What is the role of a reflector in a nuclear weapon?

The role of a reflector in a nuclear weapon, typically positioned surrounding the core of fissile material and the tamper, is to reduce the loss of neutrons from the fissioning core and increase the efficiency of the chain reaction. As the fission reaction proceeds, neutrons are released. Some of these neutrons will escape the surface of the fissile material and be lost to the surrounding environment, reducing the rate of fission reactions and therefore the explosion's energy. The reflector is made of a material with a high scattering cross-section for neutrons, meaning that neutrons colliding with the reflector's atoms are likely to be deflected, or scattered, back into the fissile core. This significantly increases the probability that these neutrons will induce further fission events, sustaining the chain reaction for a longer period. A good reflector material is typically dense and has a high atomic weight. Beryllium and uranium are common reflector materials. By reducing neutron leakage, the reflector lowers the critical mass (the minimum amount of fissile material needed to sustain a chain reaction) and increases the fraction of fissile material that undergoes fission before the weapon disassembles itself. This results in a more efficient and powerful explosion. The reflector essentially 'bounces' neutrons back into the core, maximizing their utilization in the fission process.