核裂变Nuclear fission

比结合能曲线在铁-56 附近达到最大，重核分裂为中等核时比结合能增大约 0.9 MeV/核子，质量亏损按 E = mc² 转化为能量。按液滴模型，吸收中子后的复合核发生形变，当库仑斥力超过表面张力时断裂。²³⁵U 吸收热中子即可裂变，²³⁸U 则需要快中子。裂变碎片质量分布呈双峰，常见于质量数 95 和 140 附近。

Binding energy per nucleon peaks near iron-56, so splitting a heavy nucleus into medium ones raises it by about 0.9 MeV per nucleon, and the mass difference becomes energy by E = mc². In the liquid-drop model the compound nucleus formed by neutron capture deforms, and it breaks when Coulomb repulsion overcomes surface tension. ²³⁵U fissions with thermal neutrons, while ²³⁸U needs fast ones. Fragment masses are double-peaked, around mass numbers 95 and 140.

1934 年费米用中子轰击铀，以为得到了超铀元素。1938 年 12 月，哈恩和斯特拉斯曼用化学方法证明产物中有钡。1939 年初迈特纳和弗里施用液滴模型解释为铀核分裂，并借用生物学术语命名为 fission。哈恩获 1944 年诺贝尔化学奖，迈特纳未能分享，这被认为是诺贝尔奖史上的一大遗憾。1942 年费米在芝加哥建成第一座核反应堆。

In 1934 Fermi bombarded uranium with neutrons and thought he had made transuranic elements. In December 1938 Hahn and Strassmann showed chemically that barium was among the products. Early in 1939 Meitner and Frisch explained this with the liquid-drop model as the splitting of the uranium nucleus and borrowed the biological term fission. Hahn received the 1944 Nobel Prize in Chemistry; Meitner's exclusion is widely regarded as a great injustice. Fermi built the first nuclear reactor in Chicago in 1942.

核电约提供全球 9–10% 的电力，几乎不排放 CO₂；1 kg ²³⁵U 完全裂变释放的能量约相当于 2700 吨标准煤。问题包括长寿命高放废物处置、核事故风险（切尔诺贝利 1986、福岛 2011）和核扩散。第四代反应堆、钍基熔盐堆和小型模块化反应堆是发展方向。

Nuclear power supplies about 9–10% of world electricity with almost no CO₂ emissions; complete fission of 1 kg of ²³⁵U releases about as much energy as 2,700 tonnes of standard coal. Problems include disposing of long-lived high-level waste, accident risk (Chernobyl 1986, Fukushima 2011) and proliferation. Generation IV reactors, thorium molten-salt reactors and small modular reactors are being developed.