Pit (nuclear weapon)

The pit, named after the hard core found in fruits such as peaches and apricots, is the core of an implosion nuclear weapon – the fissile material and any neutron reflector or tamper bonded to it. Some weapons tested during the 1950s used pits made with U-235 alone, or in composite with plutonium, but all-plutonium pits are the smallest in diameter and have been the standard since the early 1960s.

Pit designs

Christy pits

The pits of the first nuclear weapons were solid, with an urchin neutron initiator in their center. The Gadget and Fat Man used pits made of solid hot pressed material (at 400 °C and 200 MPa in steel dies) half-spheres of 9.2 cm diameter, with a 2.5 cm internal cavity for the initiator. The gadget's pit was electroplated with 0.13 mm of silver; the layer, however, developed blistering and the blisters had to be ground and plated with gold leaf before the test. The Fat Man pit, and those of subsequent models, were all plated with nickel. A hollow pit was considered and known to be more efficient but ultimately rejected due to higher requirements for implosion accuracy.