Editing Red mercury (section)

=== Background ===
Traditional [[Nuclear weapon design#Two-stage thermonuclear weapons|staged thermonuclear weapons]] consist of two parts, a [[Nuclear fission|fission]] "primary" and a fusion/fission "secondary". The energy released by the primary when it explodes is used to (indirectly) compress the secondary and start a [[Nuclear fusion|fusion]] reaction within it. Conventional explosives are far too weak to provide the level of compression needed.

The primary is generally built as small as possible, because the energy released by the secondary is much larger, and thus building a larger primary is generally inefficient. There is a lower limit on the size of the primary, known as the [[critical mass]]. For weapons grade [[plutonium]], this is around {{Cvt|10|kg}}. This can be reduced through the use of [[neutron reflector]]s or clever arrangements of explosives to compress the core, but these methods generally add to the size and complexity of the resulting device.

Because of the need for a fission primary and the difficulty of purifying weapons-grade fissile materials, the majority of [[arms control]] efforts to limit [[nuclear proliferation]] rely on the detection and control of the fissile material and the equipment needed to obtain it.