Fission and Fusion | 核裂变与核聚变

📚 Fission and Fusion | 核裂变与核聚变

Nuclear fission and fusion are two fundamental ways of releasing energy stored in atomic nuclei. Fission splits a heavy nucleus into smaller fragments, while fusion combines light nuclei to form a heavier one. Both processes convert a tiny fraction of mass into enormous amounts of energy, as described by Einstein’s mass–energy equation. Understanding these reactions is central to IB Physics, explaining everything from stars to nuclear power plants.

核裂变与核聚变是释放原子核中储存能量的两种基本方式。裂变使重核分裂成较小的碎片，而聚变则将轻核结合形成较重的核。两个过程都将极小的质量转化为巨大的能量，正如爱因斯坦质能方程所描述。理解这些反应是IB物理的核心，可以解释从恒星到核电站的种种现象。

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1. Nuclear Binding and the Mass Defect | 原子核结合能与质量亏损

The mass of any stable nucleus is always less than the sum of the masses of its individual protons and neutrons. This missing mass is called the mass defect (Δm), and it represents the energy required to disassemble the nucleus into its separate nucleons. The corresponding binding energy is given by Einstein’s equation.

任何稳定原子核的质量总小于其各个质子和中子质量之和。这种缺失的质量称为质量亏损 (Δm)，它代表了将原子核分解成单个核子所需的能量。对应的结合能由爱因斯坦方程给出。

E_binding = Δm c²

The binding energy per nucleon is found by dividing the total binding energy by the number of nucleons. This value peaks around iron-56, making it the most stable nucleus. Nuclei heavier than iron can release energy by splitting, while lighter ones can release energy by fusing.

每个核子的结合能由总结合能除以核子数得到。该值在铁-56附近达到峰值，使其成为最稳定的原子核。比铁重的原子核可以通过分裂释放能量，而比铁轻的则可以通过聚变释放能量。

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2. The Binding Energy per Nucleon Curve | 核子平均结合能曲线

Plotting binding energy per nucleon against mass number yields a curve that rises sharply for light nuclei, reaches a broad maximum near iron (A ≈ 56), and then gradually decreases for heavier nuclei. This curve explains why both fission and fusion are energetically favourable under different conditions.

将每个核子的结合能按质量数绘图，可以得到一条曲线：轻核区急剧上升，在铁附近（A ≈ 56）达到宽而平的峰值，然后对重核逐渐下降。这条曲线解释了为什么在不同条件下裂变和聚变在能量上都是有利的。

Fission of uranium-235, for example, produces lighter fragments with higher binding energy per nucleon, so energy is released. Similarly, fusion of deuterium and tritium produces helium-4, which sits higher on the curve.

例如，铀-235裂变产生具有更高结合能每核子的较轻碎片，因此释放能量。同样，氘和氚的聚变产生氦-4，其在曲线上位置更高。

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3. Nuclear Fission: Mechanism and Discovery | 核裂变：机理与发现

Fission occurs when a heavy nucleus, such as uranium-235 or plutonium-239, captures a neutron and becomes unstable. The nucleus then splits into two smaller fission fragments, releasing two or three free neutrons and a substantial amount of energy, typically around 200 MeV per event.

当铀-235或钚-239等重核俘获一个中子后变得不稳定，就会发生裂变。原子核随后分裂成两个较小的裂变碎片，放出两到三个自由中子并释放大量能量，每次裂变通常约200 MeV。

²³âµâ‚‰â‚‚U + ¹₀n → ¹â´Â¹â‚…₆Ba + â¹Â²â‚ƒâ‚†Kr + 3¹₀n + energy

The emitted neutrons can trigger further fissions, making a sustained chain reaction possible. The energy appears as kinetic energy of the fragments and neutrons, as well as gamma radiation.

释放出的中子可以引发进一步的裂变，使持续的链式反应成为可能。能量以碎片和中子的动能以及伽马辐射的形式出现。

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4. Chain Reactions and Critical Mass | 链式反应与临界质量

A self-sustaining chain reaction requires that at least one neutron from each fission goes on to induce another fission. The multiplication factor k describes the ratio of neutrons in successive generations; k = 1 corresponds to a steady, controlled reaction.

自持链式反应要求每次裂变至少有一个中子继续引发另一次裂变。倍增因子k描述代际中子的比率；k = 1对应于稳定、受控的反应。

Critical mass is the minimum amount of fissile material needed to maintain a chain reaction. Below this mass, too many neutrons escape without causing fission. In a nuclear weapon, a prompt supercritical state is achieved; in a reactor, the reaction is carefully controlled to remain around criticality.

临界质量是维持链式反应所需裂变材料的最小量。低于该质量，太多中子逸出而不会引发裂变。在核武器中，会达到瞬发超临界状态；在反应堆中，反应被小心控制在临界附近。

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5. Components of a Fission Reactor | 裂变反应堆的组成部分

A thermal nuclear reactor uses controlled fission to produce heat. The core contains fuel rods (enriched uranium oxide), a moderator, control rods and coolant. The moderator slows down fast neutrons so that they are more likely to be captured by uranium-235 nuclei.

热中子反应堆利用受控裂变产生热量。堆芯包含燃料棒（浓缩二氧化铀）、慢化剂、控制棒和冷却剂。慢化剂减慢快中子速度，使它们更可能被铀-235核俘获。

Control rods made of neutron-absorbing materials such as boron or cadmium are inserted or withdrawn to adjust the reaction rate. The coolant, often water or carbon dioxide, transfers heat away from the core to generate steam that drives a turbine-generator set.

由硼或镉等中子吸收材料制成的控制棒可插入或抽出以调节反应速率。冷却剂（通常是水或二氧化碳）将热量从堆芯带走，产生蒸汽驱动涡轮发电机组。

Component	Function
Fuel rods	Provide fissile nuclei (U-235)
Moderator	Slows neutrons (water, graphite)
Control rods	Absorb neutrons to control rate
Coolant	Removes heat (water, COâ‚‚, liquid sodium)

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6. Nuclear Fusion: Fundamental Process | 核聚变：基本过程

Fusion is the combining of light nuclei to form a heavier, more stable nucleus. Because the product has a higher binding energy per nucleon, energy is liberated. The most accessible reaction on Earth uses deuterium (²H) and tritium (³H).

聚变是将轻核结合形成更重、更稳定核的过程。由于产物的每个核子结合能更高，因此释放能量。地球上最容易实现的反应使用氘 (²H) 和氚 (³H)。

²â‚H + ³â‚H → â´â‚‚He + ¹₀n + 17.6 MeV

For fusion to occur, the positively charged nuclei must overcome their electrostatic repulsion. This requires extremely high temperatures (on the order of 10⸠K) to give the nuclei enough kinetic energy, creating a plasma state where electrons and nuclei are separated.

要使聚变发生，带正电的原子核必须克服静电排斥。这需要极高的温度（约10⸠K量级）赋予核足够的动能，形成电子与核分离的等离子体状态。

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7. Fusion in Stars: The Proton–Proton Chain | 恒星中的聚变：质子-质子链

Stars generate energy through fusion. In the Sun and similar stars, the dominant process is the proton–proton (p-p) chain. It starts with the fusion of two protons to form deuterium, then proceeds through a series of steps to produce helium-4 and release energy.

恒星通过聚变产生能量。在太阳和类似恒星中，主要过程是质子-质子 (p-p) 链。它从两个质子聚变成氘开始，然后通过一系列步骤产生氦-4并释放能量。

The net reaction converts four protons into one helium-4 nucleus, two positrons and two neutrinos. The extremely high pressure and temperature in the stellar core provide the necessary conditions for these reactions to occur at a sufficient rate.

净反应将四个质子转化为一个氦-4核、两个正电子和两个中微子。恒星核心极高的压强和温度为这些反应以足够的速率发生提供了必要条件。

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8. Conditions for Terrestrial Fusion and Confinement | 地球上的聚变条件与约束

To achieve practical fusion on Earth, a plasma must be heated to temperatures exceeding 100 million kelvin and confined long enough at a sufficient density. The Lawson criterion sets a required product of ion density and energy confinement time for net energy gain.

要在地球上实现实用的聚变，必须将等离子体加热到超过1亿开尔文，并在足够密度下约束足够长时间。劳逊判据规定了净能量增益所需的离子密度与能量约束时间的乘积。

Magnetic confinement uses strong magnetic fields to trap the charged plasma; the tokamak design is the most advanced, employing a toroidal chamber with helical magnetic fields. Inertial confinement uses intense laser or ion beams to compress and heat a small fuel pellet rapidly.

磁约束使用强磁场来禁锢带电等离子体；托卡马克设计是最先进的，采用环形腔室和螺旋磁场。惯性约束使用强激光或离子束快速压缩并加热小型燃料丸。

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9. Comparing Energy Output: Fission vs Fusion | 能量产出比较：裂变与聚变

Although both processes release enormous energy compared to chemical reactions, fusion releases more energy per unit mass of fuel. For equal masses, the deuterium–tritium fusion reaction yields roughly four times the energy of uranium fission.

虽然两种过程释放的能量远大于化学反应，但聚变每单位质量燃料释放的能量更多。对于相同质量，氘-氚聚变反应产生的能量大约是铀裂变的四倍。

Fission reactors produce long-lived radioactive waste that must be carefully stored. Fusion, in contrast, generates mainly helium and short-lived activation products, with no high-level long-lived waste. However, fusion reactors face immense engineering challenges and are still under development.

裂变反应堆产生必须谨慎储存的长寿命放射性废物。相比之下，聚变主要产生氦和短寿命活化产物，没有高放长寿命废物。但聚变反应堆面临巨大工程挑战，仍在开发中。

Aspect	Fission	Fusion
Fuel	U-235, Pu-239	Deuterium, tritium
Energy per kg of fuel	~80 million MJ	~300 million MJ
Waste	Long-lived high-level waste	Short-lived, low-level waste
Current status	Commercial power plants	Experimental reactors (ITER, JET)

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10. Safety, Risks and Waste Management | 安全、风险与废物管理

The risks of fission include reactor meltdown, as seen in Chernobyl and Fukushima, and the proliferation of fissile material. Spent fuel remains dangerously radioactive for thousands of years and requires deep geological disposal.

裂变的风险包括反应堆熔毁，如切尔诺贝利和福岛事故，以及裂变材料扩散。乏燃料在数千年内仍具有危险放射性，需要深地质处置。

Fusion reactors inherently have a lower risk of catastrophic failure: any loss of confinement instantly cools the plasma and stops the reaction. Tritium handling and neutron activation of the reactor structure are the main safety concerns, but do not produce long-lived actinide waste.

聚变反应堆本身具有较低的灾难性故障风险：任何约束失效都会瞬间冷却等离子体并停止反应。氚的处理和反应堆结构的中子活化是主要的安全问题，但不会产生长寿命锕系废物。

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11. The Future of Fusion Energy | 聚变能的未来

Projects like ITER (International Thermonuclear Experimental Reactor) aim to demonstrate that fusion can produce more energy than is put in, with Q > 10 (energy gain factor). Achieving practical, commercial fusion power would provide virtually limitless clean energy, fuelled by deuterium from seawater and lithium for tritium breeding.

像ITER（国际热核聚变实验反应堆）这样的项目旨在证明聚变能够产生比输入更多的能量，Q > 10（能量增益因子）。实现实用的商业聚变能几乎可以提供无限的清洁能源，其燃料来自海水的氘和用于产氚的锂。

While significant scientific and engineering hurdles remain, progress in superconducting magnets, plasma control, and materials science is bringing the goal closer. For IB Physics students, fusion exemplifies the intersection of nuclear physics, thermodynamics and cutting-edge technology.

尽管仍然存在巨大的科学和工程障碍，但超导磁体、等离子体控制和材料科学的进步正在拉近这一目标。对IB物理学生而言，聚变体现了核物理、热力学和前沿技术的交汇。

Published by TutorHao | Physics Revision Series | aleveler.com

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