Hello, I welcome you all on this course on
Power Plant Engineering and today we will
discuss Nuclear Power Plants. . Topics to
be covered in this lecture are pressurized
water reactor, we will discusspressurized
water reactors, Boiling Water Reactors; BWR
they are popularly called BWR. Canadian deuterium
uranium the candu type of reactors, gas cooled
reactor and liquid metal cooled reactor and
breeder reactors. In short, I have already
introduced you these reactors in my previous
lecture, where now we will discuss them in
details; we will start with the pressurized
water reactor.
It is simple light water cooled, light water
cooled means H 2 O; these are light water
and heavy water. So, this is a light water
cooled reactor. It is a veryunusual core design
this core design ofthisthe reactor facilitates,
forced and natural both type of cooling of
the coal forced and natural. It facilitates
both types of cooling and and natural fuel
and enriched fuel.
So, it is a very versatile type of reactor,
it is a pressure vessel, it has a thermal
shield also, fuel element, reactor pressurizer,
this is a unique thing here pressurizer. Pressure
is primarily very high I mean in this reactor
and the pressurized up to 100 bar. The primary
circuit is pressurized up to 100 bar,if you
draw thisreactor suppose this is reactor core,there
is a heat exchanger this we have done in the
previous lecture also. Then, after the heat
exchanger there is turbine condenser and then
pump and this is how the water is circulated
and this is a primary circuit this is heat
exchanger and the pressurizer is provided
here.
So, pressurize sometimes it is electrically
heated to maintain the high pressure in the
reactor. So, that steam does not form right.
And, this turbine is connected this turbine
shaft is connected to a generator todevelop
the power this is feed pump, this is condenser,
this is turbine ok. So, there are two circuits
primary circuits and secondary circuit as
I explained earlier and this is and the generation
of a steam, takes place in heat exchanger
and this steam runs the a steam turbine.
So, there are certain advantages of these
reactors. Now, advantages is the water is
used as coolant, moderator, and reflector.
All three functions are carried out by the
water and the water is cheap easily available.
So, this this is a cost I mean this reduces
the cost of the reactor power density is high
for this density is high for this this type
of reactor. And, fissured products they remain
contained in the reactor, they are not circulated
in the pipe, because they are solid fuel solid
fuel is used. So, they remain contained in
the in the reactor and small number of control
rods, control rods.
Small a very small number of control rods
are required in this type of reactor, if you
compare with the other reactor, easy for inspection
you can easily go inside and inspect the turbine
because the non-radiation area. So, and per
kg of fuel extraction energy is high power
as I said power density is high that is also
high, but per kg of fuel extraction of energy
is also high in this type of reactors. It
means less amount of fuel will be required
for the same power generation, if you compare
with theother reactors.
Now, there are certain disadvantage also.
Capital cost is high; capital cost high, because
if you remember in the primary circuit we
have to maintain temperature around 100 bar.
So, all the components are designed. So, that
they can be their operation pressure is is
remaining 100 bar for the operational pressure
of 100 bar. Now, secondly, it is using secondary
circuit circuit.
So, steam it has to this is primary circuit
this is the secondary circuit. So, for the
the the effective heat transfer at least there
has to be temperature difference of 10 degree
centigrade. More temperature difference is
better, but at least it has to be around 10
for 8 to 10, approximately 10 degree centigrade.
So, definitely this affects the efficiency
of the cycle on the other side right. So,
that efficiency is comparatively low, there
are because water is being used as a moderator
and as a coolant or as a shielding material,because
as a no shielding material as a reflector,
uh the corrosion problem is there. So, corrosion
problem has has to be dealt with in this type
of reactor.
And, the third one and the next one, which
is quite serious is that when we want to change
the fuel channels right, the reactor has to
be completely shut down. So, it takes a a
gap of 1 or 2 months' time. So, whenever there
is a replacement of the fuel or the channel
has have have to be replaced the reactor has
to be shut down for 1 or 2 month's time. The
fuel suffers the fuel in the reactor suffers
the radiation damage, damage, because it is
confined to the reactor core insertion of
control rod is also difficult this is such
a compact design of this reactor, that insertion
of fuel rod is also difficult in this reactor.
And, the fuel element fabrication fabrication
of fuel element, it is also difficult in such
type of . So, there are certain limitations
or we can say disadvantage of pressurized
water sorry this this type of reactor. So,
this is we are done with the pressurized water
reactor right, the next is boiling water reactors.
Now, the boiling water reactor as it is clear
from the name itself, the water boils in the
core right. And, this water directly is directly
used for running the turbine there is no heat
exchanger. The efficiency is certainly better
than the pressurized water reactor. So, this
simple arrangement, it is very simple arrangement
and enriched fuel is used 
is used in the this type of reactor natural
circulation is possible boiling water reactor
natural circulation is possible.
And, here in the boiling water reactor the
pressure has to be maintained constant ok,
pressure has to be remain constant. So, suppose
there is a power load there is a sudden decrease
in the load, in that case a steam bypassing
is done. So, when we are bypassing the steam
the same amount of fuel is used, but the less
power is generated.
So, if we look at the arrangement of this
boiling water reactor and this is a pump here,
which is used for forced circulation it can
be a natural circulation also, it directly
goes the some walls you can show some walls
and here there is a turbine connected to generator.
And, here there is a condenser, which is used
for taking away the heat and there is a feed
pump right.
Now, there is a certain degrees in requirement
of the power we lead this power. So, but the
reactor core continue to generate a same amount
of a steam and a s steam is bypassed steam
is bypassed and it goes to the condenser directly
bypassing the turbine, bypassing the turbine,
bypassing the turbine right. In that case
the efficiency is reduced drastically.
So, if you look at the advantages of a boiling
water reactor advantages. Now, first of all
there is no heat exchanger, no heat exchanger.
So, when there is no heat exchanger definitely
it helps in getting the efficiencies. So,
efficiency improves lower pressure vessel,
here we do not have to maintain very high
pressure of the order of 100 bar. So, lower
pressure vessel with the lower pressure vessel
definitely there is a cost cutting in the
material which is used for the fabrication
of the reactor output temperature of a steam
is high, because the inlet temperature is
high.
So, definitely output temperature is also
be high credibly high. So, it is efficiency
of this boiling water reactor water reactor
is greater than the efficiency of pressurize
water reactor right , but there are certain
disadvantages also. First of all disadvantage
disadvantages, first of all contamination
of turbines 
of turbines, because the coolant it goes to
the core and then from the core it directly
enters to the turbine, there is no heat exchange.
So, this water which enters the turbines it
contaminates the turbine. So, if there is
any failure of the fuel element. So, the fissured
material will also enter the turbine that
is that is a dangerous situation right. So,
this is the major disadvantage. So, more precautions
are needed in case of the boiling water reactor.
And, as I said earlier there is a wastage
of a steam, at part load, at part load right.
And, it is power density is also limited,
because in a boiler water reactor with the
water enters from this side and leaves from
the other side only 3 to 5 percent of mass
it is converted into steam right. So, that
is also a limitation of the boiling water
reactor and another thing is the possibility
of burnout, possibility of the burnout is
more in the case of this reactor, because
the coolant is indirectly contact with the
fuel rod. Now, what is burnout?
Now, if you take any pool of water right and
keep on increasing the temperature and you
keep on temperature in the temperature the
heat flux will also increase, because heat
flux is directly proportional to temperature
difference right, it will keep on increasing
it will reach a certain value. So, initially
the nuclear boiling will take place and when
the when there is a intense heating, in that
case on the surface will start covering with
a thin vapor film right.
So, the fuel will no longer remain in contact
with the surface. In this situation in this
situation because the heat Trans steam out
of heat has to be transmitted the delta T
shoots exorbitantly, because the heat transfer
coefficient reduces drastically and q is equal
to h delta T. So, when q is same because a
constant heat flux due to formation of film
thin film of the vapor I am just telling it
in a nutshell ok.
So, the edge goes down drastically this increases
delta T and delta T may increase up to such
an extent that the surface may burnout, that
is it is also known as this flux is known
as critical heat flux, but burn out means
physically the surface physically burns out
and in this scenario, if this happens in the
in this a boiling water reactor, the fuel
will come out and it will get mixed with the
coolant and this will lead to a very dangerous
situation ok.
Now, the next type of reactor we are going
to discuss is Canadian, deuterium, uranium
reactor, this is known as CANDU type of reactor,
Canadian, deuterium, uranium, candu type of
reactor. And, in India many of the reactors
which are work which we have in India, they
are modification of CANDU type of reactor
and they are known as pressurized heavy water
reactor, or Indian pressurized heavy water
reactor.
Now, in this reactor the heavy water is used
as coolant, a heavy water that is D 2 o with
the concentration of 98 percent with the concentration
of a 98 percent natural natural uranium as
a fuel use 0.0235. So, this is 
type of reactor is useful for the country,
which do not produce enrich uranium because
enriching uranium is also I mean it is a costly
exercise, if you artificially, if you want
to enrich uranium. So, those country which
do not have the facility or which do not do
the enriching of the uranium right, this type
of reactor are suitable for those country.
It differs from light water reactor, there
is a the number of light water reactors.
In light water reactors the water H 2 O works
as a coolant. Here it is differs from light
water reactor in that manner only and here
the heavy water used as acoolant, but the
beauty here in this reactor is that only coolant
has to be pressurized not the entire reactor
only coolant has to be pressurized. So, in
case of any accident in case of LOCA, the
severity of the damage is very less right,
if, you look at the arrangement of a typical
CANDU type of reactor.
So, there is a voluntary shell. In shell there
are number of channels maybe 300, 400, 200
depending upon the design of the reactor.
So, there are number of channels right. And,
each channel suppose I take 1 channel, each
channel has a zirconium tube of let us say
it depends upon the power generation, but
normally the thicknesses is of the order of
1 millimeter or 1.2 millimeter, zirconium
tube.
And, concentrically a zircaloy tube was also
used this is a zircaloy tube off thickness
around 4 4 mm, close to the 4 mm depends I
mean crossly it is 4 mm. And, this tube has
number of pins, fuel pins. Fuel pins are nothing,
but the, but the zirconium tubes of half approximately
half is diameter and the uranium fuel pellets
are filled in the zirconium tube right. And,
each tube is known as fuel pin and there can
be a number of fuel pins in a grid right.
Maybe 20 30 40 or 50 pins in a grid.
Now, in the void space between these pins
the heavy water flows that is D 2 o, which
flows in the void space between these pins.
And, because the fission is taking place the
fission heat which is generated in these pins
is transmitted to the heavy water and part
of the water is converted into this steam.
Now, this is steam which is generated is collected
in a header.
There is a steam header in the top part this
steam is and the for the top this steam is
taken and there is a heat exchanger again
and through heat exchanger the heat is transmitted
to the working in this fluid that is H 2 O
water, and again it goes to the turbine, and
condenser, and then pump, and again back to
the heat exchanger right.
So, the beauty of this candu type of reactor
is the pressure high pressure is there, but
high pressure is there only in this pressure
tube inside the pressure tube right. And,
this part is covered is filled with the inert
gas to in order to prevent the heat transfer
and the entire assembly there can be a let
us say 300 channels. So, 300 channels assembly
of 300 channels is placed in a in in a vessel
which is known as calandria vessel ah.
Now, the advantages, advantages of this type
of power plant first of all enriched fuel
is not required. First of all enriched fuel
is not required natural rename can work. So,
and D 2 o is used as a moderator and only
D 2 o is pressurized which is used as a moderator.
So, sorry D 2 o is also used as a moderator
not only as a as a coolant it is also used
as a moderator. So, when as a used D 2 o is
used as a moderator, it is multiplication
factor is high D 2 o has a high multiplication
factor and which results in low fuel consumption.
Right. And, this calandria vessel the calandria
vessel where the tubes are fixed it is not
it does not work on a high very high pressure,
it is works with the normal pressure. So,
that material cost is also saved second thing
is the ambient temperature is maintained between
65 to 75 degree centigrade is between this
range right. And, this because this vessel
is also filled with the water.
So, it also acts as a I mean it also serves
as a purpose of the safety also in this case,
because we have several number of safety barrier.
First of all fuel pin is filled with fuel
and it is filled in the pin, which has also
certain thickness, then there is a pressure
tube, then surrounded by the calandria tube,
calandria tube is surrounded by the calandria
, sorry pressure tube calandria tube and calandria
vessel.
So, level of the safety is more in case of
CANDU type of reactor. . Less time is needed
for construction, construction ok. And, and
I said earlier the moderator which can be
kept at as a 65 to 70 degree centigrade in
this range right. So,we can extract more work
out of theor we can. So, moderator can be
kept and another point is moderator can be
kept between the temperature of 65 to 75 degree
centigrade, which increases it is effectiveness.
It increases it is effectiveness and it slowdowns
the neutron the neutrons are also slowed down
.
Now, there are certain disadvantages also,
now very high standard of design, manufacturing
and maintenance . These things of very high
state of the art design is required for a
candu type of reactor, state of the art manufacturing
is required, because the several joints like
roller joints are also have to be provided
at the ends of the channel and high level
of maintenance is required in the candu type
of reactor.
The coolant and the moderator are D 2 o, which
is also itself is costly if you compare the
cost with the light water. There are certain
leakage problems are also there in the candu
type of reactor. And, it has low power density
if you compare the power density of candu
type reactor. So, it has low power density
if you compare with the pressurized water
reactor and boiling water reactor. So, for
the generation of the same power more space
is required by the candu type of reactor.
The next is gas cooled reactor. Now, in the
gas cooled reactor all the reactors have,
if you draw the schematic, all the reactors
have this type of the schematic. Now, this
most of the reactor have this type of the
schematic now here condenser pump. Now, here
the gas is used instead of liquid gas is used
as a coolant.
So, air can be used as a coolant,hydrogen,
helium, carbon dioxide, they can be used as
a coolant. Here the moderator is graphite.
In these type of reactor,the moderator is
graphite, but the benefit is the the problem
of corrosion, which persists in the case of
where the water is used. Here, because air
or the gases are used, the corrosion problem
is not there. This problem is not there, if
accident happens they are relatively safe
the level of safety is high, thickness of
reactor shield is much reduced, there are
2 types of gas cooled reactors.
The one is gas cooled graphite, moderator,
reactor. Graphite cooled, gas cooled, graphite
moderator reactor. And, another ishigh temperature
gas cooled reactor high temperature gas cooled
reactors. Let us say GCGM and HTGC. So, there
are two types of reactor gas cooled graphite
moderator reactor and high temperature gas
cooled reactor, both type of reactors has
graphite as moderator. But, this reactor is
uses natural uranium and this uses enriched
uranium that is a difference.
So, this natural uranium is used in gas cooled
graphite reactor and this uranium is used
in high temperature gas cooled reactor. The
pressure is 50 to 30 bars and the temperature
goes to 700 to 800 degree centigrade right.
Now, advantages of gas cooled reactors gas
cooled reactors advantage, but the advantage
is the processing of fuel is simple; process
of fuel is simple as I stated earlier there
is no problem of the corrosion inside of the
reactor and graphite remain stable under high
temperature. It does not disintegrate.
And, if we are using C O 2, it eliminates
any possibility of the exposure in the reactor
also right. And, this uranium carbide and
graphite they are able to resist high temperature
so, temperature. So, these type of reactors
can be operated on high temperatures. Now,
there are certain disadvantages also, fuel
loading. Now, fuel because here we are using
gas the density of gas is lower than the the
the solids normally.
So, the fuel loading is elaborate and costly
right, power density is low power density
is low right. . Because, critical mass is
high for this reactor criti so, high mass
has to be loaded initially because critical
mass is high, if critical mass is low initially
low mass is required, but because here the
critical mass is high high mass is required
is high mass of the fuel loading is required
initially and the another thing is if you
are using helium .
So, helium diffusivity is high for the helium.
So, they are biggest problems if you are using
helium coolant is gas. So, more power more
bulk of the volume has to be handled by the
pump. So, more power has to be consumed by
the pump in these type of reactors. And, controlled
in coolant is also complicated and because
a negative coefficient of helium and it does
not absorb the neutrons. And, gas cool next
is liquid metal cooled reactors. In previous
lectures also I I draw I have drawn the schematic
of the liquid cool where there are two heat
exchangers .
Where there are two heat exchangers right.
So, in these liquid metal type of a reactors
sodium is used at 7 bar, sodium boilsaround
800 degree centigrade temperature. So, the
very high temperature can be maintained in
such type of reactor. So, sodium can be pressurized
or without pressure also even, we do not pressurize
sodium in that case also this reactor can
be can work efficiently. Thermal efficiency
is high, definitely if if a reactor is operating
on a high temperature temperature difference
in source and sink is high. So, thermal efficiency
is going to be high.
And, graphite moderator is used graphite moderator;
even low cost graphite moderator can be used
in such type of reactor. And, is this smaller
in size smaller in size and super heating
is possible in such type of reactor, because
they are operating very high temperature.
But, the disadvantage is the sodium react
very violently is water. So, in any case in
the remotest of the possibility the mixing
of sodium and water has to be avoided.
Otherwise very violent reaction will take
place, thermal stresses because it operating
very high temperature, thermal stresses is
also an issue in these type of reactor. And,
the heat exchanger in this type of reactors
have to be leak proof, because if there is
a leakage again there will be a reaction of
sodium with the moisture with the atmosphere
or with the liquid which is circulated in
the secondarycircuit.
So, that is why shielding of first and second
stage is done right. So, the major disadvantages
of this reactor is the leakage of sodium is
very dangerous. Now, the next one is the breeder
reactor. Now, it is as it is clear from the
name itself, the breeder reactor can also
be used for generating fluid. In the breeder
reactor the mass of the fuel or the fuel which
is consumed and the end product is more fuel
is more fuel is generated at the same time
power is also generated.
So, it is used for the breeding or the generating
the fuel and breeder reactor is also used
for the power generation. So, it has a small
vessel and some some amount of enriched plutonium
is kept with moderator, enrich enrich and
there is a fissionable material, which absorbs
the neutron surrounding the vessel, which
absorbs the neutrons surrounding the vessels
the core is pulled by liquid metal .
So, liquid metal is a an additionals is because
here the gamma radiations are also there.
So, additional shielding is a very robust
shielding is provided surrounding thethe reactor
when uranium 235 is fissioned, when uranium
235 is fissioned enormous. . Heat isah liberated
and at the same time neutrons also or also
liberated. If, U 10 38 is kept in the vicinity
or a part of the reactor after the reaction
it can produce.
So, after reaction with 2 235 this uranium
238 can be converted into the fissionable
material 
this is how the first breeder reactor work.
So, it consumes the fissionable material and
end product is more fissionable material and
at the same time heat is also liberated which
can be used for the power generation that
is all for today.
Thank you very much .
