in the reactor materials
but these have relatively short half lives and only need to be stored safely
for a short time.

27. 4. 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. The process may be controlled (nuclear power) or
uncontrolled (nuclear weapons).
 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. The specific nuclear
reaction may be the fission of heavy isotopes (e.g., uranium-235, 235U).
 The concept of a nuclear chain reaction was reportedly first hypothesized
by Hungarian scientist Leó Szilárd on September 12, 1933.

28. • When the uranium nucleus splits, a number of
neutrons are also ejected. If each ejected neutron
causes another uranium nucleus to undergo fission, we
get a chain reaction The number of fissions increases
rapidly and a huge amount of energy is released.
• Uncontrolled chain reactions are used in nuclear
bombs The energy they unleash is devastating. Nuclear
power stations use the heat released in carefully
controlled fission reactions to generate electricity. They
use control rods to absorb some of the neutrons.

29. U235 + n → fission + 2 or 3 n + 200 MeV
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 x 10 23 (a mole) fissions.

30. Liquid Drop Model (water drop model)
• The liquid drop model was formulated by Niels Bohr as a theory as to
how nuclear fission takes place. Nuclear fission is the splitting of a
nucleus into several smaller parts. Bohr thought that this process
would mimic that of the molecules of a liquid drop splitting apart.
Assumptions
 The nuclei of all elements are considered to be behave like a liquid
drop of very high density.
 In an equilibrium state the nuclei of atoms remain spherically
symmetric under the action of strong attractive nuclear forces just
like the drop of a liquid which is spherical due to surface tension.
 The density of a nucleus is independent of its size just like the density
of liquid which is also independent of its size.
 The nucleons of the nucleus move about within a spherical enclosure
called the nuclear potential barrier just like the movement of the
molecules of a liquid within a spherical drop of liquid.

31.  Liquid-drop model- a description of atomic nuclei in which the
nucleons (neutrons and protons) behave like the molecules in
a drop of liquid.
 The molecules in a liquid are held together by Van der Waals force
that is only between near neighbors.
 describe the masses and binding energy of nuclei.
 If given sufficient extra energy (as by the absorption of a neutron),
the spherical nucleus may be distorted into a dumbbell shape and
then split at the neck into two nearly equal fragments, releasing
energy.
 The liquid drop model is applied to describe some basic properties
of atoms, homoatomic molecules, metallic clusters of atoms(nano
material with intermediate state of matter between molecule and
bulk) and fullerene molecules(hexagonal ring of carbon join by
covalant bond). Equilibrium atomic size, energy and polarizability of
the atom are calculated. Electromagnetic radiation by an atom,
passing through a barrier is also calculated.

32. He thought that the positive charges in the nucleus would repel from each
other, thereby splitting the nucleus apart

33. Main Achievements of liquid dropmodel (LDM)
1.It explains binding energy of large number of nuclei.
2.It explains the fusion and fission processes nicely.
3. Explains energies of radioactive decays, fission and fusion.
Applications of the water-drop model
1. Nuclear fission(very large nuclei break up)
2. Nuclear fusion(very small nuclei fuse together)
Main drawbacks of liquid dropmodel (LDM)
1. It is not able to explain excited states.
2. It is not able to calculate the nuclear spin.

34. Nuclear Stability, Mass Defect and Binding
Energy, N/Z Ratio:-
Stability of nucleus is affected by the various
factors as fallows.
1. Nuclear forces:- (strong attractive force between proton and neutron)
Nucleus has a very small size (radius 10-10 m) in
which positively charged protons and neutral
neutrons are packed together, but still nucleus is
stable. This is because some strong attractive forces
must be holding these particles together in the
nucleus and it is surrounded by electron cloud.

35. 2. Mass defect and Binding energy:-
A) Mass defect
 The difference between calculated mass and observed atomic mass is
called as mass defect.
 Mass defect of a nucleus represents mass of the energy binding the
nucleus and difference between the mass of nucleus and the sum of the
masses of the nucleons of which it is composed.
 Mathematically it can be calculated by using eqn
Δm = [ZmH + (A-Z) mn] – M
Where, Δm = mass defect,
A = mass number
ZmH = mass Z proton or hydrogen atoms,
(A-Z)mn = mass of (A-Z) neutrons,
M = observed atomic mass.
 Another formula, mass defect (Md)= (Mn+Mp) – Mo
Mn- mass of neutron
Mp- mass of proton
Mo- observed mass

36. b) Binding energy (B.E.)- always +ve, becoz nuclei require energy to separate
 It is the energy released in binding the nucleons together in the
nucleus." OR "it is the energy required to break the nucleus of an
atom into its isolated nucleons.
 Determine whether fission or fusion is the favourable process.
 This release of energy is due to loss of some mass and is given by
Einstein's equation as, (when mass defect is known)
E = Δmc2
Where, Δm = mass defect or mass lost
C = velocity of light.
 If Δm is in grams and C is in cm/sec., then Binding Energy is in ergs.
If Δm is in kg and C is in m/sec., then Binding Energy is in joules.

38. Nuclear Reactor
 A nuclear reactor produces and controls the release of energy
from splitting the atoms of certain elements. In a nuclear
power reactor, the energy released is used as heat to make
steam to generate electricity.
 The energy released from continuous fission of the atoms of
the fuel is harnessed as heat in either a gas or water, and is
used to produce steam. The steam is used to drive the
turbines which produce electricity (as in most fossil fuel
plants).
 The world's first nuclear reactors operated naturally in a
uranium deposit about two billion years ago.

39. Components of a nuclear reactor
• Fuel
Uranium is the basic fuel. Usually pellets of uranium oxide (UO2)
are arranged in tubes to form fuel rods. The rods are arranged into
fuel assemblies in the reactor core
• Moderator.
Material in the core which slows down the neutrons released from
fission so that they cause more fission. It is usually water, but may
be heavy water or graphite.
• Control rods.
These are made with neutron-absorbing material such as cadmium,
hafnium or boron, and are inserted or withdrawn from the core to
control the rate of reaction, or to halt it.
* In some PWR reactors, special control rods are used to enable the
core to sustain a low level of power efficiently. (Secondary control
systems involve other neutron absorbers, usually boron in the
coolant – its concentration can be adjusted over time as the fuel
burns up.

40.  Coolant.
A fluid circulating through the core so as to transfer the heat from
it. In light water reactors the water moderator functions also as
primary coolant. Except in BWRs, there is secondary coolant circuit
where the water becomes steam.
• Pressure vessel or pressure tubes.
Usually a robust steel vessel containing the reactor core and
moderator/coolant, but it may be a series of tubes holding the fuel
and conveying the coolant through the surrounding moderator.
• Steam generator.
Part of the cooling system of pressurized water reactors (PWR &
PHWR) where the high-pressure primary coolant bringing heat from
the reactor is used to make steam for the turbine, in a secondary
circuit. Essentially a heat exchanger like a motor car radiator.
• Containment
The structure around the reactor and associated steam generators
which is designed to protect it from outside intrusion and to
protect those outside from the effects of radiation in case of any
serious malfunction inside. It is typically a meter-thick concrete
and steel structure

41. Very high speed of the radiations
breaking the matter (until a complete stop)
Heat release
Heat removal by a coolant (a liquid or a gas) – produce stem
Transformation into energy (electrical or other, via vapor or other)

43. TYPE OF REACTOR
1-PRESSURISED WATER REACTOR (PWR)
• This is the most common type, with over 230 in use for power
generation and several hundred more employed for naval
propulsion.
• The design of PWRs originated as a submarine power plant.
PWRs use ordinary water as both coolant and moderator.
• The design is distinguished by having a primary cooling circuit
which flows through the core of the reactor under very high
pressure, and a secondary circuit in which steam is generated
to drive the turbine. In Russia these are known as VVER types
– water-moderated and -cooled.

44. 2-Boiling water reactor (BWR)
• This design has many similarities to the PWR, except that there is only a single circuit
in which the water is at lower pressure (about 75 times atmospheric pressure) so
that it boils in the core at about 285°C. The reactor is designed to operate with 12-
15% of the water in the top part of the core as steam, and hence with less
moderating effect and thus efficiency there. BWR units can operate in load-following
mode more readily then PWRs.
• The steam passes through drier plates (steam separators) above the core and then
directly to the turbines, which are thus part of the reactor circuit. Since the water
around the core of a reactor is always contaminated with traces of radionuclide's, it
means that the turbine must be shielded and radiological protection provided
during maintenance. The cost of this tends to balance the savings due to the simpler
design. Most of the radioactivity in the water is very short-lived*, so the turbine hall
can be entered soon after the reactor is shut down.
• The steam passes through drier plates (steam separators) above the core and then
directly to the turbines, which are thus part of the reactor circuit. Since the water
around the core of a reactor is always contaminated with traces of radionuclide's, it
means that the turbine must be shielded and radiological protection provided
during maintenance. The cost of this tends to balance the savings due to the simpler
design. Most of the radioactivity in the water is very short-lived*, so the turbine hall
can be entered soon after the reactor is shut down.
• A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750
assemblies in a reactor core, holding up to 140 tones of uranium. The secondary
control system involves restricting water flow through the core so that more steam
in the top part reduces moderation.

45. 3-Pressurised heavy water reactor (PHWR)
• The PHWR reactor design has been developed since the 1950s in Canada as the
CANDU, and from 1980s also in India. PHWRs generally use natural uranium (0.7% U-
235) oxide as fuel, hence needs a more efficient moderator, in this case heavy water
(D2O). The PHWR produces more energy per kilogram of mined uranium than other
designs, but also produces a much larger amount of used fuel per unit output
• The moderator is in a large tank called a calandria, penetrated by several hundred
horizontal pressure tubes which form channels for the fuel, cooled by a flow of heavy
water under high pressure in the primary cooling circuit, reaching 290°C. As in the PWR,
the primary coolant generates steam in a secondary circuit to drive the turbines. The
pressure tube design means that the reactor can be refueled progressively without
shutting down, by isolating individual pressure tubes from the cooling circuit. It is also
less costly to build than designs with a large pressure vessel, but the tubes have not
proved as durable.

46. 4-Advanced gas-cooled reactor (AGR)
• These are the second generation of British gas
cooled reactors, using graphite moderator and
carbon dioxide as primary coolant. The fuel is
uranium oxide pellets, enriched to 2.5-3.5%, in
stainless steel tubes. The carbon dioxide
circulates through the core, reaching 650°C and
then past steam generator tubes outside it, but
still inside the concrete and steel pressure vessel
(hence 'integral' design). Control rods penetrate
the moderator and a secondary shutdown system
involves injecting nitrogen to the coolant.

47. 5-Light water graphite-moderated reactor (RBMK)
• This is a Soviet design, developed from plutonium production reactors.
It employs long (7 meter) vertical pressure tubes running through
graphite moderator, and is cooled by water, which is allowed to boil in
the core at 290°C, much as in a BWR. Fuel is low-enriched uranium
oxide made up into fuel assemblies 3.5 meters long. With moderation
largely due to the fixed graphite, excess boiling simply reduces the
cooling and neutron absorption without inhibiting the fission reaction
and a positive feedback problem can arise, which is why they have
never been built outside the Soviet Union.
6-Fast neutron reactors (FNR)
• Some reactors (only one in commercial service) do not have a
moderator and utilize fast neutrons, generating power from plutonium
while making more of it from the U-238 isotope in or around the fuel.
While they get more than 60 times as much energy from the original
uranium compared with the normal reactors, they are expensive to
build. Further development of them is likely in the next decade, and
the main designs expected to be built in two decades are FNRs. If they
are configured to produce more fissile material (plutonium) than they
consume they are called Fast Breeder Reactors (FBR).