📚 Nuclear Fission and Fusion: Energy from the Nucleus | 核裂变与核聚变：来自原子核的能量
Understanding how energy can be released from the nucleus is one of the most fascinating and important topics in IGCSE Edexcel Science. This article explores the processes of nuclear fission and nuclear fusion, compares their characteristics, and explains how they are harnessed in power stations and nature. The concepts of chain reactions, critical mass, moderators, and control rods will be covered, along with the conditions required for fusion. By the end, you will clearly see why these processes are so significant for both energy generation and the universe itself.
理解原子核如何释放能量是IGCSE爱德思科学中最引人入胜且重要的主题之一。本文将探讨核裂变与核聚变的过程，比较它们的特点，并解释如何在发电站和自然界中利用这些过程。文中将涵盖链式反应、临界质量、慢化剂和控制棒等概念，以及核聚变所需的条件。阅读完毕，你将清楚地认识到这些过程对能量生成乃至整个宇宙的重要性。
1. Introduction to Nuclear Reactions | 核反应简介
ms rearrange their electrons and the total mass of reactants equals the total mass of products; energy changes come from breaking and forming bonds. Nuclear reactions, however, involve changes in the nucleus itself and can convert tiny amounts of mass into enormous amounts of energy, as described by Einstein' sics (and Combined Science), we study two main types of nuclear reactions: fission, in which a heavy nucleus splits into smaller nuclei, and fusion, in which light nuclei combine to form a heavier nucleus. Both release energy and have profound implications for power generation and the cosmos.
在普通化学反应中，原子重新排列其电子，反应物的总质量等于生成物的总质量；能量变化来自键的断裂和形成。然而，核反应涉及原子核自身的变化，可以将微小的质量转化为巨大的能量，正如爱因斯坦著名的方程E = mc²所描述。在IGCSE物理（以及综合科学）中，我们学习两种主要的核反应类型：裂变，即一个重原子核分裂成较小的原子核；聚变，即轻原子核结合形成较重的原子核。两者都释放能量，并对发电和宇宙有着深远的影响。
2. What is Nuclear Fission? | 什么是核裂变？
Nuclear fission is the process in which a large, unstable nucleus splits into two smaller, more stable nuclei, accompanied by the release of two or three free neutrons and a large amount of energy. The most commonly used fissile materials in nuclear power stations are uranium‑235 and plutonium‑239. When a slow‑moving neutron is absorbed by a uranium‑235 nucleus, the nucleus becomes highly unstable and splits almost instantly into two medium‑sized fragments—for example, krypton‑92 and barium‑141—along with extra neutrons and gamma radiation. A typical fission equation can be written as:
²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3¹₀n + energy
核裂变是指一个大的、不稳定的原子核分裂成两个较小、更稳定的原子核，同时释放出两三个自由中子和大量能量的过程。核电站中最常用的裂变材料是铀‑235和钚‑239。当一个慢中子被铀‑235原子核吸收时，原子核变得极不稳定，几乎瞬间分裂成两个中等大小的碎片——例如氪‑92和钡‑141——同时释放出额外的中子和伽马辐射。典型的裂变方程如下：
²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3¹₀n + 能量
It is the release of extra neutrons that makes a sustained chain reaction possible.
正是额外中子的释放使得持续的链式反应成为可能。
3. Chain Reaction and Critical Mass | 链式反应与临界质量
Each fission event produces, on average, 2–3 fast neutrons. If at least one of these neutrons goes on to cause another fission event in a nearby fissile nucleus, we have a chain reaction. For a chain reaction to be self‑sustaining and not die out, the sample must have a certain minimum size and mass, known as the critical mass. If the mass is below critical, too many neutrons escape without causing further fission and the reaction stops. In a nuclear bomb, an uncontrolled chain reaction is deliberately triggered; in a nuclear power plant, the chain reaction is carefully controlled to release energy at a steady rate.
每个裂变事件平均产生2到3个快中子。如果这些中子中至少有一个继续引发附近另一个易裂变核的裂变，就形成了链式反应。要使链式反应自我维持而不熄灭，裂变材料必须具有某个最小的尺寸和质量，称为临界质量。若质量低于临界值，过多中子会逃逸而不引发进一步裂变，反应就会停止。核弹中会故意引发不受控制的链式反应；而在核电站中，链式反应被小心控制，以稳定的速率释放能量。
Critical mass depends on the shape, density, and purity of the fissile material, as well as the presence of a neutron reflector.
临界质量取决于裂变材料的形状、密度和纯度，以及是否存在中子反射层。
4. Nuclear Reactors: Harnessing Fission | 核反应堆：利用裂变
A nuclear reactor is a device designed to extract useful energy from controlled fission reactions. The core contains fuel rods made of enriched uranium dioxide (with a high proportion of uranium‑235). The energy released by fission appears as kinetic energy of the fission fragments and neutrons, which heats the surrounding material. This thermal energy is carried away by a coolant (often water, heavy water, or carbon dioxide), which then passes through a heat exchanger to produce steam. The steam drives a turbine connected to an electricity generator, just as in a conventional fossil‑fuel power station.
核反应堆是一种设计用于从受控裂变反应中提取有用能量的装置。堆芯装有由浓缩二氧化铀（铀‑235含量较高）制成的燃料棒。裂变释放的能量体现为裂变碎片和中子的动能，加热周围的材料。这些热能由一个冷却剂（通常是水、重水或二氧化碳）带走，然后通过热交换器产生蒸汽。蒸汽驱动与发电机相连的涡轮机，和传统的化石燃料发电站一样。
Because the neutrons produced in fission are too fast to cause further fission efficiently, a moderator is needed to slow them down.
由于裂变产生的中子速度太快，无法高效引发进一步裂变，因此需要使用慢化剂将它们减速。
5. Controlling the Chain Reaction: Moderators and Control Rods | 控制链式反应：慢化剂和控制棒
The moderator is a substance that slows down the fast neutrons, turning them into thermal (slow) neutrons that are more likely to be captured by uranium‑235 nuclei. Common moderators include water, heavy water, and graphite. Without a moderator, the chain reaction would be very inefficient. Control rods, typically made of boron or cadmium, are excellent neutron absorbers. By raising or lowering the control rods into the reactor core, the reactor operator can adjust the number of free neutrons available to sustain the chain reaction, and thereby control the reactor's power output. In an emergency, the control rods are dropped fully into the core to shut down the reaction completely—a process called a SCRAM.
慢化剂是一种使快中子减速的物质，把它们变成更容易被铀‑235原子核俘获的热（慢）中子。常见的慢化剂包括水、重水和石墨。没有慢化剂，链式反应将非常低效。控制棒通常由硼或镉制成，是优良的中子吸收体。通过将控制棒提出或插入堆芯，操作员可以调节维持链式反应的自由中子数量，从而控制反应堆的功率输出。紧急情况下，控制棒被完全落入堆芯，彻底终止反应——这一过程称为急停。
Besides the moderator and control rods, thick concrete or steel shielding is essential to protect workers from the intense neutron and gamma radiation emitted by the core.
除慢化剂和控制棒外，厚实的混凝土或钢屏蔽对于保护工作人员免受堆芯发出的强中子和伽马辐射至关重要。
6. Nuclear Fusion: Power of the Stars | 核聚变：恒星的能量
Nuclear fusion is the process in which two light atomic nuclei combine to form a heavier nucleus, releasing a tremendous amount of energy. This is the reaction that powers the Sun and other stars. In the Sun's core, hydrogen nuclei (protons) undergo a series of fusion steps to eventually form helium‑4, with the overall net reaction being:
4¹₁H → ⁴₂He + 2⁰₁e + 2γ + energy
核聚变是两个轻原子核结合形成一个较重的原子核并释放巨大能量的过程。这是为太阳和其它恒星提供能量的反应。在太阳的核心，氢核（质子）经过一系列聚变步骤，最终形成氦‑4，总净反应为：
4¹₁H → ⁴₂He + 2⁰₁e + 2γ + 能量
On Earth, the most promising fusion reaction for commercial power generation involves deuterium and tritium, two heavy isotopes of hydrogen:
²₁H + ³₁H → ⁴₂He + ¹₀n + energy
在地球上，最具商业发电前景的聚变反应涉及氘和氚，这两种氢的重同位素：
²₁H + ³₁H → ⁴₂He + ¹₀n + 能量
Fusion releases far more energy per kilogram of fuel than fission, and the fuels (deuterium from seawater, and tritium bred from lithium) are virtually inexhaustible.
每千克燃料的聚变所释放的能量远远超过裂变，而且燃料（海水中的氘，以及由锂增殖的氚）几乎是取之不尽的。
7. Conditions for Fusion: Extreme Temperature and Pressure | 聚变的条件：极端温度和压力
For fusion to occur, the colliding nuclei must overcome their mutual electrostatic repulsion (the Coulomb barrier). This requires the fuel to be heated to incredibly high temperatures—typically over 100 million degrees Celsius—so that the nuclei move fast enough to fuse when they collide. At such extreme temperatures, the fuel becomes a plasma, a state of matter where electrons are stripped from atoms. In stars, the immense gravitational pressure provides the necessary confinement. On Earth, two main approaches are being developed: magnetic confinement (using powerful magnetic fields in a tokamak) and inertial confinement (using intense laser beams to compress a fuel pellet).
要实现聚变，相互碰撞的原子核必须克服它们之间的静电排斥力（库仑势垒）。这就需要将燃料加热到极高的温度——通常超过1亿摄氏度——以使原子核运动速度足够快，在碰撞时能够融合。在如此极端温度下，燃料变成等离子体，即电子从原子中剥离的物质状态。在恒星中，巨大的引力压力提供了必要的约束。地球上目前正在开发两种主要方法：磁约束（在托卡马克装置中使用强磁场）和惯性约束（使用强激光束压缩燃料靶丸）。
In addition to high temperature, a sufficient plasma density and a long enough confinement time are required—a combination expressed by the Lawson criterion.
除高温外，还需要足够高的等离子体密度和足够长的约束时间——这些组合条件用劳森判据来表达。
8. Comparing Fission and Fusion | 比较裂变与聚变
| Feature / 特征 | Nuclear Fission / 核裂变 | Nuclear Fusion / 核聚变 |
|---|---|---|
| Basic process / 基本过程 | Splitting of a heavy nucleus / 重核分裂 | Joining of light nuclei / 轻核结合 |
| Typical fuels / 典型燃料 | Uranium‑235, Plutonium‑239 / 铀‑235, 钚‑239 | Deuterium, Tritium / 氘, 氚 |
| Neutron production / 中子产生 | 2–3 neutrons per fission / 每次裂变产生2–3个中子 | One high‑energy neutron (in D‑T reaction) / (氘‑氚反应中)一个高能中子 |
| Energy released per kg / 每千克释放能量 | About 3.6 × 10¹⁴ J / 约3.6 × 10¹⁴ 焦 | About 3.4 × 10¹⁴ J (D‑T), potentially much higher for pure deuterium / 约3.4 × 10¹⁴ 焦(氘‑氚)，纯氘更高 |
| Conditions for reaction / 反应条件 | Moderate temperature, slow neutrons, critical mass / 中等温度、慢中子、临界质量 | Extremely high temperature and pressure (plasma) / 极高温和高压(等离子体) |
| Chain reaction possible? / 能否链式反应? | Yes (self‑sustaining) / 是(自持) | No (requires continuous external heating to maintain) / 否(需持续外部加热维持) |
| Radioactive waste / 放射性废物 | Long‑lived, high‑level waste / 长寿命高放废物 | Much less long‑lived waste; neutron activation of reactor walls / 长寿命废物少得多；反应器壁的中子活化 |
| Current use / 目前使用 | Commercial power reactors in many countries / 多国商业发电反应堆 | Experimental stage (ITER, JET, etc.) / 实验阶段(如ITER, JET) |
Although both fission and fusion release energy resulting from a mass defect (the total mass of the products is less than that of the reactants), the specific binding energies per nucleon involved dictate which process is energetically favourable for different nuclei.
尽管裂变和聚变都因质量亏损释放能量（生成物的总质量小于反应物的总质量），但其中涉及的不同核子的平均结合能决定了哪种过程对哪些原子核在能量上是有利的。
9. Radioactive Waste and Safety | 放射性废物与安全
One major drawback of nuclear fission is the production of radioactive fission products, such as caesium‑137 and strontium‑90, which have half‑lives of around 30 years and remain hazardous for centuries. High‑level waste must be stored safely, initially in cooling ponds and then potentially in deep geological repositories. Used fuel rods also contain plutonium, which could be reprocessed but raises concerns about nuclear proliferation. Fusion, in contrast, produces far less long‑lived radioactive waste. The main radioactive waste from a future fusion reactor would come from neutron activation of the structural materials, which can be chosen to have relatively short half‑lives.
核裂变的一个主要缺点是其产生的放射性裂变产物，如铯‑137和锶‑90，它们的半衰期约为30年，在数百年内仍具危害性。高放废物必须安全储存，首先在冷却池中冷却，随后可能置于深层地质处置库。用过的燃料棒还含有钚，可经后处理提取，但这引发了核扩散的担忧。相比之下，聚变产生的长寿命放射性废物要少得多。未来聚变反应堆的主要放射性废物来自结构材料的中子活化，这些材料可选择半衰期相对较短的种类。
Both fission and fusion reactors require robust safety systems. Fission reactors have multiple containment barriers to prevent the release of radioactivity; however, accidents such as Chernobyl and Fukushima highlight the risks if cooling fails.
裂变和聚变反应堆都需要强大的安全系统。裂变反应堆设有多重屏障以防止放射性物质释放；然而，切尔诺贝利和福岛等事故凸显了冷却失效时的风险。
10. Applications and Future Prospects | 应用与未来展望
Nuclear fission accounts for about 10% of the world's electricity generation and remains a reliable, low‑carbon source of baseload power. Advances in reactor design, such as small modular reactors and Generation IV reactors, aim to enhance safety and efficiency. Meanwhile, nuclear fusion research is progressing through international collaborations like ITER (International Thermonuclear Experimental Reactor), which seeks to demonstrate a net energy gain from a sustained fusion plasma. If fusion becomes commercially viable, it could provide an almost unlimited supply of clean energy, with seawater as the ultimate fuel source. Both technologies rely on the physics of the nucleus and remind us that mastering energy from the atom requires a deep understanding of scientific principles, which is exactly what we develop through the IGCSE Edexcel Science specification.
核裂变提供了全球约10%的电力，并仍是一种可靠、低碳的基荷电力来源。反应堆设计的进步，如小型模块化反应堆和第四代反应堆，旨在提升安全性与效率。与此同时，核聚变研究通过国际热核聚变实验堆（ITER）等国际合作不断推进，力求展示持续聚变等离子体的净能量增益。一旦聚变实现商业化，它几乎可以提供无限供应的清洁能源，并以海水为终极燃料来源。这两项技术都依赖于原子核物理，并提醒我们：掌控原子能需要深刻理解科学原理，而这正是我们通过IGCSE爱德思科学课程所培养的能力。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图