Development
The Polonium used in the urchin initiator was created at Oak Ridge and then extracted and purified as part of the Dayton Project under the leadership of Charles Allen Thomas. The Dayton Project was one of the various sites comprising the Manhattan Project
In 1949 Mound Laboratories in nearby Miamisburg, Ohio opened as a replacement for the Dayton Project and the new home of nuclear initiator research & development. Polonium-210 was produced by neutron irradiation of bismuth. Production and research of polonium at Mound was phased out in 1971.
Polonium from Dayton was used by the G Division of Los Alamos in initiator design studies at a test site in Sandia Canyon. The initiator group built test assemblies by drilling holes in large turbine ball bearings, inserting the active material, and plugging the holes with bolts. These test assemblies were known as screwballs. The test assemblies were imploded and their remains studied to examine how well the polonium and beryllium mixed.
The production of the beryllium-polonium TOM initiators ended in 1953. The initiators were replaced with a different design, which slightly reduced the weapon yield but its longer shelf life reduced the complexity of the logistics. The sealed neutron initiator, brought into inventory in late 1954, still required a periodic disassembly to access its capsule for maintenance checks. The capsules were phased out completely in 1962.
Urchin style initiators were later superseded by other means of generating neutrons such as pulsed neutron emitters that do not use polonium. Another possibility is a boosted fission weapon, using a hollow pit, injected with gaseous deuterium and tritium mixture; tritium, while also radioactive, has much longer half-life (13.2 years) than polonium-210, allowing longer shelf life of the weapons.
Iran was suspected of producing polonium for neutron initiators. Documents proving that have yet to be authenticated, however.
Read more about this topic: Modulated Neutron Initiator
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