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| Fig. 1: Diagram of RTG used for New Horizons (Source: Wikimedia Commons) |
Some applications require technology to be fueled for long periods of time without maintenance. Space missions, for example, require long-lasting, consistent sources of energy in order to perform reliably. Many of these missions have historically used nuclear power, which has often been produced by radioisotope thermoelectric generators, or RTGs for short. RTGs are especially useful for this application because they are relatively reliable and are able to produce energy for as long as the isotopes powering them have not completely decayed.
RTGs were first created in the mid-1950s by Kenneth Jordan and Robert Birden at the US Atomic Energy Commission (AEC). They have been used on many NASA spacecraft, including those flown for Apollo and Voyager missions. [1]
Radioisotopes are unstable elements that have an unbalanced number of protons and/or neutrons, which release radiation and energy as they break down and move towards stability. Thermoelectric generators convert heat to energy, through something called the Spin Seebeck Effect. The Seebeck effect utilizes temperature gradients within magnetic objects to generate electric power. The Spin Seebeck Effect specifically uses thermal energy to generate a "spin" that produces a charge current within conductive materials [3]. Putting these definitions together, RTGs work by utilizing heat from the decay of radioisotopes such as Plutonium-238, which is the most common element used, and then transforming the thermal energy into electric energy that fuels the spacecraft. [2,5]
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| Table 1: Development of RTGs. [1] |
Table 1 compares the properties of various RTGs over time. Earlier RTGs from the 1960s and 70s had a conversion efficiency of around 4-5%, though over time this has improved to around 6% in the modern day. Also noticeable is the significant increase in the power over time, along with mass. All of the RTGs in Table 1 use Pu-238 as a fuel.
The specific power output of Pu-238 per kilogram is calculated its half-life of 87.7 years, and mass: [4]
| λ | = | ln(2) 87.7 y × 365 d y-1 × 24 h d-1 × 3600 s h-1 |
= | 2.506 × 10-10 s-1 |
| N | = | 6.022 × 1023
atoms mole-1 0.238 kg mole-1 |
= | 2.530 × 1024 atoms kg-1 |
| Specific Activity | = | λ × N | = | 6.340 × 1014 atoms kg-1 s-1 |
| Energy per decay | = | Ed | = | 5.5 × 106 eV atom-1 × 1.602 × 10-19 J eV-1 | = | 8.811 × 10-13 J atom-1 |
| Specific Power | = | Ed λ N | = | 8.811 × 10-13 J atom -1 × 6.340 × 1014 atoms kg-1 s-1 | = | 559 Watts kg-1 |
The SNAP RTGs were developed by the AEC in the early years following the invention of the machine. There were 27 iterations, and table 1 shows the improvement that resulted over the course of a decade or so (3B was the first flown SNAP vessel). Multi-Hundred Watt (MHW) RTGs were developed soon after, using silicon germanium material and different heat sources. The General Purpose Heat Source (GPHS) RTG was created in the 1990s, improving the safety rating of RTGs, as well as producing the highest specific power at ~5 W/kg, but it is meant for operating only in vacuum. A diagram of one example of a GPHS is pictured in Fig. 1. Multi-mission RTGs were developed in the early 2000s, and have an expanded scope of function, as they can operate both in the atmosphere and in outer space. They have the most versatility of the RTGs, but the specific power is relatively low compared to the other modern mechanisms, at 2.8 W/kg. [1]
RTGs are not only used for space travel, but also for researching planets themselves, including Earth. They can be used for underwater probes and sensors, or machines running in the Arctic. [2]
One area that is being considered is changing the isotope used from Plutonium-238 to a potentially safer element. One study focused on Americium-241, Strontium-90, Polonium-208, and Polonium-210, and found that Am-241 and Polonium isotopes both have potential for use in space missions, with the latter being promising for short-term missions that require a high power density. [6]
Another study came up with a design for micro-RTGs, which can be used for small probes or sensing devices for outer space missions that require compactness. [7]
Radioisotope thermoelectric generators are an important machine that have been used to successfully power many space missions, and have great potential to develop for even more functionality and adaptability in the future.
© Divya Bhojraj. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] D. F. Woerner, The Technology of Discovery (Wiley, 2023).
[2] F. Tohidi, S. G. Holagh, and A. Chitsaz, "Thermoelectric Generators: A Comprehensive Review of Characteristics and Applications," Appl. Therm. Eng. 201 A, 17793 (2022).
[3] K.-I. Uchida et al., "Thermoelectric Generation Based on Spin Seebeck Effects," IEEE 7452553, Proc. IEEE 104, 1946 (2016).
[4] P. Dedeler, "Radioisotope Power for Fueling Space Missions," Physics 241, Stanford University, Fall 2023.
[5] S. Henriques, "Seven Things to Know about Radioisotopes," International Atomic Energy Agency, IAEA Bull. 55, No. 4, 6 (2014).
[6] R. C. O'Brien et al., "Safe Radioisotope Thermoelectric Generators and Heat Sources for Space Applications," J. Nucl. Mater. 377, 506 (2008).
[7] Z. Yuan et al., "Screen-Printed Radial Structure Micro Radioisotope Thermoelectric Generator," Appl. Energy, 225 746 (2018).
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