Nuclear fission was a human term describing a nuclear reaction in which a nucleus of a large atom split into smaller parts, often producing free neutrons as well as lighter nuclei. The following reaction with other atoms could under certain conditions cause a great release of energy for both power supply and destructive purposes.
In pre-Diaspora times on Old Earth, fission acquired a reputation as unsafe, polluting, expensive, and hard to manage. Even 2000 T-years after the Diaspora began, it still had this reputation, and was not used anywhere in the "civilized" universe. Fusion plants had replaced fission plants almost everywhere by the time of the First Havenite-Manticoran War.
On Grayson however, fission plants were common as a means of energy production, mostly because Grayson had lost the techniques needed for fusion, and because Grayson was richer in fissionable material than most habitable planets. Establishing relations with Manticore had the effect of introducing a great many lost technologies to Grayson, including fusion. (HH2)
From necessity, Grayson engineers had put considerable effort into improving their power plant designs since contact with other planets had become lost. As a result, when contact was reestablished, Grayson style fission plants could be smaller, less complex, and easier to manage than a modern fusion plant of equivalent output. Furthermore, they did not require frequent fueling – about once every decade versus about once every week or month for typical installations and usage patterns. When the Manticoran Alliance introduced its new LAC types at the end of the First Havenite-Manticoran War, they were powered by fission plants because of these virtues. (HH8)
- Detailed article about nuclear fission on Wikipedia