A neutron reflector can make an otherwise subcritical mass of fissile material critical, or increase the amount of nuclear fission that a critical or supercritical mass will undergo.
[1][better source needed] Such an effect was exhibited twice in accidents involving the Demon Core, a subcritical plutonium pit that went critical in two separate fatal incidents when the pit's surface was momentarily surrounded by too much neutron reflective material.
To obtain a 30-year life span, the SSTAR nuclear reactor design calls for a moveable neutron reflector to be placed over the column of fuel.
By reducing neutron leakage, the reflector increases reactivity of the core and reduces the amount of fuel necessary to maintain the reactor critical for a long period.
In light-water reactors, the neutron reflector is installed for following purposes: