pit

In nuclear weapon design, the pit is the core of an implosion nuclear weapon, consisting of fissile material and any neutron reflector or tamper bonded to it. Some weapons tested during the 1950s used pits made with uranium-235 alone, or as a composite with plutonium. All-plutonium pits are the smallest in diameter and have been the standard since the early 1960s. The pit is named after the hard core found in stonefruit such as peaches and apricots. 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 6.2 kg of solid hot pressed plutonium-gallium alloy (at 400 °C and 200 MPa in steel dies – and ) half-spheres of diameter, with a internal cavity for the initiator. The Gadget's pit was electroplated with 0.13 mm of silver because of plutonium's susceptibility to corrosion in an oxygen atmosphere. This layer, however, developed blisters, which had to be ground off. These gaps were then patched 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. Later designs used TOM initiators of similar design but with diameters of only about . The internal neutron initiators were later phased out and replaced with pulsed neutron sources, and with boosted fission weapons.