Pit (nuclear weapon)
[3] A hollow pit was considered and known to be more efficient but ultimately rejected due to higher requirements for implosion accuracy.
[8] During implosion of a hollow pit, the plutonium layer accelerates inwards, colliding in the middle and forming a supercritical highly dense sphere.
Following the war's end in August 1945, the laboratory focused back on to the problem of the hollow pit, and for the rest of the year they were headed by Hans Bethe, his group leader and successor to the theoretical division, with the hollow composite core being of greatest interest,[9] due to the cost of plutonium and trouble ramping up the Hanford reactors.
The efficiency of the hollow pits can be further increased by injecting a 50%/50% mixture of deuterium and tritium into the cavity immediately before the implosion, so called "fusion boosting"; this also lowers the minimum amount of plutonium for achieving a successful explosion.
The higher degree of control of the initiation, both by the amount of deuterium-tritium mixture injection and by timing and intensity of the neutron pulse from the external generator, facilitated the design of variable yield weapons.
[13] Minimizing this probability required a smaller mass of plutonium, which limited the achievable yield to about 10 kt, or using highly pure plutonium-239 with impractically low level of plutonium-240 contamination.
The advantage of the composite core was the possibility to maintain higher yields while keeping predetonation risk low, and to utilize both available fissile materials.
Such massive pits, consisting of more than one critical mass of fissile material, present a significant safety risk, as even an asymmetrical detonation of the implosion shell may cause a kiloton-range explosion.
This protects the nuclear materials from environmental degradation and helps reduce the chances of their release in case of an accidental fire or minor explosion.
Beryllium is brittle, toxic, and expensive, but is an attractive choice due to its role as a neutron reflector, lowering the needed critical mass of the pit.
An often-cited specification applicable to some modern pits describes a hollow sphere of a suitable structural metal, of the approximate size and weight of a bowling ball, with a channel for injection of tritium (in the case of boosted fission weapons), with the internal surface lined with plutonium.
[20] Beryllium-clad pits are more vulnerable to fracture, more sensitive to temperature fluctuations, more likely to require cleaning, susceptible to corrosion with chlorides and moisture, and can expose workers to toxic beryllium.
The ongoing miniaturization process led to design changes, whereby the pit could be inserted in the factory during the device assembly.
[29] The switch from solid to hollow pits caused a work safety issue; the larger surface-to-mass ratio led to comparatively higher emission of gamma rays and necessitated the installation of better radiation shielding in the Rocky Flats production facility.
By the 1996, the US Department of Energy identified more than 50 cases of chronic berylliosis among nuclear industry employees, including three dozen in the Rocky Flats Plant; several died.
[citation needed] Fire-resistant pits (FRP) are a safety feature of modern nuclear weapons, reducing plutonium dispersal in case of fire.
The class 1.1, both fire and detonation hazard, is a double-base propellant based on cross-linked polymer, containing 52% HMX, 18% nitroglycerine, 18% aluminium, 4% ammonium perchlorate, and 8% binder.
The insensitive high explosives are also less powerful, necessitating larger and heavier warheads, which reduces the missile range – or sacrificing some yield.
Other trivalent metals would also work, but gallium has a small neutron absorption cross section and helps protect the plutonium against corrosion.
Because plutonium is chemically reactive it is common to plate the completed pit with a thin layer of inert metal, which also reduces the toxic hazard.
A number of the problem-plagued W47 UGM-27 Polaris warheads had to be replaced after corrosion of the fissile material was discovered during routine maintenance.
Supergrade plutonium has less than 4% of the 240 isotope, and is used in systems where the radioactivity is a concern, e.g. in the US Navy weapons which have to share confined spaces on ships and submarines with the crews.
After several years, americium builds up in the plutonium metal, leading to increased gamma activity that poses an occupational hazard for workers.
[21] However, in around 1967 the Rocky Flats Plant stopped this separation, blending up to 80% of old americium-containing pits directly to the foundry instead, in order to reduce costs and increase productivity; this led to higher exposure of workers to gamma radiation.
[30] Metallic plutonium, notably in the form of the plutonium-gallium alloy, degrades chiefly by two mechanisms: corrosion, and self-irradiation.
Plutonium chips can spontaneously ignite; the mechanism involves formation of Pu2O3 layer, which then rapidly oxidizes to PuO2, and the liberated heat is sufficient to bring the small particles with low thermal mass to autoignition temperature (about 500 °C).
Plutonium stored at non-cryogenic temperatures does not show signs of major macroscopic structural changes after more than 40 years.
[48] However, even passive measurements of gamma spectrums may be a contentious issue in international weapon inspections, as it allows characterization of materials used e.g. the isotopic composition of plutonium, which can be considered a secret.
An Institute for Defense Analyses report written before 2008 estimated a “future pit production requirement of 125 per year at the CMRR, with a surge capability of 200.
[52] Recovery of plutonium from decommissioned pits can be achieved by numerous means, both mechanical (e.g. removal of cladding by a lathe) and chemical.
