Perhaps no isotope is more notorious in the nuclear community than 210Po. This incredibly chemically toxic, short-to-medium lived radioisotope gives off powerful alpha radiation, meaning that if the chemical toxicity doesn’t get you, the radiation sickness will. This is best exemplified by the poisoning of Alexander Litvinenko, and exposure of many others to this toxic substance, in 2006. (A report on the specific findings can be found in the “safety” section of the sources.)
However, the fact that it’s a reasonably short-lived isotope with high alpha radiation activity also makes it very attractive for short mission time radioisotope power sources, especially radioisotope thermal rockets. In that particular subtype of thruster, 210Po offers some advantages to offset the difficulties in handling and integration that would come about.
Properties of 210Po
210Po has a half-life of only 138 days, and decays 100% of the time via alpha decay into lead 206. it has a melting temperature of 255 C at 95% purity, with a specific power of a massive 144 W/g (compared to 0.56 W/g for 238Pu), and a power density of 1210 W/cm^3. However, the more common form it was used in was as a GdPo alloy, with the gadolinium increasing the melting temperature significantly while leaving an incredibly healthy specific power of 54 while increasing its melting point all the way to 1675 C, close to 244Cm but still more than 500 C less than 238Pu.
The upshot to this is that 210Po excels at short mission times that need high power density with minimal shielding impact on the overall mass budget.
To make 201Po, natural bismuth 209 is bombarded with neutrons. 209Bi undergoes neutron capture, and become 210Bi. This goes through a beta decay (with a 5.4 day half life) to become 210Po.
In the days of Mound Laboratory, the 209Bi was irradiated at the Savannah River Site, the location of many of the gaseous diffusion plants that were being used for the weapons program. It was then cast in aluminum and irradiated. After irradiation, it was shipped to Mound.
There it would undergo a complex process to be isolated as >95% pure Po, and the amount of radioactivity for each resulting foil of metal. These foils of 210Po were placed on Pt gauzes, which were then sealed in a helium-filled tube (called the “gun”). This pure metal would then be used to form different types of fuels, with different chemical compositions and physical properties, as well as experimenting with fuel shapes and other characteristics.
There were apparently issues with the separation of the Po from the Bi in the process that was used at the time of this source document (1969), and they were researching more economical methods of separating out the fuel from the unirradiated Bi to improve production cost and scale.
The most common fuel matrix seems to be an alloy of 210Po with refractory metals (predominantly gadolinium or tantalum) to form a high thermal conductivity, high melting temperature fuel which requires minimal shielding for the fuel element.
However, according to a summary report from the Mound Lab,
High temperature compounds were prepared of polonium-210 with all fifteen of the rare earth elements, plus yttrium, scandium, uranium, and thorium.
Mixed oxide compounds of polonium and group II elements were prepared and studied.
X-ray crystallographic structure and density determinations were carried out on most of the rare earth and actinide polonides that were synthesized.
Melting points were determined for all the polonides that were synthesized.
The phase diagram was determined for the gadolinium polonide system as a function of lead concentration, or in other words, time after preparation.
The vaporization rates of gadolinium
polonide, … were studied with a
time-of-flight mass spectrometer. The
vapor pressure of gadolinium polonide
was estimated from measurements made on
analogous selenide and telluride systems.
Emissivity measurements were carried
out on gadolinium and dysprosium polonides
in the 1000 to 2000°K temperature range.
Neutron emission and spectra and gamma
emission and spectra were measured on
numerous polonium fuel samples.
Fabrication parameters and techniques
were developed for preparing a rare earthtantalum matrix fuel form. The variation
of the mechanical and physical properties
of the tantalum matrix fuel form was determined.
Superalloy and other metal matrix fuel
forms were studied and their compaction
Compatibility tests have been completed on polonium and gadolinium
polonide with refractory metals and
alloys, noble metals, and superalloys
at temperatures up to 1500°C and for times
up to 200 days. …
Three tests were carried out with the
metal fuel form to determine its distribution characteristics inside various geometry containers.
Mound Laboratory, 1969
This laid the groundwork for 210Po to be an available, well-characterized material. It still needed improvement in the refinement side, but as a material for a designer to work with it was mature and ready for application.
Not much research into 210Po has occurred recently in regards to astronuclear engineering outside a design concept level, but it remains a valuable alpha source for some day to day health physics and industrial applications. The environmental and health regulations surrounding it make it more difficult to deal with than some other sources, though, so it’s not as common as some.
Fuel Element Design
While the pure Po metal is used on occasion in reference to RPS fuel elements, the melting point of the pure metal is inconveniently low, at 256 C. This can be improved greatly by alloying the Po with gadolinium, which increases the melting temperature to a quite acceptable 1675 C. While this ends up reducing the specific power from 144 to 82 W/g, this is still higher than any other practical radioisotope in its pure form, much less a workable fuel form.
For more information on RHU fuel elements, check out the Radioisotope Fuel Selection page and the Radioisotope Fuel Element design pages, available soon!
Designs Incorporating 210Po
210Po was far more popular in the early space race than it is today, due to a combination of factors such as health and environmental regulation changes, mission timeline increases, and other contributing forces. However, as the half-life dictaties, 210Po was used as a short-term, high-power heat source for RHU use, primarily directed out of the Mound Laboratory on the fuel synthesis and fabrication side.
Perhaps the best known in nuclear circles is Project Poodle, which you can find out more about here. This radioisojet thruster was a supplementary propulsion system, sort of like a cross between a kick stage and a JATO bottle. The short (by mission timeline standards) half-life means that it has to be used fairly quickly, and its efficiency is always going down, but leaving the current gravity well to go to the next is one of the most thrust intensive times, and a booster with all the functional simplicity of a solid rocket booster have their attractions.
Other short-timeline mission power sources, such as the SNAP-3, SNAP-7, SNAP-21, SNAP-23, and SNAP-29 RTGs, also used 210Po. More information (somewhat sparing, but there’s a links list to primary sources) can be found on the SNAP RTGs page here.
Further Reading and Sources
Isotope Production and Nuclear Characteristics
Polonium-210 Program Issued: January 22,1969, Mound RL https://www.osti.gov/servlets/purl/934541
A COMPILATION OF SOME RADIATION PROPERTIES OF VARIOUS PLUTONIUM-238 AND POLONIUM-2 IO HEAT SOURCES 1969 https://www.osti.gov/servlets/purl/4455640
Fuel Element Geometry, Clad, and Manufacture
SNAP RTG Page (SNAP-3 and SNAP-29)
Radioisotope Thermal Rockets page (coming soon!)
The polonium-210 poisoning of Mr Alexander Litvinenko John Harrison et al 2017 https://www.osti.gov/servlets/purl/1346669