Table of Contents
INTRODUCTION (What is Nuclear Fission)
Nuclear fission is the process of splitting large atomic nuclei into smaller atomic nuclei in order to release a significant quantity of energy. The process involves forcing nuclei to absorb neutrons, which are typically contained in the atomic nucleus alongside protons. Humans have used the phenomenon to provide energy through nuclear power plants as well as to power nuclear weapons.
PERIPHERAL REACTIONS
In most nuclear reactions, the resulting particle is no heavier than the α-particle. That is, the mass number and atomic number of the target nucleus change little. These are called peripheral reactions, because the change in the nucleus is almost exclusively confined to its outer layer, with no effect on the interior. For example,
147N + 42H → 178O + 11H ; 147N + 10n → 146C + 11H
COLLISION OF THERMAL NEUTRON WITH U-235
When U-235 extracted from uranium ore is hit with thermal neutrons, this heavy nucleus splits into two medium-sized nuclei. For example,
- Xe134 and Sr100 or
- Ba141 and Kr92
92U235 + 0n1 → 54Xe134 + 38Sr100 + 2 0n1……..(1)
92U235 + 0n1 → 56Ba141 + 36Kr92 + 3 0n1……..(2)
Obviously, this is no longer a marginal reaction; This type of nuclear reaction is commonly called nuclear fission. This method was discovered in 1939 (Otto Hahn and Strassmann; Frisch and Mittner).
Definition: The splitting of a heavy nucleus into two medium-sized nuclei is called nuclear fission.
ENERGY RELEASED IN NUCLEAR FISSION
The loss of mass in nuclear fission is converted into energy according to mass-energy principle.
Primary Mass = U-235 and the combined mass of neutrons
= 235.0439 + 1.0087 = 236.0526 u
Final mass = Xe-134, Sr-100 and the combined mass of two neutrons
= 133.9054 + 99.9354 + 2 X 1.0087 = 235.8583 u
So, mass-loss = 0.1944 u
Since the equivalent energy of 1u mass is about 931 MeV, the free energy in this fission is 181 MeV. Thus, the amount of free energy released when only 1g of U-235 atoms split is 7.4218 X 10¹⁰ J; This is equivalent to the energy produced by burning about 3000 tonnes of coal.
MODERATOR
Most of the energy released in nuclear fission (about 186 MeV) is converted into kinetic energy of the 3 neutrons produced. So, these neutrons are very fast. These need to be thermally controlled by sufficiently reducing them to be used in subsequent separations. Materials through which high-speed neutrons can be decelerated into thermal neutrons are called moderators. The two most common moderators are: a. Deuterium oxide (D20) and b. Graphite.
CHAIN REACTION
In the nuclear fission shown in equation no. (2), the number of neutrons incident on U-235 is 1, but the number of neutrons produced is 3. These three neutrons are decelerated into thermal neutrons. By fissioning them again, the next step will be the fission of 3 U-235 nuclei. Fission of 3 U-235 nuclei produces 9 neutrons again. This successive splitting phenomenon is called a chain reaction.
In the case of series reactions occurring according to equation No (2). 1, 3, 9, 27, 81, …….—the number of divisions in this multiplicative class increases rapidly. As a result, a large amount of energy is generated in the entire sample of U-235 in a very short time. This is the principle of atomic bomb.
CRITICAL SIZE
Neutrons produced in nuclear fission tend to escape through the outer surface of the radioactive sample. If a large number of neutrons are released, there is a risk that the number of neutrons will gradually decrease rather than increase. The chain reaction then ceases and nuclear division ceases. Two methods are adopted to prevent this phenomenon:
- The radioactive sample is taken as a sphere, so that the outer surface area is less than the volume,
- The mass of the sample is kept above a pre-calculated minimum mass. The Critical size is the minimum size that a radioactive sample must take to continue fission while maintaining a chain reaction.
CONTROLLED FISSION: NUCLEAR REACTOR
Measures need to be taken to convert the thermal energy generated from nuclear fission into electrical energy, so that the entire process does not get out of control by producing large amounts of energy in a very short period of time, and so that a long-term supply of energy is available. This is called controlled fission. Nuclear reactors in nuclear power plants are designed to facilitate this controlled fission.
The number of usable thermal neutrons from one step of the chain reaction to the next step is called the neutron reproduction factor. In the case of nuclear bombs this factor is 2.5 or more, so that the rate of energy production cannot be kept under control. On the other hand, if this coefficient is exactly 1 or slightly greater, then the chain reaction is in control, the rate of energy production does not increase uncontrollably with time. This is the principle of nuclear reactor. One of the different types of reactors is the pressurized water reactor.
CORE
Nuclear reactions take place in the core. This part contains fuel rod, control rod, moderator and coolant. U-235 is used as the fuel rod. Thermal energy is produced by hitting U-235 with neutrons. Boron coated steel rod is used as control rod. Excess thermal neutrons are absorbed by boron. Heavy water is usually used as the moderator. Moderators slow down high-speed neutrons produced by nuclear reactions. The decelerated neutrons cause nuclear reactions again. Water is used as coolant these days. The heat generated from the nuclear reaction is absorbed by the coolant. Note that the hot water is kept at a very high pressure with a pressurizer; So, this water does not boil.
HEAT EXCHANGER
The hot water released from the core reaches the heat exchanger. This water is radioactive. So, this water is not allowed to come outside the concrete shielding. But we need the heat energy stored in water. The heat exchanger circulates the thermal energy stored in this water and the radioactive water.
TURBINE
The superheated radioactive water is then brought outside the concrete shielding and turned into steam. This steam enters the turbine and turns the turbine to generate heat energy.
THE CHERNOBYL ACCIDENT
Nuclear power facilities cannot be fully discussed without addressing nuclear accidents. There is no comparison to Chernobyl in terms of vividness or sobering. The Chernobyl nuclear power facility, located about 80 kilometres north of Kyiv in the former Soviet Union, had an RBMK-1000 reactor. The world’s worst nuclear catastrophe occurred on April 26, 1986, at the Chernobyl power station. An explosion and fire in the No. 4 reactor sent radioactivity into the environment. With the flow reduced, the reactor’s cooling water began to boil and produce steam.
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