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 is 19.329 g/cm3.

Heat

One gram of 238Pu corresponds to 1/238 mole, which is 2.53×1021 plutonium atoms. Considering its half-life t1/2 = 87.7 years, its activity is
A is the number of 238Pu decays per second per gram. Each of the emitted alpha particles has kinetic energy, which is quickly converted to heat when the particle decelerates in the material. Therefore, each gram of 238Pu spontaneously generates 0.568 W of heat.

History

Initial production

Plutonium-238 was the first isotope of plutonium to be discovered. It was synthesized by Glenn Seaborg and associates in December 1940 by bombarding uranium-238 with deuterons, creating neptunium-238. The reaction involves a β+ decay of a proton to a neutron, and the escape of another neutron.
+ → + 2 + +
The neptunium isotope then undergoes β- decay to plutonium-238, with a half-life of 2.12 days:
→ + +
Plutonium-238 naturally decays to uranium-234 and then further along the radium series to lead-206. Historically, most plutonium-238 has been produced by Savannah River in their weapons reactor, by irradiating with neutrons neptunium-237.
+ →
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.
The first application was its use in a weapons component made at Mound for the Weapons Design Agency Lawrence Livermore 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 LLL in joint development while the Special Metallurgical Building was constructed at Mound to house the project. Meanwhile, the first sample of plutonium-238 came to Mound in 1959.
The weapons project was planned for 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 a 3-year period 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 120 LWRHU's 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

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. When it was recognized that the heat source would not remain intact through cremation, the program was cancelled because 100% assurance could not be guaranteed that a cremation event would not occur.
As of 2007, there were nine living people with nuclear powered pacemakers, of 139 original recipients. When one of these individuals dies, 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, some $23,000 in 2007 dollars, the follow-up costs have been about $19,000 compared with $55,000 for a battery-powered pacemaker.

Production

from spent nuclear fuel contains various isotopes of plutonium. 238Pu makes up only one or two percent, but it may be responsible for much of the short-term decay heat because of its short half-life relative to other plutonium isotopes. Reactor-grade plutonium is not useful for producing 238Pu for RTGs because difficult isotopic separation would be needed.
Pure plutonium-238 is prepared by neutron irradiation of neptunium-237, one of the minor actinides that can be recovered from spent nuclear fuel during reprocessing, or by the neutron irradiation of americium in a reactor. The targets are purified chemically, including dissolution in nitric acid to extract the plutonium-238. A 100 kg sample of light water reactor fuel that has been irradiated for three years contains only about 700 grams of neptunium-237, and the neptunium must be extracted selectively. Significant amounts of pure 238Pu could also be produced in a thorium fuel cycle.
The United States 238Pu inventory supports both NASA and other national security applications. The Department of Energy maintains separate inventory accounts for the two categories. As of March 2015, a total of of 238Pu was available for civil space uses. Out of the inventory, remains in good enough condition to meet NASA specifications for power delivery; it is this pool of 238Pu that will be used in a multi-mission radioisotope thermoelectric generator for the 2020 Mars Rover mission and two additional MMRTGs for a notional 2024 NASA mission. will remain after that, with approximately barely meeting the NASA specification. This 21 kilograms can be brought up to NASA specifications if it is blended with a smaller amount of newly produced 238Pu having a higher energy density.

U.S. production ceases and resumes

The United States stopped producing bulk 238Pu with the closure of the Savannah River Site reactors in 1988. Since 1993, all of the 238Pu used in American spacecraft has been purchased from Russia. In total, have been purchased, but Russia is no longer producing 238Pu, and their own supply is reportedly running low.
In February 2013, a small amount of 238Pu was successfully produced by Oak Ridge's High Flux Isotope Reactor, and on December 22, 2015, they reported the production of of 238Pu.
In March 2017, Ontario Power Generation and its venture arm, Canadian Nuclear Partners, announced plans to produce 238Pu as a second source for NASA. Rods containing neptunium-237 will be fabricated by Pacific Northwest National Laboratory in Washington State and shipped to OPG's Darlington Nuclear Generating Station in Clarington, Ontario, Canada where they will be irradiated with neutrons inside the reactor's core to produce 238Pu.
In January 2019, it was reported that some automated aspects of its production were implemented at Oak Ridge National Laboratory in Tennessee, that are expected to triple the number of plutonium pellets produced each week. The production rate is now expected to increase from 80 pellets per week to about 275 pellets per week, for a total production of about 400 grams per year. The goal now is to optimize and scale-up the processes in order to produce an average of per year by 2025.

Applications

The main application of 238Pu is as the heat source in radioisotope thermoelectric generators. The RTG was invented in 1954 by Mound scientists Ken Jordan and John Birden, who were inducted into the National Inventors Hall of Fame in 2013. They immediately produced a working prototype using a 210Po heat source, and on January 1, 1957, entered into 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.
RTG technology was first developed by Los Alamos National Laboratory during the 1960s and 1970s to provide radioisotope thermoelectric generator power for cardiac pacemakers. Of the 250 plutonium-powered pacemakers Medtronic manufactured, twenty-two were still in service more than twenty-five years later, a feat that no battery-powered pacemaker could achieve.
This same RTG power technology has been used in spacecraft such as Pioneer 10 and 11, Voyager 1 and 2, Cassini–Huygens and New Horizons, and in other devices, such as the Mars Science Laboratory, for long-term nuclear power generation.