Neutron reflector
A neutron reflector is a material that reflects neutrons . This means more a scattering than a reflection. The materials used are e.g. B. graphite , beryllium , lead , steel , tungsten carbide or others. A neutron reflector can make an otherwise subcritical mass of fissile material critical or increase the number of nuclear fission that occurs in a critical or supercritical mass. One example of this is the Demon Core . This was a subcritical plutonium core that became supercritical in two different fatal accidents because the core was surrounded by too much reflector material for a short time.
Nuclear reactors
In a graphite-moderated uranium reactor , the critical mass can be reduced considerably by a surrounding graphite layer, as this reflects many neutrons back into the core.
A reflector made of a lightweight material such as graphite or beryllium simultaneously serves the kinetic energy to reduce the neutrons (see Host (physics) ). Heavy materials, such as lead or a lead bismuth - eutectic have only a small influence on the neutron speed.
Nuclear weapons
A comparable shell can be used to reduce the critical mass of a nuclear weapon . There, the shell also has the task of delaying the expansion of the gap material through its inertia . For this reason, such a cover is also called a tamper . The weapon tends to burst into pieces as the reaction progresses. This interrupts the reaction prematurely. The tamper ensures a longer lasting, more energetic and more efficient explosion. The most effective tamper is the one with the highest density . Tensile strength does not matter as no material can withstand the pressure of a core explosion. In addition, heavy materials are usually good neutron reflectors. The first nuclear weapons used uranium or tungsten carbide tamper reflectors.
The disadvantage of heavy tampers is that the explosion-implosion system increases. The first stage of a modern nuclear weapon can use a light beryllium reflector. This is transparent to gamma rays when it is ionized. This enables the first burst of energy to leave the core quickly in order to support the compression by heating the outer layers.
The effect of a tamper on the efficiency is only small, however, as the reflection process takes some time.