What is the primary purpose of the tamper in an implosion-type nuclear weapon?

The primary purpose of the tamper in an implosion-type nuclear weapon is two-fold: to increase the efficiency of the nuclear chain reaction and to contain the fissioning material. First, the tamper is a dense layer of material, such as uranium or tungsten, surrounding the core of fissile material (like Plutonium-239). Its high density reflects neutrons back into the core, increasing the likelihood that these neutrons will cause further fissions, and thus accelerating and sustaining the chain reaction. Second, the tamper's inertia provides containment; as the core begins to fission and expand rapidly, the tamper resists this expansion, holding the fissile material together for a longer period. This extended confinement allows a larger fraction of the fissile material to undergo fission before the weapon disassembles itself, thus significantly increasing the weapon's yield and overall efficiency. Without the tamper, the core would expand too quickly, prematurely halting the chain reaction and resulting in a drastically reduced yield. Therefore, the tamper acts as both a neutron reflector and an inertial confinement layer, maximizing the efficiency of the fission process in an implosion-type weapon.