Nuclear Fission and Types of Reactors | Geography for UPSC CSE PDF Download

Nuclear Fission

Discovery and historical context

• James Chadwick discovered the neutron in 1932 (England).
• The experimental observation of nuclear fission of heavy elements was made in 1938 by Otto Hahn and Fritz Strassmann (Germany).
• The process was explained theoretically in 1939 by Lise Meitner and Otto Robert Frisch.

What is nuclear fission?

• Nuclear fission is a radioactive decay or reaction process in which the nucleus of a heavy atom splits into two or more lighter nuclei (fission products).
• Fission typically releases free neutrons, prompt and delayed gamma radiation, and a large amount of energy.
• Fission can be spontaneous in some nuclei or induced by the absorption of a particle such as a neutron, proton, deuteron, alpha particle, or by high-energy photons (gamma rays).
• When neutrons emitted by one fission event cause further fission in nearby fissile nuclei, a chain reaction can occur.

Illustrative chemistry note: Energy changes in reactions are described as exothermic when heat is released and endothermic when heat is absorbed. For example, dissolving urea in water is an endothermic process (the solution cools). The hydration of quicklime is exothermic: CaO + H₂O → Ca(OH)₂ + heat.

Fissile versus fissionable

• Fissionable - a nuclide that can undergo fission (either spontaneously or when struck by suitably energetic neutrons).
• Fissile - a nuclide that can sustain a chain reaction with low-energy (thermal or slow) neutrons and therefore can be used in controlled or self-sustained fission chain reactions (reactors or weapons).
• If the chain reaction is controlled (reactor), it can generate power; if uncontrolled, it can produce an explosive yield (nuclear weapon).

Important uranium isotopes and their behaviour

• Natural uranium is a mixture of isotopes: about U-238 (≈ 99.27%), U-235 (≈ 0.72%), and a trace of U-234 (≈ 0.0055%).
• Uranium-235 (U-235) is a fissile isotope that readily undergoes fission on absorption of a slow (thermal) neutron.
• Uranium-238 (U-238) is fissionable primarily with fast neutrons; it has a small probability of spontaneous fission and is not directly useful as a primary nuclear fuel for thermal reactors.
• Uranium-233 is another fissile nuclide that can be produced from thorium in a thorium fuel cycle.
• Other heavy nuclei (for example, thorium-232) are fertile: they absorb neutrons and transmute into fissile nuclides (thorium-232 → uranium-233 after neutron absorption and beta decays).

How nuclear fission releases energy

• Atomic nuclei are composed of protons and neutrons (nucleons); the mass of a bound nucleus is less than the sum of the masses of its constituent nucleons.
• This difference in mass is the mass defect, which corresponds to the nuclear binding energy according to Einstein's relation E = mc².
• When a heavy nucleus splits into lighter daughter nuclei with higher total binding energy per nucleon, the binding-energy difference appears as kinetic energy of the fragments, emitted neutrons, and electromagnetic radiation - the liberated energy in fission.

Common fissile materials

• Typical fissile nuclides used in reactors and weapons include uranium-233, uranium-235, plutonium-239, plutonium-241.
• Natural uranium's ~0.72% U-235 is usually insufficient to sustain a self-sustaining thermal chain reaction in light-water reactors without enrichment.
• For commercial light-water reactors, uranium fuel is typically enriched to about 2.5-3.5% U-235.
• Plutonium-239 can be produced (bred) in reactors from U-238 by neutron capture and subsequent beta decays.
• Thorium-232 acts as a fertile material that can be transmuted to fissile U-233; this forms the basis of the thorium fuel cycle.

Uranium enrichment

• Enrichment is the process of increasing the fraction of U-235 in uranium feedstock.
• Natural uranium contains ≈ 0.7% U-235; light-water reactors require enrichment to roughly 2.5-3.5% U-235.
• Centrifuge cascades are the common industrial technology for enrichment.
• Fuel enriched to a few percent U-235 for reactor use is not suitable for a nuclear weapon; weapons require enrichment to about 90% or more (weapons-grade U-235).
• Intermediate enrichments (e.g., 15-30%) may be used in certain reactor types, including some fast or breeder reactor concepts.

Nuclear Reactor

A nuclear reactor is an engineered system that contains and controls sustained nuclear fission chain reactions to produce useful energy, to breed nuclear fuels, or to produce neutron fluxes for research and isotope production.

• Reactor fuel (commonly enriched U-235 or plutonium-bearing fuel) is assembled into a core where fission events generate heat.
• Fission neutrons can sustain a chain reaction if enough fissile material and the correct geometry and moderation exist.
• The heat produced in the reactor core is removed by a coolant and transferred to a secondary system where steam drives turbines to generate electricity.
• Major reactor functions are achieved by the interaction of fuel, moderator (if needed), coolant, control rods, and containment structures.
• The containment structure (steel-reinforced concrete) separates the reactor from the environment and provides protection against release of radioactivity.

Nuclear reactor coolant

• The coolant circulates through the core to remove heat; common coolants include light water, heavy water, liquid sodium, helium gas, and molten salts.
• The coolant transfers thermal energy to a heat-exchange system where steam is produced to drive turbines.

Neutron moderator

• A neutron moderator reduces the speed (kinetic energy) of fast neutrons, converting them into thermal neutrons that have a higher probability of inducing fission in certain fissile nuclides (e.g., U-235).
• Common moderators are light water (ordinary H₂O), graphite, and heavy water (D₂O).
• The choice of moderator strongly influences reactor design, fuel requirements, and neutron economy.

Control rods and reactivity control

• Power and reactivity are controlled by control rods, which contain neutron-absorbing materials (neutron poisons).
• Inserting control rods deeper into the core absorbs more neutrons and reduces the fission rate; withdrawing them raises power.
• Common control rod materials include compounds of boron, silver, indium, and cadmium.

Moderators slow neutrons - Control rods absorb neutrons.

Critical mass and criticality

• Critical mass is the minimum quantity of fissile material, in a given configuration, required to support a sustained chain reaction.
• Critical mass depends on nuclear properties, density, shape, enrichment, temperature, and surrounding materials (reflectors/absorbers).
• Subcritical - neutron losses exceed production; the neutron population decreases with time.
• Critical - neutron production equals losses; the neutron population and power remain steady.
• Supercritical - neutron production exceeds losses; the neutron population and power increase.
• During reactor start-up operators pass through supercritical conditions carefully until the desired operating point is reached and the reactor is returned to critical (steady) operation; shutdown places the reactor subcritical.

Neutron poison

• A neutron poison (nuclear poison) is any substance with a large neutron absorption cross-section that reduces the number of neutrons available to sustain fission; poisons may be used intentionally (control rods, soluble poisons) or may accumulate as fission products.

Types of Nuclear Reactors

Reactors are classified by the energy of neutrons that sustain the chain reaction (thermal or fast), by the coolant and moderator used, and by technological choices. All commercial power reactors use nuclear fission. Fuels most commonly are uranium and plutonium, though thorium-based cycles are possible.

Thermal reactors and fast neutron (breeder) reactors

• Thermal reactors rely on moderated (thermal) neutrons to sustain fission; they are the most common commercial reactors.
• Fast neutron reactors operate without a moderator and use fast neutrons; certain fast-reactor designs are intended as breeder reactors to convert fertile isotopes (U-238, Th-232) into fissile ones (Pu-239, U-233).

Light-water reactor (LWR)

• An LWR is a thermal-neutron reactor that uses ordinary (light) water as both coolant and moderator.
• LWRs are the most common reactor type worldwide.
• Three principal varieties of LWRs are the Pressurized Water Reactor (PWR), the Boiling Water Reactor (BWR), and the Supercritical Water Reactor (SCWR) (the latter is an advanced concept under development).

Pressurized Water Reactor (PWR)

• The PWR uses light water as coolant in a pressurised primary circuit so that water does not boil in the core.
• Heat from the primary coolant is transferred via a steam generator to a secondary loop where steam is produced to drive turbines.
• PWRs form the majority of Western commercial power reactors and were originally developed for naval propulsion (submarines).

Advantages of PWR

• Stable operational behaviour: many designs have negative temperature coefficients that reduce power as temperature increases, aiding stability.
• Secondary loop isolates turbine and balance-of-plant from primary radioactivity; less radioactive contamination outside the primary system.
• Control rods are often held by electromagnets and drop by gravity under power failure, enabling reliable scram (shutdown).
• Compact designs suitable for marine propulsion.

Disadvantages of PWR

• Primary circuit must operate at high pressure to keep water liquid; this requires thick pressure vessels and high-strength piping, increasing construction cost.
• High-pressure systems increase potential consequences of loss-of-coolant accidents if they occur.
• Use of boric acid in the coolant for reactivity control can increase corrosion concerns and tritium production in some conditions.
• PWR fuels require enrichment to a few percent U-235, which increases fuel-cycle cost and has proliferation implications.

Boiling Water Reactor (BWR)

• In a BWR the reactor core boils the coolant water directly inside the reactor vessel to produce steam that drives the turbine; there is no separate steam generator.
• BWRs are the second most common type of power reactor after PWRs.

Advantages of BWR

• Primary system operates at lower pressure than a PWR (no pressurizer and no steam generators), simplifying some aspects of design.
• Fewer components inside the primary circuit (no steam generator) can reduce complexity and cost.
• BWRs typically do not use soluble boron for reactivity control, reducing some corrosion and tritium-production concerns.
• Can operate at lower core power density and in some cases use natural circulation under certain operating conditions.

Disadvantages of BWR

• Because steam from the core goes directly to the turbine, some turbine and balance-of-plant equipment is exposed to small amounts of radioactivity.
• Control rods in many BWR designs are inserted from below, which can complicate certain safety scenarios; reliable insertion under loss-of-power conditions must be ensured.
• BWR containment issues were raised by events such as the Fukushima I accidents; containment and cooling strategies require careful design and backup systems.

Supercritical Water Reactor (SCWR)

• The SCWR operates with water above its thermodynamic critical point, using supercritical water as the working fluid.
• SCWR concepts combine aspects of PWR and BWR technology: high pressure and temperature (like PWR) with a direct once-through cycle (like BWR).
• SCWRs aim for higher thermal efficiency and simpler plant designs; they are under research and development and not yet widely deployed commercially.

Advantages of SCWR

• High thermal efficiency and potential for compact core and containment due to excellent heat transfer properties of supercritical water.
• Simpler internal reactor vessel configuration: no steam separators, steam dryers, or complex recirculation inside the vessel for certain designs.
• Smaller stored thermal and radiological inventory in some designs compared with large LWR cores.
• Potential options for fast-spectrum SCWRs that could act as breeders, or heavy-water SCWRs that could support thorium-based cycles with higher proliferation resistance than plutonium breeders.

Pressurised Heavy-Water Reactor (PHWR)

• PHWRs use heavy water (D₂O) as both moderator and often as coolant; the heavy water is kept under pressure in many designs.
• Heavy water provides an excellent neutron economy that permits the use of natural (unenriched) uranium as fuel in many PHWR designs.

Advantages of PHWR

• Ability to use natural uranium without enrichment reduces dependency on enrichment infrastructure and associated costs.
• High neutron economy due to low neutron absorption by D₂O; moderators located at lower temperatures can improve thermalisation and fuel utilisation.
• On-line refuelling is possible in some PHWR designs, allowing continuous operation and flexible fuel management.

Disadvantages of PHWR

• Natural uranium has a lower fissile content per unit mass, requiring higher fuel throughput and more frequent fuel changes compared with enriched-fuel reactors.
• PHWRs typically produce larger volumes of spent fuel for a given energy output, requiring more fuel handling and storage capacity.

Nuclear proliferation considerations and PHWRs

• Heavy-water reactors can be used to produce plutonium-239 from U-238 without enrichment; if fuel is changed frequently and reprocessed, weapons-grade plutonium can be separated.
• Historical examples show that materials from heavy-water research reactors have been used to obtain fissile material for weapons development.
• Heavy-water moderation also produces small quantities of tritium when deuterium captures neutrons; tritium is a material of relevance to certain weapon designs.

Applications of fission reactors

• Electricity generation on utility grids (commercial power reactors).
• Naval propulsion: compact reactors for ships and submarines.
• Research reactors: neutron sources for scientific experiments and materials testing.
• Isotope production for medicine, industry, and research (e.g., medical radioisotopes).
• Breeding reactors and fuel-cycle research aimed at utilising fertile resources (U-238, Th-232) more effectively.

Safety and containment summary

• Modern reactor design emphasises defence-in-depth: multiple, independent, redundant safety systems and robust containment structures to limit release of radioactivity.
• Accident mitigation includes reliable coolant systems, automatic and manual shutdown (scram) systems, passive safety features, and emergency core cooling systems to prevent core damage.
• Regulatory frameworks and international safeguards address safety, security, and non-proliferation aspects of civil nuclear technology.

If further technical detail is required (neutron cross-sections, reactor kinetics, thermal-hydraulics, fuel-cycle chemistry, or specific reactor designs and their performance parameters), these topics can be presented as separate focused sections with stepwise derivations and example calculations.