So good morning everybody!
In the last lecture, we just saw what are
the different sources of energy from which
we get our electricity.
We saw the coal, the hydro generation and
nuclear power and we also had a look at the
carbon emissions, from these which are very
responsible for Reno's effect, then we went
into some other aspects.
Utilization has been there like in the field
of medicine in Environmental Protection in
agriculture, then sterilization of most of
the articles used in surgery then Radio pharmaceutical,
and it’s utilization in cancer diagnosis,
and treatment then of course industrial radiography,
food preservation, and a variety of uses which
we come across in our day to-day life .
Finally we saw an aspect of course, the story
of three men and a tiger I told you that there's
a risk perception of everybody but there is
nothing like absolute risk zero risk, you
have to compare the risks of different things
and as you know returns you get more when
you take risks in the investment business.
However in the last lecture.
I got some questions from some of the students.
One was, I talked about treatment of waste
water and Treatment of sewage water.
So the question was what is the difference
between the two?
Waste water refers to the water that is discharged
from the industries like chemical industries
could be leather industry, textile industry,
mining or any other manufacturing process
industry and this water it contains a lot
of bacteria, chemicals and quite good number
of contaminants and when you do treatment
with radiation, it reduces the level of the
contaminants to acceptable level then you
can discharge it to the environment.
Whereas, the sewage water is that water which
is contaminated with human fixes the urine,
and not only which is harmful, it contains
a lot of pathogens, lot of bacteria, so essentially
it includes domestic waste, the municipal
waste and waste products which are disposed
of usually through your sewer pipes so that's
why the name sewage.
So, this I hope clarifies the difference between
the sewage water, and the wastewater.
Now there is one more clarification asked
talking about the radioactive releases from
the thermal and nuclear power plants.
I had made a mention that coal contains through
thorium and uranium when you dig it out, and
this gets released when you burn it also gets
to the fly bash etc.
So, basically how doe s this get into our
system?
This release is basically due to the radio
nuclides in the fly ash which are present
in high concentration and they are inhaled.
So inhalation is the main route by which these
radioactive products get into our system and
radium, of course, it has got a affinity for
the lungs, it will l get sedated in the lungs,
if it is iodine it will get in the thyroid.
So this is the issue which needs to be kept
in mind.
So overall for a thermal power plant and nuclear
power stations in India, a research has been
carried out and there are lot of measurements
have been taken based on the IAEA’s guidelines
and these studies -- to summarize have led
to the following conclusions that the doses
which you are receiving whether there is a
nuclear power plant or a thermal power plant
around that they are all around a natural
background level but the collective effective
doze if you compute, for a 80 kilometer radius
around thermal power station, that is about
10 times than those of the nuclear power plant.
It doesn’t mean that okay thermal power
plant is more hazardous than this thing, it
is higher.
The only message which I would like to convey
is there is nothing to fear about living near
a nuclear plant that you are going to get
radiations.
So that’s the main thing which I want to
convey.
Now, having had an idea about the different
uses of radiations, now let us go to the main
thing about which our topic is on a nuclear
power production.
Now to produce power basically we have one
of the radioactive materials as Uranium-235
that is available in nature.
There is only one element which can be fissioned,
and that fission process produces heat and
that heat is used to convert water into steam
and run a turbine.
Now let us have a look where is uranium come
from?
As I said it is naturally available but perhaps
the Earth's uranium was produced in one or
more supernova.
What is a supernova?
It is an explosive brightening of a star in
which the energy radiated increases by a factor
of ten billion, it's a huge release of energy
and then when does a supernova occur?
When a star has burned up all its available
fuel, in fact this earth itself was formed
by the Big Bang Theory in which the part of
the sun exploded and formed the different
planets.
So the earth’s uranium was produced through
one such process and it's a part of our earth,
it is not in the whole solar system has, when
I said the Sun there was a big bang.
Sun is a fusion reactor unlike fission.
We will talk about fusion and fission later
in our lectures.
So this uranium is not man-made, uranium has
existed in a natural way in the earth.
Now let us see what does uranium does, you
know we are living on the earth whereas people
are not able to live on planets beyond the
earth, Mercury, Venus may be the very hot.
Mars, we are thinking that some people may
live, Jupiter, Saturn, why?
That means the earth’s temperature is very
much suitable for us to live.
So you require maintaining that temperature.
If you look at the heat loss from the earth
which it dissipates is about 42 to 44 terawatt-hours,
terawatts of heat it is happening from there.
But as the heat is lost surely the planet
should be cooling but that is not happening.
So what is happening?
The heat is there because of the radioactive
decay of the uranium, thorium and potassium
which is in the Earth's mantle and this energy
is getting transferred to the surface of the
earth through the our soil and other areas.
So this way you can see and the measurements
of this heat which is coming out have shown
that they are somewhere between 30 to 44 terawatts
and if you just compare this number with the
heat loss, you can get an idea maybe that
is why we are able to survive on this earth
and the radioactive decay of uranium, the
presence of uranium and its radioactive decay
is one of the reasons we are able to survive
on this earth.
Look surprising but it's a fact.
Then one might think okay the uranium is present
there and this natural uranium in general
contains 0.7% of Uranium-235 and 99.3% this
uranium 238.
This Uranium-235 is the only fissionable element
which can be efficient by a neutron.
So apparently in West Africa, there's a place
called Oklo in Gabon people did a survey of
the natural uranium deposits which are there
and they were surprised to find that the it
contained 3.7% of Uranium-235 and not 0.7%.
So apparently when they did further explorations,
there appears to have been a natural reactor
present and not only one reactor something
like 17 natural reactors have been operating
there and these have been like a spontaneous
reaction water has been there and the water
has acted as a moderator and has been able
to give Philip to the chain reaction.
And they just continued for some million years
and many of the radioactive products that
have decayed from these elements have been
found in West Africa.
And one more thing which they found, they
did find plutonium but this plutonium has
not moved, it has been immobile this was again
one of the -- what you call inputs that even
though you may put a radioactive material
in the earth in a place, the earth itself
acts like a filter, it doesn't allow the radioactive
products to move about.
So this concept has been very useful in waste
management.
Coming to the nuclear power again, as I said
it is produced from the fission of Uranium-235
and if you ask how much of uranium is there,
it is 500 times more abundant than gold and
it is present in rocks, soils and everywhere,
in water also.
And in granite it is about 4 parts per million
and you remember granite makes up nearly 60%
of the Earth's crust.
If you take fertilizers uranium concentrations
can be as high as 400 ppm and coal deposits
as I mentioned they have concentrations greater
than 100 ppm that is because all are ducked
from under the earth and all of all these
the uranium or thorium and potassium.
Now since the concentration of uranium as
Uranium-235 to say it is only about 0.7%,
there has been need to separate it from the
other constituents which are not fissionable.
So we have to dig out the uranium deposits
and this where the concentrations are high
level we call them as ore and then we start
processing them to get the uranium.
Now the enrichment of uranium varies in the
in the heavy water reactors, we use natural
uranium itself as a fuel which contains about
0.7%.
But most of the reactors in the world which
are light water reactors, they use somewhere
between 2.5% to 5% of Uranium-235 while the
fast reactors use something like about 80%
to 85%.
So we have for developed unit of process to
increase the content of Uranium-235 and they
have all been developed then after its utilization
in the fuel of course in the reactor it could
be -- the fuel could be Uranium Oxide, Uranium
Carbide or Uranium Metal.
Uranium Metal was used in the initial reactors
but due to its low melting point it was felt
for -- to go to high temperatures we should
use a ceramic so people went to Uranium Oxide
which has been very widely used, Uranium Carbide
also has been used.
Now once the fuel has been utilized in the
reactor it will surely contain some unused
uranium, surely not all the uranium would
have been utilized, and then Plutonium 239,
which is produced by conversion of the Uranium
238 and some fission products.
So this Uranium 238, which is 99.3% gets converted
to plutonium 239, and this plutonium 239 luckily
is again fissionable.
So this is a man-made fissile material.
They recovering of this plutonium 239 and
unused uranium is what you call as reprocessing,
these are what very frequently used, reprocessing.
So now we can get an idea about the full cycle
through which the fuel goes.
You start with the mining, get the mined,
get the uranium ore then you go through a
process of crushing and refining, then you
have an a stage of enrichment, then in case
you need to have to more than 0.7% you are
enriched it, then you have to convert it into
the form of oxide, then these fuel which are
in the powder form they have to be put into
pellets, and then pelletizing is one and then
made into fuel elements and used in the nuclear
power plant.
And from the nuclear power plant it goes to
the reprocessing where the plutonium 239 and
unused uranium are extracted and they are
sent back for reconversion and again fabrication
to be used in the reactor.
And whatever is not the fission products and
all which are after the reprocessing, they
have to be called as a radiation waste and
stored properly.
So this is the nuclear fuel cycle in a very
brief manner.
To get an idea of the quantities involved
because, that is very much important, you
must know what you are material amount of
material you are dealing with.
Let us take a 1000 megawatt electrical nuclear
power plant.
Now for that you have to -- suppose we consider
a ore containing about 1% uranium you require
about 20,000 tons of uranium ore you have
to mine and 230 tons of uranium oxide finally
you will get and when you convert it, we have
the uranium fluoride process it gets converted
to 288 tons of uranium UF6 then this when
enriched gives about 35 tons and when you
fabricate it, it leads to about 27 tons and
this 27 tons of uranium oxide when you put
in the reactor it gives you some 8,640 million
kilowatt hours and after all the thing what
is left is the use of fuel which contains
about 27 tons.
So this gives you an idea of from 20,000 tons
what is finally we are losing about 27 tons.
So this is an idea of the material used involved
in the nuclear cycle.
Just to get a bit more idea about how we do
the mining, we generally have open pit mines
where deposits are close to the surface, and
in case they are deep, they we go for underground
mining like any other coal plant.
And, we do a process of in situ leaching wherein
water is circulated through the ore to dissolve
the uranium and bring it to the surface and
we take of course special precautions basically
ventilation so that the people working there
should not be put to any airborne exposure
from the radiation.
Then we have the enrichment process.
The enrichment consists of two methods, one
is the gas diffusion process and the other
is a centrifuge process.
Now in the case of gas diffusion process,
we use basically principle of diffusion and
it has been quite effective.
The other process is basically using the centrifugal
force, what we call is a centrifuge running
at about 70,000 rpm wherein the Uranium-235,
and uranium 238 gets separated a then you
are able to take out the enriched uranium
but the centrifuge process is quite power
intensive.
So there is a reason why many countries have
not been able to go ahead with this, already
they are in need of power.
Then this fuel has to be transported.
When I mentioned to you about the carbon dioxide
emissions from the new -- in case of nuclear
power plants, whenever we manufacture and
transport a component, it has to come on a
diesel vehicle which is going to have carbon
dioxide emission.
So transport is very important stage in which
the nuclear fuel will move between different
parts of the cycle.
So you have to be very careful that during
the movement this radiation exposure should
not affect the people who are involved, it
should not affect the public.
So we have to see that they are packed properly,
shielded properly .We use the word shield,
radiation shielding materials we use so that
effect of the radiation is not felt outside.
So these are the safety measures, which need
to be involved in the transportation.
Just to give you an idea, how a cask containing
radioactive material looks like, this as you
see the cask which is mounted on the train
you see it's a goods train.
It is mounted and this contains waste or raw
materials also.
This particular one contains waste, nuclear
waste which is also radioactive and this design
is done in such a way, that these casks even
if they fall down they won't break.
You will be surprised in the United Kingdom,
in the 90s there was one person raised the
question, suppose this train carrying this
radioactive material or radioactive waste
meets with an accident, what will happen?
Believe it, they took a train with one or
two containers and containing the radioactive
material and the train really was meet to
have an accident and the cask fell down and
nothing happened.
So one thing is sure the methodologies which
we are adopting for radioactive materials
are good, and we have to need to -- the need
is that safety at every level of this we should
be safe so that radioactive releases or the
reactive contamination to the public and workers
is minimal.
Then the spent fuel, let us take the spent
fuel the fuel has been used now you have to
take it out but it is at a high temperature,
there is not -- temperature in which you can
handle immediately.
So it needs to be cooled, it has to be put
in a -- with proper of cooling then only,
if you don't cool it that fuel itself can
melt.
So normally we have a spent fuel way in which
we put the fuel, and you can see a simple
system wherein from the reactor core the fuel
is handled, and then put to the spent fuel
bay.
One more thing is important that the geometry
of the fuel bay and water should be such that
it should not become critical.
We will see about this later.
So this storage, it has to be kept for some
time until the temperature comes down.
Then in a dry cask you can transfer it to
the reprocessing and during that time and
you transfer it even natural consumption of
air could be sufficient to cool the cask.
Then as I mentioned reprocessing and the spent
fuel is about 95% uranium-238 because all
the uranium-238 doesn't get converted into
plutonium, only a part of it gets converted.
So also it contains about 1% of uranium-235
which is not fission, 1% plutonium and about
3% fission products.
Besides you have Neutron poisons, like xenon
etc which may be there and they are highly
reactive.
So in the reprocessing you separate the components
basically we are looking for taking out uranium,
plutonium and the waste containing the fission
products and based on the reprocessing output
we can use the uranium and plutonium into
fresh fuel and so that it is effectively we
are reducing the waste.
Now you might wonder that in the media there
has been always thing that a reprocessing
should not be there.
Reprocessing that can lead to proliferation,
and people can take the plutonium.
Now USA has adopted this attitude that whatever
is coming out of the reactor it is not used
again.
It is just kept as a waste but in this process
the waste activity is high because of the
presence of uranium and plutonium.
If you can separate it and then use it in
another reactor you are effectively replacing
the resources.
In fact this type of approach without reprocessing
is called generally, as open fuel cycle whereas
what I was talking about the nuclear fuel
cycle which is used back, it is called a closed
fuel cycle.
And India per say has gone in for a closed
fuel cycle, because we want to effectively
utilize all our natural resources of uranium.
There is a process which is a well known Purex
Process which is used in reprocessing and
just to get an idea, what is the components
of the Purex Process you can have a look at
this figure, it's a schematic figure in which
involves the disassembly of the fuel, the
decladding, remove the cladding material of
the fuel, then dissolve it using nitric acid,
then to extract the fission products they
use TBP plus Kerosene, It is Tributyl phosphate
and Kerosene along with nitric acid then whatever
is removed then goes further uranium and plutonium
come in solution form, then you remove the
plutonium and uranium separately.
Whatever is the rest in the second stage after
the TBP, what is doesn't get dissolved is
sent the high level waste and of course the
rest of the process is as shown.
Okay now the waste which is coming out from
the reprocessing plant, what we do?
It’s a very important part of a fuel cycle.
We normally try to categorize them as high
level, medium level or low level based on
the amount of radiation.
Now low level wastes are practically produced
at all stages like right from mining, then
your fuel fabrication, everywhere they are
produced.
Then intermediate level wastes are produced
during reactor operation or by reprocessing,
and the final high level waste is basically
from the fission products which are taken
out in the reprocessing plant and that is
which is called as a high level waste.
Now this high level waste is the one which
we are saying that we can use back in the
reactor, so that we don't have any high level
waste.
To get an feel of this high level waste, how
much would be the quantity of high level waste,
very simple example I can give you.
Suppose one person's requirement of electricity
throughout his life was to be produced only
through nuclear power then the high level
waste will be equal to one fist that's all,
and that itself if you again put in to the
reactor there is no high level waste.
So there is a fear that this high waste management
is a very difficult and things, it is not
the quantity is very much less.
Now how do you manage these wastes?
Low level wastes contain radioactivity but
this radioactivity is having a very short
life, so it is not very much -- it does not
require a shielding and it can be jus buried
under the earth.
But before burying up we just try to compact
it so that and we also incinerate so that
those material which can be burnt they will
all become ash and become very compact.
So it will have only about 1% of the radioactivity
of the whole waste, 1% would be present in
intermediate waste.
Then coming to the intermediate level waste
it will be higher, surely it requires shielding
you can't do without shielding then it makes
up about 7% of the volume and has about 4%
of the radioactive wastes again in this.
So this again has to be dealt with, again
stored for some time then the high level waste
about we should be really worried, it is highly
radioactive.
As I mentioned it requires cooling and lot
of amount of shielding also and it contains
about 95% of the total radioactivity.
So this waste is the one which we need to
be concerned about.
So what we do?
We have to immobilize the waste that is -- we
have to see that the waste is not able to
move.
So how to make it immobile?
Put it in some other material matrix in which
it can it get bonded and it doesn’t move
and that is where we find this borosilicate
glass has been very good as a medium and borosilicate
glass mixed with the fuel is able to hold
the high level waste fuel very well, bonded
very well.
So this is remain stable for a very long time
because they contain some of the long-lived
waste.
This is just to give a figure of the process.
This process is called the verification.
There is mixing glass and the fuel, high level
waste and then trying to make this thing . So
the glass is molten at about 1200* centigrade
and we add the waste high level waste and
then we pour it and then make it into a solid
form and this solid form we put in radiation
shielded casks and put it under the ground.
Now coming to the waste disposal there are
different types as one is near-surface disposal
and as I mentioned this is done for low-level
waste in all countries, it has been done then
deep geological disposal we are looking for
basically for the high level waste because
they are going to remain for a very long time.
So we are looking at what you call geological
areas where sites are such that they are not
going to be approachable for a very long time.
In the USA that you come Yucca Mountain was
the place chosen for putting these high-level
waste casks, but they have been delaying due
to so many reasons.
In fact because of that the high level wastes
are lying in the plants and it is not a good
thing.
In our own country we have looked at some
geological repositories where the Colar gold
fields where there now practically its all
– there’s lot of place available where
so -- this is one of the areas which we think
could be utilized but till now even though
our power program is not a very big now, it
has not come to a level where it have to be
-- that need to be a concern.
In brief, I can say that this part of the
lecture we covered mining, we covered about
the processing of the ore, then the enrichment
of Uranium-235 using the centrifuge and the
diffusion process.
We also had a look at the transportation aspect
that we need to transport things properly
in a radiation shielded casks so that it does
not affect.
So at the mining level also we need to be
safe processing everywhere every step we need
to be safe reprocessing.
It is quite a bit of higher activity.
Wherein ,we have to be very careful, we deal
with solutions also.
Then finally the waste management wherein
high-level waste is involved and the waste
disposal.
So this lecture practically I have taken you
through the different stages of how the fuel
from its inception on its birth to its -- I
shouldn't say death, and it's reuse.
So this gives an idea about the thing.
So safety is involved at each and every step
of this fuel cycle.
Many of you might be interested to read through
some of the literature.
So I just given the Bibliography related to
my these first two lectures and there are
books, many of them published by IAEA on sea
water desalination, hydrogen production, then
on nuclear safety by Gianni Petrangeli and
Lewis and the World Health Organization has
produced some booklets on Irradiated Food,
it is not that we say that okay irradiated
food is not bad, it is all the result of research
done over decades.
So be sure irradiation not only results food,
it avoids food wastage, it avoids bacteria
in the food and you are able to have food
at any time you want.
Maybe it would be of interest to have a small
assignment on this which I think you should
take it up very simple, what are the different
electricity generating technologies?
What are the different applications of nuclear
energy?
Besides power, I mentioned you so many other
applications and what is its use of radiation
in medicine and industry?
And important how do we treat the flue gas
by radiation and make it safe, there is sulfuric
oxide and nitrous oxide, how it is absorbed
and then you don't get it put it into the
environment, the sewage treatment etcetera
and last but not how you perceive risk.
Thank you for a patient listening.
Now in my next lecture, I shall try to give
you some insight into the structure of the
atom.
So that let us get back okay we talked about
fission but how does fission happen and how
that concept of the fission and hopefully
with that background you would be in a better
position to follow the further lectures.
Have a nice day.
Thank you.
