Marie and Pierre Curie discovered the radioactive elements radium and polonium. Before this discovery, uranium and thorium were the only known radioactive elements. While studying uranium minerals. Marie Curie noticed two minerals were much more radioactive than uranium itself. She and her husband, Pierre, chemically separated the compounds in the minerals and found a substance 400 times more radioactive than uranium. Marie named this substance polonium, after her native country of Poland.
In the summer of 1942, the Corps of Engineers organized the Manhattan Engineer District. The purpose of the District's Manhattan Project was to build an atomic bomb. Polonium-210 was vital to this program, because it was to be used in a neutron source that would ensure initiation of a chain reaction. An initiator is a device that produces a timed burst of neutrons to initiate a fission chain reaction in a nuclear weapon. Initiators made of polonium-210 and beryllium were located at the center of the fissile cores of early atomic weapons. The highly radioactive isotope of Polonium (Po-210) is a strong alpha emitter. Beryllium will absorb alphas and emit neutrons. This isotope of polonium has a half life of almost 140 days, and a neutron initiator using this material needs to have the polonium, which is generated in a nuclear reactor, to be replaced frequently. To supply the initiation pulse of neutrons at the right time, the polonium and the beryllium need to be kept apart until the appropriate moment and then thoroughly and rapidly mixed by the implosion of the weapon.
The polonium project was undertaken by MCC at the company's Central Research Department in Dayton, Ohio, in September 1943, and became known as the Dayton Project. Subsequently, polonium-based neutron sources were produced for other industrial and research applications.
Prior to 1944, polonium had not been isolated in pure form or in any appreciable quantity. Therefore, any program involving the recovery, purification, and fabrication of polonium metal from a variety of sources required an understanding of the chemical and physical properties and the metallurgy of polonium-210. The Dayton Project's goal was to develop an understanding of the properties of polonium and its metallurgy.
Initially, the recovery of polonium was attempted from naturally occurring sources such as lead-containing wastes from uranium, vanadium, and radium refining operations. Upon investigation, it became apparent that sufficient quantities of polonium could not be recovered from these sources without processing prohibitively large amounts of material. To obtain polonium in the quantities needed, other approaches to its production were investigated, and the transmutation of bismuth metal to polonium-210 by neutron irradiation was selected for production scale operations.
In February 1949, the polonium operations were transferred from Dayton to Mound (Moyer 1956). At this time, the process for producing polonium-210 had been decided upon. Polonium-210 would be produced by the transmutation of bismuth by neutron bombardment.
In 1954, the Mound began a program using polonium-210 to convert nuclear energy to useable electric energy. This application of nuclear energy, using a thermoelectric principle, was demonstrated that same year, and in February, Mound received a directive to fabricate a polonium-powered model steam-electric plant. A model was built and demonstrated in 1954. In 1956, a conceptual design to produce a mercury boiler fueled with polonium was described. By 1958, an RTG powered by polonium-210 was built.
The power density of polonium is unique and made it attractive as a power source. One pound of polonium-210 occupies a volume of approximately 3 cubic inches and produces heat at the rate of 3.6 x 108 British Thermal Units (BTUs) per minute or about 64 kilowatts of electric power. With a thermal energy output of 120 watts per g, polonium-210 was selected initially for use in the RTG.
Known as SNAP, these generators convert the thermal energy generated by radioactive decay to electrical energy. The first SNAP-3A, fueled with polonium-210, provided power to a satellite radio transmitter. The use of satellites powered by SNAP for global communication was first demonstrated under President Eisenhower in 1961, at which time the President's peace message was broadcast via a satellite containing a radio transmitter powered by the SNAP-3A RTG.
Because polonium-210 has a short half-life (138 days), its usefulness was limited for application on long duration satellite and space probe missions. Polonium research and production at Mound were eventually phased out in 1971.
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