17 Nuclear Energy From Fission

Dr. Dhanya M.S M.S, Dr. Sandeep Kumar, and Dr. S. Prasad Prasad

1. Introduction

The nuclear fission is one type of nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller nuclei along with release of huge amounts of energy. Fission is a form of nuclear transmutation with unsimilar resulting particles which differ from parent nuclei.

The fission occurs in heavy nuclei after the capture of a neutron. The slow or thermal neutrons cause fission only in those isotopes of uranium (U) and plutonium (Pu) whose nuclei having odd numbers of neutrons (e.g. U-233, U-235, and Pu-239). The fission in nuclei with even number of neutrons occurs only when the incident neutrons have energy above around one million electron volts (MeV).

The two nuclei products from common fissile isotopes are comparable differing in sizes with a mass ratio of products of nearly 3 to 2. The binary fissions that produce two charged particles are common but three positively charged particles (ternary fission) are also produced occasionally.

The electricity produced by nuclear fission in a self-sustaining nuclear chain reaction that releases a significant amount of energy at a controlled rate in a nuclear reactor had wide energy potential.

1.1. Discovery of nuclear fission

In 1917, Rutherford was able to perform transmutation of nitrogen into oxygen, using alpha particles directed at nitrogen

14N + α → 17O + p

That was the first observation of a nuclear reaction, in which particles from one decay are used to transform another atomic nucleus. Finally, in 1932, a fully artificial nuclear reaction and nuclear transmutation were concluded by Rutherford’s colleagues Ernest Walton and John Cockcroft, who applied artificially accelerated protons against lithium-7 (Li-7), to split this nucleus into two alpha particles.

Fermi and his colleagues studied the results of bombarding uranium with neutrons in 1934. The nuclear fission of heavy elements was identified on December 17, 1938, by German Otto Hahn and his colleague Fritz Strassmann, and described theoretically in January 1939 by Lise Meitner and her nephew, Otto Robert Frisch.

3. Principle of nuclear fission

Nuclear fission occurs when a neutron hits with a nucleus of a large atom such as U and is absorbed into it causing the nucleus to become unstable and thus break down into two smaller and more stable atoms with the release of more neutrons and an enormous amount of energy. Nuclear fission can happen naturally with the spontaneous decay of radioactive material, or it can be induced by bombarding the fuel consisting of fissionable atoms with neutrons (Fig. 1).

1. a) Fission Energy Release

The fission of naturally occurring U isotope induced by thermal neutrons is U-235 which splits into Ba-141 and Kr-92 and emits excess free neutrons.

When nucleus fission occurs, it breaks into several smaller fragments with release of two or three neutrons. The sum of the masses of fission products is less than the original mass. On an average near about 0.1% of the original mass has been transformed into energy according to Einstein’s equation. And that reduction in mass comes off in the form of energy according to the Einstein equation E = mc2.

The fission of U-235 in nuclear reactors is triggered by the absorption of a low energy neutron, usually termed a “slow neutron” or a “thermal neutron.” Other fissionable isotopes which can be induced to fission by slow neutrons are Pu-239, U-233, and Th-232.

1.2. Fissionable isotopes

The nuclear fission reactions start when isotopes are hit by either high energy (fast moving) neutrons or thermal (slow-moving) neutrons at the optimum speed. For examples, U-235 and Pu-239 are fissionable isotopes. U-235 is the naturally occurring fissionable isotope, and Pu-239 can be produced by “breeding” from U-238. U-238 and Th-232 are the chief naturally occurring fertile isotopes. U-238, which makes up 99.3% of natural uranium, is not fissionable by slow neutrons. U-238 has a little probability for spontaneous fission and also a small chance of fission when bombarded with fast neutrons. However, it is not useful as a nuclear fuel source. Th-232 is fissionable, so could conceivably be employed as a nuclear fuel. The other isotope of natural thorium which is known to undergo fission upon slow neutron bombardment is U-233. Pu-241 and Pu-241 are fissile materials. The isotopes as mentioned earlier can be created artificially in a nuclear reactor, from the fertile nuclei of Th-232, U-238, and Pu-240. U-235 is only naturally found isotope which is thermally fissile, and present in natural uranium at a concentration of 0.7%.

1.3. Principle of Nuclear Fission

When U-235 or fissionable isotopes capture a neutron, the total energy is distributed amongst the nucleons (protons & neutrons) present in the compound nucleus. These nucleuses are relatively unstable, and it is likely to break into two fragments of near about half of the mass. Production of these fission fragments is followed almost immediately by the emission of some neutrons (commonly 2 or 3, average 2.45), which facilitate the chain reaction to be sustained (Fig. 3).

About 85% of the energy released is initially the kinetic energy of the fission fragments (Fig. 2). The balance of the power comes from gamma rays emitted during or quickly following the fission process and from the kinetic energy of the neutrons. The immediate neutrons are called prompt neutrons, and there is a small portion of delayed neutrons, as these are linkedwith the radioactive decay of certain fission products. The highest delayed neutron group has a half-life of nearly 56 seconds.

The number of neutrons and the particular fission products from any fission case are guided by statistical probability, in that the precise break up of a single nucleus cannot be predicted. Nonetheless, conservation laws require the total number of nucleons and the total energy to be conserved during the reaction. The fission reaction in U-235 creates fission products like Ba, Kr, Sr, Cs, I and Xe with atomic masses distributed nearly 95 and 135. The following reactions are representing typical fissionable products:

In above reactions, the number of nucleons (protons + neutrons) is balanced. The total binding energy released in nuclear fission reaction of an atomic nucleus varies with the precise break up (near about 200 MeV for U-235). Near about 6% of the heat produced in the reactor core originates from radioactive decay of fission products and transuranic elements composed by neutron capture. That must be allowed for when the reactor is shut down since heat generation continues after fission stops. Even after one year, commonly used fuel generates about 10 kW of decay heat per ton, decreasing to about 1 kW per ton after ten years.

1.4. Nuclear Chain Reactions

Nuclear chain reactions are series of nuclear fissions, each reaction initiated by a neutron produced in a preceding fission. This process may be controlled (nuclear power) or uncontrolled (nuclear weapons). If each neutron releases two more neutrons, then the number of fissions doubles each generation. In that case, in 10 generations there is 1,024 fission and in 80 generations about 6 x 10 23 (a mole) fissions (Fig. 4).

The energy released from the kinetic energy of each fission reaction is nearly 200 MeV, which includes,165 MeV from fission, 6 MeV from neutrons. 7 MeV from gamma rays, 7 MeV from energy from fission products, 6 MeV from gamma rays and 9 MeV from anti-neutrinos from fission products (1 MeV = 1.609 x 10 -13 joules).

1.4.1. Critical Mass (K)

Although two to three neutrons are created for every fission, not all of these neutrons are available for continuing the fission reaction. If the situations are such that the neutrons are lost at a faster rate than they are created by fission, the chain reaction will not be self-sustaining. At the point where the chain reaction can become self-sustaining, this is referred to as critical mass.

In an atomic bomb, a mass of fissile matter greater than the critical mass must be assembled instantaneously and held together for about a millionth of a second to allow the chain reaction to propagate before the bomb explodes. The amount of a fissionable material’s critical mass depends on lots factors; the shape of the material, the degree of enrichment of the fuel, its composition and density, the level of purity and whether it is contained within a neutron-reflective matter. The minimum critical mass of U-235 and Pu-239 is a 52 Kg and 17 cm in sphere diameter and 10 Kg in a 9.9 cm sphere diameter respectively.

Critical mass (K) is the effective multiplication factor K is defined as the ratio of the number of neutrons formed by fission in one generation to the number in the preceding generation. It refers to the conditions of the population of neutrons within the reactor core. This is not the same as the average numbers of neutrons produced by the fission reaction (2.4 in the case of U-235) as some neutrons are absorbed in non-fission reactions, and others escape from the system without being absorbed. The way in which fission chain reaction proceeds depends on the value of K.

• Sub-criticality (K < 1): There is no chain reaction. External neutrons may start a reaction, but it dies out fairly after some time.
• Criticality (K = 1): Every fission cause on average one more and the reaction proceeds at a steady This is the operating state of an energy reactor.
• Super-criticality (K > 1): With a much higher concentration of fissile material, each fission cause K more fission and the number of neutrons escalates exponentially in an uncontrollable chain reaction a possible explosive discharge of energy (in atom bomb).

1.4.2. Critical Size

The actual size of material which permits the escape of neutrons to such an extent that at least one neutron is positively left behind per fission reaction is known as the critical size of the particular material.

1.5. Neutron Capture: Transuranic elements & activation products

Non-fissile nuclei may capture neutrons, and some energy is produced by this mechanism in the form of gamma rays as the compound nucleus de-excites. The resultant new nucleus may become more stable by emitting alpha or beta rays.

The atomic number of an element is greater than 92 (uranium) known as transuranic elements except for the plutonium isotopes Pu 244 and Pu 239 detected in very small quantities. These elements are unstable and decay radioactively into other elements. Transuranium elements that can be found in nature now are artificially-generated, synthetic elements made via nuclear reactors or particle accelerators. The half-lives of these elements show a general trend of decreasing as atomic numbers increase.

Transuranium elements may be utilized to synthesize other super heavy elements. The potential uses of transuranium are very broad presently, for example, the element americium (Am) is employed in devices like smoke detectors and spectrometers.

1.6. Control of nuclear fission

During fission of U-235 nuclei, usually, releases 2 or 3 neutrons (average of almost 2.5). These neutrons are needed to sustain the chain reaction at a steady level of controlled criticality. If this ratio is less than one, then the reaction will lower down; if it is greater than one, it will become uncontrolled. Neutron absorbing rods are used to adjust the power output of a reactor during fission reaction. These typically use boron and cadmium (both are strong neutron absorbers) and are inserted among the fuel assemblies. The neutrons usually have too much kinetic energy. These fast neutronsare slowed through the use of a moderator (heavy water, ordinary water, and graphite). When they are slightly withdrawn from their position at criticality, the number of neutrons available for ongoing fission exceeds unity and the power level increases. When the power reaches the desired level, the control rods are returned to the critical position, and the power stabilizes (Fig. 5). The capacity to control the chain reaction is entirely due to the presence of the small proportion of delayed neutrons resulting from fission (0.66% for U-235, 0.27% for U-233, 0.23% for Pu-239. Without these products, any change in the critical balance of the chain reaction would lead to a virtually instantaneous and uncontrollable rise or fall in the neutron population.

1. Time of chain reaction

The liberated neutron travels at speeds of about 10 million meters per second, or about 3% the speed of light (almost 3 ×108 m/s). The characteristic time for a generation is roughly the time required to cross the diameter of the sphere of fissionable material. A critical mass of uranium is nearly the size of a baseball (0.1 meters). The time, T, the neutron would take to cross the sphere is:

The U-235 nuclide has a binding energy of approximately 1786 MeV. The total binding energy of the nuclides of Ba and Kr, which remain after fission amount to around 1952 MeV. The difference of 166 MeV corresponds to the energy discharged in the fission process. Apart from this, there will also be various small energy releases totaling nearly 33 MeV connected with the ejection of the neutrons and another particle as well as beta and gamma radiation.

Thus, the total energy liberated by the fission of 1 atom of U-235 is almost 200 MeV (166+33 MeV) which corresponds to 3.2 X 10-11 Joules. The energy released from practical amounts of fuel can be calculated as 1 Atom of U-235 weighs 235 AMU (atomic mass units) = 3.92 X 10-25 Kg. thus, 1 kg of U-235 contains 1/235amu atoms = 2.54 X 1024 atoms. The energy released from 1 kg of fuel is (3.2 X 10-11) X (2.54 X 1024) = 8.1 X 1013 Joules. The mass consumed in the transformation is presented by Einstein’s formula E = mc2. From one atom of U-235, the mass of U converted into the 200 MeV of Energy is 3.56 X 10 -28Kg, an almost insignificant amount. From the one kg of U-235 fuel consumed the mass of U actually transformed into energy is (3.56 X 10 -28) X (2.54 X 1024) = 9.04 X 10 -4Kg = 0.9 grams.

By contrast, the fission of one atom of uranium produces 10 million times the energy generated by the combustion of one atom of carbon from coal. Only 0.6 grams of U-235 were consumed by the atomic bomb which devastated Hiroshima in 1945.

1.7. Differences between nuclear fission and nuclear fusion

Both fission and fusion are nuclear reactions that generate energy, but the applications are not the same. Fission is the splitting of a heavy, unstable nucleus into two lighter nuclei, and fusion is the process where two light nuclei combine and discharging large amounts of energy.

Table 2: Differences between nuclear fission and nuclear fusion

A nuclear power plant or nuclear power station is a thermal power station in which the heat source is a nuclear reactor. As of 23 April 2014, the IAEA report there are 449 nuclear power reactors in operation operating in 31 countries.

• Nuclear power is the fourth-largest source of electricity in India after thermal, hydroelectric and renewable sources of electricity.
• As of 2016, India has 22 nuclear reactors in operation in 8 nuclear power plants, having an installed capacity of 6780 MW (3.5% of total installed base) and producing a total of 30,292.91 GWh of electricity while 6 more reactors are under construction and are expected to generate an additional 4,300 MW.
• Large deposits of thorium (518,000 tonnes) in the form of monazite in beach sands and very modest reserves of low-grade uranium (92,000 tonnes) are nuclear reserves in India.

Components of a Nuclear Power Plant

• Nuclear Reactor
• Heat Exchanger
• Steam Turbine
• Alternator
• Condenser
• Heat Exchanger

In the heat exchanger, the primary coolant transfers heat to the secondary coolant (water). Thus water from the secondary loop is converted into steam. The primary system and secondary system are closed loop, and they are never allowed to mix up with each other. Thus, heat exchanger helps in keeping secondary system free from radioactive stuff. Heat exchanger is absent in boiling water reactors.

• Steam Turbine

Generated steam is passed through a steam turbine, which runs due to pressure of the steam. As the steam is passed through the turbine blades, the pressure of steam gradually decreases and it expands in volume. The steam turbine is coupled to an alternator through a rotating shaft.

• Alternator

The steam turbine rotates the shaft of an alternator thus generating electrical energy. Electrical output of the alternator is the delivered to a step up transformer to transfer it over distances.

• Condenser

The steam coming out of the turbine, after it has done its work, is then converted back into water in a condenser. The steam is cooled by passing it through a third cold water loop.

Energy Release by Nuclear Fission

• The only naturally occurring isotope in which fission can be induced by thermal neutrons is U-235 which splits into Ba-141 and Kr-92 and emits excess free neutrons.
• When nucleus fission occurs, it breaks into several smaller fragments. These fragments (fission products) are about equal to half the original mass. Two or three neutrons are also released. The sum of the masses of these particles (fission products) is less than the original mass. This ‘missing’ mass (near about 0.1% of the original mass) has been transformed into energy according to Einstein’s equation.
• If a massive nucleus such as U-235 breaks apart (fissions), then there will be a net yield of energy because the sum of the masses of the fragments will be less than the mass of the U nucleus.
• If the mass of the particles is equivalent to or greater than that of iron (Fe) at the peak of the binding energy curve, then the nuclear particles will be more tightly bound than they were in the U nucleus.
• And that reduction in mass comes off in the form of energy according to the Einstein equation E = mc2.
• The fission of U-235 in nuclear reactors is triggered by the absorption of a low energy neutron, usually termed a “slow neutron” or a “thermal neutron.”

Other fissionable isotopes which can be induced to fission by slow neutrons are Pu-239, U-233, and Th-232.

• The U-235 nuclide has a binding energy of approximately 1786 MeV.
• The total binding energy of the nuclides of Ba and Kr, which remain after fission amount to around 1952 MeV.
• The difference of 166 MeV corresponds to the energy discharged in the fission process.
• Apart from this, there will also be various small energy releases totaling nearly 33 MeV connected with the ejection of the neutrons and another particle as well as beta and gamma radiation.
• Thus, the total energy liberated by the fission of 1 atom of U-235 is almost 200 MeV (166+33 MeV) which corresponds to 3.2 X 10-11
• The energy released from practical amounts of fuel can be calculated as 1 Atom of U-235 weighs 235 AMU (atomic mass units) = 3.92 X 10-25
• thus, 1 kg of U-235 contains 1/235amu atoms = 2.54 X 1024
• The energy released from 1 kg of fuel is (3.2 X 10-11) X (2.54 X 1024) = 8.1 X 1013
• The mass consumed in the transformation is presented by Einstein’s formula E = mc2.
• From one atom of U-235, the mass of U converted into the 200 MeV of energy is 3.56 X 10 -28 kg, an almost insignificant amount.
• From the one kg of U-235 fuel consumed the mass of U actually transformed into energy is (3.56 X 10 -28) X (2.54 X 1024) = 9.04 X 10 -4Kg = 0.9 grams.
• By contrast, the fission of one atom of uranium produces 10 million times the energy generated by the combustion of one atom of carbon from coal. Only 0.6 grams of U-235 were consumed by the atomic bomb which devastated Hiroshima in 1945.

Nuclear reactor

A nuclear reactor is a special apparatus used to perform nuclear fission. a protective shield  covers the reactor since the nuclear fission is radioactive.

• fuel rods,
• control rods and
• A fuel rod contains small round fuel pellets (uranium pellets).
• Control rods are of cadmium (which absorb neutrons)- are inserted into reactor and moved in or out to control the reaction.
• The moderator used is graphite rods or the coolant itself.

Moderator slows down the neutrons before they bombard on the fuel rods.

Types of nuclear reactors

Two types of nuclear reactors that are widely used

• Pressurised Water Reactor (PWR)
• Boiling Water Reactor (BWR)
• PWR- This type of reactor uses water as coolant.

The coolant (water) is kept at very high pressure so that it does not boil.The heated water is transferred through heat exchanger where water from secondary coolant loop is converted into steam. Thus the secondary loop is completely free from radioactive stuff.

In a PWR, the coolant water itself acts as a moderator. Due to these advantages, pressurised water reactors are most commonly used.

• Boiling Water Reactor (BWR) –

In this type of reactor only one coolant loop is present. The water is allowed to boil in the reactor. The steam is generated as it heads out of the reactor and then flows through the steam turbine.

One major disadvantage of a BWR is that, the coolant water comes in direct contact with fuel rods as well as the turbine. So, there is a possibility that radioactive material could be placed on the turbine.

https://commons.wikimedia.org/w/index.php?curid=14617356  –

In this type of reactor only one coolant loop is present. The water is allowed to boil in the reactor. The steam is generated as it heads out of the reactor and then flows through the steam turbine. One major disadvantage of a BWR is that, the coolant water comes in direct contact with fuel rods as well as the turbine. So, there is a possibility that radioactive material could be placed on the turbine.

• Pressurised Water Reactor (PWR)

PWR- This type of reactor uses water as coolant.The coolant (water) is kept at very high pressure so that it does not boil.The heated water is transferred through heat exchanger where water from secondary coolant loop is converted into steam.Thus the secondary loop is completely free from radioactive stuff.In a PWR, the coolant water itself acts as a moderator. Due to these advantages, pressurised water reactors are most commonly used.

1.8.   Advantages

• Since the requirement of fuel is very small, so the cost of fuel transportation, storage etc. is small.
• Nuclear power plant needs less space as compared to any other power station of the same size. Example: A 100 MW nuclear power station needs 38 – 40 acres of land whereas the same capacity coal based thermal power plant needs 120-130 acres of land.
• This type of power plant is very economical to produce large electric power.
• Nuclear power plant can be located near load centre because bulk amount of fuel (like water, coal) is not required.
• Nuclear power is most economical to generate large capacities of power like 100 MVA or more. It produces huge amount of energy in every nuclear fission process.
• Using a small amount of fuel, this plant produces large electrical energy.
• This plant is very reliable in operation.
• Since, the large number of nuclear fuel is available in this world. So, a nuclear power plant can generate electrical energy thousands of years continuously.
• Nuclear Power Plant is very neat and clean as compared to a steam power plant.

The operating cost is low at this power plant but it is not affected for higher load demand. Nuclear power plant always operates a base load plant and load factor will not be less than 0.8.

Disadvantage

• Initial installation cost is very high as compared to the other power station.
• Nuclear fuel is very much expensive and it is difficult to recover.
• Capital cost is higher in respect of other power station.
• Good technical knowledge is required to operate such type plant. The maintenance cost will also be higher to operate such of a plant.
• There is a chance to spread of radioactive pollution from this type of plant.
• Nuclear Reactor does not response efficiently with the fluctuating load demand. So, it is not suited for varying the load.
• Cooling water requirement is twice than a coal based steam power plant.

1.10. Conclusion

Nuclear fission offers the opportunity of a next-generation alternative to fossil fuels. Nuclear fission, an abundant and low-carbon source, has a tremendous proven potential to provide reliable base-load electricity and displace coal or gas power plants directly. The prospect of nuclear power concerns many people who worry about sustainability, spent-fuel disposal and radiation release from accidents. To achieve the long-term sustainable goal, India must actively invest in the science, technology, and infrastructure of next-generation nuclear fission for electricity production.