A presentation on Nuclear Reactor and the physics behind it .

2. Prepared By :- Rudra Pratap Sen
Roll No :- 3029
Student Id :- 2020-574
Section :- Day
Paper

3.  Nuclear Reactor
 Types of Nuclear Reactor
 Nuclear Fission Reaction
 U-235 Fission
 Basic Elements of a Nuclear Reactor
 Dangers Of Nuclear Reactor
 Acknowledgement

4. What is a Nuclear Reactor ?
 A Nuclear Reactor is essentially a device wherein a neutron-induced self-sustained chain
reaction involving fission of heavy elements take place. The purpose of the reactor is (i)
initiate nuclear fission reaction, (ii) control these reactions, and (iii) extract the energy
produced by fission. The control of neutrons is the key to the functioning of a nuclear
reactor.
 Nuclear Reactors are used at Nuclear Power Plants , Nuclear Marine Propulsions .
 Chicago Pile-1 (CP-1) was the world's first artificial nuclear reactor. On 2 December 1942,
the first human-made self-sustaining nuclear chain reaction was initiated in CP-1, during an
experiment led by Enrico Fermi. It was then called a uranium-carbon pile.
 India’s first Research Nuclear Reactor became operational in August 1956 at Trombay
Campus of Bhaba Atomic Research Centre . It was decommissioned in 2009. The first
nuclear power reactors built in India were two BWRs at Tarapur, constructed by GE as
turnkey projects through Indo-US cooperation.
Fig-1:- First Reactor
construction in 1942
Fig-2:- ‘APSARA’ , India’s
first research nuclear
reactor

5. There are several types of Nuclear Reactors which are used to
generate heat energy at power plants . Main types of reactors
follows as
 Pressurized water reactor (PWR)
 Boiling water reactor (BWR)
 Advanced gas-cooled reactor (AGR)
 Light water graphite-moderated reactor (LWGR)
 Fast neutron reactor (FNR)
 Pressurized heavy water reactor (PHWR)
 High temperature gas-cooled reactor (HTGR)
Among these various types of Reactors PWR is widely used . Fig-3:- Schematic of a Pressurized
Water Reactor

6.  The phenomenonof the division or disintegration of a heavy nucleus into two nuclei of
comparable masses is termed nuclear fission. It is analogous to cell division in biology. Nuclear
Fission can either be rapid or controlled . Controlled nuclear fission is used in Nuclear Reactor.
 Nuclear fission of heavy elements was discovered on 19th December 1938 by German chemist
Otto Hahn & his assistant Fritz Strassman in co-operationwith Austrian Swedish physicist Lise
Meitner .
 The chemical element isotopes that can sustain a fission chain reaction are called nuclear fuel,
are said to be fissile. The most common nuclear fuels are 92
235𝑈 & 94
239𝑃𝑢 . In a nuclear reactor or
nuclear weapon, the overwhelming majority of fission events are induced by bombardment with
another particle, a neutron which is itself produced by prior fission events.
 Nuclear fission can occur without neutron bombardment as a type of radioactive decay. This
type of fission ( Called spontaneous fission) is rare except in a few heavy isotopes.
Fig-4:- Uranium metal
highly enriched in
uranium-235
Fig-5:- A 99.96% pure ring
of plutonium

7. 92
235
𝑈 FISSION
Uranium-235 is the most used nuclear fuel . When a slow neutron ( having
therefore a high value for capture cross-section ) bombardedinto a
Uranium-235 atom , a highly unstable isotope of Uranium , 92
236
𝑈 is created .
Instantly this isotope breaks apart and creates two daughter nuclei which
follows as 56
141
𝐵𝑎 & 36
92
𝐾𝑟 . Three neutron is also created & a huge amount of
energy which is nearly 200 MeV is released.That three neutrons also take
part into the reaction & bombards another Uranium-235 atoms. This process
takes place rapidly and a chain reactions starts. As a result we get a
overwhelmingamount of energy in a fractions of seconds.
The Reaction follows as :-
92
235
𝑈 + 0
1
𝑛 -> 92
236
𝑈 -> 56
141
𝐵𝑎 + 36
92
𝐾𝑟 + 30
1
𝑛 + 200 MeV
This reactions happens in the core of a nuclear reactor in a controlled way.
Natural Uranium contains 0.7% of U-235 isotope. The remaining 99.3% being
U-238, which is not fissionableby thermal neutrons.One of the common
methods to enrich natural uranium with U-235 isotope is the gaseous
diffusion through a porous barrier. Naturally enriched uranium by U-235
isotope is also used in many reactors. Fig-6:- Chain Reaction of U-235 Fission
Fig-5:- U-235 fission by thermal neutron

8. All types of nuclear reactors contain the following basic essential elements or component
I. Fuels:- Fuel undergoes fission and thereby supplies neutrons for inducing further fissions. The commonly used
reactor fuels are the Uranium isotope U-235 , Thorium isotope Th-232 , Plutonium isotope Pu-239 .
II. Reflector:- Some of the neutrons produced by fission will leak out of the reactor and so not be part of the chain
reaction. Leakage is a surface effect, its magnitude is proportional to the squarer of a typical reactor dimension. A
reflector is placed around the reactor core to prevent neutrons from escaping from the core. Efficiency of a
reflector increases rapidly with its thickness. Good moderators are usually good reflectors.
III. Moderator:- The neutrons produced by fission are fast, with kinetic energy of about 2 MeV. However, Fission is
induced most effectively by thermal neutrons. The fast neutrons can be slowed down by mixing the uranium fuel
with a substance – called Moderator –that has two properties: It is effective in slowing down neutrons via elastic
collisions, and it does not remove neutrons from the core by absorbing them so that they do not result in fission.
Good moderators should have a large scattering cross section and small neutron capture cross section. The usual
moderators are : Graphite , Heavy Water , Beryllium Oxide , Hydrides of metal and organic liquids. The nuclei of
these materials hardly absorbs neutron.
IV. Cooling System:- The cooling system in a reactor helps to control the temperature of the fuel element and
transport the heat generated by fission to the heat engine. There are four types of possible coolants. These are (i)
Gases: air , carbon-di-oxide, helium or steam, (ii) Water type liquids: water or heavy water, (iii) Molten Metals: Hg,
Na, K, Pb, Bi & (iv) fused salts. Each type has its own merits & demerits.
V. Safety System:- Safety of reactor is accomplished by surrounding the reactor with massive layers of concrete and
lead and by providing completely closed coolant circuits.

9. VI. Control Rods:- An important reactor parameter is the ratio of the number of neutrons present at the conclusion of a
particular generation to the number present at the beginning of that generation. In Fig-7 the multiplication factor is
1000/1000, or exactly unity. For k=1 , the operation of reactor is said to be exactly critical, which is what we wish it to be for
steady power operation. Reactors are actually designed so that they are inherently supercritical (k>1) . The multiplication
factor is then adjusted to critical operation by inserting control rods into the reactor core . These rods, containing a material
such as cadmium that absorbs neutrons readily, can be inserted farther to reduce the operating power level and withdrawn
to increase the power level or to compensate for the tendency of reactors to go subcritical as (neutron absorbing) fission
products build up I the core during continued operation.
Fig-7:- Neutron Bookkeeping in a reactor
*NOTE* The Neutron Capture Problem:- As fast (2 MeV) neutrons generated by
fissions are slowed down in the moderator to thermal energies (about 0.04 MeV)
they must pass through acritical energy interval (from 1 to 100 MeV) in which
they are particularly susceptible to nonfission capture by U-238 nuclei. Such
resonance capture , which results in the emission of a gamma ray, removes the
neutron from the fission chain. To minimize such nonfission capture, the uranium
fuel and the moderator are not intimately mixed but rather are placed in
different regions of the reactor volume.
VII. Steam Generator:- It is the part where heat is exchanged with water and water become vapor.
Pressurized vapor is send to turbine through a narrow channel . Steam turbines are connected to
dynamos which generates electricity.

10. Nuclear Power is no doubt a greener energy option . We often see smoke coming out of big
chimneys of nuclear power plant . But that is vaporized water which is used to cool the reactor
core . Most of the water used in reactor is reused in the process. Only a small amount of fuel in
enough to generate sufficient amount of energy . But like other energy resources it has its own
demerits. Though we can tell Nuclear Energy pollution free to most extent but it is hazardous if it
is not handled properly. Nuclear Reactors produces radioactive plutonium isotopes as nuclear
waste . Till today nuclear waste disposal is major concern for nations worldwide. Mishandled
nuclear waste can easily contaminate environment which can have a far reaching bad effects on
the living organism of earth.
Fig-8:- Nuclear waste disposal
in a nuclear power plant
Another concern for nuclear reactor is nuclear disasters . Sudden failure of
safety system , uncontrolled power surge , failure of cooling system , even human errors
can cause a severe nuclear disaster . The Chernobyl disaster was a nuclear accident that
occurred on 26 April 1986 at the No. 4 reactor in the Chernobyl Nuclear Power Plant, near
the city of Pripyat in the north of the Ukrainian SSR in the Soviet Union. It is one of only
two nuclear energy accidents rated at seven—the maximum severity—on the International
Nuclear Event Scale, the other being the 2011 Fukushima nuclear disaster in Japan.
Fig-9:- Chernobyl Nuclear Disaster
Keeping in mind that fossil fuels will not be available at near future we
have to make efforts to make nuclear energy as safe as possible .

11.  Principles of Physics ; International Student
Version ; Tenth Edition by Haliday, Resnick,
Walker
 Modern Atomic and Nuclear Physics ; Fourth
Edition by A.B.Gupta
 https://en.Wikipedia.org
 https://www.google.com
 https://www.barc.gov.in
 https://world-nuclear.org