They were expecting the lithium-6 to split and contribute to the boom. They were expecting the lithium-7 to be inert. They just didn't consider it worth the i

They were expecting the lithium-6 to split and contribute to the boom. They were expecting the lithium-7 to be inert. They just didn't consider it worth the isotope separation work to use just lithium-6. Likewise, they didn't realize how much energy the U-238 jacket would add.