📚 Nuclear Power | 核能
Nuclear power harnesses the energy released from nuclear reactions, primarily fission, to generate electricity. It offers a low-carbon alternative to fossil fuels but poses challenges in safety, waste disposal, and public perception. In IB Physics, understanding nuclear power requires knowledge of mass defect, binding energy, fission chain reactions, and reactor design.
核能利用核反应（主要是裂变）释放的能量来发电。它为化石燃料提供了一种低碳替代方案，但也带来了安全、废物处置和公众接受度方面的挑战。在IB物理中，理解核能需要掌握质量亏损、结合能、裂变链式反应以及反应堆设计等知识。
1. Mass Defect and Binding Energy | 质量亏损与结合能
The mass of an atomic nucleus is always less than the sum of the masses of its individual nucleons (protons and neutrons). This mass difference, Δm, is called the mass defect. It arises because energy is released when nucleons bind together, locking away some mass according to Einstein’s equation E = mc².
原子核的质量总是小于其各个核子（质子和中子）质量的总和。这个质量差Δm称为质量亏损。它的产生是因为核子结合时会释放能量，根据爱因斯坦方程E=mc²，一些质量被“锁定”了起来。
The binding energy (BE) of a nucleus is the energy required to separate it into its constituent nucleons. It is calculated as BE = Δm c². The average binding energy per nucleon peaks around iron-56, which explains why both fusion of light nuclei and fission of heavy nuclei can release energy.
原子核的结合能（BE）是将其分离为组成核子所需的能量，计算公式为BE = Δm·c²。每个核子的平均结合能在铁-56附近达到峰值，这解释了为什么轻核聚变和重核裂变都能释放能量。
BE = Δm · c²
2. Nuclear Fission | 核裂变
Fission occurs when a heavy nucleus, such as uranium-235 or plutonium-239, absorbs a neutron and splits into two lighter nuclei (fission fragments), releasing a large amount of energy and typically 2 or 3 additional neutrons.
当一个重原子核，如铀-235或钚-239，吸收一个中子并分裂成两个较轻的核（裂变碎片）时，就会发生裂变，同时释放大量能量，通常还释放2到3个额外的中子。
A typical fission reaction is ²³⁵U + ¹n → ¹⁴¹Ba + ⁹²Kr + 3¹n + energy. The energy released per fission is about 200 MeV, mostly as kinetic energy of the fragments and neutrons.
一个典型的裂变反应是：²³⁵U + ¹n → ¹⁴¹Ba + ⁹²Kr + 3¹n + 能量。每次裂变释放的能量约为200 MeV，主要表现为碎片和中子的动能。
3. Chain Reactions | 链式反应
The neutrons released in fission can go on to induce further fissions, creating a self-sustaining chain reaction. The neutron multiplication factor, k, is defined as the ratio of neutrons in one generation to the next. For a steady power output, k = 1 (critical). If k > 1, the reaction is supercritical and can lead to a runaway reaction; if k < 1, the reaction dies out.
裂变释放的中子可以继续引发更多裂变，形成自持链式反应。中子倍增因子k定义为后一代中子数与前一代中子数之比。要获得稳定的功率输出，需保持k = 1（临界状态）。若k > 1则处于超临界状态，可能导致反应失控；若k < 1则反应逐渐停止。
To sustain a chain reaction, a minimum mass of fissile material, known as the critical mass, is required. The shape and purity of the material also affect the critical mass.
要维持链式反应，需要一定的最低易裂变材料质量，称为临界质量。材料的形状和纯度也会影响临界质量。
4. Nuclear Reactor Components | 核反应堆组件
The core contains fuel rods, a moderator, and control rods. The fuel is typically uranium dioxide enriched to 3–5% U-235.
堆芯包含燃料棒、慢化剂和控制棒。燃料通常是富集到3–5%铀-235的二氧化铀。
Control rods made of neutron-absorbing materials like boron or cadmium can be inserted to regulate the chain reaction. The moderator slows fast neutrons to thermal energies to increase the probability of fission.
由硼或镉等中子吸收材料制成的控制棒可以插入以调节链式反应。慢化剂将快中子减速为热中子，以提高裂变概率。
A coolant, often light water, circulates through the core to remove heat, which is then used to generate steam and drive turbines. The entire system is housed within a robust containment structure.
冷却剂（通常是轻水）在堆芯中循环，将热量带出，然后利用这些热量产生蒸汽驱动涡轮机。整个系统置于坚固的安全壳内。
5. Moderator and Control Rods | 慢化剂与控制棒
Fission neutrons are born with high kinetic energies (~2 MeV). To induce further fission in U-235, these neutrons must be slowed to thermal energies (~0.025 eV), where the fission cross-section is much larger. The moderator accomplishes this via elastic scattering collisions. Common moderators are water (H₂O), heavy water (D₂O), and graphite.
裂变产生的中子具有很高的动能（约2 MeV）。要引发铀-235的进一步裂变，必须将这些中子慢化到热能范围（约0.025 eV），此时裂变截面要大得多。慢化剂通过弹性散射碰撞来实现减速。常用的慢化剂有轻水（H₂O）、重水（D₂O）和石墨。
Control rods provide fine adjustment of the neutron multiplication factor. By absorbing excess neutrons, they maintain criticality and allow for power changes or emergency shutdown (SCRAM).
控制棒可以精细调节中子倍增因子。通过吸收多余中子，它们维持临界状态，并允许功率变化或紧急停堆（SCRAM）。
6. Coolant and Heat Exchange | 冷却剂与热交换
The coolant must efficiently remove thermal energy from the reactor core and transfer it to a steam generator or directly to a turbine. In pressurized water reactors (PWR), water serves as both moderator and coolant, kept under high pressure to prevent boiling. In boiling water reactors (BWR), water is allowed to boil in the core.
冷却剂必须有效地将热能从堆芯带走，并传递给蒸汽发生器或直接驱动涡轮机。在压水堆（PWR）中，水同时作为慢化剂和冷却剂，并保持高压以防止沸腾。在沸水堆（BWR）中，水在堆芯中直接沸腾。
Other coolants include heavy water, carbon dioxide gas, and liquid sodium in fast reactors. The choice of coolant affects thermal efficiency, safety, and cost.
其他冷却剂包括重水、二氧化碳气体以及快堆中的液态钠。冷却剂的选择影响热效率、安全性和成本。
7. Safety and Risk Assessment | 安全与风险评估
Nuclear reactor safety relies on multiple independent barriers: fuel cladding, reactor vessel, and containment building. Defense-in-depth ensures that no single failure leads to a release of radioactivity.
核反应堆的安全依赖多重独立屏障：燃料包壳、反应堆压力容器和安全壳。纵深防御确保单一故障不会导致放射性物质释放。
Key risks include loss-of-coolant accidents (LOCA), core meltdown, and external hazards. The Three Mile Island (1979), Chernobyl (1986), and Fukushima (2011) accidents highlighted the importance of design, operator training, and natural disaster resilience. Passive safety systems, which rely on natural circulation or gravity, are now standard in advanced reactor designs.
主要风险包括冷却剂丧失事故（LOCA）、堆芯熔毁和外部灾害。三里岛（1979年）、切尔诺贝利（1986年）和福岛（2011年）事故凸显了设计、操作员培训和自然灾害抗御的重要性。依靠自然循环或重力的非能动安全系统现已成为先进反应堆设计的标准配置。
Probabilistic risk assessment (PRA) is used to quantify accident likelihood and inform regulatory decisions.
概率风险评估（PRA）用于量化事故可能性，并为监管决策提供依据。
8. Radioactive Waste Management | 放射性废物管理
Spent nuclear fuel contains highly radioactive fission products and transuranic elements. High-level waste (HLW) generates significant heat and remains hazardous for tens of thousands of years. It is initially stored in cooling pools, then transferred to dry cask storage. Permanent disposal options, such as deep geological repositories, are under development (e.g., Onkalo in Finland).
乏燃料含有高放射性裂变产物和超铀元素。高放废物（HLW）会产生大量热量，并在数万年中保持危险性。它首先储存在冷却池中，然后转移到干式贮存容器。永久处置方案，如深层地质处置库，正在开发中（例如芬兰的昂卡洛）。
Low and intermediate level waste (LILW) from reactor operations and decommissioning is volume-reduced and stored in near-surface facilities. Reprocessing can recover plutonium and uranium for reuse, but raises proliferation concerns.
反应堆运行和退役产生的中低放废物（LILW）经减容后储存在近地表设施中。后处理可以回收钚和铀以便再利用，但引发了核扩散担忧。
9. Nuclear Fusion: An Alternative? | 核聚变：另一种选择？
Nuclear fusion powers the Sun by combining light nuclei. The most promising reaction for terrestrial fusion is deuterium–tritium (D–T): ²H + ³H → ⁴He + n + 17.6 MeV.
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