Fission & Fusion | 核裂变与核聚变

📚 Fission & Fusion | 核裂变与核聚变

Nuclear energy is one of the most powerful and controversial topics in physics. In your Edexcel IGCSE Physics course, you need to understand two fundamental nuclear reactions: fission and fusion. Both processes release enormous amounts of energy by rearranging the particles inside the nucleus, but they do so in opposite directions. This article explains both concepts in depth, with equations, comparisons, and exam-style tips.

核能是物理学中最强大也最具争议的话题之一。在 Edexcel IGCSE 物理课程中，你需要理解两种基本的核反应：核裂变和核聚变。这两个过程都通过重新排列原子核内部的粒子来释放巨大能量，但方向正好相反。本文将深入讲解这两个概念，包括方程、比较和考试技巧。

1. Atomic Structure Recap | 原子结构回顾

Every atom is built from three types of subatomic particles: protons, neutrons, and electrons. Protons carry a positive charge, neutrons are neutral, and electrons carry a negative charge. The protons and neutrons are packed tightly together in the nucleus at the centre of the atom, while electrons orbit around it in shells. The number of protons determines which element the atom is, and the total number of protons plus neutrons is called the mass number (or nucleon number).

每个原子都由三种亚原子粒子构成：质子、中子和电子。质子带正电，中子不带电，电子带负电。质子和中子紧密地排列在原子中心的原子核中，而电子在核外壳层中绕行。质子数决定原子属于哪种元素，质子数加中子数的总和称为质量数（或核子数）。

The nucleus is held together by the strong nuclear force, which operates over extremely short distances and overcomes the electrostatic repulsion between positively charged protons. Without this force, a nucleus containing several protons would fly apart. When nuclear reactions rearrange these particles, the energy associated with the strong force is released as kinetic energy and radiation. This is the fundamental basis of both fission and fusion.

原子核依靠强核力将粒子束缚在一起。强核力在极短距离内起作用，并能克服带正电质子之间的静电斥力。没有这种力，含有多个质子的原子核就会飞散。当核反应重新排列这些粒子时，与强核力相关的能量就会以动能和辐射的形式释放出来。这就是核裂变和核聚变的根本基础。

2. What Is Nuclear Fission? | 什么是核裂变？

Nuclear fission is the splitting of a large, unstable nucleus into two smaller nuclei, together with the release of energy. The process is usually triggered when a slow-moving neutron is absorbed by a heavy nucleus such as uranium-235 (²³⁵U) or plutonium-239 (²³⁹Pu). Once the nucleus absorbs the neutron, it becomes highly unstable and splits apart almost instantly into two daughter nuclei called fission fragments, plus two or three additional free neutrons.

核裂变是指一个大的不稳定原子核分裂成两个较小的原子核，同时释放能量的过程。该过程通常由一个慢速运动的中子被铀-235（²³⁵U）或钚-239（²³⁹Pu）等重原子核吸收所触发。原子核吸收中子后变得极不稳定，几乎立即分裂成两个称为裂变碎片的子核，同时释放出两到三个多余的自由中子。

The energy released appears as the kinetic energy of the fission fragments and the emitted neutrons, as well as gamma radiation. The fragments fly apart at tremendous speeds because the electrical repulsion between the two positively charged daughter nuclei is enormous. In a nuclear reactor, this kinetic energy is converted into heat, which is used to generate steam and drive turbines.

释放的能量表现为裂变碎片和中子的动能，以及伽马辐射。由于两个带正电的子核之间的电斥力巨大，碎片以极快的速度飞离。在核反应堆中，这些动能被转化为热能，用于产生蒸汽并驱动涡轮机。

3. The Fission Equation and Energy Release | 裂变方程与能量释放

For your Edexcel IGCSE exam, you should be able to interpret and complete a fission equation. A common fission reaction of uranium-235 can be written as:

在 Edexcel IGCSE 考试中，你需要能够解读和完成裂变方程。铀-235 的一个典型裂变反应可写成：

²³⁵U + ¹₀n → ¹⁴⁴Ba + ⁸⁹Kr + 3¹₀n + energy

Notice that the reaction produces three new neutrons. This is critical because those neutrons can go on to trigger further fission events in other uranium-235 nuclei, creating a chain reaction. The energy released comes from a small loss of total mass. The total mass of the products is slightly less than the total mass of the reactants. According to Einstein’s equation E = mc², this mass deficit is converted into a large amount of energy.

注意反应产生了三个新中子。这一点至关重要，因为这些中子可以继续触发其他铀-235 核发生裂变，形成链式反应。释放的能量来源于总质量的微小亏损。生成物的总质量略小于反应物的总质量。根据爱因斯坦方程 E = mc²，这部分质量亏损转化为大量能量。

A single fission event releases roughly 200 MeV (million electron volts) of energy. Although this seems small for one nucleus, consider this: one kilogram of uranium-235 contains about 2.5 × 10²⁴ atoms. Fissioning all of them releases millions of times more energy than burning one kilogram of coal. This explains why nuclear fuel is so energy-dense.

单次裂变释放约 200 MeV（百万电子伏特）能量。对于单个原子核来说这似乎微不足道，但请想一下：一千克铀-235 含有约 2.5 × 10²⁴ 个原子。将它们全部裂变所释放的能量是燃烧一千克煤的数百万倍。这就是核燃料能量密度如此之高的原因。

4. Chain Reactions | 链式反应

A chain reaction occurs when the neutrons released by one fission event cause further fission events, each of which releases more neutrons, and so on. Since every fission event releases two or three neutrons, and each of those neutrons can cause one more fission, the number of fissions grows extremely quickly. In an uncontrolled chain reaction, the energy released multiplies in a fraction of a second, resulting in a nuclear explosion.

链式反应是指一次裂变事件释放的中子引发更多裂变事件，而每次都释放更多中子，以此类推的过程。由于每次裂变释放两到三个中子，而每个中子又可以引发另一次裂变，裂变次数增长极快。在不受控的链式反应中，能量在不到一秒的时间内倍增至巨大规模，最终导致核爆炸。

For a chain reaction to be sustained, at least one neutron from each fission must go on to cause another fission. The minimum mass of fissile material needed to sustain a chain reaction is called the critical mass. If the mass is below critical, the chain reaction dies out because too many neutrons escape from the surface before finding another nucleus. For a controlled chain reaction, as used in power stations, we insert materials that absorb excess neutrons so that on average exactly one neutron per fission continues the chain.

链式反应得以持续的条件是：每次裂变产生的至少一个中子必须继续引发另一次裂变。维持链式反应所需的最小裂变材料质量称为临界质量。如果质量低于临界值，链式反应就会终止，因为有太多中子在找到另一个原子核之前从表面逃逸。对于发电站中使用的受控链式反应，我们插入吸收多余中子的材料，使每次裂变平均恰好有一个中子延续链式反应。

5. Controlling Fission in a Nuclear Reactor | 核反应堆中裂变的控制

In a nuclear power station, the fission chain reaction is carefully controlled to produce a steady output of heat. Three essential components work together to achieve this:

在核电站中，裂变链式反应受到严格控制，以产生稳定的热量输出。三个关键部件协同完成这一任务：

Moderator (graphite or water): slows down the fast neutrons produced during fission. Slow-moving (thermal) neutrons are far more likely to be captured by uranium-235 nuclei than fast neutrons, so the moderator helps sustain the chain reaction.
Control rods (boron or cadmium): absorb neutrons. By inserting the control rods deeper into the core, more neutrons are absorbed and the reaction rate decreases. Pulling them out increases the reaction rate.
Coolant (water or carbon dioxide): removes heat from the reactor core so that a steam generator can produce electricity, and prevents the core from overheating.

慢化剂（石墨或水）：使裂变产生的快中子减速。慢速（热）中子被铀-235 核捕获的概率远比快中子高，因此慢化剂有助于维持链式反应。
控制棒（硼或镉）：吸收中子。将控制棒向堆芯插入更深，就能吸收更多中子，从而降低反应速率。拔出控制棒则提高反应速率。
冷却剂（水或二氧化碳）：从反应堆堆芯带走热量，使蒸汽发生器能够发电，并防止堆芯过热。

In a pressurised water reactor, the fission heat boils water into steam, which spins a turbine connected to a generator. The generator converts the kinetic energy of the turbine into electrical energy. This same basic principle links nuclear physics to the practical generation of electricity. You may also be asked to describe the energy transformations: nuclear energy → thermal energy → kinetic energy → electrical energy.

在压水堆中，裂变热量将水加热成蒸汽，蒸汽推动连接发电机的涡轮旋转。发电机将涡轮的动能转化为电能。这一基本原理解释了核物理与实用发电之间的联系。考试中你也可能被要求描述能量转化链：核能 → 热能 → 动能 → 电能。

6. What Is Nuclear Fusion? | 什么是核聚变？

Nuclear fusion is the process in which two small, light nuclei join together to form a single larger nucleus. This process is the opposite of fission. Fusion releases energy because the combined nucleus has a smaller mass than the sum of the two original nuclei, and this mass difference is converted into energy according to E = mc².

核聚变是两个小的轻原子核结合形成一个更大的原子核的过程。这一过程与裂变相反。聚变释放能量的原因是：结合后的原子核质量小于两个原始原子核质量之和，这部分质量差根据 E = mc² 转化为能量。

The most important fusion reaction for energy production involves the hydrogen isotopes deuterium (²H) and tritium (³H):

对能源生产而言，最重要的聚变反应涉及氢的同位素氘（²H）和氚（³H）：

²H + ³H → ⁴He + ¹₀n + energy

Fusion requires extraordinarily high temperatures of around 100 million °C. At these temperatures, atoms are stripped of their electrons and exist as a plasma. The nuclei must travel fast enough to overcome the electrostatic repulsion between their positive charges, and get close enough for the strong nuclear force to bind them together. This is why fusion is extremely difficult to achieve on Earth. No material container can survive at 100 million degrees, so scientists use powerful magnetic fields to confine the plasma in devices called tokamaks.

聚变需要大约 1 亿摄氏度的极高温度。在此温度下，原子被剥离电子，以等离子体状态存在。原子核必须以足够快的速度运动，以克服正电荷之间的静电斥力，并靠得足够近，使强核力能将它们束缚在一起。这就是为什么在地球上实现聚变极其困难。没有任何材料容器能

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Fission & Fusion | 核裂变与核聚变