Edexcel A-Level Combined Science: Nuclear Fission and Fusion | 爱德思 A-Level 综合科学：核裂变与核聚变

📚 Edexcel A-Level Combined Science: Nuclear Fission and Fusion | 爱德思 A-Level 综合科学：核裂变与核聚变

Nuclear fission and nuclear fusion are two contrasting processes that release energy from atomic nuclei. Fission splits a heavy nucleus into smaller fragments, while fusion joins light nuclei together. Both are central to Edexcel A-Level Combined Science, linking nuclear physics, energy calculations, and real-world applications.

核裂变和核聚变是从原子核释放能量的两种相反过程。裂变使重核分裂成较小的碎片，而聚变使轻核结合。两者都是爱德思 A-Level 综合科学的核心内容，将核物理、能量计算和实际应用联系起来。

1. The Nuclear Landscape: Nuclides and Binding Energy | 核素与结合能

A nuclide is a species of atom characterised by its proton number Z and nucleon number A. The nucleus contains Z protons and (A − Z) neutrons, held together by the strong nuclear force. The total mass of a nucleus is always slightly less than the sum of the masses of its individual protons and neutrons.

核素是由质子数 Z 和核子数 A 表征的原子种类。原子核含有 Z 个质子和 (A − Z) 个中子，由强核力维系。原子核的总质量总是略小于其各个质子和中子质量之和。

This missing mass is called the mass defect. The energy equivalent of the mass defect is the binding energy of the nucleus. Binding energy is the minimum energy needed to separate a nucleus into its individual nucleons.

这个缺失的质量称为质量亏损。质量亏损的能量当量就是原子核的结合能。结合能是将原子核分离成单个核子所需的最小能量。

2. Mass Defect and E=mc² | 质量亏损与质能方程

Einstein’s mass–energy equivalence is written as:

爱因斯坦的质能等价方程可写为：

E = mc²

For nuclear changes, the energy released ΔE is related to the mass defect Δm by:

对于核变化，释放的能量 ΔE 与质量亏损 Δm 的关系为：

ΔE = Δm c²

If mass is measured in kilograms and c = 3.00 × 10⁸ m s⁻¹, then ΔE is in joules. In nuclear physics, mass is often given in atomic mass units u, where 1 u = 1.66 × 10⁻²⁷ kg and 1 u is equivalent to 931.5 MeV of energy.

如果质量以千克为单位，c = 3.00 × 10⁸ m s⁻¹，则 ΔE 的单位为焦耳。在核物理中，质量常以原子质量单位 u 给出，其中 1 u = 1.66 × 10⁻²⁷ kg，1 u 相当于 931.5 MeV 的能量。

3. Nuclear Fission: Splitting Heavy Nuclei | 核裂变：重核分裂

Nuclear fission occurs when a heavy nucleus, such as uranium-235, absorbs a thermal neutron and becomes unstable. The nucleus then splits into two smaller daughter nuclei, releasing two or three neutrons and a large amount of energy.

核裂变发生在重核（如铀-235）吸收一个热中子并变得不稳定时。随后原子核分裂成两个较小的子核，释放出两到三个中子以及大量能量。

A typical fission equation for uranium-235 is:

铀-235 的典型裂变方程为：

²³⁵U₉₂ + ¹n₀ → ¹⁴¹Ba₅₆ + ⁹²Kr₃₆ + 3¹n₀

The products are not fixed; many different pairs of daughter nuclei are possible. However, in every fission event, the total proton number and total nucleon number are conserved.

产物并不是固定的；可能存在多种不同的子核组合。然而，在每次裂变事件中，总质子数和总核子数都是守恒的。

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

Each fission event releases neutrons that can go on to cause further fission events in nearby uranium nuclei. This self-sustaining process is called a chain reaction. If each fission produces on average one further fission, the reaction is controlled; if more than one, it is supercritical.

每次裂变事件释放出的中子可以继续引发附近铀核的进一步裂变。这个自我维持的过程称为链式反应。如果每次裂变平均产生一次后续裂变，则反应是受控的；如果多于一次，则处于超临界状态。

The minimum mass of fissile material required to sustain a chain reaction is called the critical mass. Below this mass, too many neutrons escape without causing further fission, and the reaction dies out.

维持链式反应所需的可裂变材料的最小质量称为临界质量。低于这一质量时，过多中子逃逸而不能引发进一步裂变，反应便会停止。

5. Nuclear Reactors: Control and Safety | 核反应堆：控制与安全

In a nuclear reactor, the fission rate is controlled using control rods made of materials such as boron or cadmium, which absorb neutrons. Lowering the rods further into the core reduces the number of neutrons available and slows the reaction.

在核反应堆中，通过使用由硼或镉等材料制成的控制棒来控制裂变速率，这些控制棒能吸收中子。将控制棒进一步插入堆芯会减少可用中子的数量，从而减慢反应。

A moderator, often water or graphite, slows down fast neutrons so they are more likely to cause further fission of uranium-235. The heat produced by fission is removed by a coolant and used to produce steam that drives turbines.

慢化剂（通常是水或石墨）使快中子减速，从而更容易引发铀-235 的进一步裂变。裂变产生的热量由冷却剂带走，并用于产生蒸汽来驱动涡轮机。

Control rods absorb neutrons to regulate power output.
控制棒吸收中子以调节功率输出。
Moderator slows neutrons to increase fission probability.
慢化剂使中子减速以提高裂变概率。
Coolant transfers thermal energy to the generator.
冷却剂将热能传递给发电机。

6. Nuclear Fusion: Joining Light Nuclei | 核聚变：轻核结合

Nuclear fusion is the process in which two light nuclei combine to form a heavier nucleus. The mass of the product nucleus is less than the total mass of the reactants, so energy is released according to ΔE = Δm c².

核聚变是两个轻核结合形成一个较重核的过程。产物核的质量小于反应物的总质量，因此根据 ΔE = Δm c² 释放能量。

A common fusion reaction involves deuterium and tritium, two isotopes of hydrogen:

一个常见的聚变反应涉及氘和氚，这两种氢的同位素：

²H₁ + ³H₁ → ⁴He₂ + ¹n₀

This reaction releases about 17.6 MeV of energy per event. Fusion is the source of energy in stars, including the Sun.

该反应每次事件释放约 17.6 MeV 的能量。聚变是包括太阳在内的恒星的能量来源。

7. Conditions for Fusion: Temperature and Pressure | 聚变条件：高温与高压

For fusion to occur, two positively charged nuclei must overcome their electrostatic repulsion. This requires an extremely high temperature, typically tens of millions of kelvin, so that nuclei have enough kinetic energy to collide.

要发生聚变，两个带正电的原子核必须克服它们之间的静电排斥力。这需要极高的温度，通常为数千万开尔文，以便原子核有足够的动能发生碰撞。

High pressure is also required to bring nuclei sufficiently close together for the strong nuclear force to take over. In stars, gravitational confinement provides this pressure; on Earth, magnetic or inertial confinement is used in experimental reactors.

还需要高压使原子核足够靠近，以便强核力发挥作用。在恒星中，引力约束提供了这种压力；在地球上，实验反应堆采用磁约束或惯性约束。

Very high temperature gives nuclei enough kinetic energy.
极高的温度使原子核具有足够的动能。
Very high pressure forces nuclei close together.
极高的压力迫使原子核彼此靠近。

8. Fusion in Stars and Nucleosynthesis | 恒星中的聚变与核合成

In the cores of main-sequence stars, hydrogen nuclei fuse to form helium through a series of reactions known as the proton–proton chain. This releases energy that maintains the star’s internal pressure against gravitational collapse.

在主序星的核心，氢核通过一系列称为质子-质子链的反应聚变形成氦。释放出的能量维持了恒星内部压力，抵抗引力坍缩。

In more massive stars, heavier elements are produced by successive fusion processes, a process called nucleosynthesis. Elements up to iron-56 can be formed in stellar cores, as iron has the highest binding energy per nucleon.

在质量更大的恒星中，较重的元素通过连续的聚变过程产生，这一过程称为核合成。恒星核心中可以形成直到铁-56 的元素，因为铁具有最高的比结合能。

9. Comparing Fission and Fusion | 裂变与聚变的比较

Fission and fusion both release nuclear energy, but they operate at opposite ends of the binding energy curve. Fission uses heavy nuclei such as uranium, while fusion uses light nuclei such as hydrogen isotopes.

裂变和聚变都释放核能，但它们在结合能曲线的两端运作。裂变使用铀等重核，而聚变使用氢同位素等轻核。

Feature	Fission	Fusion
Starting nucleus	Heavy, e.g. U-235	Light, e.g. H-2 and H-3
Key condition	Neutron absorption	Very high temperature and pressure
Typical products	Two medium nuclei and neutrons	One heavier nucleus and a neutron
Energy per event	About 200 MeV	About 17.6 MeV
Waste products	Radioactive fission fragments	Helium, low radioactivity

Although fusion releases less energy per reaction than fission, it releases far more energy per unit mass of fuel.

尽管每次聚变反应释放的能量少于裂变，但单位质量的燃料释放的能量要多得多。

10. Energy Calculations and Binding Energy per Nucleon | 能量计算与比结合能

Binding energy per nucleon is the total binding energy of a nucleus divided by its nucleon number A. It is a measure of nuclear stability; a higher value means a more stable nucleus.

比结合能是原子核总结合能除以其核子数 A。它是核稳定性的量度；数值越高，原子核越稳定。

Binding energy per nucleon = Total binding energy ÷ A

Fission and fusion both increase the average binding energy per nucleon of the products compared with the reactants. This is why energy is released in both processes.

裂变和聚变都使产物相对于反应物的平均比结合能增加。这就是两种过程都释放能量的原因。

Example calculation: If a deuterium nucleus has a mass defect of 0.002388 u, its binding energy is:

计算示例：如果氘核的质量亏损为 0.002388 u，其结合能为：

E = 0.002388 × 931.5 = 2.224 MeV

Since A = 2, the binding energy per nucleon is 1.112 MeV per nucleon.

由于 A = 2，比结合能为每核子 1.112 MeV。

11. Environmental and Ethical Issues | 环境与伦理问题

Nuclear fission produces long-lived radioactive waste, which must be stored safely for thousands of years. Accidents, such as those at Chernobyl and Fukushima, highlight the risks of reactor failure and radiation release.

核裂变产生长寿命的放射性废物，必须安全储存数千年。切尔诺贝利和福岛等事故凸显了反应堆故障和放射性释放的风险。

Fusion produces much less radioactive waste and cannot undergo a runaway chain reaction. However, achieving controlled fusion on Earth remains a major scientific and engineering challenge.

聚变产生的放射性废物少得多，且不会发生失控的链式反应。然而，在地球上实现受控聚变仍然是一个重大的科学和工程挑战。

Fission: radioactive waste, reactor safety, nuclear proliferation concerns.
裂变：放射性废物、反应堆安全、核扩散担忧。
Fusion: cleaner in principle, but not yet commercially viable.
聚变：原则上更清洁，但尚不具备商业可行性。

12. Exam Skills: Graphs and Data Analysis | 考试技能：图表与数据分析

Edexcel exams often provide a graph of binding energy per nucleon against nucleon number. You need to identify the most stable nucleus as iron-56, near the peak of the curve, and use the graph to explain why fission and fusion release energy.

爱德思考试经常给出比结合能对核子数的图像。你需要识别最稳定的核是铁-56，位于曲线峰值附近，并利用该图解释裂变和聚变为什么释放能量。

When reading data on mass defect or energy released, always check units. Convert atomic mass units to kilograms or MeV using the given conversion, and show all steps in calculations.

在读取质量亏损或释放能量的数据时，务必检查单位。使用给定的换算将原子质量单位转换为千克或 MeV，并在计算中展示所有步骤。

In written answers, link the sign of Δm to energy release. A decrease in total mass means energy is released to the surroundings, so the reaction is exothermic in the nuclear sense.

在书面回答中，将 Δm 的符号与能量释放联系起来。总质量减少意味着能量释放到周围环境中，因此该反应在核意义上是放热的。

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Edexcel A-Level Combined Science: Nuclear Fission and Fusion | 爱德思 A-Level 综合科学：核裂变与核聚变