Complex sentence: Natural lithium is a mixture of lithium-6 and lithium-7 isotopes (with 7.5% of the former); the enriched lithium used in Bravo was approximately 40% lithium-6.
Split into simpler sentences: Natural lithium is a mixture of lithium-6 and lithium-7 isotopes (with 7.5% of the former). The enriched lithium used in Bravo was approximately 40% lithium-6.