How does a tamper increase the efficiency of a fission weapon?

In the context of nuclear weapons, a tamper—also referred to as a reflector—is a layer of material surrounding the fissile core of an implosion-type device. It increases the efficiency of a fission weapon primarily by acting as a neutron reflector.12

By surrounding the core, the tamper scatters neutrons that would otherwise leak out of the fissile material, reflecting them back into the core. This process increases the number of neutrons available to sustain and propagate the chain reaction, allowing for a more efficient use of the fissile material and enabling the device to achieve a higher explosive yield. Additionally, the physical presence of the tamper can help contain the core for a longer duration during the initial stages of the explosion, further contributing to the efficiency of the fission process.21