Nuclear FissionEdit

Nuclear fission is a nuclear reaction in which an atomic nucleus splits into smaller parts, releasing energy and neutrons. The phenomenon was first observed in 1938 when Otto Hahn and Fritz Strassmann detected barium after bombarding uranium with neutrons, and it was explained by Lise Meitner and Otto Frisch as the splitting of a heavy nucleus into lighter fragments. The energy released per fission is on the order of 200 MeV, which means that a small amount of fissile material can produce a large amount of heat. Because each fission can release additional neutrons that may induce further fissions, a chain reaction can be sustained in a controlled manner in a reactor or can run away in a weapon if not properly managed. These basic dynamics underpin both civilian electricity generation and military applications, making the technology central to modern energy policy and geopolitical considerations. See Nuclear fission for a broader treatment of the process and its history.

In civilian uses, the goal is to harness the heat from fission to boil water or otherwise drive turbines, producing electricity with high reliability and low direct carbon emissions. Modern reactors largely rely on moderated, controlled chain reactions, with heat transferred to a secondary system that drives turbines while containing radioactive materials. The vast majority of reactors around the world use light water as a moderator and coolant, but other designs employ heavy water, gas, or molten salt in different configurations. See Nuclear power plant and Nuclear reactor for overviews of how these systems are engineered and operated.

Fundamental principles

Mechanism of fission

In fissile materials such as Uranium-235 and Plutonium-239, a thermal neutron can be absorbed by a nucleus, causing it to become unstable and split into two lighter fragments plus one or more neutrons. The distribution of fission fragments and the energy released are highly characteristic of the isotope involved. The neutrons produced can then continue the reaction, which is why a reactor design aims to control the number and timing of fissions. See Nuclear fission and Fissile material.

Chain reactions and criticality

A key concept is whether the system is subcritical, critical, or supercritical. In a subcritical state, the reaction dies out; in a critical state, it is self-sustaining at a steady rate; in a supercritical state, the reaction grows more rapidly. Reactor designers manage criticality with a combination of fuel geometry, material properties, and neutron moderators to achieve a stable, controllable heat output. See Critical mass and Chain reaction.

Fuel, moderators, and control

Fissile isotopes are typically housed in assemblies that intertwine fuel with materials that slow neutrons (moderators) and absorb excess neutrons (control rods). The choice of moderator and coolant determines reactor behavior, efficiency, and safety characteristics. The dominant civilian designs use Light-water reactor, which employ ordinary water as both moderator and coolant; other options include Heavy-water reactors and various gas-cooled or molten-salt approaches. See Moderator (nuclear) and Nuclear reactor design for further details.

Reactor technology and the fuel cycle

Reactor designs and operation

The most common civilian reactors are pressurized water reactors (PWRs) and boiling water reactors (BWRs). PWRs keep water under pressure to prevent boiling in the primary loop, while BWRs allow some boiling within the core to supply the secondary loop. Other designs explore different moderator materials or coolants, including heavy-water systems and gas-cooled reactors. Small modular reactors (SMRs) are gaining attention as a way to reduce upfront costs and shorten construction times. See Pressurized-water reactor, Boiling water reactor, and Small modular reactor.

Fuel cycle and waste

Nuclear fuel typically starts as uranium ore, which must be mined and then enriched to increase the fraction of fissile Uranium-235. After fuel has been used, it becomes spent nuclear fuel and must be managed as high-level radioactive waste or reprocessed in some systems to recover usable materials. Reprocessing can separate plutonium and uranium for reuse, but it raises proliferation concerns and costs. Long-term waste disposal remains a central policy issue, with sites and proposals such as Yucca Mountain or other geological repositories debated in many countries. See Nuclear fuel reprocessing, Spent nuclear fuel, and Nuclear waste management.

Safety systems and regulation

Nuclear safety relies on multiple layers of defense: robust reactor design, rigorous quality control, redundant cooling systems, containment structures, and institutional oversight. International and national regulators, such as Nuclear Regulatory Commission in some jurisdictions and the broader framework of the International Atomic Energy Agency, establish licensing, safety standards, and emergency response planning. Public confidence hinges on transparency, ongoing safety culture, and credible incident analysis. See Nuclear safety.

Economics, policy, and energy security

Nuclear power offers high-capacity, low-direct-emission electricity suitable for base-load demand. Its costs are dominated by upfront capital, long planning horizons, regulatory requirements, and long-lived waste management obligations. The industry relies on clear, predictable policy signals—such as reasonable loan guarantees, stable incentives, nonproliferation safeguards, and efficient permitting—to mobilize private capital and compete with other generation technologies. Proponents emphasize reliability, fuel diversity, and the ability to decarbonize power grids while maintaining grid stability, especially when paired with energy storage and flexible demand. See Nuclear energy policy and Capital costs of nuclear power.

Controversies and debates

Nuclear energy remains controversial because of perceived risks and tradeoffs. Critics point to high upfront costs, long construction times, and challenges in financing large plants, which can deter private investment. They also highlight waste management questions, potential for catastrophic accidents, and the risk of nuclear material diversion. Proponents respond that advances in reactor safety, passive cooling, and smaller modular designs reduce many of these concerns, and that the carbon-free nature of nuclear power makes it a critical option for reducing greenhouse gas emissions alongside renewables. They argue that with strong export controls, verified enrichment limitations, and robust international safeguards, the proliferation risk can be managed effectively. In this debate, some critics from broader environmental movements have at times opposed nuclear expansion on principles that many observers view as impractical in the face of climate goals; supporters contend that a balanced mix of energy sources, including nuclear, provides reliable, affordable energy while advancing decarbonization. See Non-Proliferation Treaty, Proliferation risk, and Nuclear safety.

Where discussions emphasize energy security and cost, a market-based approach paired with credible regulation and predictable policy can unlock private investment in new reactors, including SMRs, while maintaining strict safeguards against misuse. The controversies surrounding nuclear energy illustrate a broader political and economic debate about how best to supply reliable, affordable electricity while meeting environmental and national-security objectives. See Energy policy and Nuclear proliferation for related topics.