Nuclear Reactors and Their Types | Science & Technology for UPSC CSE PDF Download

Nuclear Power Reactors

• Nuclear reactors use the energy released by splitting atoms (nuclear fission) of certain elements to produce heat that is usually converted into steam to drive turbines and generate electricity.
• Most commercial nuclear electricity today is produced by a small number of reactor families developed from designs of the 1950s and improved since; early (first-generation) designs have been retired and the majority of operating plants are second-generation designs.
• New reactor designs are under development and deployment, including both large reactors and small modular reactors (SMRs).
• Approximately 10% of the world's electricity is supplied from nuclear energy.

This document covers the main conventional types of nuclear reactor and their principal features, components, fuels and coolants. It summarises operational characteristics, design generations, and selected advanced concepts. It does not attempt to cover detailed reactor physics or plant-specific design drawings, but includes all facts and examples necessary for a clear conceptual understanding.

How Does a Nuclear Reactor Work?

• A nuclear reactor controls the release of energy from the fission of heavy atomic nuclei (commonly isotopes of uranium or plutonium). In power reactors, heat from continuous fission is transferred to a coolant and used to produce steam that drives turbines to generate electricity.
• The basic operating principle common to most reactors is a sustained nuclear chain reaction in a core containing fuel. Neutrons produced by fission induce further fissions; control systems regulate the neutron population so the chain reaction remains steady (critical) or can be reduced or stopped as required.
• Reactors used for research or isotope production are designed to maximise neutron flux for experiments or radioisotope generation rather than to produce maximum thermal power. Naval reactors are designed primarily for propulsion, using steam or direct turbine drive for ship systems.
• Natural examples of self-sustaining fission reactions occurred about two billion years ago at Oklo (Gabon). These natural reactors operated intermittently in rich uranium ore bodies moderated by percolating water and illustrate that sustained fission is possible under natural conditions when geometry, moderation and fuel enrichment permit.
• The two most widely used commercial reactor families derived from naval and submarine reactor development are the pressurised water reactor (PWR) and the boiling water reactor (BWR). Both use water as moderator and coolant but differ in circuit arrangement: PWRs keep the primary coolant under high pressure and produce steam in a secondary circuit; BWRs allow boiling in the core and send steam directly to the turbine.

Components of a Nuclear Reactor

Most reactor types share common component groups. Brief descriptions follow with key functions and notable design features.

Fuel

• Fuel material: Uranium is the primary fuel for most reactors. Commonly used form is ceramic uranium dioxide (UO2).
• Fuel geometry: Fuel pellets (approximately 1 cm diameter and 1-1.5 cm length in many designs) are stacked in long tubes of zirconium alloy (zircaloy) to form fuel rods; rods are grouped into fuel assemblies that are loaded into the core.
• Quantity example: A 1000 MWe class PWR may contain tens of thousands of fuel rods and millions of fuel pellets; a large reactor core may contain tens to hundreds of fuel assemblies and several tens to a few hundred tonnes of uranium in the core.
• Start-up neutrons: A new reactor core may require an external neutron source (for example, beryllium mixed with an alpha emitter) for initial criticality; used fuel or residual neutron sources may sometimes be sufficient when refuelling.
• Fuel behaviour: During operation some U-238 in the fuel transmutes to plutonium isotopes (for example Pu-239), which contribute significantly to the total energy produced over the fuel cycle.
• Cladding and structural material: Zirconium alloys are widely used for cladding because of low neutron absorption, corrosion resistance and mechanical strength; nuclear-grade zirconium must be hafnium-free as hafnium is a strong neutron absorber.
• Advanced fuels: Development of accident-tolerant fuels and alternative fuel forms (metal, carbide, nitride, TRISO particles, molten salt fuels) aims to improve high-temperature behaviour and reduce risks in beyond-design-basis events.
• Burnable absorbers: Materials such as gadolinium or zirconium diboride (ZrB2) are used as burnable poisons to flatten core reactivity over the fuel cycle and allow higher average burn-up. Incorporation methods include mixing into pellets or coating pellet surfaces (IFBA).

Moderator

• The moderator is the material in the core that slows down fast neutrons produced in fission to thermal energies where they are more likely to cause further fission in fissile isotopes. Common moderators are light water, heavy water (D2O) and graphite.

Control Rods or Blades

• Control rods contain neutron-absorbing materials such as cadmium, hafnium or boron and are inserted into or withdrawn from the reactor core to regulate the fission rate or to shut the reactor down.
• Control rod geometry and insertion direction vary by design (for example, many PWRs insert rods from the top, whereas BWRs typically insert cruciform blades from the bottom).
• Delayed neutrons (neutrons emitted by certain fission product precursors some time after fission) are essential for controllability; the presence of delayed neutrons gives the reactor a time scale on which control systems and human operators can act.
• Secondary control systems may include soluble neutron absorbers (e.g., boron dissolved in PWR primary coolant or gadolinium added to the moderator in some designs) for longer-term reactivity management.

Coolant

• The coolant is the fluid that removes heat from the reactor core and transfers it to the turbine system or intermediate heat exchangers; in many light water reactors the moderator also acts as the primary coolant.
• PWRs typically have multiple primary coolant loops, each with pumps; example: some modern designs have three primary pumps driven by multi-megawatt electric motors.

Pressure Vessel or Pressure Tubes

• The reactor core is contained by a robust steel pressure vessel in many designs; alternate designs use pressure tubes or channels that house the fuel and allow coolant flow through a larger surrounding moderator (for example, PHWR/CANDU designs use pressure tubes).

Steam Generator

• In pressurised water reactors and some heavy-water designs the high-pressure primary coolant transfers heat to a secondary circuit through steam generators (large shell-and-tube heat exchangers) where water in the secondary side turns to steam for the turbine.
• Steam generators contain many tubes (hundreds to thousands) carrying radioactive primary coolant; tube failure is managed by plugging and designing surplus capacity. Nitrogen-16, an activation product of oxygen in water, causes high gamma levels in the primary coolant; its short half-life (≈7 seconds) means direct radiation levels fall quickly after reactor shutdown.

Containment

• A containment structure surrounds the reactor pressure vessel, steam generators and associated systems to protect the plant from external hazards and to protect the public in the event of internal release of radioactive material. Typical containment is thick reinforced concrete and steel.
• Some modern designs include core-melt localisation devices (core catchers) beneath the vessel to collect and cool molten core material in severe accident scenarios.

Fuelling a Nuclear Reactor

• Refuelling strategy depends on reactor design: many reactors are shut down for refuelling at intervals such as 12, 18 or 24 months, replacing a fraction (for example a quarter to a third) of the fuel assemblies.
• Some designs allow on-power refuelling: reactors with pressure tubes (for example CANDU/PHWR and RBMK) can be refuelled under load by isolating and replacing individual channels; AGRs are also designed for on-load refuelling.
• Natural vs enriched uranium: Reactors using efficient moderators (heavy water or graphite) can operate on natural uranium (≈0.7% U-235). Reactors using ordinary (light) water normally require enriched uranium (commonly 3.5-5% U-235 for many LWRs). New small reactors may use high-assay low-enriched uranium (HALEU) enriched up to ~20% U-235.
• Plutonium produced in operation (from neutron capture in U-238 and subsequent decay) typically contributes a substantial fraction of the energy produced in later stages of the fuel cycle.
• Typical fuel assembly masses and uranium contents vary by reactor type (example: a BWR assembly ~320 kg, a PWR assembly ~655 kg; these figures include zircaloy structural mass and uranium mass as cited for reference designs).
• Zirconium alloys are controlled materials because nuclear-grade zirconium requires low hafnium content; zircaloy formulations include small alloying additions such as tin, iron and chromium for strength.
• Burnable poisons (gadolinium, IFBA coatings) are used to manage reactivity over time. Gadolinium requires slightly higher initial fuel enrichment and retains some residual absorptive effect after burn-up; ZrB2 IFBA tends to burn away more steadily and is widely used in modern fuels.
• Average fuel burn-up has increased over time; for example, US reactor fuel burn-up averaged near 50 GWd/t in recent decades, up from roughly half that in the 1980s.

Main Types of Nuclear Reactor

Pressurised Water Reactor (PWR)

The PWR is the most common commercial reactor type worldwide and the predominant naval-propulsion reactor. It uses light water as both moderator and primary coolant. The primary circuit is held at very high pressure to prevent boiling while the coolant reaches temperatures of roughly 300-330°C; heat is transferred to a secondary steam circuit via steam generators.

Pressurised Water Reactor

• Fuel assemblies typically contain 200-300 rods each; a large reactor core may contain 150-250 assemblies and tens of tonnes of uranium.
• Primary coolant is kept under high pressure (for example, on the order of 150 atmospheres for many designs) using a pressuriser; a negative reactivity feedback operates if primary water turns to steam, reducing fission rate and aiding safety.
• Secondary circuit steam drives the turbine; the primary circuit remains radioactive and isolated from the turbine plant by steam generators.
• Soluble boron in the primary coolant provides an additional long-term reactivity control method in many PWR designs.

Boiling Water Reactor (BWR)

• The BWR uses light water as coolant and moderator but operates with a single circuit in which water is allowed to boil in the core; steam generated in the core is directed to the turbine.
• Typical core operating temperatures are somewhat lower than PWR primary temperatures, with boiling taking place at high pressure (for example ~75 atmospheres in many BWR designs) to reach steam temperatures in the core region.
• The reactor is designed to operate with a fraction of the coolant in the core as steam; steam separators/dryers remove moisture before steam goes to the turbine.
• Turbine systems must be radiologically shielded and maintained with appropriate protection because the primary coolant that becomes steam is radioactive (although most radioactivity is short-lived such as N-16 with 7 s half-life).
• Typical fuel assembly sizes and counts differ from PWRs (for example, a BWR assembly may have 90-100 rods and a core may contain up to ~750 assemblies; uranium quantities vary accordingly).
• Reactivity control additional to control rods includes manipulation of coolant flow rates to change void fraction in the core and thus moderation.

Pressurised Heavy Water Reactor (PHWR / CANDU)

PHWR designs (notably the Canadian CANDU and Indian PHWR variants) use heavy water (D2O) as moderator and sometimes as coolant. Heavy water is a superior moderator, allowing use of natural uranium fuel.

Pressurised Heavy Water Reactor

• CANDU reactors use a calandria - a large moderator tank - penetrated by many horizontal pressure tubes that contain fuel bundles and primary coolant. Primary coolant in the pressure tubes transfers heat to steam generators in a secondary circuit.
• Because fuel is in discrete tubes, on-power refuelling is possible by isolating and replacing channels, allowing continuous operation while individual channels are changed.
• CANDU fuel bundles are typically shorter than LWR assemblies (example: 37-element bundles ~0.5 m long laid end-to-end in channels). Secondary shutdown systems may use soluble neutron absorbers such as gadolinium in the moderator.
• PHWRs can use a range of fuels, including natural uranium, recycled uranium, blends with depleted uranium, and thorium-containing fuels; this flexibility aids resource utilisation strategies.
• Advanced CANDU variants (ACR) have been developed using light water cooling and slightly enriched fuel to modify economics and performance.

Advanced Gas-Cooled Reactor (AGR)

The AGR is the United Kingdom's second-generation gas-cooled reactor using graphite as moderator and carbon dioxide as primary coolant. Fuel is enriched UO2 in stainless steel cladding and steam is generated in heat exchangers outside the core but within the reactor pressure vessel. AGRs operate at higher temperatures than LWRs and achieve relatively high thermal efficiency (around 41% in typical designs). On-power refuelling is possible.

Advanced Gas Cooled Reactor

The AGR evolved from the earlier Magnox reactors, which used magnesium-alloy cladding and metal natural uranium fuel; the last UK Magnox units have been retired.

Light Water Graphite-Moderated Reactor (LWGR / RBMK)

• The Soviet RBMK family is a graphite-moderated design with long vertical pressure tubes through the graphite and water coolant that may boil in the core. Fuel assemblies are longer than in many LWRs (for example ~3.5 m long in RBMK). The fixed graphite moderator and localised boiling behaviour can produce positive reactivity feedback under certain conditions, which created safety challenges in some early RBMK units.

Fast Neutron Reactors (FNR) and Fast Breeder Reactors (FBR)

• Fast neutron reactors operate without a moderator and use fast neutrons to sustain the chain reaction; they typically burn plutonium and can convert (breed) U-238 into more plutonium in the surrounding fertile material.
• Fast reactors extract far more energy from natural uranium resources (potentially over an order of magnitude more) but are more expensive and technologically demanding to construct and operate.
• When configured to produce more fissile material than they consume they are referred to as fast breeder reactors (FBRs).
• Fast reactors generally use liquid metal coolants (sodium, lead, or lead-bismuth) or molten salts to maintain low moderating properties and to operate at useful temperatures and heat-transfer characteristics.

Operable Nuclear Power Plants

Information on reactors under construction and national/utility deployment plans is maintained by international organisations and industry bodies; specific project details should be consulted from authoritative current sources.

Advanced Reactors and Generations

• Reactor designs are commonly grouped in generations: Generation I (early prototype and first commercial plants from 1950s-60s), Generation II (most current operating plants), Generation III (evolutionary improvements with enhanced safety and performance), and conceptual Generation IV designs (innovative systems with closed fuel cycles and higher outlet temperatures).
• Generation III reactors improve safety systems, economics and lifetime performance over Generation II; several are in operation or under construction in Japan, China, Russia, UAE and other countries.
• Generation IV concepts emphasise closed fuel cycles, actinide burning, low-pressure coolants and high outlet temperatures; international collaboration has prioritised seven candidate designs, many fast-spectrum and using liquid metal or fluoride salt coolants.
• Advanced small modular reactors (SMRs) and a range of novel fuel forms (TRISO, molten salt fuels, metal fuels) are part of the longer-term development landscape.
• TRISO fuel particles consist of a small uranium-containing kernel surrounded by carbon and silicon carbide layers to contain fission products and to withstand very high temperatures (stable above 1600°C in design), and are used in high-temperature gas-cooled reactor concepts.

Floating Nuclear Power Plants

• Floating nuclear power plants (FNPPs) are ship- or barge-mounted reactor units designed to provide electricity and heat (including desalination) to remote coastal or arctic locations. Russia has led early deployment with units based on icebreaker reactor technology.
• Examples include KLT-40S units (derivative of icebreaker reactors) and proposed RITM-200M units; typical configurations operate as paired reactors on a barge, with refuelling intervals measured in years and planned offsite refuelling/overhaul cycles.

Power Rating of a Nuclear Reactor

Reactor output is quoted in three commonly used ways:

• Thermal MWt - the thermal power produced in the reactor core; it determines the amount of steam and the thermodynamic resource available to the turbine cycle.
• Gross electrical MWe - the electrical power produced by the turbine-generator set before subtracting plant internal consumption; this depends on turbine efficiency and ambient conditions affecting condenser performance.
• Net electrical MWe - the electrical power delivered to the grid after subtracting the plant's own electrical loads (pumps, auxiliary systems). This is the practical export capacity.

Two derived performance metrics are:

• Thermal efficiency (%) - ratio of gross electrical output to thermal input (MWt → MWe). Light water reactors typically achieve around 33-37% thermal efficiency, with some newer PWR designs reaching ≈38%.
• Net efficiency (%) - ratio of net electrical output to thermal input; it is slightly lower than thermal efficiency because it accounts for internal plant usage.

Gross and net electrical outputs vary with site conditions (for example, cooling-water temperature) and operational choices; seasonal variations can be significant and are sometimes reflected in published plant ratings.

Lifetime of Nuclear Reactors

• Many existing nuclear plants were originally licensed for 30-40 years. Licence extensions and major component replacements (e.g., steam generators, pumps, digital I&C upgrades) can extend operating life to 60 years or more; in some cases plants may be operated with appropriate investments and reviews for 80 years.
• Ageing issues include mechanical wear, corrosion, irradiation effects on materials, obsolescence of instrumentation and control systems, and loss of original design knowledge over multi-decadal lifecycles. Periodic safety reviews and investments are required to maintain safety margins.
• Knowledge management and data portability across plant lifecycle phases and generations of software/hardware are important for long-term plant safety and economics; international standards and guidance address information handover and lifecycle data management.

Primary Coolants

Primary coolant selection affects reactor engineering, operating pressure, thermal efficiency and safety characteristics. Major coolant classes and their salient features are:

• Water (light or heavy) - widely used, good heat capacity and familiarity. Light water requires high pressure to operate at temperatures well above 100°C (e.g., up to ≈345°C in PWRs). Heavy water (D2O) is a better moderator permitting natural uranium fuel in PHWRs/CANDU designs.
• Helium - used in high-temperature gas-cooled reactors; it is chemically inert but low density requires high pressure to provide useful heat transfer. Helium allows coupling to Brayton cycles for higher thermal efficiency and direct gas-turbine applications.
• Carbon dioxide - used historically in UK Magnox and AGR designs; allows higher core temperatures than light water but chemical stability at very high temperatures is limited.
• Sodium - common in fast reactors for its high thermal conductivity and heat capacity; it is liquid over a wide temperature range but reacts chemically with water and air. Sodium produces short-lived radioactive Na-24 under neutron activation, requiring shielding and careful radiological management.
• Lead or lead-bismuth eutectic (Pb or Pb-Bi) - used in some fast reactor concepts; operate at near-atmospheric pressure and allow high outlet temperatures. Lead-based coolants do not react violently with water or air, but corrosion and polonium production (from Pb-Bi) are technical issues to manage.
• Molten fluoride salts (e.g., FLiBe, FLiNaK) - used in molten salt reactors (MSRs) and some high-temperature reactor concepts. They have high boiling points, low vapour pressure, good heat capacity and chemical stability, and can serve as both coolant and fuel carrier in MSR designs.
• Chloride salts - candidates for fast-spectrum molten salt reactors with high actinide solubility; radiological activation (Cl-36) and material compatibility must be considered.
• Low-pressure liquid coolants deliver heat at higher temperatures with smaller temperature drops in heat exchangers, enabling efficient conversion to electricity and effective passive decay heat removal strategies when designed appropriately.
• Decay heat removal: After reactor shutdown, fission product decay continues to produce heat (decay heat). Immediately after shutdown decay heat is typically about 6.5% of prior full power, falling to ≈1.5% after an hour, ≈0.4% after a day and ≈0.2% after a week. Reliable removal of decay heat is a critical design and safety requirement (passive or active systems), as failure to remove residual heat can lead to core damage.
• Activation of coolant (for example N-16 formation in water) produces short-lived high-energy radiation and influences operational access rules around reactor systems while operating.

Load-Following Capability

• Nuclear plants are most economical when operated continuously at high capacity to serve baseload demand because capital costs are large and marginal fuel costs are low. However, load following (varying output with grid demand) is sometimes required and technically achievable.
• BWRs can adjust output by changing coolant flow and void fraction, giving relatively straightforward daily load following without severe core effects. PWRs can also perform load following, often with design and operational features such as special control rods and soluble boron adjustments; operator strategy across a fleet of reactors can also provide effective daily load management.
• Advanced control systems have been developed to allow automatic load following (for example, systems capable of modulating output between 50% and 100% without operator intervention). Utility and regulatory requirements may specify capability and rates of change (for example, some guidelines expect 3-5% per minute electric output ramping capability between specified bounds).
• Load following has economic consequences due to reduced capacity factor of capital-intensive plants and may increase operational wear, but it provides flexibility in grids with high variable renewable penetration or particular daily demand patterns.

Nuclear Reactors for Process Heat

• Beyond electricity generation, reactors can be designed to provide high-temperature heat for industrial processes including hydrogen production, chemical processes and district heating. Reactor outlet temperature requirements depend on the end use.
• Typical outlet temperature ranges cited for advanced reactor concepts include roughly 500°C for some liquid-metal cooled fast reactors, 750-950°C or higher for gas-cooled or molten salt systems targeted at process heat or thermochemical hydrogen production. Higher outlet temperatures enable higher thermodynamic efficiencies and broaden industrial applications.

Primitive Reactors (Oklo Natural Reactors)

• Natural self-sustaining nuclear chain reactions operated about 2 billion years ago in rich uranium ore at Oklo (Gabon). At that time the natural abundance of U-235 was higher (~3.6%) than today (~0.7%), allowing criticality in appropriate geological and hydrogeological conditions.
• Oklo reactors operated intermittently over long periods (estimated overall operation for hundreds of thousands to a million years) and demonstrate natural feedback mechanisms: when groundwater boiled away, moderation decreased and reactions ceased, resuming when water returned.
• Study of Oklo shows that significant quantities of fission products and transuranics were generated but remained relatively immobile in the orebody over geological timescales - important evidence for natural containment processes relevant to repository science.

Notes on Terminology and Key Concepts

• Criticality - a reactor is critical when the neutron population remains steady from generation to generation, sustaining a controlled chain reaction at constant power. Subcritical means the reaction will die away; supercritical means power increases unless controlled.
• Moderator - material that reduces neutron energy by collisions so that thermal neutrons are available to sustain fission in fissile isotopes.
• Burn-up - a measure of the energy produced per unit mass of fuel, typically expressed in gigawatt-days per tonne (GWd/t).
• Decay heat - heat produced by radioactive decay of fission products after reactor shutdown; an important factor in post-shutdown cooling requirements.
• Containment - engineered barrier to prevent or minimise release of radioactive material to the environment in normal operation and accident conditions.

Concluding Remarks

This chapter has presented the principal reactor types, components, fuels and coolants, generation classification and key operational considerations. The material collected here focuses on concepts and facts essential for a foundational understanding of nuclear power reactors-suitable as a syllabus-aligned reference for engineering and general science students. For detailed technical design, regulatory, licensing and site-specific information consult design-specific documentation and current technical references maintained by regulatory authorities and international nuclear organisations.