My understanding is that the yield was halved by the use of a lead tamper, rather than a uranium one. The tamper is a shield around the core that slows down the

My understanding is that the yield was halved by the use of a lead tamper, rather than a uranium one. The tamper is a shield around the core that slows down the explosion in order to consume the reactants more fully. In production they commonly make the tamper out of waste uranium, which undergoes fission during the blast and contributes a significant amount of energy, but if you don't need the boost then lead works fine, or anything else heavy. The other thing about uranium is it greatly multiplies the radioactive fallout (fusion bombs are actually pretty clean when lead is used), which may have contributed to the decision not to use it.