At first glance, this might seem like the same process that takes place in nuclear reactors, but it isn’t. Nuclear reactors generate heat by fission, a process in which atoms are split by a bombardment of neutrons. In an RTG, heat is generated only by the natural radioactive emissions of the isotope contained in it. This is a much slower process, but the advantage is its stability; it can never go out of control and melt down.
Another advantage of RTGs is that they contain no moving parts. This is beneficial because moving parts tend to break and wear out over time. RTGs make their electricity by using thermocouples. To understand how a thermocouple works, lets take a trip back to 1821 to the great country of Estonia. There, scientist Thomas Seebeck found that a temperature difference between two dissimilar metals produces a voltage. Like a true scientists, he discovered this accidentally, and named the phenomenon after himself , the “Seebeck Effect”.
The “Seebeck Effect” is quite useful in an RTG. If one end of a thermocouple is placed next to a hot radioisotope and the other is connected to a heat sink in the cold, hostile void of space, a temperature difference is created, providing a continuous source of energy limited only by the isotope’s decay rate.
This brings up an important point: What isotopes are good candidates for powering an RTG? Firstly, a good isotope should decay at such a rate that it gives off a usable amount of heat, but does not decay so fast that it disappears quickly. The decay rate is measured by a factor called half- life, the time it takes for half of an isotope’s mass to disappear . The half-life for isotopes used in most RTGs is between 20 and 100 years.Secondly, the isotope should have a high energy density. This is especially important in space, when every gram of weight is at stake. The higher the energy density, the smaller the RTG can be.Thirdly, the isotope should emit only alpha-type radiation. Alpha radiation is easily absorbed and can be stopped. This means that it requires the least amount of shielding to protect people and the environment from the ionizing radiation. Since shielding equals weight, the less the better. Some isotopes emit beta and gamma radiation, both of which are more penetrating than alpha particles. This makes such isotopes less than ideal candidates, even if they have good energy output.
But a major setback with RTG’s is that their prefered fuel ( 238Pu) is becoming a rare sight. The United States of America stopped producing its own 238Pu in 1988, and started buying its supply from Russia. in total 16.5 kg has been purchased, but Russia is no longer producing 238Pu and their own supply is reportedly running low.
With these principles in mind, a continuous, reliable source of energy can be created that will last for decades. The Voyager 1 and 2 satellites, which were launched in 1977 to Jupiter and Saturn, were only designed to operate for five years, but the RTGs however have been running for about 28 years and they’re projected to run for much longer. Their RTG’s were successfully used to power scientific instruments and communications, which today has become normal for an RTG to do.
Engineers have realized with this kind of potential, they don’t have to limit the use of RTGs to the outer reaches of space. Many RTGs have been used to power remote lighthouses, buoys, and other hard to reach places because of their longevity .
Nevertheless, NASA and the Department of Nuclear Energy continue to improve the design to (the still in development) ASRGs or advanced stirling radioisotope generators. These new generators will use less plutonium cells and weigh less when successfully deployed.
Who knows? Someday we might have RTGs powering our MP3 players and cell phones. Just make sure to slip them in your lead-lined pants pocket.
