… hydroelectric power plants and the next
topic will be nuclear power plants.
But, before we go, it will be good to give
you a flavour of the kind of problems that
one has to solve in planning a hydroelectric
plant.
So, take down a problem.
Let me write the problem, so that you can
solve it later.
The catchment area of a river has an annual
rainfall of 2 into 10 to the power 7 or m
cube, out of which about 60% reaches and flows
through the river.
If you have any difficulty in understanding
the problem just tell me.
Now, 10 to the power 7 meter cube is precipitated
during the monsoon months of July and August,
while the rest of the rainfall is distributed
evenly over ... So, the catchment area of
a river has an annual rainfall of 2 into 10
to the power 7 meter cube, out of which about
60% reaches and flows through the river.
Now, half of it that means 10 to the power
7 meter cube is precipitated during the 2
months and other half is evenly distributed
over the other 10 months.
Though that is not generally true, but it
is a good approximation.
It is necessary to construct a dam on that
river with two purposes.
1: supplying water for irrigation for the
month of May - June and December - January.
Why are these two times?
As you know that May - June and December - January
are cropping seasons in which there are standing
crops.
If there is no rain you have to supply water
and 2: using the rest of the water for peak
power generation for 2 hours per day throughout
the year.
So, there will be some amount of necessary
for irrigation and the rest of it should be
made available for peak power generation for
2 hours a day throughout the year.
If the available head is 100 meters and 
the irrigation water demand is 0.5 into 10
to the power 6 meter cube per month, calculate
1: the turbine power rating and 2: the retaining
capacity of the dam.
Is the point clear?
Is the problem clear?
So, the water that is available has to be
used for two purposes, irrigation and power
generation and then on that basis you have
to calculate what should be the reservoir
capacity and what should be the turbine rating.
Turbine rating should be in kilowatts or megawatts
and the water retaining capacity should be
in meter cube.
How much has to be this, the capacity, clear.
So, solve this problem.
This is a typical problem.
I mean, I have taken it from one of the earlier
end semester examinations question papers.
So, you may expect similar style of questions.
Now, let us go on to the next topic which
is nuclear power.
Through your education in the school, probably
you have got some exposure to how nuclear
power is generated.
For example, there are heavy nuclei which
breakup on their own, which are the heavy
nuclei, fissile nuclei, radioactive nuclei.
Do you, do you know which they are? Uranium
yes, plutonium yes, thorium yes; so, these
are the standard, there are many others.
Not only the heavy ones breakdown, but also
lighter ones that are unstable also can breakdown.
So, they can also be sources of nuclear radiation,
but in nuclear power generation, in general
uranium is used.
So, we will consider in the main, uranium.
India has uranium mines mainly in Orissa,
Jaduguda mines and there have been some other
resources found in the North East.
So, India has a sufficient deposit of uranium.
A larger deposit is available, of thorium
mainly in Kerala beach.
So, these are two resources that India has,
but all the power plants run on uranium and
therefore let us concentrate our discussion
on uranium only and as you know, the essential
physical principle that goes behind it is
that energy and mass are equivalent and when
mass is converted into energy, it follows
the relationship E is equal to MC square;
that you all know.
Now, uranium or any other such material comes
in many isotopes.
For uranium for example, uranium 233 is there,
235 is there, 238 is there.
So, all these possible variants of the same
thing are available in nature.
Now uranium, when it comes, natural uranium,
most of it is uranium 238.
How do you write?
You write U here 238.
So, U 238 is the abundant variety of uranium
and the other variety is U 235.
Now, abundant means in a naturally occurring
resource, deposit of uranium, we will find
uranium 238 about 99%, but uranium 238 is
not naturally fissile.
It does not breakup on its own; 235 is.
So, the actual resource is uranium 235, but
when it occurs in nature, it is mixed up with
not only clay, but also a lot of uranium 238
which does not really participate in the fission
process.
Now, let us first understand what happens?
What is the nuclear reaction that takes place
when uranium 235 breaks up?
Now, when uranium 235 breaks up, it breaks
up by the addition of a neutron.
So, a neutron hits the uranium 235 and makes
it break down.
So, you will write it as U 235 plus a neutron.
That will lead to krypton 92 plus barium 141
plus, these are the material products of the
fission process, but it also yields three
neutrons.
So, one neutron is used up in the process
of the fission and three neutrons are create
in the, created in the process of fission.
Now, if at least one of these three neutrons
hit another uranium 235, it will also make
it break.
So, imagine it statistically that three neutrons
emerging in various directions and in all
probability, in a larger probability they
will hit either a non-fissile nucleus or a
uranium 238 nucleus, but not a uranium 235
nucleus.
So, only if you can ensure that at least one,
on an average, hits another uranium 235 nucleus,
then only the fission process will continue.
In other words, chain reaction will take place.
So, in order for ensuring the chain reaction
therefore, you need to ensure that if the
three nuclei, three neutrons go in various
directions, there will be a probability that
at least one will hit 235.
What will be necessary in order ensure that?
That there has to be sufficient amount of
uranium 235 around; if it is insufficient,
it will go out without hitting anybody.
So, there has to be a sufficient quantity
of uranium 235 that is called the critical
mass; there has to be sufficient quantity.
Not only that, it has to be in sufficient
concentration, because if most of the things
that the three neutrons see around it are
uranium 238, obviously nothing will happen;
it has be in sufficient concentration.
So, these two are the major requirements and
that is what actually consisted of all these
years of the effort of the Manhattan project,
as you know.
The whole thing was started through the effort
to make bombs in the Manhattan project and
much of it essentially was the efforts to
concentrate, enrich uranium to a process,
to a state where it can be used for fission.
Now if, so you have the uranium 235 and uranium
238 in a mixture and if you can enrich it
to say 5% uranium 235, then you would need
one critical mass; if you can enrich it to
10% of uranium 235, it will require less critical
mass; that stands to reason.
But, one thing is clear that these fellows
also breakdown on their own.
That means here I have depicted it as, it
requiring a neutron to impinge in order to
breakdown and cause this fission.
But, in actuality, they are naturally fissile
substances which mean that some will always
break down.
That means always some neutrons will be created
and therefore if there is a critical mass
in some place it will automatically explored.
So the, I mean, I should not be taken as a
terrorist, but I can tell you that the making
of a nuclear bomb is not a very big deal,
engineering wise.
Once you have a sufficient amount of critical
mass of uranium, making them, just putting
them together makes a bomb.
But then, here our objective is to control
that, so that we can get energy out of it
in a controlled way for power production.
So, we need to control that.
Now, how can you control that?
You have understood that if you put together
sufficient amount of or the critical mass
of uranium 235, it is itself a bomb; obviously
you cannot do that.
If you then try to do that, it will explode
and you will die.
There is no point in it.
So, what is done is that rods are made out
of uranium oxide; you make rods, each rod
does not contain the critical mass.
When if you put a large number of rods together,
then only the critical mass is reached and
that will require neutrons to pass from one
rod to another.
So, neutrons naturally occurring or emanating
from one rod must be able to pass to the other
in order to have that chain reaction.
Now, you can have some material to absorb
those electrons in between and a naturally
occurring such substance is steel, chromium
steel for example.
So, if you have steel rods that go between
the fuel rods and if you can have a mechanism
of either pulling them up or putting them
down, then obviously, by that you can control.
Imagine a situation something like this.
You have got the fuel rods something like
this.
I am drawing only a few of them, but there
will be hundreds in a reactor assembly.
Suppose these are the fuel rods and you can
have the steel rods something like this, so
that by allowing these to go up or down, if
it goes up, you can see that neutrons can
pass through from one rod to the other.
If it goes down, it absorbs more and therefore,
you can control the process of fission.
This is what exactly is done in order to control,
clear.
But in addition to that, something more is
necessary.
One is that it has been found that the neutrons
that naturally emanate from a reaction, these
neutrons are fast neutrons.
They move very fast.
They have a large amount of kinetic energy
and if they move very fast, they are not readily
absorbed by uranium 235.
So, they need to be slowed down.
They need to be slowed down and the process
of slowing down is to use another material
called moderator.
So, there should be another material called
moderator, so that while going through that
moderator the neutrons slow down and therefore
when it hits another uranium 235, it will
have a large probability of causing fission.
So, you need, we have now come to the understanding
that, one you need fuel rods, two you need
control rods.
Fuel rods are made of rods of uranium oxide
and control rods are made of steel and you
need some moderator; you need some moderator.
What can be the moderators?
Normal water is the moderator.
Heavy water is a better moderator.
Graphite is another possible moderator.
So, you can have various things as a moderator
- water, heavy water, heavy water is D 2 O,
deuterium oxide and graphite.
In addition to that as heat is generated in
this assembly if you allow the heat to accumulate
after sometime this will all melt.
So, there has to be some way of taking the
heat out; so, there has to be some coolant,
something that cools down this thing, something
that takes the heat out to a place where the
power is generated and there are various choices
of the coolant substance.
So, you see, the various types of nuclear
power plant that you hear about are nothing
but different combinations of the possible
fuel rods, control rods, moderators and coolants,
nothing but, nothing but that.
So, let us now try to understand the various
types of nuclear power plants that you have.
India has many nuclear power plants and therefore
we need to understand what exactly is inside.
First let us come to, okay, let me, let me
just write down what these are.
One, you need the fuel rods.
This is, what are the options?
Uranium 238 plus 235, of course; it can also
be made of plutonium.
Plutonium is also a fissile substance.
Two, number 2 is control rods.
Control rods is generally made of steel; various
types of steel is there, but in general, the
choice is nothing but steel.
3: moderator, H 2 O, D 2 O and graphite.
I cannot write C; that will not be clear,
what is carbon or graphite and four, coolant.
Now, coolant can be various things.
I will list them as we go along, but the easiest
coolant obviously is water.
So, the simplest possible nuclear power plant
is where is used fuel rod uranium, control
rod steel, moderator water, coolant is also
water; that is the simplest.
So, in that case what will be the structure
of that plant?
Let me draw with some different colours.
First, as I showed you, there has to be those
fuel rods.
So, these are the fuel rods say and in between
there have to be the control rods.
This can be pulled up or down and you can
see this structure that if something fails,
it automatically goes down by its own weight.
If it goes down it stops the reaction.
So, it sort of, it is a natural safety, but
it, I will show you later that it does not
always offer that safety, but some safety
is built into the system.
Then you have the whole thing encased within
a structure, in which you allow water to be
there; that water acts as the moderator as
well as it absorbs the heat and boils.
So, what will go out of here is steam.
So, you allow it to boil to steam and then
that goes to the turbine.
So, from here it will go to the turbine.
I will draw schematically, not showing like
exactly turbines and then what will be there?
Next, you have already done the thermal power
plant; this is the boiler then.
If this assembly is the boiler, then what
will be the next?
You need, after the turbine you need a condenser,
right.
So, here is a turbine, the condenser and you
have to allow cooling water to go in and water
to go out which is different from this water
and here you have the electric generator.
Then, after that what will you have?
After turbine and condenser, what will you
have?
Remember, it also works on the same fuel cycle.
No, no, no; after condenser it has become
water.
It has to go, it has go to the pump; yes,
it has go to the pump.
So, it has to go to the pump and pumping,
pump water must go into this.
So, that is the essential structure of what
is known as the boiling water reactor where
you hear about the abbreviation BWR.
It is nothing but boiling water reactor, because
inside the chamber it actually boils.
Now, this whole assembly needs to be encased
in a container and that is what you see as
a dome, right; that is what is seen as a dome,
but I am drawing it like this, so that radiation
cannot escape.
Obviously there will be nuclear radiation
generated inside and so, this obviously is
the simplest possible thing.
But, what are the problems?
Can you identify?
You see, the water that is here is always
coming in contact with the fissile material,
the control, the fuel rods.
As a result, it is radioactive and the same
radioactive water is going into the turbine,
same radioactive water is going to the condenser,
same radioactive water is going to the pump.
So, all these things will after sometime become
radioactive.
So, in this system, the problem is that it
is not possible to contain the radiation,
contain the contamination within this black
coloured chamber.
That is why boiling water reactors are no
longer in favour, though they are the cheapest
and simplest.
Most of the countries are now moving away
from the boiling water reactors.
What happens is that the people handling the
turbine, the condenser, there has been people
around; they always are exposed to radiation,
because the turbine itself becomes radioactive,
the condenser itself becomes radioactive.
So, you understand the natural problem, clear.
What is the next possible step?
Next possible step is, there is another problem.
Because you are allowing the water to boil
here, because you are allowing the water to
boil here the temperature is low.
The temperature is not very high, because
here it is, it is actually boiling.
Depending on the pressure that you allow,
it will be not 100 degrees; it will be higher,
but not much higher.
The water that goes out of this is allowed
to expand a bit.
That means the pressure is brought down.
As a result, it goes from the saturated steam
stage to superheated steam stage.
In a boiler what happens?
In a normal coal fired boiler, you had the
super heater where you add more heat to it.
But here, the place where heat is generated
is inside here where water is coming in contact
with that which at that time is boiling and
therefore, you have to do something else to
get into the superheated stage and what is
generally done is to bring down the pressure
little bit, so that it goes into superheated
stage, clear.
Now, here the thing that is coming in contact
with the fuel rod is water and something,
water, that water that time is boiling into
steam and therefore, water steam mixture and
steam is not a very good absorber of the heat,
right.
Water can absorb the heat well, but not steam
and therefore, since it is a mixture of water
and steam, the overall heat transfer capability,
the coolant action is not very strong.
This is another problem of this design of
the boiler.
So, the next possible stage therefore, is
to allow water to be there; water will be
used in the substance, in this place as the
moderator as well as coolant, but pressurize
the water such that it does not boil.
If it is not allowed to boil, obviously it
will extract heat, act as the coolant more
efficiently.
So, the next step is that within this chamber
it is not allowed to boil.
So, the structure would be the same chamber;
water is being fed in and the water is going
out, but the whole thing is water.
It does not, it is not allowed to boil into
steam.
But later, after this there will be heat exchanger
in which that pressurized water gives the
heat to normal water allowing it to boil,
clear.
So, let use draw that schematic diagram.
You have the control rods.
So, these are the control rods, these are
the fuel rods and these are the control rods
and you have got a chamber.
In this chamber what you have is pressurized
water which is entering through this and going
out through this and you have a 
heat exchanger in which this thing goes around
like this 
and here you have water coming in and steam
going out.
The rest of it will be the same and the whole
thing will be encased in a steel casing.
So, here in this part you have, sorry, you
have pressurized water.
So, here you have pressurized water, so that
it is not allowed to boil.
Naturally, it acts as a very good coolant
and then that passes through this.
So, here is the water path, pressurized water
path and here you have the normal water coming
after the condenser, taking the heat from
this heat exchanger and going out as steam
to the turbine.
Of course after that, flushing stage, flushing
means where you bring down the pressure a
bit, so that it becomes superheated steam,
then to the turbine.
The advantage is, you can see, first that
the water that is contaminated with radioactive
substance is only within this zone.
It does not come in direct contact with this
steam, this water and therefore, this part,
the turbine, the condenser, the pump, everything
becomes uncontaminated with radiation, radioactive
substances for a longer time; remember for
a longer time, you cannot really say that
it never comes, becomes radioactive because
there are leakages and everything.
So, this is called the pressurized 
water reactor or PWR.
So, here also the same substance, the normal
H 2 O, water is being used both as the coolant
and as the moderator.
Now, in the next stage, we can think of improving
the moderator to D 2 O, the heavy water, in
which case it is the same cycle, but it is
pressurized heavy water that circulates through,
in which case the moderation will be better
and since you are not needing a large amount
of that substance, because it is only this,
it remains encased within this zone, right,
so you can use deuterium for that purpose.
Many of the Indian reactors are of this type
where it is pressurized heavy water that circulates
through, that gives heat to the steam and
of course … You may have heard the name
of the CANDU type reactors Canada deuterium
uranium reactors which India has installed
in the first stage.
These are of this type.
That means these are pressurized heavy water
reactor that means inside it is heavy water,
clear.
What other improvements can we think of?
So far we have used, in the last one that
I have just discussed what is the moderator?
D 2 O. What is the coolant?
That is also D 2 O; come on because here what
is circulating is D 2 O, so coolant is also
D 2 O, right.
Now you have, you can see that there is also
an option of using graphite as a moderator,
of using graphite as a moderator.
That comes as the next stage.
Graphite is a solid substance, is not a liquid
and therefore, you cannot use then graphite
also as a coolant.
So, if you use graphite as a moderator, then
you will have to use something else as the
coolant.
Normally, some gas is used, inert gas is used
as coolant.
Helium for example, argon for example, this
kind of gases can be used here as a coolant
and here there would be graphite plates acting
as the moderator.
So, I do not probably need to draw the figure
again; only I will …, so that you do not
get confused.
In those cases here inside it will be some
kind of coolant, gaseous coolant and the graphite
rods used as the moderator.
Gaseous coolant obviously is a bad coolant,
right.
In order to cool it you have to get heat out
fast and heat transfer rate for a gaseous
substance is obviously low.
So, you cannot use that as a good coolant.
So, what can be good coolant?
Something that is liquid, but we have already
explored the possibilities of water and heavy
water.
The other good coolant is sodium, liquefied
sodium.
So, the other possible coolant is … So,
in many of the modern reactors you have liquid
sodium acting as a coolant in a similar structure,
where here it will be liquid sodium that we
will be circulating.
Sodium is not a, not a good moderator.
So, you have to have graphite as the moderator
substance, get graphite plates, which is a
moderator substance and sodium gives the heat
to the water and sodium can be raised to a
larger temperature.
See, in case of pressurized water or a pressurized
heavy water reactor, you cannot really bring
this temperature to a very high level.
Why?
Because, it starts boiling and there is a
limit to the pressure that you can allow it
to withstand, because these are all made of
steel and there is a limit.
So, the other advantage of sodium is that
even though sodium is a more expensive substance
and in fact, it is explosive, also you cannot
really put sodium anywhere, but the advantage
is that it has a larger heat transfer coefficient.
It acts as a good coolant.
It gives the heat fast, so that is also one
advantage.
The other advantage is that here you can get
the temperature very high.
So, that is the other type of reactor you
have.
Is that clear?
Now, the material that is inside the rod,
as I told you, is mainly uranium 238, a bit
of it say 5% uranium 235.
After all that concentration enrichment it
is only that.
So, what happened to the rest of the uranium
238?
All the neutrons that are coming out they
would be hitting them and by hitting them,
much of that will be transformed to something
that has number 239.
Because uranium 238 was there, it is hit by
another neutron, something is added.
What will happen?
It will turn into something that has an atomic
number 239.
What has atomic number 239?
Plutonium.
So, the material that is here that was not
fissile slowly turns into fissile material,
plutonium 239 and that is one of the major
concerns globally, because plutonium 239 is
the main component of modern bombs.
So, that is why people are so very concerned,
because this is the essential point that as
you process the used fuel rods you get material
for the bomb.
I will, I will come to the these aspects a
littler later, but one of the suggestion that
people are now somewhat rigorously pursuing
is that this wall, this wall on which much
of the neutron really hits, neutrons that
escape through this hits the wall and the
wall if it is made of steel, simply gets brittle
after sometimes, because continuously being
hit by neutrons.
Instead if you line them up by a lining of
uranium 238 what will happen?
That lining will slowly become plutonium 239,
as a result it will become a fissile material.
India has a lot of thorium.
That can also be used in the lining, so that
that also becomes converted to some fissile
material.
So, what happens is that while it uses the
available amount of uranium 235 and converts
it into energy, it also generates more fuel
and in order for that process to be fast,
that process to work fast, the neutrons have
to be, have to go fast and hit the thorium
or uranium 238 nucleus, because here the objective
is different.
Here the objective is not to fission the uranium
235, rather to convert the uranium 238 into
239, so you do not use the moderator.
So, there in that case, the kind of reactors
would be called the fast breeder reactor;
fast, because the neutrons are fast, you are
not allowing them to slow down, breeder means
they breed more fuel than they consume.
So, these are the FBR, the fast breeder reactors.
In India for example, there is an experimental
FBR in Kalpakkam.
So, India is experimenting with this possibility
of breeding more fuel while you generate energy.
For today let us end here and we will continue
in the next class.
