- OFF-LINE Reference to a procedure or measurement done in conjunction with an accelerator or reactor but neither at the reaction site nor at the exact time the reaction occurs.
- OKLO REACTORS Naturally occurring nuclear reactors which operated at the Oklo mine in Gabon, Africa nearly 2 X 109 years ago. Geological uranium deposits partially depleted of fissile uranium. In the past, the ores were substantially more enriched in fissile uranium. The combination of the richness of the ore, its higher 235U content (~3%) at the time, and presence of water (to act as moderator ) caused the formation to behave as a natural fission reactor; burning fuel, producing fission radionuclides , and boiling off water, the latter resulting in "shut-down". This behavior can cycle.
- ON-LINE Reference to a procedure or measurement done in conjunction with an accelerator or reactor and performed at the reaction site or at the exact time the reaction occurs.