Radioisotope thermoelectric generator

A radioisotope thermoelectric generator (RTG, RITEG), or radioisotope power system (RPS), is a special kind of nuclear battery. It uses tiny parts called thermocouples to turn the warmth from a radioactive material into useful electricity. This works because of something called the Seebeck effect. Because there are no moving parts, these generators can work for a very long time without breaking, even in tough places.

RTGs are often used when we need power in places that are hard to reach or very far away. They can give a few hundred watts of power for many years. This makes them perfect for things like satellites and space probes, where we can’t visit to fix or change the power source. They have also been used in far-off places on Earth, like lighthouses built by the Soviet Union in the cold Arctic Circle.

It’s important to keep the radioisotopes — the materials that give off the heat — safely contained, even after the generator stops working well. Because making these generators costs a lot, they are only used in special cases where other power sources wouldn’t work well.

History

The RTG was created in 1954 by scientists Kenneth Jordan and John Birden. They worked on a project to turn heat from radioactive materials into electricity.

RTGs were first used in space in 1961 on a United States spacecraft. They are very useful for powering spacecraft that travel far from the Sun, where solar panels don’t work well. RTGs have powered many famous missions, including Voyager, Cassini, and the Mars rovers Curiosity and Perseverance. They have also been used in lighthouses, remote Arctic equipment, and even in some heart pacemakers in the past.

Design

The design of an RTG, or radioisotope thermoelectric generator, is very simple. It has a strong container filled with a special radioactive material that gives off heat. Inside the container walls are tiny devices called thermocouples. These thermocouples use the heat from the radioactive material to make electricity.

A thermocouple is a special tool that can turn heat directly into electricity. It is made from two different metals or materials joined together. When one end is hot and the other end is cool, it creates an electric current. Many thermocouples are linked together to produce more power. Unlike nuclear reactors, RTGs do not need control to change how much power they make. The heat comes from the natural breakdown of the radioactive material and cannot be turned up or down. This means extra batteries might be needed for extra power, and the RTG must stay cool at all times. While RTGs are very safe and cannot explode, there is a small chance of spreading radioactive material if damaged.

Developments

Because there isn't enough of a special material called plutonium-238, scientists suggested a new way to make RTGs. This new method uses the energy from the material's decay in special reactions to create a long-lasting source of tiny particles called neutrons. This idea was shared with NASA in 2012 and studied further in 2013.

RTGs have also been suggested for very far-reaching space missions, like sending probes into deep space. For example, a plan called the Innovative Interstellar Explorer, which started in 2003, considered using RTGs for these missions. Some ideas include using different materials to power these space probes for many years. The electricity from RTGs could help run scientific tools and send messages back to Earth. There have also been ideas to use this electricity to help move the probes through space. Scientists are always looking for ways to make RTGs even better.

Models

A typical RTG uses the heat from radioactive decay to make electricity. Some systems work in a similar way but are a bit different.

Space

Main articles: Nuclear power in space and List of nuclear power systems in space

Spacecraft sometimes use these systems for power. For example, some parts left on the Moon and rovers on Mars have small heaters that use this idea. Different spacecraft use different amounts of material, like the Curiosity rover, which has about 4.8 kg of plutonium-238 dioxide.

Some systems are not exactly like RTGs. One used a special reactor with thermocouples to turn heat into electricity. Another used a different kind of fuel and coolant to make power. And another uses a device called a Stirling engine with radioisotope heat.

Terrestrial

Fuels

The radioactive material used in radioisotope thermoelectric generators (RTGs) must have special qualities:

1. Its half-life should be long enough to release energy steadily over time. Isotopes with longer half-lives release energy more slowly. Common isotopes for RTGs last several decades, but shorter ones can be used for special jobs.
2. For space use, the fuel must produce a lot of power for its weight and volume. The amount of energy released can be figured out if we know how much the material’s mass changes during decay.
3. The radiation should be easy to turn into heat, preferably alpha radiation. Some types of radiation need heavy shielding, so the fuel should not produce too much of these.

These rules limit the choices to fewer than thirty possible materials.

Plutonium-238, curium-244, strontium-90, and americium-241 are often mentioned, but many more were considered in the 1950s.

The table below does not list power densities for the pure material but for a chemically inert form.

²³⁸Pu

Plutonium-238 lasts 87.7 years, produces 0.57 watts per gram, and has very low levels of certain types of radiation. It needs very little shielding. It is commonly used in RTGs, made as plutonium(IV) oxide (PuO₂).

⁹⁰Sr

Strontium-90 was used in some RTGs. It lasts 28.8 years and produces 0.95 watts per gram. It is usually changed into a stable form called strontium titanate (SrTiO₃) for safety.

²¹⁰Po

Polonium-210 was used in early RTG prototypes. It produces a lot of power but only lasts 138 days. It needs very little shielding but can be dangerous if swallowed.

²⁴¹Am

Americium-241 could be used in RTGs. It lasts much longer than plutonium-238 but produces less power and needs more shielding. It is used in smoke detectors.

²⁵⁰Cm

Curium-250 produces energy through a process called spontaneous fission. It lasts about 8300 years but needs special shielding for a type of radiation called neutrons.

Life span

Most radioisotope thermoelectric generators use a special material that slowly loses its power over time. This material loses just a tiny bit of its strength each year, so the generator makes a little less power every year.

One famous example is the power system used on the Voyager space probes. Even after many years, these generators still work well, though they make less power than when they started. Scientists are always finding new ways to make these generators last even longer and work better.

Safety

Radioactive materials in RTGs can be dangerous and might be used for harmful purposes. Even though they aren’t useful for making real nuclear weapons, they could still be used in a "dirty bomb". The Soviet Union made many uncrewed lighthouses and navigation beacons using RTGs with strontium-90 (⁹⁰Sr). These are very reliable and give a steady power supply. Most don’t have any protection, and some places where they were put are now unknown because records weren’t kept well.

RTGs can also pose risks if the container holding the fuel leaks, and the radioactive material might spread into the environment. For spacecraft, there is worry that an accident during launch could release harmful material into the atmosphere. However, with current designs, this is very unlikely. The fuel is stored safely in strong materials to minimize these risks.

There have been a few accidents with RTG-powered spacecraft, but the designs have been made safer because of these events. For example, when a satellite failed to reach orbit, the RTG burned up in the atmosphere, but no harmful material was released because of the strong design. These accidents helped improve future RTG safety.