As of April 2020 a total of 440 Nuclear power plants are commercially operational around the world. Additionally, there are 55 reactors under construction and 109 reactors planned, with a combined capacity of 63 GW and 118 GW, respectively. All these use Nuclear fission as their primary process for power generation in a Nuclear Reactor.
Let's have a look what goes inside the core of a nuclear reactor.
A nuclear reactor depends on sustained nuclear chain reaction for continual generation of heat energy. When highly fissile, heavy atomic nuclei are bombarded with free neutrons, known as neutron induced reaction the nuclei may undergo fission. You will find my previously mentioned statement probabilistic as phenomena in subatomic ranges are probabilistic (according to our understanding of quantum mechanics). A subatomic interaction mainly depends upon the reaction cross-section(probability), in our present case cross-section of neutrons and particles of the medium in which the nuclear fission is undergoing .
Cherenkov radiation (Blue) in the core of a nuclear reactor
And when fission occurs the heavy nuclei split into two or more lighter nuclei, releasing free neutrons and Gamma radiation. These free neutron can further induce more nuclear fission reactions and so on. This is known as Chain Reaction. (I am repeatedly mentioning a heavy nuclei, as per the binding energy curve the Ebn is lower for A>170 than for 30<A<170. So, when a heavy nucleus splits into lighter nuclei they release energy)
Most nuclear reactors around the world use isotopes of Uranium and its product plutonium as fuels. Thorium fuel cycles can also be used. Different breeding methods are being used for preparing fissile isotopes from a fertile isotope of Uranium and Thorium.
Uranium-235, plutonium-239, and plutonium-241 are mostly used in reactors as fissile elements.
Neutron Sources:-
All nuclear reactors operate on a neutron induced or initiated nuclear fission process. For starting up the chain reaction in a reactor first we need a reliable neutron source, which can obtained from below mentioned processes.
1.Based on (α, n) reactions:-
It can take place if the kinetic energy of alpha-particle is above the coulomb barrier of the target nucleus. The excited energy of the nucleus after capture is higher than the binding energy of a neutron in the compound state. So, as in the above example the unstable carbon emits energy and a neutron, and this becomes the source for a nuclear reactor.
2.Based on (γ ,n) reactions:-
The abnormally low binding energy of hydrogen and helium makes them perfect for gamma-neutron reactions, which can also be used as a neutron source.
3.Based on (p,n)reaction:-
(Eth=1.019 MeV)
(Eth=1.88 MeV)By bombardment of proton it is possible to obtain a monochromatic neutron source. Light nuclei are appropriate, because of the need of penetration through the coulomb barrier.
The above mentioned sources are known as Startup neutron source. After sometime these neutron sources are removed (usually in a few months). Because neutron capture resulting from the thermal neutron flux in a functioning reactor changes the composition of the isotopes used, and thus reduces their useful lifetime as neutron sources.
Nuclear Fission Reaction:-
Fissile isotopes:- Uranium(233 & 235),Plutonium(239 & 241)
These fissile isotopes have greater probability of fission with thermal neutrons (energy range in eV) rather than fast neutrons (energy range in MeV).
Fertile isotopes:- Uranium(238 & 234), Thorium(232), Plutonium(240)
These are efficiently broken by fast neutrons to get fissile isotopes.
Three typical fission reactions are shown below with fission fragments and number of neutrons ejected:
Enriched uranium is a critical component for both nuclear power generation and military nuclear weapons. Enriched uranium is a type of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (with 99.2739–99.2752% natural abundance), uranium-235 (0.7198–0.7202%), and uranium-234 (0.0050–0.0059%). U-235 is the only nuclide existing in nature that is fissile with thermal neutrons and widely used in nuclear reactors.
Apart from using Uranium, Plutonium-239 can also be used for fission reactions. During the operation of a nuclear reactor with Uranium-238, some Plutonium will also accumulate in the reactor core. Plutonium-239 present in reactor fuel can absorb neutrons and fission just as uranium-235.Since, Plutonium is produced constantly in the core, it can be used as fuel in nuclear power plants without reprocessing of spent fuel.
The average energy released and number of neutrons ejected is a function of the incident neutron speed. Also, these equations exclude energy from neutrinos since these subatomic particles are extremely non-reactive and, therefore, rarely deposit their energy in the system.
(A neutrino is a subatomic particle similar to an electron but with no electrical charge and negligible mass. Both electrons and neutrinos do not participate in the strong nuclear force, but both participate equally in the weak nuclear force.)
Thorium Fuel Cycle:-
Some modern Nuclear reactors mostly developed in India use, Thorium fuel cycle. Breeder Reactors use mainly Uranium-238, a fertile isotope of uranium to produce fissile Plutonium-239.
Similar to above breeding process where Uranium-238 absorbs a neutron to produce fissile Plutonium-239, Thorium cycle uses isotope Thorium-232 (fertile). In the reactor in-situ transmutation of Thorium-232 occurs to fissile isotope Uranium-233, which is used as a nuclear fuel.
Thorium fuel cycle has several advantages over the Uranium breeding:-
- Thorium's greater abundance .
- Superior nuclear properties.
- Reduced Plutonium and actinide production.
- Resistance to nuclear weapon proliferation when used in a light water reactors.
Because of the small amount of uranium-235 that exists in Earth crust , it is considered a non-renewable energy source despite being found in rock formations. The second most common isotope used in nuclear fission is Pu-239 or plutonium-239. This is due to its ability to become fissile with slow neutron interaction. Plutonium was once found naturally in the earth's crust but only trace amounts remain. The only way it is accessible in large quantities for energy production is through the neutron capture method. But our foremost target should be developing on renewable sources, which can be apparently achieved by Thorium Cycle Nuclear reactors.
"Thorium is like wet wood , it needs to be turned into fissile uranium just as wet wood needs to be dried in a furnace."
— Ratan Kumar Sinha, former Chairman of the Atomic Energy Commission of India.
In a nutshell, the post tells us about :-
- Startup-neutrons from different sources.
- Nuclear fission of fissile isotopes.
- Transmutation of fertile to fissile isotopes.
We will be discussing in details on functioning of Nuclear Reactors and their types used for controlled nuclear fission for power generation purposes in the upcoming section.
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