Plutonium-238

Perhaps the best known radioisotope for RTG use is plutonium 238, or 238Pu. Its practical use after discovery was pioneered by the Mound Laboratories in the 1950s, who were the providers of many different radioisotopes, as well as radioisotope powered systems of many sorts.

238Pu is rare in that it only emits alpha radiation, meaning that the fuel pretty much shields itself. It also emits a useful amount of heat for either producing electricity or heat, and over a useful timeframe. There are many other options for radioisotope fuel for RTGs, but of the flown missions the vast majority have used 238Pu for fuel. We wont go into the other fuel types here, but I’m currently working on a page on RTGs which will go into the different options.

238Pu has a half-life of 87.7 years, and when it decays it emits a single 5.593 MeV alpha particle to become the geologically stable uranium 234. Its emissions of gamma and beta radiation are incredibly small as a pure metal. However, other properties of plutonium dictate that it be formed into an oxide. This is fine, and can easily be worked with, but other options (such as the use of CERMET) have been proposed for other radioisotopes (241Am), and could possibly be used with 238Pu as well.

Like many of the transuranic isotopes that aren’t considered fissile, 238Pu does undergo some spontaneous fission, causing a small amount of gamma and neutron radiation, but these are among the lowest (if not the lowest) gamma and neutron fluxes of a short-lived transuranic isotope.

The following tables show the radiation flux of 238Pu in the various spectra, from Stoddard et al, Savannah River, in 1965 (while the fuel was still under development). If you want more detailed information, that source (linked first in the References section) is an excellent starting point.

Manufacture of 238Pu

238Pu is produced by irradiating specially prepared targets of Neptunium 237. This can either be extracted from spent nuclear fuel (or removed during batch reprocessing in a fluid fueled reactor), or be created by first bombarding a target of 235U for long enough that it will undergo double neutron capture to produce the Np target.

The initial production of 238Pu was done at the Savannah River Site (SRS), with much of the research and development into its conversion into radioisotope heat sources occurring at Mound Laboratory. While 238Pu is a low-occurrence but significant poison for nuclear weapons fissile components, it is in such low (about 3-6%) concentrations as to be impractical to isotopically separate from the Pu being generated to meet the needs of a space program.

The targets would be irradiated at SRS, then transported to Mound, where they would be chemically seaparated and prepared for formation into the final PuO2 fuel form. There were a wide variety of fuel forms in the early days of RTG design, everything from cylinders of every size to spheres to some experimental shapes that quickly disappeared as too unworkable. Both cylinders and spheres remained popular fuel forms until the current era of RTGs.

Current US Production of 238Pu

The current American stocks of 237Np are produced and held at Idaho National Laboratory. This is then shipped to Oak Ridge National Laboratory, where it’s converted into a CERMET of NpO2 and aluminum. This is then placed in the High Flux Isotope Reactor (HFIR), where it undergoes one final neutron capture before going through a rapid beta decay and becoming 238Pu. This is then easily chemically separated, formed into PuO2, and prepared for the next stage in the fuel pellet formation process.

The formation of the 237Np target into the appropriate form factor has just recently been automated at Oak Ridge NL, which promises to lower the cost of producing 238Pu significantly. This also means that the staff at ORNL are able to focus on other aspects of the supply chain, increasing availability of 238Pu for future missions.

Concerns in the US 238Pu Supply Chain

Unfortunately, due to a number of organizational and planning difficulties within NASA and the DOE, who supply the 238Pu oxide fuel, the radioisotope itself is becoming scarce. With less fuel available, and more powerful sensors needing support from more powerful transmitters to get all the information back to Earth, the increased use of electric propulsion, and other power requirements, the transition is almost being forced on NASA’s mission planners.

Fortunately, recent advances in the precursor “target” for fuel production (https://www.scientificamerican.com/article/new-supply-could-prevent-deep-space-plutonium-shortage/ ), as well as a widely-cited Government Accountability Office report (https://www.gao.gov/products/GAO-18-161T ), seem to offer cautious promise that the fuel supply will begin to flow again soon.

Oak Ridge completed their first round of 238Pu production in late 2015 after a gap of over 30 years in the domestic program. Current reporting on the 238Pu supply seems to indicate that there will not be a significant shortage of 238Pu for NASA’s missions that would require 238Pu, but it’s not clear if this would be true with current technology (the MMRTG and GPHS-RTG), or if this is assuming the use of the ASRG and Next-Gen RTG, which are either under development or waiting for a mission to be assigned.

238Pu Fuel Element Design

The only use to date of 238Pu in RTGs is as Pu(IV)O2, usually shortened to PuO2. The IV indicates the crystalline structure of the oxide, being a face-centered cubic crystal rather than the more common body-centered cubic structure. This means that the thermal conductivity is slightly higher, allowing for greater heat transfer, and is slightly more dense, allowing for higher power densities in the fuel. In order to ensure the proper crystalline structure, the temperature, pressure, and stoichometric ratio of the materials being reacted are carefully controlled, and then the resulting molten oxide is cast into the proper shape.

PuO2 is a well-understood, stable chemical structure, but complicates the radiation situation due to a reaction with two isotopes of oxygen in the chemical structure of the fuel: 17O and 18O both produce neutron radiation when bombarded with alpha particles, and as such they increase the neutron radiation amount by ten times! However, this flux is still extremely low, and isotopic enrichment brings the neutron production well within an acceptable dosage.

Similarly, there is a gamma flux from brehmsstralung radiation as the alpha particles are quickly slowed by the oxide and surrounding clad materials. This flux is very low compared to other materials, however, and is well within comfortable flux levels for an RTG.

While the most common form factor today (the GPHS fuel element) is a domed cylinder, a host of different fuel element shapes have been used, with a variety of clad materials. Iridium alloys were quickly selected as the best material to be in direct contact with the 238Pu, but the shape was in a state of high flux for a long time.

The SNAP-19 fuel was a plain cylinder, while the SNAP-27 used microspheres of 238Pu poured into an annular cylinder with a void for helium buildup to occur in and minimize the damage to the fuel pellets and mechanical structure. These microspheres became ~2cm spheres in the GPHS-RTG that powered Voayger, but they weren’t robust enough to survive severe mechanical trauma, and also had a lot of excess empty space in the fuel canister – beyond what was needed for He buildup. This spurred the creation of the GPHS program, which was meant to provide greater volumetric efficiency, mechanical integrity, and applicability to a host of RTG designs, rather than designing a new fuel element each time you wanted to design a new RTG.

This evolution will be looked at more in depth on the Radioisotope Fuel Forms page, coming soon!

Use of 238Pu

238Pu has fueled many of the RTGs that have been flown over the years, as well as those that have been used in naval and air defense capacities. The first satellite to be powered by an RTG was fueled with 238Pu oxide, as were almost all of the missions to the outer solar system (Juno being the single exception). It is also currently powering Voyagers 1 and 2, New Horizons, the Curiosity rover, and will power the Mars 2020 rover as well.

The most well-known of the 238Pu-fueled RTGs that have been used are the SNAP-19 (used by the Viking landers and Pioneers 10 &11), the SNAP-27 (used by the lunar astronauts for the ALSEP experiment package), the MHW-RTG (used by the Voyager spacecraft), the GPHS-RTG (which replaced the MHW-RTG), and the MMRTG (which powers the Curiosity and Mars 2020 rovers). While there was significant diversity in the shape and size of early RTG fuel pellets, including microspheres, several-inch-wide spheres, and codes of various sizes, later designs focus on the GPHS fuel element in the GPHS-RTG and MMRTG, which is a cylinder that is domed at top and bottom.

For more information on fuel element geometry, check out the Radioisotopes fuel page, here.

The advantages that 238Pu offers are not going to diminish with time, and as interest in deep space and outer solar system exploration increases, 238Pu will remain a valuable and popular power source for spacecraft for the indefinite future. New, smaller RTG designs offer more efficient utilization of 238Pu in ever-smaller mission packages, which allow more science to be collected, and by reviving older designs with modern materials and engineering for greater power and efficiency, the age of 238Pu’s contribution to exploration and science may just be beginning

Beyond NERVA

Manufacture of 238Pu

Current US Production of 238Pu

Concerns in the US 238Pu Supply Chain

238Pu Fuel Element Design

Use of 238Pu

Further Reading and References

Fuel Element Design

Radioisotope Surrogate Testing

Isotope Production

Precursor Production

Irradiation

Preparation for Fuel Element Manufacture

Fuel Matrix

Oxides

Alloys

Carbides

Composite

Fuel Element Manufacture

Environmental and Radiological Impacts

Aging and End of Life Considerations

Facilities for 238Pu Production