📚 IB Physics: Nuclear Fission Principles & Reactions in Nuclear Power Plants | IB物理：核裂变原理与核电站中的反应
Nuclear fission is the process by which a heavy nucleus splits into two lighter nuclei, releasing a tremendous amount of energy. In this article, we explore the physics of fission, the chain reaction, and how this energy is harnessed in a nuclear power plant—an essential topic for the IB Physics curriculum.
核裂变是指重原子核分裂为两个较轻原子核并释放巨大能量的过程。本文围绕裂变物理、链式反应以及核电站如何利用这一能量展开，是IB物理课程中的核心内容。
1. The Fundamentals of Nuclear Fission | 核裂变的基本原理
When a heavy nucleus such as uranium-235 absorbs a slow (thermal) neutron, it becomes an excited compound nucleus. This nucleus oscillates and deforms until it splits into two smaller nuclei—the fission fragments—along with 2 to 3 free neutrons and a great deal of energy.
当铀-235等重原子核吸收一个慢（热）中子后，会形成高激发的复合核。该复合核发生振荡与形变，最终分裂为两个较小原子核（即裂变碎片），同时释放出2至3个自由中子和大量能量。
A typical fission reaction of uranium-235 is written as:
n + ²³⁵U → ¹⁴¹Ba + ⁹²Kr + 3n + Energy
The total mass of the products is smaller than that of the reactants. This mass difference Δm is converted into kinetic energy of the fragments and neutrons, as described by Einstein’s famous equation ΔE = Δmc².
生成物的总质量小于反应物的总质量，这一质量亏损Δm转化为碎片和中子的动能，正符合爱因斯坦著名方程ΔE = Δmc²。
An important concept behind fission is the binding energy per nucleon. Heavy nuclei such as ²³⁵U lie on the right side of the binding-energy curve, where nucleons are less tightly bound. When the nucleus splits into two mid-sized fragments, the products sit higher on the curve, meaning the released energy is the difference between these binding energies.
理解裂变的关键概念是“比结合能”。像²³⁵U这样的重核位于比结合能曲线的右侧，核子束缚较弱。当原子核分裂为两个中等质量的碎片后，生成物在曲线上位于更高处，释放的能量正是两者结合能之差。
2. Energy Release from Fission | 裂变的能量释放
For every fission event of a uranium-235 nucleus, approximately 200 MeV (3.2 × 10⁻¹¹ J) of energy is released. Most of this energy appears as kinetic energy of the fission fragments; the rest is carried by neutrons, gamma rays, and later by radioactive decay products.
每个铀-235原子核发生裂变时释放约200 MeV（即3.2 × 10⁻¹¹ J）的能量。其中大部分表现为裂变碎片的动能，其余由中子、伽马射线以及随后的放射性衰变产物带走。
The energy released can be calculated from the mass defect:
Q = (m_initial − m_final) × c²
Using the fission channel above:
m_initial = 1.008665 + 235.0439 = 236.0526 u
m_final = 140.9144 + 91.9262 + 3 × 1.008665 = 235.8666 u
Δm = 0.1860 u → Q = 0.1860 × 931.5 ≈ 173 MeV
Since 1 u = 931.5 MeV/c², the mass defect of 0.186 u corresponds to about 173 MeV for this particular channel. The average over all fission channels is about 200 MeV. In the IB exam, you may be given mass values in u or in kg and asked to find ΔE.
因为1 u = 931.5 MeV/c²，该裂变道0.186 u的质量亏损对应约173 MeV的能量。所有裂变道的平均值约为200 MeV。在IB考试中，题目可能给出以u或kg为单位的质量，要求计算ΔE。
3. The Chain Reaction | 链式反应
Each fission event releases on average 2-3 neutrons. If at least one of these neutrons goes on to induce another fission, a self-sustaining chain reaction is achieved. This gives rise to the concept of the multiplication factor k.
每次裂变平均释放2至3个中子。如果其中至少一个中子能引发下一次裂变，便可形成自持的链式反应，由此引出倍增因子k的概念。
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