📚 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
m 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
ost 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 − mfinal) × c²
Using the fission channel above:
m_initial = 1.008665 + 235.0439 = 236.0526 u
mfinal = 140.9144 + 91.9262 + 3 × 1.008665 = 235.8666 u
Δm = 0.1860 u → Q = 0.1860 × 931.5 ≈ 173 MeV
ct 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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