Unit 3 nuclear power plants

ME 6701
POWERPLANT ENGINEERING
UNIT 3
NUCLEAR POWER PLANTS
S.PALANIVEL ASSOCIATE PROF./MECH
ENGG KAMARAJ COLLEGE OF ENGG & TECH
(NEAR) VIRUDHUNAGAR

Basics of Nuclear Engg.
S.PALANIVEL ASSOCIATE PROF./MECH ENGG
KAMARAJ COLLEGE OF ENGG & TECH (NEAR) VIRUDHUNAGAR

• All matters are composed of unit particles called ATOMs
• Atoms consists of +vely charges NUCLEUS & -vely charged
Electrons orbiting around Nucleus
• Nucleus consists of PROTONs (+vely charged) & Neutrons
(neutrally Charged)
• No.of Protons equal to no. of electrons.
• No. of protons in the nucleus is ATOMIC Number
• Total no.of nucleons (Protons+Neutrons) is MASS Number
• Neutron mass = mn = 1.008665 amu = 1.674 x 10-27
kg.
• Proton mass = mp = 1.007277 amu = 1.673 x 10-27
kg.
• Electron mass = me = 0.0005486 amu = 9.109 x 10-31
kg.

Chemical & Nuclear REACTIONS
Atoms are combined or separated in a Chemical reactions
C + O2 = C O2 an energy of 4 electron volt (ev) is released. 1 e.v =
1.6021 x 10-19
J
It is the energy acquired by an electron when it is accelerated
across a potential difference of 1 V.
In a Chemical reactions although molecules change, each
participates as a whole and retains its identity. Only valance
electrons are shared or exchanged. Nuclei does not change.
No. of atoms on each elements in the products is equal to
the no. in the reactants.
In Nuclear reactions, the products do not have the reactant
Nuclei but some other Nuclei. No. of nucleons in the
products are the same as those in reactants.

MASS DEFECT
• The sum of masses of the Protons and Neutrons that
comprise the nucleus EXCEEDS the mass of the atomic
Nucleus. This difference is called “Mass Defect’
• This can be found by adding all the individual particle
weights and subtracting the actual mass of the atom.
• Mass defect is converted to energy in a Nuclear reaction
• Energy associated with mass defect is known as ‘binding
energy’ (BE) of the Nucelus.
• It acts as a glue which binds the Protons and Neutrons
together in the Nucleus.
• Binding energy per nucleon can be calculated for all
isotopes. Higher the binding energy per nucleon, higher is
the stability of the nucleus
• If elements of low mass no. are fused together it will lead to
more stable elements

• The elements of higher mass number are less stable and if they
are fissioned, they would form elements of less mass number,
which are more stable.
• Light isotopes like hydrogen, deuterium are good for fusion &
heavier isotopes like Uranium are suitable for fission reaction.
• Most medium & heavy nuclei have binding energy per nucleon
between 7.5 and 8.7 Mev.
• If a nucleus is to expel one nucleon (say neutron) it should first
have a minimum excitation energy between 7.5 and 8.7 Mev.
Only in such an excited state a nucleus can emit a neutron.
• It was found that nuclei of the even- even type, having even no.
of protons and even no. of neutrons are very stable.
• U238
atom having 92 protons and 146 neutrons is quite stable and
requires very high energy neutrons for fissions
• U235
having 92 protons and 143 neutrons can be fissioned even
by low energy neutrons.S.PALANIVEL ASSOCIATE PROF./MECH ENGG

Radioactive Decay
• Most Isotopes that occur in nature are stable.
• Some Isotopes of heavy elements like thallium (Z=81), lead
(Z=82) and bismuth (Z=83) and all isotopes of heavier elements
starting with Polonium(Z=84) are not stable since the binding
energy per nucleon being small and emit radiation till a more
stable nucleus is reached. Thus spontaneous disintegration
process called Radioactivity decay occurs.
• Resulting Nucleus is called daughter and the original Nucleus is
called the Parent. Daughter product may be stable or
radioactive.
• Lower mass Isotopes such as K40
, Rb87
& ln115
are also naturally
radio active.
• Radio active Isotopes both natural and man made are called as
“radio Isotopes” S.PALANIVEL ASSOCIATE PROF./MECH ENGG

An example of radioactivity is
49ln115
50Sn115
+ -1e0
Where ln115
is a naturally occurring radioisotope and its
daughter Sn115
is stable.
Radio activity is always accompanied by a decrease of mass or
liberation of energy. Energy thus liberated are in the form of
KE of the emitted particles and as electro- magnetic
radiation ( rays)Ƴ
Naturally occurring radioisotopes emit
1. ἁ particles 2. ᵝ particles , 3. radiation : undergoƳ
4. Positron decay, 5.orbital electron absorption called ‘K”
capture and 6.emit neutrons and neutrinos.

• (ἁ) Alpha decay : Alpha particles are helium nuclei,
commonly emitted by heavier radioactive nuclei and are
accompanied by radiationƳ
• (ᵝ) Beta decay : It is equivalent to the emission of an
electron and raises the atomic number by one, while the
mass number remains the same. It is usually accompanied
by the emission of neutrino (v) and radiation.Ƴ
• 82Pb214
ᵝ 83Sn214
+ -1e0
+ v
• Penetration of power of ᵝ particles is small compared to Ƴ
rays but it is larger than that of ἁ particles.

• ( ) Gamma RadiationƳ : This is an electro magnetic radiation
of extremely short wave length and very high frequency and
hence, high energy (C= hv) . raysƳ originate from the
nucleus, while X- rays originate from the atom. By emitting Ƴ
radiation an excited daughter nucleus falls back into its stable
ground (lowest) energy state. Ƴ radiation is highly
penetrating and does not affect either the atomic or mass
number.
(ZXA
)*
ZXA
+ Ƴ
Positron Decay : When a radioactive nucleus contains an excess
of protons, positron (positive beta) decay occurs. Converting
a proton into a neutron in the process.
7N13
6C13
+ -1e0
15P30
14Si30
+ -1e0

Positron decay (Contd.)
• The two particles, positron and electron are thus
annihilated producing energy equivalent to sumƳ
of their rest masses, (ie) 2 x 0.00055 x931 = 1.024
Mev.
• The parent nucleus must be at an energy level of
atleast 1.02 Mev above the ground state to
facilitate positron decay .
• The reverse of annihilation process is called pair
production. A photon having an energyƳ
exceeding 1.02 Mev can form positron- electron
pair, converting energy into mass in an endothermic
process.

K capture& Neutron emission
• When a nucleus posses an excess of protons but does not
have the threshold energy of 1.02 Mev to emit a positron, it
captures an orbital electron from the rear most or K shell.
• A proton in the nucleus changes into a neutron by K capture.
The vacancy in the K shell is filled by another electorn falling
from a higher orbit. Thus k capture is accompanied by X- ray
emission from the atom.
29Cu64
+ -1e0
28Ni64
Neutron emission : If a nucleus possess an extremely high
excitation energy, it may emit a neutron. The binding energy
of a neutron varies with mass number, but is on an average
above 8 Mev. If the excitation energy is 8 Mev or more, the
nucleus could decay by emission of a neutron
54Xe137
54SXe136
+ 0n1

Neutron emission (contd.)
In a nuclear decay the daughter is an isotope of the
parent. Though it occurs rarely, it however comes
about in nuclear reactors yielding delayed fission
neutrons which greatly influence the reactor control.
The rate of decay is a function only of the number of
radioactive nuclei present at a time, provided that
the number is large. It does not depend on temp,
pressure or physical and chemical states of matter.
(ie.) whether it is in solid , liquid or gaseous state or
in chemical combinations.

Half Life
• If N be no. of radioactive nuclei of one species at any time Ѳ,
the rate of decay - dN = λN , where λ is constant of
dѲ
proportionality called decay constant having different values for
different isotopes with dimension s-1
By integrating we get N= N0 e-λѲ
Where N= radioactive atoms present at time Ѳ,
N0=Radioactive atoms present at time =0
Rate of decay - dN is also called as Activity A : A = λN = λN0 e-λѲ
dѲ A = A0 e-λѲ
Decay rate is often expressed in the form of half life, time during
which one half of number of radioactive species decays.
N/N0 = A/ A0 = ½ = e-λѲ/2
: Ѳ/2 = ln 2/ λ =0.6931/ λ.
Thus the half life is inversely proportional to decay constant.S.PALANIVEL ASSOCIATE PROF./MECH ENGG KAMARAJ COLLEGE OF ENGG & TECH (NEAR) VIRUDHUNAGAR

ISOTOPES HALF LIFE ACTIVITY ISOTOPES HALF LIFE ACTIVITY
Tritium (H3
) 12.26yrs. ᵝ Thorium 233 22.1 min ᵝ
Carbon 14 5730 yrs. ᵝ Protactinium
233
27 days ᵝ & Ƴ
Krypton 87 76 min. ᵝ Uranium 233 1.65 x 105
yr ἁ & Ƴ
Strontium90 28.1 yr ᵝ Uranium 235 7.1 x 108
yr ἁ & Ƴ
Xenon 135 9.2 hour ᵝ & Ƴ
Barium 139 82.9 min. ᵝ & Ƴ
Radium
223 11.43 days ἁ & Ƴ
Thorium 232 1.41 x 106
yr ἁ & Ƴ

Finger Prints
• No two radio isotopes have exactly the same
lives. Thus, half lives are considered as
“finger prints’ to identify a radio isotope.
• Readily fissionable isotopes U- 233, U-235,
and Pu-239 have extremely high half lives,
showing that they can be stored practically
indefinitely.

• The unit of radio activity is Curie (Ci)
• 1 Curie = 3.615 x 1010
dis/s
• It was based on measurement of activity of 1 gm of
radium 226. Curie is now been superseded by the SI
unit Becquerel (Bq) which is defined as one
disintegration per second.
• Since this is very small, the level of radio activity are
expressed in KBq (or) MBq
• Another unit, called roentgen® is used to provide
some measure of the extent of biological injury say
due to X- rays and rays.Ƴ

Nuclear Fission
• Fission can be caused by bombarding with high
energy ἁ particles, protons, deuterons, X- rays as
well as neutrons.
• However neutrons are most suitable for fission.
They are electrically neutral and thus require no
high KE to overcome electrical repulsion from
positively charged nuclei
• Two or three neutrons are usually released for each
one absorbed in fission, and thus keep reaction
going.
92U235
+ 0n1
54Xe140
+38Sr94
+2 0n1

• Isotopes like U-233, U-235 and Pu-239 can be fissioned by
neutrons of all energies, where as isotopes U-238, Th-232 and
Pu-240 are fissionable by high energy (14Mev) only.
• The immediate (prompt) products of a fission reaction, such as
Xe140
& Sr94
are called fission fragments
• Other decay products (ἁ , ᵝ , etc.) called fission products.Ƴ
• When a neutron collides with and is absorbed by a fissionable
nucleus, the latter is transformed into a compound nucleus in
an excited state. Which then undergoes fission
92U235
+ 0n1
92U236
+ fission
If the excitation energy is not sufficiently large, the nucleus may
not undergo fission and emit only radiation or eject aƳ
particle.
Such absorption of a neutron in a non- fission reaction occurs
about 16 % of the time in all neutron absorptions by U-235.S.PALANIVEL ASSOCIATE PROF./MECH ENGG

Fission Chain
• Two fission fragments (Xe & Sr) are not equal in size and are
radioactive. The original nuclei U235 have neutron proton
ratio of 1.55.
• Their fission fragments have also similar n- p ratios, which
for stable nuclei are however 1.2 to 1.4.
• The fission fragments, therefore undergo several stages of
beta decay (converting neutrons into protons, first instance
for 16 sec., 2nd
instance for 66 sec., 3rd
instance for 12.8 days
and lastly 4th
instance 40 hours) until a stable product is
formed in the following case
54Xe140 ß
55Cs140 ß
56Ba140
58Ce140
57La140

Energy from fission and fuel burnup
• There are many fission reactions which release different
amounts of energy.
92U235
+ 0n1
56Xe137
+36Sr97
+2 0n1
has the mass balance.
235.0439 + 1.00867 138.9061+ 96.9212 + 2x1.00867
236.0526 235.8446 amu . Thus a reduction in mass which
appears in the form of energy (exothermic)
Mass defect Δm = 235.8446- 236.0526 =- 0.2080 amu
There fore ΔE = -0.2080 x 931 = 193.6 Mev.
The fission of U -235 yields on an average about 193 Mev, which is
same for fission of u-233 and Pu- 239. This amount of energy is
prompt ie. Released during fission process. More energy is
however produced due to
(i) slow decay of fission fragments and
(ii)The non- fission capture of excess neutrons.S.PALANIVEL ASSOCIATE PROF./MECH ENGG KAMARAJ COLLEGE OF ENGG & TECH (NEAR) VIRUDHUNAGAR

• The total energy released per fission reaction is about 200 Mev.
The complete fission of 1 gm, of U-235 nuclei thus produces
energy = Avogadro No./mass of U 235 isotope x 200 Mev.
=6.023x1023
x 200 /235.0439 = 5.126 x 1023
=8.19 x1010
J
= 2.276x 104
kwh = 0.984 Mw- day.
Thus . A reactor burning 1 gm of U-235, generates nearly 1 Mw day
of energy. This referred to by the term “FUEL BURNUP” which is
the amount of energy in Mw- days produced by each MT of fuel.
The complete fission of all U-235 nuclei in a fuel mass is impossible,
since many of the fission products capture neutrons in a non-
fission reaction.
In time, the no. of neutrons thus captured becomes great enough
because of the accumulation of products and the fission chain no
longer be sustained.
Depending upon fuel enrichment often less than 1 5 of fissionable
nuclei in the fuel has been consumed. Further use of this
poisoned fuel can only be made by removing the fission products
and reprocessing

Chain Reaction
• The no. of newly born fission neutrons in a single fission for U-
235 nuclei is either 2 or 3 and on an average 2.47.
• In a reactor where controlled and sustained energy production
is desired, conserving neutrons is vital.
• There are two reasons why not all the fission neutrons cause
further reaction –
• 1. Non – fission capture (or) absorption of some neutrons by the
fission products non fissionable nuclei in the fuel. Structural
material, coolant, moderator and so on.
• 2. leakage of neutrons escaping from core.
• The smaller the surface – volume ratio of the core ie. The larger
its size, the lower is the percentage of leakage of neutrons. The
core size is increased to the point where a chain reaction is
possible. This size is called the “critical size’. The mass of fuel in
such a core is called the “ critical mass”S.PALANIVEL ASSOCIATE PROF./MECH ENGG

Chain Reaction (contd.)
• In a reactor using U-235 as a fuel, 100/2.47 or about
40.5 of each 100 fission neutrons must ultimately
engage in fission to keep the reactor critical. However
only about 84 % of the neutrons that get absorbed in u-
235 cause fission. The remaining 16 % neutrons reacting
with its produce U-236 (non- fission capture), an
isotope of non importance.
• Therefore a total of about 40.5/0.84 or 48 neutrons
must be absorbed in u-235 to cause fission.
• Thus , a maximum of about 52 neutrons may be
allowed to leak out of the core and be absorbed in
other core materials.`

Neutron Energies
• The newly born fission neutrons have energies varying
between 0.075 to 17 Mev. As these neutrons travel through
matter, they collide with other nuclei and get slowed down.
This process is called “scattering”. The neutron gives up
some of its energy with each successive collision.
• Neutrons are classified into 3 general categories according
to their energy as fast, intermediate and slow.
Classification Neutron
Energy(ev)
Corresponding
Velocity
(m/s)
Fast > 105
> 4.4 x 106
Intermediate 1 - 105
(1.38 to 4.4) x 106
Slow < 1 < 1.38 x 104

Neutron Energies (Contd.)
• Newly born fission neutrons carry, on an average,
about 2 5, of reactor fission energy in the form of KE
(kinetic Energy).
• Fission neutrons can be
(1) prompt neutrons, emitted within 10-14
seconds
after fission occurs from the fission fragments and
(2) Delayed neutrons, produced in radio active decay
reactions of fission fragments and their products.
Though the energies of delayed neutrons are
relatively small, they play a vital role in nuclear
reactor control.

Neutron scattering
• In the scattering process, the energy balance of colliding
particles before and after collision gives
(En+ K Ec)1 = (En+ K Ec + Ec
*
)2
Where n= neutron and c = nucleus and 1 & 2 denote before and
after collision. Ec
*
= excitation energy of the struck nucleus.
• Scattering can be of two types as (i) Inelastic scattering and (2)
Elastic scattering
• Inelastic scattering : In which momentum and total energy of
the particles before and after collision are conserved. However
KE before collision is absorbed by the nucleus to have the
excitation energy Ec
*
. Thus, for a neutron to engage in inelastic
scattering, it has to posses an initially high KE (>Ec
*
)
• Elastic scattering : In which a neutron does not posses the
necessary minimum KE and Ec
*
is zero. Both momentum and KE
of the colliding particles are conserved.S.PALANIVEL ASSOCIATE PROF./MECH ENGG KAMARAJ COLLEGE OF ENGG & TECH (NEAR) VIRUDHUNAGAR

Neutron scattering (Contd.)
• In each scattering process, a part of KE of the neutron is
transferred to the initial relatively stationery and heavier nucleus,
there by slowing down the neutron.
• The amount of energy lost by a neutron in each collision depends
on the mass of the nucleus and angle of scatter.
• Maximum energy is lost in a head on collision. If a neutron
possess an initial KE En,i , its KE after head on collision En.min is given
by En.min = En,i ( M- Mn / M+ Mn )2
where M= mass of nucleus and Mn =
Mass of neutron.
• It can be approximately expressed as En.min / En,i =(A-1/A+ 1)2
where
A= mass no. of nucleus.
• A neutron may lose a maximum of less than 2 % in a collision with
U-238 nucleus, but about 28% with carbon nucleus and all its
energy in a single collision with a hydrogen nucleus (A=1)S.PALANIVEL ASSOCIATE PROF./MECH ENGG

Neutron scattering (Contd
.)
• The average neutron energy lost per elastic collision is expressed
in terms of a quantity called the logarithmic energy decrement ξ,
defined by ξ = ln En,i - ln En,av
Where En,av = the average energy of neutron after a single collision
ξ = 1 – ( (A-1)2
/2A x ln(A+1)/(A-1)) : where A = mass no. of the struck
nucleus (moderator). From this equation it is seen that if A > 1,
ξ=1
Thus if a neutron collides with a hydrogen nucleus (A=1), the average
neutron energy after one collision is 1/e of its initial energy. This is
the max. possible decrease of neutron energy.
Thus , the no. of collisions, ‘n’ required to slow down a neutron from
initial energy En,i , to a final energy En,f in elastic scatter is given by
N = ln(En,i / En,f )/ξ .
A moderator is used to slow down the neutron in a reactor. Thus,
smaller the nucleus, better the moderator.

• Table gives the value of ‘n’ to bring down the neutron
energies from 2 Mev to 0.025ev in elastic collisions.
However it is not the sole criterion of moderator
effectiveness. Other aspects, like probability of collision,
probability of absorption and scattering as well as the no. of
moderator nuclei in a given volume also influence the
moderator effectiveness.
Nucleus A ξ n
H 1 1.000 18
D 2 0.725 25
Be 9 0.208 86
C 12 0.158 114
Al 27 0.074 246
Fe 56 0.038 472
Zr 91 0.021 866
U 238 0.004 4480

Nuclear Breeding
• In nuclear reactors, as fuel is spent, neutrons are released. A single neutron
for every fission event is necessary to sustain the reaction, i.e. to cause
another fission event. However, as many fission reactions release more than
one neutron, it is possible for the other neutrons to cause more fission
events, as in conventional nuclear power plants and also nuclear weapons.
These neutrons can also be absorbed by non-fissile isotope atoms, causing
them to transmute to a higher isotope. This higher isotope atom then
undergoes beta-decay to form a fissile isotope atom.
• The number of fissile isotope atoms generated per fission event is known as
the breeding ratio [5]. A breeding ratio of greater than unity implies that for
every fission reaction, more than one fissile atom can be generated, and the
fuel can be bred from non-fissile material. Nuclear reactors are being
developed to make use of this to breed fissile material from relatively much
more abundant materials [6].
• Research has shown that breeding in nuclear reactors can generate the same
amount of power using less nuclear fuel (and more abundant fuel) than
conventional reactors. Further, as breeders can reduce the risk of nuclear
proliferation, they present a viable technology for power generation.

Atomic structure – Atoms are fundamental subunits of matter. Matter is anything
that takes up space and has mass. Air, water, trees, cement, and gold are examples
of matter.
Figure 4.2 diagram of oxygen
All atoms have a central region know as the
nucleus, which is composed of two kinds of
relatively heavy particles: positively charged
particles called protons and uncharged
particles called neutrons. Surrounding the
nucleus is a cloud of relatively light weight,
fast moving, negatively charged particles
called electrons. The atoms of each element
differ in the number of protons, neutrons,
and electrons present.
H2O

Isotopes
• All atoms of the same element have the same
number of protons and electrons but the number
of neutrons may differ.
• Atoms of the same element that differ in the
number of neutrons are called isotopes.
• Since the positively charged protons in the
nucleus repel one another energy is needed to
hold the protons and neutrons together.
• However, some isotopes of some atoms are
radioactive, that is the nucleus of these atoms are
unstable and decompose. Neutrons, electrons,
and protons are released during this
decomposition releasing a great deal of energy.
Half-life… ½ of radioactive material to decompose

Only certain kinds of atoms are suitable for the development of a nuclear chain
reaction. The two materials most commonly used are uranium-235 and plutonium-
239.

To appreciate the consequences of using nuclear fuels to
generate energy it is important to recognize the nuclear fuel
cycle. Mining produces low grade uranium ore. The ore
contains 0.2 % uranium by weight. After it is mined, the ore
goes through a milling process. It is crushed and treated with
a solvent to concentrate the uranium. Milling produces
yellow-cake, a material containing 70-90% uranium oxide.S.PALANIVEL ASSOCIATE PROF./MECH ENGG

Naturally occurring uranium ore contains about 99.3%
nonfissionable U-238 and only 0.7% fissionable U235 (the
U235 is the uranium isotope needed in nuclear reactors).
This concentration of U-235 is not high enough for most
types of reactors, so the amount of U-235 must be increased
by enrichment. Since the masses of the isotopes U-235 and
U-238 vary only slightly, enrichment is a difficult and
expensive process. However, enrichment increases the U-
235 content from 0.7% to 3%.
Fuel fabrication converts the enriched material into a
powder, which is then compacted into pellets about the size
of a pencil eraser. These pellets are sealed in metal rods
about 4 meters (13.2 feet) in length, which are then loaded
into the reactor (of course the enrichment and fabrication
generally do not occur at the reactor so these enriched rods
have to be transported to the site of the reactors).S.PALANIVEL ASSOCIATE PROF./MECH ENGG

As fission occurs, the concentration of U-235 decreases.
After about three years a fuel rod does not have enough
radioactive material to sustain a chain reaction so the
rods must be replaced by new ones. The spent rods are
still very radioactive, containing about 1 percent of the U-
235 and 1 percent plutonium. These rods are the major
source of radioactive waste material.
You may have heard about the US and nuclear
inspectors looking for aluminum rods in Iraq. Some
countries with nuclear reactors try, through
centrifugation, to extract the plutonium so it can be used
in nuclear weapons. This is a current concern with North
Korea. The US has known that the North Koreans had
used rods in a pool at one of their plants. Apparently,
during the recent visit by the group of citizens they were
shown the pool but it contained no spent rods.

Nuclear Reactors
A nuclear reactor is a device that permits a controlled fission chain reaction. In
the reactor, neutrons are used to cause a controlled fission of heavy atoms such as
Uranium 235 (U-235). U-235 is a uranium isotope used to fuel nuclear fission
reactors.

In addition to fuel rods containing uranium, reactors contain control rods of
cadmium, boron, graphite, or some other non-fissionable material used to
control the rate fission by absorbing neutrons. Lowering the rods decreases
the rate of reaction.
The heat
generated by
the fission of or
uranium
releases energy
that heats water
to produce
steam to turn
turbines to
generate
electricity.
Cooling Tower

The light water
reactors (LWR) make
up 90% of the reactors
operating today, use
ordinary water as the
moderator and as the
coolant. The BWR and
PWR are light water
reactors. In a BWR (20%
of reactors in the world).
Steam is formed within
the reactor and
transferred directly to
the turbine.The steam must be treated and the generating building must be shielded. In the
PWR (70% of reactors in the world) the water is kept under high pressure so that
steam is not formed in the reactor. Such an arrangement reduces the risk of
radiation in the steam but adds to the cost of construction by requiring a secondary
loop for the steam generator.
Emergency core
cooling system

The energy that would be released by combining the deuterium in
one cubic meter of ocean water would be greater than that
contained in all of the world’s entire fossil fuels. Even though in
theory fusion promises to furnish large amounts of energy, technical
difficulties appear to prevent its commercial use in the near future.
Even the governments of nuclear nations are budgeting only modest
amounts of money for fusion research. And, as with nuclear fission
and the breeder reactor, economic costs and fear of accidents may
Nuclear Fusion
S.PALANIVEL ASSOCIATE PROF./MECH ENGG KAMARAJ COLLEGE OF ENGG & TECH (NEAR) VIRUDHUNAGAR

Chernobyl is a small
city in Ukraine near
the border with
Belarus, north of
Kiev. At 1 A.M. on
April 25, 1986, at
Chernobyl Nuclear
Power Station-4, a
test was begun to
measure the amount
of electricity that a still
spinning turbine
would produce if the
steam were shut off.
This was importantinformation because the emergency core cooling system required
energy for its operation and the coasting turbine could provide some of
that energy until another source became available. But the test was
delayed because of a demand for electricity, and a new shift of workers
came on duty.

The operators failed to program the computer to maintain power at
700 megawatts, and output dropped to 30 megawatts. This
presented an immediate need to rapidly increase the power, and
many of the control rods were withdrawn. Meanwhile, an inert gas
(xenon) had accumulated on the fuel rods. The gas absorbed the
neutrons and slowed the rate of power increase. In an attempt to
obtain more power, operators withdrew all the control rods. This
was a second safety violation.
At 1 AM on April 26, the operators shut off most emergency
warning signals and turned on all eight pumps to provide adequate
cooling for the reactor following the completion of the test. Just as
final stages of the test were beginning, a signal indicated an
excessive reaction in the reactor. In spite of the warning, the
operators blocked the automatic reactor shut down and began the
test.
As the test continued, the power output of the reactor rose beyond
its normal level and continued to rise. The operators activated the
emergency system designed to put the control rods back into the

The core had already
deformed, and the rods would
not fit properly: the reaction
could not be stopped. In 4.5
seconds the energy level of the
reactor increased 2000 times.
The fuel rods ruptured, the
cooling water turned into
steam, and a steam
explosion occurred. The lack
of cooling water allowed the
reactor to explode. The
explosion blew the 1102 ton
concrete roof off the reactor
and the reactor caught on fire.
In less than 10 seconds, Chernobyl became the
scene of the world’s worst nuclear accident.
It took 10 days to bring the runaway reaction
under control. By November, the damaged
reactor was entombed in a hastily built concrete
covering that may have critical flaws. A 2nd
containment is planned.

The immediate consequences were 31 fatalities, 500 persons hospitalized, including
237 with acute radiation sickness; and 116,000 people were evacuated. More than a
year after the disaster at Chernobyl, the decontamination of 27 cities and villages was
considered finished. That does not mean they were safe just that all practical
measures had been completed. Some areas were simply abandoned. The largest city
to be affected was Pripyat which had a population of 50,000 and was only 4
kilometers from the reactor. A new town was built to accommodate those displaced
by the accident and Pripyat remains a ghost town. Seventeen years after the accident
some scientists believe the worst is yet to come. Compared to the general public
(control) the rates of some noncancer diseases, endocrine disorders, and stroke for
instance appears to be rising disproportionately among the roughly 600,000
“liquidators” who cleaned up the heaviest contamination in the plant’s vicinity.
Whether people who live in the shadow of Chernobyl remain at risk is an intensely
debated question now.

One impact of Chernobyl is that it deepened public concern about the safety of
nuclear reactors. Even before Chernobyl, between 1980 and 1986, the governments
of Australia, Denmark, Greece, Luxembourg, and New Zealand had officially adopted
a “no nuclear” policy. Since 1980, 10 countries have cancelled nuclear plant orders
or mothballed plants under construction. Argentina canceled 4 plants, Brazil 8,
Mexico 18, and the US, 54. There have been no orders for new plants in the US
since 1974. Sweden, Austria, Germany, and the Phillipines have decided to phase
out and dismantle their nuclear power plants.
Decommissioning Costs
Decommissioning a a fossil fuel plant is relatively easy a wrecking ball is about all
that is required. Nuclear power plants are not demolished they are decommissioned.
Decommissioning involves removing the fuel, cleaning the surfaces, and
permanently preventing people from coming in contact with the contaminated
buildings and equipment.
S.PALANIVEL ASSOCIATE PROF./MECH
ENGG KAMARAJ COLLEGE OF ENGG & TECH
(NEAR) VIRUDHUNAGAR

S.PALANIVEL ASSOCIATE PROF./MECH. ENGG
KAMARAJ COLLEGE OF ENGG & TECH. (NEAR) VIRUDHUNAGAR
Principles of Classification of Nucelar
Reactors
• Energy spectrum utilized for fission
• Prime purpose
• Fuel
• Coolant
• Coolant system
• Moderator

Energy
Thermal (0.5 eV) Intermediate (103
eV) Fast (105
eV)
Power reactors:
BWR, PWR, VVER,
RBMK; thermal
breeder,…
Spaceship
reactors (accident
in Can.)
BN350, BN600,
Super Fenix,
Naval reactors
Energy Spectrum Classification

Purpose
Power Ship propulsion Production Research Specialized
LWR
PHWR (CANDU)
HTGR
AGR
LMR
LGR (RBMK)
LMFBR
HWLWR
OLR
Submarines
Navy
Aircarrier
Icebreakers
PWR-cargo
Plutonium
Tritium
WPR TRIGA
Studsvik
Isolated areas
Space propul.
Heat for chem

Fuel
UO2 (3-4%)
sintered pellets
zirconium alloy
Nat. U
pellets
tubes
UO2
spherical part.
carbon, silicon
U-Pu-Zr alloy
pellets
steel cladding
U-Al alloy
plates
Al cladding + Al-Si
LWR CANDU HTGR LMR TRIGA

Coolant
Light Water
H2O
Heavy Water
D2O
Gas
air, CO2, He
Liquid Metal
Na, Na-K, Pb-Bi
LWR CANDU HTGR
Pebble bed
LMR

Moderator
Light Water
H2O
Heavy Water
D2O
Graphite
C
LWR CANDU LGR (RBMK)
GCR
HTGR

1-loop BWR
2-loop PWR
3-loop LMR

Steam Separator in BWR

World Inventory of
Reactor Types (Dec. 1994)
Type Number Capacity (GWe) Usual
Fuel
Moderat
or
Coolant First
devel
Oper. Cons Oper. Cons
PWR 245 39 215.7 36.8 UO2 enr H2O H2O USA
BWR 92 6 75.9 6.0 UO2 enr H2O H2O USA
PHWR 34 16 18.6 7.9 UO2 nat. D2O D2O Canada
LGR
(RBMK)
15 1 14.8 0.9 UO2 enr C H2O USA/
USRR
GCR 35 0 11.7 0 U, UO2 C CO2 UK
LMFBR 3 4 0.9 2.4 UO2+
PuO2
None Liq. Na Various
Total 424 66 338 54

PWR BWR HTGR LMFBR GCFR PHW
MWt
3400
(2700)
3600
(2900)
3000 2400 2500 1600
MWe 1150 1200 1170 1000 1000 500
Eff 33.7 33.5 39.0 39.0 39.5 31.0
Fuel UO2
UO2
UC,ThO2
PuO2
, UO2
PuO2
, UO2
UO2
Cool H2
O H2
O He Na He D2
O
Moder. H2
O H2
O Graphite – – D2
O
Height 366 376 634 91 148 410
Diam. 377 366 844 222 270 680
Burnup 33,000 27,500 98,000 100,000 100,000 10,000
Typical Data

CANDU Reactor
• CANDU Reactors : Heavy water moderated and cooled reactors
have been extensively developed in Canada and form the basis
of nuclear power programme in Canada. They are called
CANDU-PHW ( Canadian Deutrium Uranium pressurised heavy
water).
• The CANDU reactors have several features that distinguish
them from other types. The moderator is contained in a
cylindrical steel vessel called calandria, with large number of
Zircalloy tubes through it parallel to its axis, which is horizontal.
• The active core region is approximately 6m high with a
diameter of 7 to 8 m. D2O coolant enters the regular array of
pressure tubes ar 260 deg. C and 110 bar, flows through the
fuel elements and leaves the pressure tubes at 320 deg. C. Net
efficiency is about 29%. Like PWR there is no bulk boiling of
coolant.

CANDU reactors

LGR (RBMK). The light-water-cooled, graphite-moderated reactor uses water
as a coolant and graphite for moderation. The world’s only currently
operating LGRs are the RBMK reactors in the former Soviet Union. There
were four such units at the Chernobyl plant at the time of the accident
there, two of which are still in operation.
Ignalina

GCR. The gas-cooled, graphite-moderated reactor uses a CO2
coolant and a graphite moderator. Its use is largely limited to
the United Kingdom; it is sometimes known as the Magnox
reactor. A larger second-generation version is the advanced gas-
cooled, graphite- moderated reactor (AGCR).

HTGR. The high-temperature gas-cooled reactor uses
helium coolant and a graphite moderator. The only
operating HTGR in the United States (Fort St. Vrain) has
been shut down, and there are no operating HTGRs
elsewhere.
HWLWR. The heavy-water-moderated, light-water-
cooled reactor is an unconventional variant of the heavy
water reactor, and only one has been in recent
operation, a 148-MWe plant in Japan.

LMFBR. The liquid-metal fast breeder reactor uses fast
neutrons and needs no moderator. A liquid metal is
used as coolant, now invariably liquid sodium. There are
2 liquid- metal reactors in operation + 2 newly shut-
down (France – Phenix and Superphenix, and one each
in Kazakhstan- BN-600 and Russia BN-60) and several
others under construction or reconstruction, including
Monju reactor in Japan, which suffer sodium leakage
accident in 1994.

Monju Reactor

LIGHT WATER REACTORS
PWRs and BWRs
The two types of LWR in use in the world are the
pressurized water reactor (PWR) and the boiling water
reactor (BWR). In the PWR, the water in the reactor
vessel is maintained in liquid form by high pressure.
Steam to drive the turbine is developed in a separate
steam generator. In the BWR, steam is provided
directly from the reactor.

PWR

BWR

Typical conditions in a PWR, temperatures of the cooling
water into and out of the reactor vessel are about 292o
C and
325o
C, respectively, and the pressure is about 155 bar. For
the BWR, typical inlet and outlet temperatures are 278 o
C and
288o
C, respectively, and the pressure is only about 72 bar.
The high pressure in the PWR keeps the water in a
condensed phase; the lower pressure in the BWR allows
boiling and generation of steam within the reactor vessel.
Neither the PWR or BWR has an overwhelming technical
advantage over the other, as indicated by the continued
widespread use of both.
The future is not clear-cut, however, and in Japan 3 BWRs
were under construction at the end of 1994, compared to
only 1 PWR.

Components of a Light Water Reactor
The reactor pressure vessel is a massive cylindrical steel tank. Typically
for a PWR, it is about 12 m in height and 4.5 m in diameter. It has thick
walls, about 20 cm, and is designed to withstand pressures of up to 170
atmospheres.
A major component, or set of components, is the system for converting
the reactor’s heat into useful work. In the BWR, steam is used directly
from the pressure vessel to drive a turbine. This is the step at which
electricity is produced. In the PWR, primary water from the core is
pumped at high pressure through pipes passing through a heat
exchanger in the steam generators.
Water fed into the secondary side of the steam generator is converted
into steam, and this steam is used to drive a turbine. The secondary
loop is also closed. The exhaust steam and water from the turbine enter
a condenser and are cooled in a second heat exchanger before
returning to the steam generator.

The pressure vessel and the steam generators are contained
within a massive structure, the containment building, commonly
made of strongly reinforced concrete. In some designs, the
concrete containment is lined with steel; in others, there is a
separate inner steel containment vessel. The containment is
intended to retain activity released during accidents and is also
believed capable of protecting a reactor against external events
such as an airplane crash.
In the Three Mile Island accident, the containment very
successfully retained the released radioactivity, although it may
be noted that the physical structure was not put fully to the test,
since there was no explosion or buildup of high pressures. At
Chernobyl there was no containment, with disastrous results. In
principle, if a reactor is sufficiently protected against accidents, a
containment is unnecessary. Nonetheless, it is widely thought to
be an essential safety feature, providing an important
redundancy.

Nuclear Waste…Why?
•Recently nuclear power has entered many discussions as world energy needs rise and
oil reserves diminish.
•Most opponents of nuclear power point to two main arguments: meltdowns and
nuclear waste.
•Nuclear waste is any form of byproduct or end product that releases radioactivity.
•How to safely dispose of nuclear waste is pivotal for the continued operation of
nuclear power plants, safety of people living around dump sites, and prevention of
proliferation of nuclear materials to non-nuclear states.
S.PALANIVEL ASSOCIATE PROF./MECH.
ENGG KAMARAJ COLLEGE OF ENGG &
TECH. (NEAR) VIRUDHUNAGAR

Nuclear Fuel Cycle

Classifications
• Nuclear waste is segregated into several
classifications.
• Low level waste is not dangerous but sometimes
requires shielding during handling.
• Intermediate level waste typically is chemical
sludge and other products from reactors.
• High level waste consists of fissionable elements
from reactor cores and transuranic wastes.
• Transuranic waste is any waste with transuranic
alpha emitting radio nuclides that have half-lives
longer than 20 years.

Low Level Waste (LLW)
• Low level waste is any waste that could be
from a high activity area.
• 90% volume of waste
• It does not necessarily carry any
radioactivity.
• Split into four categories: A, B, C, and GTCC.

Intermediate Level Waste (ILW)
• Intermediate level waste requires shielding
when being handled.
• Dependent on the amount of activity it can
be buried in shallow repositories.
• Not recognized in the United States.

High Level Waste (HLW)
• High level waste has a large amount of
radioactive activity and is thermally hot.
• 95% of radioactivity
• Current levels of HLW are increasing about
12,000 metric tons per year.
• Most HLW consists of Pu-238, 239, 240, 241,
242, Np-237, U-236

Transuranic Waste (TRUW)
• Transuranic waste consists of all waste that
has radio nuclides above uranium.
• TRUWs typically have longer half-lives than
other forms of waste.
• Typically a byproduct of weapons
manufacturing.
• Only recognized in the United States.

•Nuclear waste is generated at all points of
the fuel cycle.
Creation of Nuclear Waste
wasimarilof low level alpha emission waste.
•Service period waste typically includes LLW
and ILW such as contaminated reactor
housings and waste from daily operation.
•Back end waste normally is the most
radioactive and includes spent fuel rods and
reactor cores.S.PALANIVEL ASSOCIATE PROF./MECH. ENGG

Service Period Waste
• Consists of mostly ILW.
• Mostly waste produced at the plant
during normal operation.
• Spent fuel rods are the most dangerous
waste produced during the service
period.
S.PALANIVEL ASSOCIATE PROF./MECH.
ENGG KAMARAJ COLLEGE OF ENGG &
TECH. (NEAR) VIRUDHUNAGAR

Back End Waste
• Nuclear waste developed
during the back end of the
fuel cycle is the most
dangerous and includes
most of the HLW
produced.
• Most back end waste
emits both gamma and
beta particles.
• Also uranium-234,
neptunium-237,
plutonium-238 and
americium-241are found
in back end waste.
Spent nuclear fuel in a cooling pond in North
Korea.

Waste Management (LLW)
• There are several
options available for
the disposal of LLW
due to its lack of
radioactivity.
• Waste Isolation Pilot
Plant
• On-site disposal Map of WIPP Facility

Treatment (LLW)
• Filtration
• Ion Exchange
• Evaporation
• Incineration
• Compaction
• Solidification
Typical LLW treatment facility.

Waste Management (HLW)
• Most common utilized option are reactor
pools and dry cask storage.
• Other Options for waste management
include:
–Deep Geological Storage
–Transmutation
–Reuse
–Launching it into spaceLocations of storage sites for nuclear
waste in the U.S.

Treatment
• Most common initial treatment of waste is
vitrification.
– Waste is first mixed with sugar and then passed
through a heated tube to de-nitrite the material.
– This material is then fed into a furnace and
mixed with glass.
– The molten glass mixture is poured into steel
cylinders and welded shut.

Treatment (Cont.)
• Mid level active waste is commonly treated
with ion exchange
• Process reduces the bulk volume of
radioactive material.
• Typically, mixed with concrete for a solid
storage form.

Treatment (Cont.)
• Synroc is a new method for storing nuclear
waste developed in 1978 by Ted Ringwood.
• Attempts to hold radioactive material in a
crystalline matrix.
• Currently in use for military waste
management at Savannah River Site.
• Can hold 50%-70% volume of waste.

Deep Geological Repository
• Most common method
for handling nuclear
waste.
• Typically kept separate
from actual plants and
buried far below
ground.
• First used in 1999 in the
US.
• Current research is
focusing on Yucca
Mountain.
Yucca Mountain Site

Transmutation of Nuclear Waste
• Reduces transuranic waste.
• Integral Fast Reactor
• Banned 1977-1981 (U.S.)
• MOX Fuel
–Behaves as low-enriched uranium
• Research now in subcritical reactors.
• Fusion also being researched.

Reuse of Nuclear Waste
• Research is being performed to find uses for
nuclear waste.
• Caesium-137 and strontium-90 already used
in industrial applications.
• Some waste can be used for radioisotope
thermoelectric generators (RTGs).
• Overall can reduce total HLW but not
eliminate it.

Launch it into Space
• Near infinite storage
space
• Completely removes
waste from biosphere
• Technical risks and
problems
• Political
entanglements

Unit 3 nuclear power plants

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