"Bravo", "Yankee (II)", and "Union" used lithium enriched in the Li-6 isotope ("Bravo" and "Yankee" used lithium enriched to 40% Li-6, while the lithium used in "Union" was enriched to 95% Li-6), while "Romeo" and "Koon" were fueled with natural lithium (92% Li-7, 7.5% Li-6).