临界质量Critical mass

裂变产生的中子一部分引起新裂变，一部分从表面逃逸或被杂质吸收。体积越大，表面积与体积之比越小，逃逸比例越低。球形表面积最小，所以临界质量最小。裸露的高浓铀-235 金属球临界质量约 52 kg（直径约 17 cm），钚-239 约 10 kg；加上铍或天然铀反射层、或通过压缩提高密度，临界质量可显著降低。

Some fission neutrons cause new fissions, while others escape through the surface or are absorbed by impurities. The larger the volume, the smaller the surface-to-volume ratio and the lower the escape fraction. A sphere has the least surface, so it has the smallest critical mass. A bare sphere of highly enriched uranium-235 metal needs about 52 kg (about 17 cm across), and plutonium-239 about 10 kg; a beryllium or natural-uranium reflector, or compression to higher density, lowers these substantially.

1939 年佩兰首次提出临界质量的概念。1940 年弗里施和派尔斯在英国估算出纯铀-235 的临界质量只需几千克，远小于此前预想的数吨，这份备忘录促成了英国和美国的原子弹计划。洛斯阿拉莫斯曾发生多起临界事故，1945 年和 1946 年达格利安和斯洛廷都因“恶魔核心”实验遭受致命辐射。

Francis Perrin first proposed the concept of critical mass in 1939. In 1940 Frisch and Peierls in Britain estimated that pure uranium-235 would need only a few kilograms, far less than the tonnes previously assumed; their memorandum spurred the British and American bomb projects. Several criticality accidents occurred at Los Alamos: in 1945 and 1946 Daghlian and Slotin each received fatal doses in experiments with the so-called demon core.

核燃料加工、运输和储存必须严格进行临界安全控制：限制单个容器中的质量和浓度、使用细长或扁平的几何形状、加入中子吸收剂（如硼）、避免含氢物质（水会慢化中子）。1999 年日本东海村临界事故就是因工人违规把过量铀溶液倒入沉淀槽引起的。

Fuel processing, transport and storage are governed by strict criticality safety: limiting mass and enrichment per container, using slender or slab geometries, adding neutron absorbers such as boron and keeping out hydrogen-rich materials, since water moderates neutrons. Japan's 1999 Tokaimura criticality accident happened when workers improperly poured too much uranium solution into a precipitation tank.