Plutonium-238

Plutonium-238 is a radioactive isotope of plutonium that has a half-life of 87.7 years.
Plutonium-238 is a very powerful alpha emitter; as alpha particles are easily blocked, this makes the plutonium-238 isotope suitable for usage in radioisotope thermoelectric generators and radioisotope heater units. The density of plutonium-238 at room temperature is about 19.8 g/cc. The material will generate about 0.57 watts per gram of ²³⁸Pu.
The bare sphere critical mass of metallic plutonium-238 is not precisely known, but its calculated range is between.

History

Initial production

Plutonium-238 was the first isotope of plutonium to be discovered. It was synthesized by Glenn Seaborg and his associates in December 1940 by bombarding uranium-238 with deuterons, creating neptunium-238.
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The neptunium isotope then undergoes β⁻ decay to plutonium-238 with a half-life of 2.099 days.
Plutonium-238 naturally decays to uranium-234 and then continues, after a long period of time, along the radium series to lead-206. Historically, most plutonium-238 has been produced by Savannah River in their weapons reactor, by irradiating neptunium-237 with neutrons.
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Neptunium-237 is a by-product of the production of plutonium-239 weapons-grade material, and when the site was shut down in 1988, ²³⁸Pu was mixed with about 16% ²³⁹Pu.

Manhattan Project

Plutonium was first synthesized in 1940 and isolated in 1941 by chemists at the University of California, Berkeley.
The Manhattan Project began shortly after the discovery, with most early research carried out using small samples manufactured using the large cyclotrons at the Berkeley Rad Lab and Washington University in St. Louis.
Much of the difficulty encountered during the Manhattan Project regarded the production and testing of nuclear fuel. Both uranium and plutonium were eventually determined to be fissile, but in each case they had to be purified to select for the isotopes suitable for an atomic bomb.
With World War II underway, the research teams were pressed for time. Micrograms of plutonium were made by cyclotrons in 1942 and 1943. In late 1943 Robert Oppenheimer is quoted as saying "there's only a twentieth of a milligram in existence."
By his request, the Rad Lab at Berkeley made available 1.2 mg of plutonium by the end of October 1943, most of which was taken to Los Alamos for theoretical work there.
The world's second reactor, the X-10 Graphite Reactor built at a secret site at Oak Ridge, would be fully operational in 1944. In November 1943, shortly after its initial start-up, it was able to produce a minuscule 500 mg. However, this plutonium was mixed with large amounts of uranium fuel and destined for the nearby chemical processing pilot plant for isotopic separation. Gram amounts of plutonium would not be available until early 1944.
Industrial-scale production of plutonium only began in March 1945 when the B Reactor at the Hanford Site began operation.

Plutonium-238 and human experimentation

While samples of plutonium were available in small quantities and being handled by researchers, no one knew what health effects this might have.
Plutonium handling mishaps occurred in 1944, causing alarm in the Manhattan Project leadership as contamination inside and outside the laboratories was becoming an issue. In August 1944, chemist Donald Mastick was sprayed in the face with a solution of plutonium chloride, causing him to accidentally swallow some. Nose swipes taken of plutonium researchers indicated that plutonium was being breathed in. Lead Manhattan Project chemist Glenn Seaborg, discoverer of many transuranium elements including plutonium, urged that a safety program be developed for plutonium research. In a memo to Robert Stone at the Chicago Met Lab, Seaborg wrote "that a program to trace the course of plutonium in the body be initiated as soon as possible... the very highest priority." This memo was dated January 5, 1944, prior to many of the contamination events of 1944 in Building D where Mastick worked. Seaborg later claimed that he did not at all intend to imply human experimentation in this memo, nor did he learn of its use in humans until far later due to the compartmentalization of classified information.
With bomb-grade enriched plutonium-239 destined for critical research and for atomic weapon production, plutonium-238 was used in early medical experiments as it is unusable as atomic weapon fuel. However, ²³⁸Pu is far more dangerous than ²³⁹Pu due to its short half-life and being a strong alpha-emitter. It was soon found that plutonium was being excreted at a very slow rate, accumulating in test subjects involved in early human experimentation. This led to severe health consequences for the patients involved.
From April 10, 1945, to July 18, 1947, eighteen people were injected with plutonium as part of the Manhattan Project. Doses administered ranged from 0.095 to 5.9 microcuries.
Albert Stevens, after a terminal cancer diagnosis which seemed to include many organs, was injected in 1945 with plutonium without his informed consent. He was referred to as patient CAL-1 and the plutonium consisted of 3.5 μCi ²³⁸Pu and 0.046 μCi ²³⁹Pu, giving him an initial body burden of 3.546 μCi total activity. The fact that he had the highly radioactive plutonium-238 contributed heavily to his long-term dose. Had all of the plutonium given to Stevens been the long-lived ²³⁹Pu as used in similar experiments of the time, Stevens's lifetime dose would have been significantly smaller. The short half-life of 87.7 years of ²³⁸Pu means that a large amount of it decayed during its time inside his body, especially when compared to the 24,100 year half-life of ²³⁹Pu.
After his initial "cancer" surgery removed many non-cancerous "tumors", Stevens survived for about 20 years after his experimental dose of plutonium before succumbing to heart disease; he had received the highest known accumulated radiation dose of any human patient. Modern calculations of his lifetime absorbed dose give a significant 64 Sv total.

Weapons

The first application of ²³⁸Pu was its use in nuclear weapon components made at Mound Laboratories for Lawrence Radiation Laboratory. Mound was chosen for this work because of its experience in producing the polonium-210-fueled Urchin initiator and its work with several heavy elements in a Reactor Fuels program. Two Mound scientists spent 1959 at Lawrence in joint development while the Special Metallurgical Building was constructed at Mound to house the project. Meanwhile, the first sample of ²³⁸Pu came to Mound in 1959.
The weapons project called for the production of about 1 kg/year of ²³⁸Pu over a 3-year period. However, the ²³⁸Pu component could not be produced to the specifications despite a 2-year effort beginning at Mound in mid-1961. A maximum effort was undertaken with 3 shifts a day, 6 days a week, and ramp-up of Savannah River's ²³⁸Pu production over the next three years to about 20 kg/year. A loosening of the specifications resulted in productivity of about 3%, and production finally began in 1964.

Use in radioisotope thermoelectric generators

Beginning on January 1, 1957, Mound Laboratories RTG inventors Jordan & Birden were working on an Army Signal Corps contract to conduct research on radioactive materials and thermocouples suitable for the direct conversion of heat to electrical energy using polonium-210 as the heat source.
In 1961, Capt. R. T. Carpenter had chosen ²³⁸Pu as the fuel for the first RTG to be launched into space as auxiliary power for the Transit IV Navy navigational satellite. By January 21, 1963, the decision had yet to be made as to what isotope would be used to fuel the large RTGs for NASA programs.
Early in 1964, Mound Laboratories scientists developed a different method of fabricating the weapon component that resulted in a production efficiency of around 98%. This made available the excess Savannah River ²³⁸Pu production for Space Electric Power use just in time to meet the needs of the SNAP-27 RTG on the Moon, the Pioneer spacecraft, the Viking Mars landers, more Transit Navy navigation satellites and two Voyager spacecraft, for which all of the ²³⁸Pu heat sources were fabricated at Mound Laboratories.
The radioisotope heater units were used in space exploration beginning with the Apollo Radioisotope Heaters warming the Seismic Experiment placed on the Moon by the Apollo 11 mission and on several Moon and Mars rovers, to the 129 LWRHUs warming the experiments on the Galileo spacecraft.
An addition to the Special Metallurgical building weapon component production facility was completed at the end of 1964 for ²³⁸Pu heat source fuel fabrication. A temporary fuel production facility was also installed in the Research Building in 1969 for Transit fuel fabrication. With completion of the weapons component project, the Special Metallurgical Building, nicknamed "Snake Mountain" because of the difficulties encountered in handling large quantities of ²³⁸Pu, ceased operations on June 30, 1968, with ²³⁸Pu operations taken over by the new Plutonium Processing Building, especially designed and constructed for handling large quantities of ²³⁸Pu. Plutonium-238 is given the highest relative hazard number of all 256 radionuclides evaluated by Karl Z. Morgan et al. in 1963.

Nuclear powered pacemakers

In the United States, when plutonium-238 became available for non-military uses, numerous applications were proposed and tested, including the cardiac pacemaker program that began on June 1, 1966, in conjunction with NUMEC. The last of these units was implanted in 1988, as lithium-powered pacemakers, which had an expected lifespan of 10 or more years without the disadvantages of radiation concerns and regulatory hurdles, made these units obsolete.
, there were nine living people with nuclear-powered pacemakers in the United States, out of an original 139 recipients. When these individuals die, the pacemaker is supposed to be removed and shipped to Los Alamos where the plutonium will be recovered.
In a letter to the New England Journal of Medicine discussing a woman who received a Numec NU-5 decades ago that is continuously operating, despite an original $5,000 price tag equivalent to $23,000 in 2007 dollars, the follow-up costs have been about $19,000 compared with $55,000 for a battery-powered pacemaker.
Another nuclear powered pacemaker was the Medtronics “Laurens-Alcatel Model 9000”. Approximately 1600 nuclear-powered cardiac pacemakers and/or battery assemblies have been located across the United States, and are eligible for recovery by the Off-Site Source Recovery Project Team at Los Alamos National Laboratory.