核裂变与核聚变
物理· 7.17–7.26，第7节 放射性与粒子· 12 分钟阅读
1. 核裂变基础★★☆☆☆⏱ 3 min
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核裂变
重的不稳定原子核吸收慢中子后发生分裂，生成两个更小的放射性子核，裂变产物的动能会以能量形式释放，同时产生2-3个快中子。
例:
Uranium-235 is the most common fuel used in nuclear fission reactors.
包括裂变、聚变和放射性衰变在内的所有核反应都是巨大的能量来源，单位质量释放的能量远高于燃烧化石燃料这类化学反应。当一个慢（热）中子撞击铀-235原子核时，会被原子核吸收，使其变得极不稳定。这个不稳定的原子核几乎会立刻分裂为两个质量大致相等的小子核，同时产生2到3个高速中子。所有裂变产物都具有很高的动能，这些动能会在反应堆堆芯中转化为热能，用于发电。
写出铀-235吸收一个中子后发生裂变，生成钡-141、氪-92和3个中子的配平核反应方程式。
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步骤1：写出左侧反应物：铀-235加一个中子
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步骤2：写出右侧产物：钡-141、氪-92和3个中子
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步骤3：检查两侧的质量数（上方数值）和质子数（下方数值）是否守恒
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左侧质量数：235 + 1 = 236；右侧：141 + 92 + 3 = 236 ✔️
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左侧质子数：92 + 0 = 92；右侧：56 + 36 + 0 = 92 ✔️
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最终配平的方程式：
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Exam tip:
书写核裂变方程式时务必同时检查质量数与质子数是否守恒，正确的符号标注与配平均可获得分数。
2. 链式反应与核反应堆组件★★★☆☆⏱ 4 min
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链式反应
自持反应：单次裂变事件释放的中子撞击其他铀-235原子核，触发更多裂变事件，释放更多中子使反应持续进行。
如果每次裂变事件释放的中子中至少有一个可以触发下一次裂变，反应就可以自持。如果有超过一个中子可以触发裂变，反应速率就会指数级上升，原子弹就是基于这个原理工作的。在核反应堆中，反应会被精准控制，维持稳定的速率：每次裂变恰好有一个中子可以触发下一次裂变。
Moderator: Typically made of graphite or water, slows fast neutrons released from fission to thermal speeds so they are easily absorbed by U-235 nuclei (fast neutrons bounce off U-235 without being absorbed).
Control rods: Made of boron or cadmium, absorb excess neutrons. Lowering the rods further into the reactor core absorbs more neutrons, slowing or stopping the reaction; raising them increases the reaction rate.
Shielding: Thick layers of lead and concrete surrounding the reactor core absorb alpha, beta and gamma radiation released during fission, protecting plant workers and the surrounding public from harmful radiation exposure.
核反应堆操作员发现裂变反应速率的增长速度超过了安全限值。请说明操作员应采取的措施，并解释该操作如何校正反应速率。
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步骤1：说明操作：将控制棒更深地插入反应堆堆芯中。
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步骤2：解释作用机制：控制棒由硼等中子吸收材料制成。将其插入更深会增加它们吸收的过剩中子数量，减少可触发后续裂变事件的中子数，从而将反应速率减慢回安全的稳态水平。
Exam tip:
不要混淆慢化剂和控制棒的功能：慢化剂的作用是减速中子，控制棒的作用是吸收中子。如果将二者的作用搞混会丢失分数。
3. 核聚变基础★★★☆☆⏱ 3 min
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核聚变
两个小的轻原子核结合形成单个更大更重的原子核的过程。该过程中会损失少量质量，这些质量会转化为极其巨大的能量。
例:
Fusion of hydrogen isotopes deuterium and tritium is the energy source for all stars including our Sun.
单位质量下核聚变释放的能量远高于核裂变，且产生的长寿命放射性废物少得多，因此它正被研究作为未来的清洁能源。然而，核聚变无法在常温常压下发生，因为所有原子核都因质子而带正电，会在静电力作用下相互排斥。
尽管存在技术挑战，科学家仍在研究核聚变作为发电的裂变替代方案，请解释其原因。
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步骤1：说明核聚变相较于核裂变的优势：
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- 核聚变燃料（氢的同位素）在海水中储量丰富，而用于裂变的铀235是有限的不可再生资源。
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- 核聚变几乎不产生长寿命放射性废物，不同于核裂变会产生高放射性废物，需要安全储存数千年。
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- 核聚变不存在灾难性熔毁的风险，因为如果条件无法维持，反应会立即停止，不像裂变链式反应如果管理不当会失控。
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步骤2：说明挑战：在地球上复刻维持持续核聚变所需的极端温度和压力难度极大、成本极高，因此商用聚变反应堆目前尚未问世。
4. 核裂变与核聚变的对比★★☆☆☆⏱ 2 min
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考试中你经常会被要求说明核裂变和核聚变的关键区别，因此请确保你能清晰区分这两个过程。
Feature | Nuclear Fission | Nuclear Fusion |
|---|---|---|
Process | Splits large heavy nuclei | Joins small light nuclei |
Fuel used | Uranium-235 (or plutonium) | Hydrogen isotopes (deuterium, tritium) |
Natural occurrence | Does not occur naturally on Earth | Occurs naturally in star cores |
Waste product | Highly radioactive long-lived waste | Very little low-level radioactive waste |
Reaction conditions | Room temperature/pressure, requires slow neutrons | Extremely high temperature and pressure required |
Energy released per kg | Very large | ~4x larger than fission |
Which process powers the Sun?
Fission
Fusion
Radioactive decay
Chemical combustion
显示答案
Fusion —Correct! Fusion of hydrogen nuclei into helium in the Sun's core releases the energy that reaches Earth as light and heat.
What is the role of a moderator in a nuclear reactor?
Absorb excess neutrons
Slow down fast neutrons
Shield workers from radiation
Heat water to make steam
显示答案
Slow down fast neutrons —Correct! Moderators like graphite or water slow neutrons so they can be absorbed by U-235 nuclei to trigger fission.
5. 常见陷阱
错误做法:
Stating that moderators absorb neutrons
原因:
Confusing the role of moderators and control rods. Examiners regularly test this distinction, and mixing the two loses marks.
正确做法:
Remember: Moderators slow neutrons, control rods absorb neutrons. Use the mnemonic 'Slow Mo, Absorb Rod' to recall.
错误做法:
Saying fusion only requires high temperature, not pressure
原因:
Both high temperature and pressure are needed: temperature gives nuclei speed to overcome repulsion, pressure forces nuclei close enough to collide. Missing either loses a mark.
正确做法:
Always state both very high temperature AND pressure are required for fusion to occur.
错误做法:
Balancing only mass number, not proton number, in fission equations
原因:
Marks are awarded for both correct mass number and proton number balance, as well as correct particle notation.
正确做法:
Check both top (mass number) and bottom (proton number) values add up to the same total on both sides of the equation before finalizing your answer.
错误做法:
Stating that fusion occurs naturally on Earth
原因:
There are no natural conditions on Earth with high enough temperature and pressure to sustain fusion, unlike star cores.
正确做法:
Neither occurs naturally on Earth under normal conditions — fusion occurs naturally in stars, and on Earth fission happens only in engineered reactors.
错误做法:
Claiming chain reactions occur in fusion reactors
原因:
Chain reactions are specific to fission, as they rely on neutrons released from fission triggering further reactions. Fusion does not produce neutrons in a way that creates a self-sustaining chain reaction the same way.
正确做法:
Only associate chain reactions with nuclear fission, not fusion.
6. 速查表
Key Concept | Quick Facts |
|---|---|
Fission of U-235 | Trigger: slow neutron; Products: 2 daughter nuclei + 2-3 neutrons + energy |
Chain reaction | 1+ neutron per fission triggers another fission; controlled in reactors, uncontrolled in bombs |
Moderator | Graphite/water: slows neutrons for U-235 absorption |
Control rods | Boron/cadmium: absorb neutrons to control reaction rate |
Shielding | Thick lead/concrete: absorbs radiation to protect workers |
Fusion | Joins light nuclei; releases more energy than fission; needs high T + pressure to overcome proton repulsion |
Fusion in stars | Sun fuses H to He; gravity provides required pressure/temperature |
Fission vs Fusion | Fission = split heavy nuclei; Fusion = join light nuclei |
7. 常见问题
核裂变和核聚变的区别是什么？
核裂变将大质量重核（例如铀-235）分裂为更小的碎片，而核聚变将小质量轻核（例如氢的同位素）结合为更大的原子核。二者都会释放大量能量。
为什么核聚变无法在室温下发生？
所有原子核都因内部的质子带正电，因此彼此之间存在静电斥力。极高的温度（数百万摄氏度）和压强可以为原子核提供足够的动能，克服这种斥力，使它们碰撞到足够近的距离，让强核力将它们结合在一起。
深入阅读
下一步
Now that you have mastered nuclear fission and fusion for Edexcel IGCSE Physics, you can connect this knowledge to other parts of the syllabus to build a complete understanding. This content directly links to energy resources in Unit 4, where you will compare nuclear power to other electricity generation methods, and to the astronomy unit (Unit 8) where you learn about stellar life cycles powered by fusion. You should also practice answering past paper questions on this topic to familiarize yourself with exam phrasing and mark scheme expectations, as questions on reactor components and fusion conditions are very common. Make sure you can confidently balance fission nuclear equations and clearly distinguish between fission and fusion processes before moving on to more advanced topics.