Why Aren't We Using Actinium-227 for RTGs?

Radiation-emitting compounds, known as radioisotopes, are used in radioisotope thermoelectric generators (RTGs) to provide long-lasting energy for spacecraft and other applications. One such radioisotope is Actinium-227 (Ac-227). Despite its properties, it is less commonly used for RTGs in comparison to Strontium-90 (Sr-90). This article explores the advantages and disadvantages of using Ac-227 in RTGs and why Strontium-90 is more prevalent.

The Advantages of Actinium-227

Actinium-227 has a significant advantage over other radioisotopes like Strontium-90 in terms of power density. It is known to produce a higher power density, which can be a critical factor in space missions where efficiency and energy output are paramount. Additionally, Ac-227 is mostly an alpha emitter, which has fewer complications in shielding and containment compared to beta or gamma emitters.

However, there are some drawbacks to consider. Ac-227 can also emit beta rays and, in some cases, gamma rays. While this might seem appealing for its higher energy output, the presence of gamma rays poses challenges for the design of shielding and containment systems for RTGs. This factor makes it less convenient for practical use in RTGs, where the emission of gamma rays needs to be carefully controlled to ensure safety and protect electronic components.

Availablity of Actinium-227

The main reason for the limited use of Actinium-227 in RTGs is its availability. Sr-90 is readily available as a common nuclear fission product from the fission of U-235. On the other hand, Ac-227 can only be produced through neutron activation of Radium-226 (Ra-226). This process works fine because Ra-226 has a high neutron absorption cross section, but there is a significant challenge in acquiring Ra-226.

Radium is extremely rare, approximately 1/10,000,000th as abundant as uranium. This scarcity makes it difficult to obtain sufficient amounts of Radium to produce Actinium-227 on a large scale. The production of Strontium-90, in contrast, is more straightforward as it can be extracted from spent nuclear fuel, which further contributes to its availability and practical use.

Cost and Availability

Another key factor in the selection of radioisotopes for RTGs is cost. Actinium-227 is more expensive to produce and procure due to the limited availability of Radium. Strontium-90, being a byproduct of nuclear fission, is much more abundant and therefore less expensive. This cost and availability issue further limits the practical use of Ac-227 in RTGs.

In conclusion, while Actinium-227 offers some advantages such as high power density, its limited availability and the challenges posed by its gamma-ray emissions make it less suitable for RTGs compared to Strontium-90 and other more readily available radioisotopes. However, research and development in producing and using Actinium-227 in RTGs could potentially change this landscape in the future.