Monday, November 12, 2012

Nuclear Energy - In a Nutshell !

Before dealing with the nuclear technologies, lets just discuss important basics behind it. S&T questions for prelims could be framed from such basics and people gets stuck, where they know everything about the Kudankulam Nuclear Plant, but gets confused with an Isotope :)

Important lines are marked in blue

1. Structure of an Atom 
Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom. The electrons are arranged in energy levels, or shells, around the nucleus.

Properties of sub-atomic particles
particlerelative massrelative charge
electronalmost zero–1

2. Atomic Number & Mass Number 
The number of protons in the nucleus of an atom is called its atomic number. 

Hence two key things emerge:
> Atoms of same element will have same number of protons.
> Atoms of different element will have different number of protons. 

The total number of protons and neutrons in an atom is called its mass number.

3. Isotopes
Isotopes are the atoms of an element with different numbers of neutrons. They have the same proton number(because its the same element), but different mass numbers.
E.g: Uranium has couple of isotopes like U-238, U-235, U-234 etc

4. Radioactive Decay
The nuclei of some isotopes are unstable. They can split up or ‘decay’ and release radiation (radioactive particles + energy). Such isotopes are called radioactive isotopes or radioisotopes

Types of radioactive decay - Alpha, Beta, Gamma 

Alpha/Beta - When an atom emits alpha or beta radiation, its nucleus changes. It becomes the nucleus of a different element. This is because the number of protons in the nucleus determines which element the atom belongs to. 

So for, 
> alpha decay the mass number (-4) and atomic number(-2) decreases. 
> beta decay there is no change in mass number but atomic number increases by 1.
>beta decay converts a neutron into proton and electron. See -1 against neutrons and +1 against protons, in above table. This would mean free neutrons will always decay into electrons and protons.   

Since in both the decays the number of protons changes and hence one element converts into another. e.g. Uranium --> Thorium etc.  

Gamma Decay - This is the last stage of radioactive decay when nucleus from earlier decays (alpha/beta) remain in higher energy state. They emit electromagnetic radiation and come to a lower energy state. This does not change number of protons or neutrons in a nuclei. 

Few key points emerge from here:
When a radioactive isotope decays, it forms a different atom with a different number of protons - only alpha and beta decays forms different elements. 
> The radioactivity of an object is measured by the number of nuclear decays it emits each second – the more it emits, the more radioactive it is.
> Nucleus with more number of neutrons will be usually more unstable and hence radioactive. We can also get this clue that all radioactive elements have higher atomic number. 
> Alpha decay emits neutrons and protons and beta decay emits electrons(and forms protons inside nucleus).
> Gamma decays occurs only after alpha and beta decays. 

[Note: Helium we fill in gas balloons is formed during the natural radioactive decay of elements such as Uranium and Thorium.   These heavy elements were formed before the earth but they are not stable and very slowly, they decay. One mode of decay for uranium is to emit an alpha-particle.   This alpha-particle is actually just the heart of a helium atom - its nucleus. It then grabs some electrons and forms a helium atom.]

5. Half Life 
We measure rate of speed in a car in km/h. This method of measuring a rate won't work for radioactive decay, however, we know that radioactive substances disintegrate at a known rate. We call this rate the isotope's half-life. It is the length of time required for the disintegration of half of a given number of nuclei of a radioactive element.
E.g: Beryllium-11(11Be) has a half-live of 13.81 seconds. So it would mean that if we take say 16 grams of 11Be and wait for 13.81 seconds, we will be left with 8 grams 11Be; the rest will have decayed to Boron-11. Another 13.81 seconds go by, and we'll be left with 4 grams of 11Be; 13.81 seconds more, and we have 2 grams etc. 

>Half-live rate of radioactive decay follows an exponential curve. 
>Half-live is isotope specific and NOT element specific. So different isotopes of same element might have different half lives. 

More simplistic questions can be asked on half-lives of radioactive isotopes. As we know, radioactivity decreases with time. It is possible to find out the half-life of a radioactive substance from a graph of the count rate against time. The graph shows the decay curve for a radioactive substance.
In the graph above, the count rate drops from 80 to 40 counts a minute in two days, again we see that in the next two days, it drops from 40 to 20 - i.e. it halves. So the half-life of the radioactive substance is two days.  

Nuclear Fission is a reaction when the nucleus of an atom, having captured a neutron, splits into two or more nuclei, and in so doing, releases a significant amount of energy as well as more neutrons. These neutrons then go on to split more nuclei and a chain reaction takes place. 

Nuclear Reactor 
Nuclear reactors work on the principle of Nuclear Fission. 

(a) Mining of Ore: Uranium is scarcely available on Earth. Naturally occurring Uranium isotopes are U-238 and U-235. Mined Uranium contains 99.3% U-238 isotope and 0.7% U-235. It’s U-235 which is radioactive and used in nuclear reactors and bombs.
Difference in mass between U-235 and U-238 enables us to enrich U-238 into U-235. Its mixed with acid and converted to gaseous form and passed through a centrifuge to separate U-235 from U-238. Based on concentration, uranium ore has its uses. Nuclear Power Plants uses 4% enriched U-235 and Nuclear Bomb requires 90% enriched. [Remember the famous Netanyahu General Assembly speech on red line for Iran's Uranium enrichment programme. Read here]

(b) Nuclear Reaction Process
Induced fission reaction takes place inside a nuclear reactor when a neutron is bombarded into Uranium and the nuclear fission reaction takes place. 
Nuclear reaction(fission) --> Generates heat energy --> converts water into steam --> steam drives turbine --> turbine generates electricity 
Control Rods – are used to absorbs neutrons and control the nuclear fission and thereby the heat generated by the reactor.
Fuel Rods – contains U-235 pellets 
[Remember at the end, its these fuel rods which will contain the spent fuel waste and will need to be safely disposed.]
(c) Spent Nuclear Fuel (SNF)
As the nuclear reactor operates, Uranium atoms fission (split apart) and release energy. When most of the usable Uranium has fissioned, the "spent" fuel assembly is removed from the reactor. This is called Spent Nuclear Fuel (SNF). SNF contains various U-238, unfissioned U-235 and transuranic (elements with higher atomic number than Uranium i.e. more than 92, like Plutonium etc). Its is this transuranic elements which are most dangerous for human health as they have very high half-lives of thousands of years, which means they will take a very long time to decay. 

Hence these SNF are:

> Stored in some remote locations, underground safe, far from human reach 
> Reprocessed to get unfissioned uranium and plutonium.
[Note: This reprocessing of SNF has got KKNPP into trouble with lot of people filing for public interest litigations in Madras High Court. We will see later, why?]

Kudankulam Nuclear Power Plant (KKNPP)
Its a VVER reactor. VVER is an acronym for Russian designed “water cooled, water moderated energy reactor”. The VVER reactors belong to the family of Pressurized Water Reactors (PWRs) which are predominantly in operations, world over.

> Signed by India-Russia in 1988.
> Environmental clearances was given to KKNPP in 1989 based on two conditions
  • Dangerous ‘Spent Nuclear Fuel’ would be shipped back to Russia
  • Freshwater which will be used to cool the plant will be supplied from Pechiparai Dam in Kanyakumari District, TN
> No action from 1989-1997, in which two separate notifications was effected
  • Coastal Regulation Zone (CRZ) notification which bans all industrial activities within 500m of the high tide zone. Exceptions were given to the Dept of Atomic Energy (DAE) projects. KKNPP is not DAE project but a company, registered under the Companies Act.
  • Environment Impact Assessment (EIA) notification in 1994 which mandates any notified company (Nuclear power is a notified company) to submit EIA report which inter alia includes record of public hearing i.e. discussion with local community and getting their buy-in. This EIA is then appraised by an independent Environmental Appraisal Committee, before an approval is given. 
> Two new major developments took place post 1998
  • SNF will NOT be transported to Russia, but will instead be stored, transported and reprocessed in India. 
[It is this change in provisions that drew most flak for KKNPP and was challenged in M-HC, seeking a fresh environmental clearance as the one granted in 1989 was based on the original agreement stipulating that all SNF would be sent away to Russia. However M-HC dismissed all such petitions and agreed with Govt. that SNF is not a waste but can be reprocessed and used further.]
  • 5-6 desalination plants to be setup near KKNPP to provide for fresh water instead of transporting water from Pechiparai Dam.
Thorium Power - Future for India
Bhabha Atomic Research Centre in Mumbai is working on a long-range program to convert India to thorium-based power over the next fifty years, making the most of India’s modest uranium reserves and vast thorium reserves.(India along with Australia account for 25% of World's Thorium reserves)
Thorium is nonfissile but can be converted to fissile Uranium-233(another fissible isotope of Uranium, apart from U-235). The chemistry, economics, safety features and non-proliferation aspects of the thorium->uranium fuel cycle gives hope to us that this element can be leveraged for safe and reliable Nuclear energy.

Remember, one of the major issues with using Nuclear energy is the SNF(nuclear waste, see above) generated at the end. 

Now, lets look at figure below:
If you see figure above, Thorium(Th-232) captures a neutron to become Th-233, then undergoes beta decay—emission of an electron with the transformation of a neutron into a proton (See basics above). With the increase in proton number, Th-233 transmutes into Pa-233, then beta decay of Pa-233 forms fissile U-233. Most U-233 in a reactor will absorb a neutron and undergo fission; some will absorb an additional neutron before fission occurs, forming U-234 and so on up the ladder. 

Important thing to note here is that, transmutation routes to Plutonium (PU-239)[harmful element in SNF] in thorium based reactors, requires many more absorption and decay events when starting from Th-232, compared to U-238 (used in Nuclear reactors now) thus leaving far less Plutonium to be managed. Moreover if you remember, in a normal Uranium ore only 0.7% of Uranium are U-235 and rest all are U-238. This would mean to get a desired level of U-235, we need more fuel. That problem is also lessened in a Thorium based nuclear reaction, which would mean less of SNF to handle. 

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.