Unit 10: Nuclear Reactions & Energy
Exploring the immense power released from the splitting and merging of atomic nuclei.
10.13 Nuclear Fission (Principles, Chain Reactions)
Besides radioactive decay, there are other types of nuclear reactions. Nuclear fission is the process in which a large, unstable nucleus splits into two or more smaller, lighter nuclei. This process does not typically occur spontaneously but is induced by bombarding the nucleus with a neutron.
The fission of heavy elements is a highly exothermic process, releasing enormous amounts of energy, as well as gamma rays and several neutrons.
Example: Fission of Uranium-235
$$^{235}_{92}U + ^1_0n \rightarrow ^{141}_{56}Ba + ^{92}_{36}Kr + 3^1_0n + Energy$$
Chain Reaction
Crucially, the neutrons released from the initial fission event can go on to strike other U-235 nuclei, causing them to undergo fission as well. This creates a self-sustaining chain reaction. If uncontrolled, this chain reaction grows exponentially, releasing a massive amount of energy in a fraction of a second, which is the principle behind an atomic bomb. In a nuclear reactor, this chain reaction is carefully controlled.
Solved Examples:
- What is nuclear fission?
Solution: The splitting of a large atomic nucleus into smaller ones, accompanied by the release of energy and neutrons. - What particle is typically used to initiate a fission
reaction?
Solution: A neutron. - What is a chain reaction in the context of nuclear
fission?
Solution: A self-sustaining process where the neutrons released from one fission event trigger further fission events in other nuclei. - Are the products of a fission reaction stable or
radioactive?
Solution: The smaller nuclei produced (fission fragments) are typically highly radioactive. - Besides smaller nuclei and energy, what else is released during
fission?
Solution: Two or three neutrons. - Why does fission release so much
energy?
Solution: The total mass of the products is slightly less than the mass of the reactants. This "missing" mass is converted into a large amount of energy according to Einstein's equation, E=mc². - What is the fuel used in many nuclear
reactors?
Solution: Uranium-235. - Complete the fission reaction: $^{235}_{92}U + ^1_0n \rightarrow
^{139}_{54}Xe + ^{95}_{38}Sr + ?$.
Solution: Balance mass number: 235 + 1 = 139 + 95 + A, so A = 2. Balance atomic number: 92 + 0 = 54 + 38 + Z, so Z = 0. The missing particles are two neutrons ($2^1_0n$). - What is the difference between a controlled and an uncontrolled chain
reaction?
Solution: A controlled chain reaction (in a reactor) proceeds at a steady rate. An uncontrolled chain reaction (in a bomb) escalates exponentially. - Is fission a spontaneous process like alpha
decay?
Solution: No, it is typically an induced process, started by neutron bombardment.
10.14 Nuclear Reactors & Electricity Generation
A nuclear reactor is a device designed to initiate and control a sustained nuclear chain reaction. The primary purpose of most reactors is to generate electricity.
How a Nuclear Power Plant Works:
- Fission: Inside the reactor core, enriched uranium-235 fuel rods undergo fission, releasing a tremendous amount of heat energy.
- Control: Control rods (made of neutron-absorbing materials like boron or cadmium) are raised or lowered into the core. Lowering the rods absorbs more neutrons, slowing the chain reaction, while raising them speeds it up. This allows the reaction rate to be precisely controlled.
- Heat Transfer: The heat generated by fission is absorbed by a coolant (e.g., water), which is pumped through the reactor core.
- Electricity Generation: This superheated coolant is used to boil water in a separate circuit, creating high-pressure steam. The steam turns a turbine, which is connected to a generator that produces electricity.
Nuclear power is a powerful source of electricity that does not produce greenhouse gases, but it creates radioactive waste that must be stored safely for thousands of years.
Solved Examples:
- What is the function of a nuclear
reactor?
Solution: To control a nuclear fission chain reaction to generate energy, usually in the form of heat. - What is the purpose of control rods in a
reactor?
Solution: To absorb neutrons and thereby control the rate of the fission chain reaction. - What material are control rods typically made
from?
Solution: A neutron-absorbing material like boron or cadmium. - How is the heat from a fission reaction converted into
electricity?
Solution: The heat is used to boil water, creating steam that turns a turbine connected to a generator. - What is meant by "enriched" uranium?
Solution: Natural uranium has been processed to increase the percentage of the fissile isotope, uranium-235. - What is the role of the coolant in a nuclear
reactor?
Solution: To absorb the heat generated in the reactor core and transfer it to the water that will become steam. - What is a major advantage of nuclear
power?
Solution: It generates a large amount of electricity without producing greenhouse gases like carbon dioxide. - What is a major disadvantage of nuclear
power?
Solution: It produces highly radioactive nuclear waste that is difficult and expensive to store safely for long periods. - What happens if the control rods are completely removed from the reactor
core?
Solution: The chain reaction would become uncontrolled, leading to a rapid release of energy and a potential meltdown. - Is the steam that turns the turbine
radioactive?
Solution: No, in most common reactor designs, the coolant that passes through the reactor is in a separate, closed loop from the water that is turned into steam.
10.15 Nuclear Fusion (Principles, Sun's Energy)
Nuclear fusion is the process in which two or more light atomic nuclei combine to form a single, heavier nucleus.
This process releases even more energy per nucleon than fission. For fusion to occur, extremely high temperatures and pressures are required to overcome the electrostatic repulsion between the positively charged nuclei and force them close enough to fuse.
Fusion in the Sun:
Nuclear fusion is the power source of stars, including our Sun. In the Sun's core, immense gravitational pressure and temperatures of millions of degrees Celsius cause hydrogen nuclei (protons) to fuse together in a series of steps to form helium nuclei.
Example: A key fusion reaction involving isotopes of hydrogen (deuterium and
tritium).
$$^2_1H + ^3_1H \rightarrow ^4_2He + ^1_0n + Energy$$
Fusion power is a major area of research, as it promises a clean and virtually limitless source of energy, producing little long-lived radioactive waste. However, achieving and sustaining the required conditions on Earth is incredibly challenging.
Solved Examples:
- What is nuclear fusion?
Solution: The process of combining two light nuclei to form a heavier nucleus, releasing energy. - What conditions are required for nuclear fusion to
occur?
Solution: Extremely high temperatures and pressures. - Where does nuclear fusion occur
naturally?
Solution: In the cores of stars, like the Sun. - Which process releases more energy per nucleon: fission or
fusion?
Solution: Fusion. - Why are high temperatures needed for
fusion?
Solution: To give the positively charged nuclei enough kinetic energy to overcome their electrostatic repulsion and get close enough for the strong nuclear force to bind them. - What are the reactants in the main fusion cycle of the
Sun?
Solution: Hydrogen nuclei (protons). - What is the main product of the Sun's fusion
reactions?
Solution: Helium nuclei. - Complete the fusion reaction: $^2_1H + ^2_1H \rightarrow ? +
^1_0n$.
Solution: Balance mass number: 2 + 2 = A + 1, so A = 3. Balance atomic number: 1 + 1 = Z + 0, so Z = 2 (Helium). The product is Helium-3, $^3_2He$. - What is a major advantage of fusion power over fission
power?
Solution: It produces very little long-lived radioactive waste, and its fuel (isotopes of hydrogen) is abundant. - What is the main technological challenge in achieving fusion power on
Earth?
Solution: Creating and containing a plasma at the extremely high temperatures (over 100 million °C) required for fusion.
10.16 Atomic and Hydrogen Bombs
The immense energy released by nuclear reactions can be used for destructive purposes in nuclear weapons.
Atomic Bombs (Fission Bombs)
An atomic bomb is based on an uncontrolled nuclear fission chain reaction. A critical mass of a fissile material, such as uranium-235 or plutonium-239, is rapidly brought together. Conventional explosives are used to compress the material, initiating the chain reaction, which releases a devastating amount of energy in an instant.
Hydrogen Bombs (Thermonuclear Bombs)
A hydrogen bomb is far more powerful and is based on nuclear fusion. It is a multi-stage device. A primary fission bomb is detonated first. The intense heat and radiation from this fission explosion provide the extreme temperatures and pressures needed to trigger a secondary fusion reaction in isotopes of hydrogen (like lithium deuteride). This fusion stage releases vastly more energy than the initial fission bomb.
Solved Examples:
- What type of nuclear reaction powers an atomic
bomb?
Solution: An uncontrolled fission chain reaction. - What type of nuclear reaction powers a hydrogen
bomb?
Solution: Nuclear fusion. - Which type of bomb is more powerful?
Solution: A hydrogen bomb (thermonuclear bomb). - What is meant by a "critical mass"?
Solution: The minimum amount of fissile material needed to sustain a nuclear chain reaction. - Name a fissile material used in atomic
bombs.
Solution: Uranium-235 or Plutonium-239. - How is the chain reaction started in a fission
bomb?
Solution: By rapidly assembling a critical mass of fissile material, often using conventional explosives. - What is the role of the fission bomb inside a hydrogen
bomb?
Solution: It acts as a trigger, providing the incredibly high temperatures and pressures needed to initiate the fusion reaction. - What is another name for a hydrogen
bomb?
Solution: A thermonuclear bomb. - Does a hydrogen bomb produce radioactive
fallout?
Solution: Yes, primarily from the fission trigger, although the fusion reaction itself is relatively "clean". - Why is it called a "hydrogen" bomb?
Solution: Because the fusion fuel consists of heavy isotopes of hydrogen (deuterium and tritium).