a) Nuclear Fission:

• Nuclear fission is the process where a heavy nucleus splits into two (or more) lighter nuclei, usually accompanied by:
• – Release of energy
• – Emission of neutrons
• – Sometimes gamma radiation
• This process can occur:
• – Spontaneously in some unstable heavy nuclei.
• – Induced by a neutron, usually in nuclear reactors or weapons.
• Energy is Released in Spontaneous and Neutron-Induced Fission
• ⇒ Energy Released:
• The energy released in fission comes from the mass defect — that is, the difference in mass between:
• – The original heavy nucleus and
• – The total mass of the fission products + emitted neutrons.
• 
• Figure 1 Nuclear fission
• According to Einstein’s mass-energy equivalence:
• [math]E = \Delta mc^2[/math]
• Where:
• – E = energy released
• – Δm = mass lost (mass defect)
• – c = speed of light
• Even a tiny mass difference yields a large amount of energy because [math]c^2[/math] is a huge number.
• ⇒ Spontaneous Fission:
• Some very heavy nuclei (like uranium-238, plutonium-240) can undergo fission without being struck by a neutron.
• – This is a rare process compared to induced fission.
• – The nucleus splits into two smaller nuclei (fission fragments) and several neutrons.
• 
• Figure 2 Spontaneous fission
• ⇒ Characteristics:
• Occurs naturally (but infrequently).
• Releases energy and neutrons.
• Contributes slightly to background radiation.
• ⇒ Neutron-Induced Fission
• Occurs when a slow-moving neutron strikes a fissile nucleus, such as Uranium-235 or Plutonium-239.
• The nucleus becomes unstable and splits into two lighter nuclei, releasing:
• – Large amounts of energy
• – 2 or 3 more neutrons
• – Gamma radiation
• Example: Fission of Uranium-235
• [math]{}^{235}_{92}\mathrm{U} + {}^{1}_{0}n \rightarrow {}^{144}_{56}\mathrm{Ba} + {}^{89}_{36}\mathrm{Kr} + 3\,{}^{1}_{0}n + \text{energy}[/math]
• – This reaction releases around 200 MeV of energy per fission.
• 
• Figure 3 Spontaneous fission and neutron emission
• Where Does the Energy Go?
• – Kinetic energy of the fission fragments (~85% of energy)
• – Kinetic energy of emitted neutrons
• – Gamma radiation
• – Energy from beta decay of fission products
• ⇒ Chain Reaction:
• In neutron-induced fission, the emitted neutrons can:
• Cause more fission reactions
• – Lead to a chain reaction, which can be:
• – Controlled (in nuclear power plants)
• Uncontrolled (in nuclear weapons)

• b) Chain Reaction in Nuclear Fission:

• A chain reaction is a self-sustaining sequence of nuclear fission reactions, where:
• 1. A neutron induces fission in a heavy nucleus like uranium-235 or plutonium-239.
• 2. That fission releases more neutrons (usually 2 or 3).
• 3. These new neutrons go on to cause more fission reactions in nearby fissile nuclei.
• 4. The cycle repeats, releasing more and more energy.
• ⇒ Chain Reaction:
• Because like links in a chain, each fission triggers the next — and it can grow very rapidly.
• If 1 neutron causes 1 more fission, you have a steady state. If 1 neutron causes >1 more fission, the reaction grows (exponentially). If <1, the reaction dies out.
• 
• Figure 4 Chain reaction in nuclear fission
• ⇒ Chain Reaction Example (Uranium-235)
• A neutron hits a uranium-235 nucleus:
• [math]{}^{235}_{92}\mathrm{U} + {}^{1}_{0}n \rightarrow \text{Fission Products} + \text{2–3 neutrons} + \text{energy}[/math]
• Each new neutron can:
• – Cause more U-235 atoms to fission,
• – Or escape without reacting,
• – Or be absorbed without causing fission (by control rods or non-fissile material).
• ⇒ Types of Chain Reactions
• 1. Uncontrolled Chain Reaction
• – Happens in nuclear bombs.
• – No regulation — each fission leads to more and more.
• – Massive energy is released in a tiny fraction of a second.
• 2. Controlled Chain Reaction
• – Used in nuclear reactors.
• – Uses moderators (like water or graphite) to slow neutrons.
• – Uses control rods (made of boron or cadmium) to absorb excess neutrons.
• – Keeps the reaction going at a steady, safe rate.
• ⇒ Chain Reaction and Critical Mass
• For a chain reaction to be sustained:
• – There must be a critical mass of fissile material.
• – If the mass is too small, too many neutrons escape → reaction stops.
• – If the mass is just right, you get a sustained chain reaction.
• – More than the critical mass = supercritical, leading to rapid energy release.
• ⇒ Outcomes of Chain Reactions

• c) Nuclear Power Plant Components and Their Roles

• ⇒ Control Rods
• Purpose: To control the rate of the nuclear chain reaction.
• Work:
• Control rods are made of materials that absorb neutrons very effectively (e.g., boron, cadmium, or hafnium).
• By inserting the rods deeper into the reactor core, they absorb more neutrons, reducing the number of neutrons available to cause further fission.
• By withdrawing the rods, more neutrons are available to sustain the chain reaction, increasing the reaction rate.
• In an emergency, control rods are fully inserted to immediately shut down the reaction (a process called SCRAM).
• Analogy:
• – Think of control rods like a brake pedal in a car — they slow down or stop the reaction when needed.
• ⇒ Moderator
• Purpose:
• To slow down neutrons so they are more likely to cause further fission in uranium-235 or plutonium-239.
• Works:
• – Neutrons released in fission are fast-moving.
• – However, slow (thermal) neutrons are much more effective at sustaining the chain reaction.
• – A moderator slows these neutrons down without absorbing them.
• – Common moderators: heavy water (D₂O), normal water (H₂O), graphite.
• Without a moderator, the chain reaction would not sustain efficiently, as fast neutrons are less likely to cause fission.
• 
• Figure 5 Nuclear power Plant
• ⇒ Heat Exchanger
• Purpose: To transfer heat generated in the reactor core to a working fluid (usually water) that turns turbines to generate electricity.
• Works:
• – Fission in the reactor core generates heat.
• – This heat is used to heat water or another coolant.
• – The heated coolant passes through a heat exchanger, where it transfers heat to a secondary water circuit.
• – This secondary water becomes steam, which drives turbines, which spin generators to produce electricity.
• – After the steam passes through the turbines, it’s cooled and condensed back into water and recirculated.
• This setup isolates radioactive coolant from the turbine system, ensuring safety and efficiency.
• ⇒ Shielding
• Purpose: To protect workers and the environment from harmful ionizing radiation.
• Works:
• – The reactor core emits gamma rays, neutrons, and beta particles — all forms of ionizing radiation.
• – Shielding materials such as concrete, lead, or water absorb or block this radiation.
• – The shielding surrounds the reactor vessel and other radioactive components.
• – In large plants, multiple layers of shielding are used for enhanced safety.
• Without shielding, radiation exposure could lead to serious health risks for workers and the public.

• d) The Properties of the Products of Nuclear Fission and Their Management

• ⇒ Nuclear Fission:
• In nuclear fission, a heavy nucleus (like uranium-235 or plutonium-239) absorbs a neutron and splits into two smaller nuclei, known as fission fragments, along with:
• – 2–3 neutrons
• – A large amount of energy
• – Gamma radiation
• This process releases energy due to the mass defect, explained by Einstein’s equation:
• [math]E = mc^2[/math]
• ⇒ Properties of Fission Products
• Fission products fall into two main categories:
• Fission Fragments (Daughter Nuclei)
• Highly radioactive: These are unstable isotopes of elements like iodine-131, cesium-137, strontium-90, etc.
• Short to medium half-lives: Some decay quickly, others persist for years.
• Emit beta and gamma radiation: Making them hazardous to health.
• High thermal energy: Continue to generate decay heat even after the reactor is shut down.
• 
• Figure 6 Nuclear fusion and fission
• These are the main contributors to nuclear waste.
• Free Neutrons
• On average, 5 neutrons are released per fission event.
• Some are used to sustain the chain reaction.
• Gamma Rays
• Emitted both during fission and from the decay of fission products.
• Highly penetrating and pose serious radiation risks.
• ⇒ Heat Energy
• Around 200 MeV is released per fission event.
• This energy is used to generate electricity by producing steam to spin turbines.
• Management of Fission Products
• Proper management of fission products is crucial for environmental safety, human health, and nuclear plant operation.
• Short-Term Management (After Reactor Operation)
• ⇒ Cooling Pools
• Spent fuel rods are immediately submerged in cooling pools after removal.
• Purpose:
• – To remove decay heat
• – To shield radiation.
• Water provides cooling and radiation protection.
• ⇒ Dry Cask Storage (After Pool Cooling)
• Once cooled (after several years), fuel rods may be moved to dry cask storage.
• Stored in sealed, shielded containers above ground.
• Used as interim storage for decades.
• ⇒ Long-Term Management
• Radioactive Waste Disposal
• Fission products include high-level radioactive waste (HLW). They must be:
• – Isolated from the environment,
• – Shielded to prevent radiation exposure,
• – Stored securely to account for their long half-lives.
• Two main strategies:
• ⇒ Geological Disposal
• – Long-term solution.
• – Waste is buried deep underground in stable geological formations.
• – Prevents leaks for thousands to millions of years.
• ⇒ Reprocessing (for Some Countries)
• – Some fission products can be reprocessed to extract usable uranium or plutonium.
• – Reduces waste volume.
• – Used in countries like France and Japan.
• – However, this raises proliferation concerns due to plutonium separation.
• ⇒ Environmental and Health Concerns
• If not managed properly:
• – Fission products can leak into soil, water, and air.
• – Radiation exposure can cause cancer, genetic damage, and contamination of ecosystems.
• – Proper containment and monitoring are critical.