链式反应（核）Nuclear chain reaction

有效增殖因数 k 定义为一代中子引起的下一代中子数之比。k < 1 为次临界，反应逐渐熄灭；k = 1 为临界，反应稳定持续；k > 1 为超临界，中子数指数增长。反应堆中约 0.65% 的缓发中子（来自裂变产物的衰变，延迟数秒）使功率变化足够缓慢，控制棒（含硼、镉）有时间调节，这是反应堆可控的关键。

The effective multiplication factor k is the ratio of neutrons in one generation to the previous one. If k < 1 the system is subcritical and the reaction dies away; k = 1 is critical and steady; k > 1 is supercritical, with exponential growth. In a reactor about 0.65% of neutrons are delayed by seconds because they come from decaying fission products, which slows power changes enough for boron or cadmium control rods to respond; this is what makes reactors controllable.

1933 年西拉德在伦敦过马路时想到：如果某种核反应吸收一个中子能放出两个，就可能形成链式反应，并于 1934 年申请专利。1939 年发现铀裂变每次放出 2–3 个中子后，这一设想成为现实。1942 年 12 月 2 日，费米领导的团队在芝加哥大学建成芝加哥一号堆，首次实现人工可控链式反应。

In 1933, crossing a London street, Leo Szilard realised that a nuclear reaction absorbing one neutron and emitting two could sustain a chain, and he filed a patent in 1934. Once uranium fission was found in 1939 to release two or three neutrons, the idea became real. On 2 December 1942 Fermi's team at the University of Chicago started Chicago Pile-1, the first man-made controlled chain reaction.

热中子堆用水、重水或石墨作慢化剂，把快中子减速为热中子以提高 ²³⁵U 的裂变截面。水在压水堆中既是慢化剂又是冷却剂，具有负温度反馈：温度升高时水密度降低，慢化减弱，k 下降。切尔诺贝利 RBMK 堆采用石墨慢化，存在正空泡系数，是 1986 年事故的重要原因之一。

Thermal reactors use water, heavy water or graphite as a moderator to slow fast neutrons and raise the fission cross-section of ²³⁵U. In a pressurised water reactor water is both moderator and coolant and gives negative temperature feedback: hotter water is less dense, moderates less and lowers k. Chernobyl's graphite-moderated RBMK had a positive void coefficient, a major factor in the 1986 accident.