An introduction to promethium

Promethium demand and end-uses

Promethium is a chemical element with the atomic number 61, meaning that all atoms of promethium contain 61 protons in their nucleus. This number uniquely identifies the element on the periodic table. However, unlike most other lanthanides, promethium does not have any stable isotopes, meaning all of its forms are radioactive and decay over time. The different isotopes of promethium are distinguished by their mass number, which is the sum of protons (61) and neutrons in the nucleus.

One of the most significant isotopes of promethium is promethium-147 (Pm-147), which has a mass number of 147, meaning it contains 61 protons and 86 neutrons. This isotope is particularly notable due to its half-life of 2.62 years, allowing it to be used in practical applications before it decays. Other isotopes, such as Pm-145 or Pm-148, exist but are either too short-lived or not as viable for industrial or commercial applications. Because promethium has no stable isotopes, those with the best balance of longevity and predictable radioactive emissions are the most valuable.

The reason Pm-147 is widely used lies in its radioactive decay properties. As it breaks down, it emits beta radiation (high-energy electrons) but does not produce dangerous gamma radiation, making it relatively safer to handle compared to other radioactive materials. This characteristic allows promethium-147 to be used in a range of specialised applications, particularly where a stable and predictable emission of energy is required.

One of the most significant applications of promethium-147 is in nuclear batteries, specifically beta voltaic cells. These batteries convert beta radiation into electricity, offering long-lasting power for applications where maintenance is difficult or impossible. Promethium-based batteries have been used in space probes, military equipment, and medical devices such as pacemakers, although tritium-based alternatives have become more common in recent years. Their long-term reliability and compact design make them invaluable in deep-space missions, defence technology, and medical implants.

Another key use of promethium is in luminous paints and signal markers. Historically, promethium was used in military equipment, such as aircraft dials, watch faces, and emergency exit signs, due to its ability to emit a steady glow without the need for an external power source. However, concerns about radiation exposure and the availability of safer alternatives, such as tritium, have led to a decline in this application. Despite this, its ability to provide stable luminescence for extended periods remains an area of interest for some specialised applications.

Promethium also plays a role in portable X-ray sources, particularly in non-destructive testing and certain medical imaging technologies. Its radioactive properties allow for compact and efficient X-ray generation, which is useful in industrial inspection and medical diagnostics. Additionally, there is ongoing research into promethium’s potential as a heat source for radioisotope thermoelectric generators (RTGs), which could be useful for deep-space missions and remote power generation. RTGs rely on the steady release of heat from radioactive decay to generate electricity, making Pm-147 a potential energy source for spacecraft and planetary exploration where conventional solar power is impractical.

The widespread use of promethium-147 in these applications highlights its unique advantages despite its challenges. While its supply is constrained due to its short half-life and reliance on nuclear reactor by-products, ongoing research continues to explore new possibilities, including potential roles in advanced medical imaging, security sensors, and next-generation energy storage solutions. The continued development of promethium-based technologies may further expand its role in aerospace, defence, and scientific research, ensuring its relevance in highly specialised fields.

Promethium supply

Promethium is an extremely rare element that does not occur in significant quantities in naturally occurring minerals. Unlike other lanthanides, it has no stable isotopes, and any promethium that forms in nature quickly decays due to its short half-life. While it is not found in extractable concentrations, trace amounts can sometimes be detected in certain rare earth minerals.

One such mineral is monazite, a phosphate mineral that contains a mix of rare earth elements. While monazite deposits are a significant source of other lanthanides such as neodymium and cerium, they may also contain tiny traces of promethium. However, due to its rapid radioactive decay, any naturally occurring promethium in monazite is usually undetectable. Similarly, bastnäsite, a carbonate-fluoride mineral rich in rare earth elements, may contain minute amounts of promethium, but its presence is negligible due to the element’s instability.

Another mineral associated with promethium is uraninite, also known as pitchblende. This uranium-rich mineral is a natural source of radioactive decay products, including promethium. As uranium undergoes fission, it can produce small amounts of promethium, but due to its short half-life, any naturally occurring promethium in uraninite is constantly replenished and decayed. Beyond minerals, promethium is also produced through the spontaneous fission of uranium and thorium. In minerals that contain these radioactive elements, such as thorite or uraninite, the continuous decay process generates trace amounts of promethium. However, because the element decays rapidly into other elements, it is not found in any significant, extractable quantity.

Despite its limited demand, promethium's supply is highly constrained, as it is found only in trace amounts in uranium ores and must be extracted from spent nuclear fuel. This rarity, combined with strict regulatory controls due to its radioactive nature, makes promethium a strategically significant element. While its current market remains niche, advancements in nuclear energy, space exploration, and specialised battery technologies may drive future interest in this rare element. Rather than being extracted from minerals like other rare earth elements, promethium is primarily obtained as a by-product of nuclear reactor operations, where it is produced through the fission of uranium and plutonium isotopes. This dependency on nuclear fuel processing makes its availability highly controlled and subject to strict regulations.

Production of Promethium-147

Promethium-147 (Pm-147) does not naturally occur in significant amounts on Earth due to its short half-life of 2.62 years. Any promethium that originally formed when the Earth was created has long since decayed. As a result, Pm-147 must be artificially produced as a by-product of nuclear reactions, primarily in nuclear reactors. The process involves either fission reactions in nuclear fuel or neutron activation of precursor elements.

The most common method of obtaining Pm-147 is through nuclear fission in reactors. When uranium-235 (U-235) or plutonium-239 (Pu-239) undergoes fission, they break apart into various smaller isotopes, one of which is promethium-147. However, Pm-147 is not directly extracted from uranium ores or minerals, as its short half-life means it does not persist in natural deposits.

Historically, Russia dominated the global production of promethium-147 through fuel reprocessing methods. However, recent geopolitical developments have disrupted this supply chain, creating opportunities for new market entrants. In response, the Department of Energy Isotope Program (DOE IP) in the United States has begun entering the promethium-147 market to address critical supply needs.

To extract promethium-147 from nuclear reactor waste, several steps are required. First, the fission of uranium-235 or plutonium-239 in a nuclear reactor produces a variety of radioactive isotopes, including rare earth elements such as neodymium, samarium, and promethium. After a period of irradiation, spent nuclear fuel is removed and chemically processed to extract useful by-products. Promethium is then separated from other lanthanides such as neodymium and samarium using techniques like solvent extraction and ion exchange, which take advantage of its distinct chemical properties. Finally, the extracted promethium undergoes purification and isolation to obtain high-purity Pm-147, which is then used in nuclear batteries, luminous materials, and industrial measurement tools.

Promethium-147’s reliance on nuclear reactor operations for its production means that its availability is closely tied to the nuclear energy industry. Its unique beta radiation emission properties and lack of significant gamma radiation make it valuable in nuclear power sources, industrial applications, and space technology. However, its production remains strictly regulated due to radioactive handling requirements and international safety protocols.