Plutonium-238 for Deep Space Exploration

A radioisotope thermoelectric generator (RTG) is a type of power generator which converts heat produced from the decay of a suitable radioactive material into electricity. Plutonium-238 is normally employed in RTGs because it has a long half-life of 87.7 years and is a very powerful alpha emitter that does not emit other forms of more penetrating radiation. Alpha radiation can be easily blocked by something as thin as a sheet of paper. RTGs are commonly used to power spacecraft that travel to places in the Solar System where solar cells are not practical and where the mission duration is too long for batteries or fuel cells to be used. One kilogram of plutonium-238 produces 560 watts of power in the form of heat. Examples of spacecraft powered by RTGs include the Cassini spacecraft in orbit around Saturn, the Curiosity rover on Mars and the New Horizons spacecraft on its way to Pluto and beyond. All these missions are made possible by the availability of Plutonium-238.

Figure 1: Artist’s impression of NASA’s Curiosity rover on the surface of Mars.

Figure 2: Artist’s impression of NASA’s Cassini spacecraft in orbit around Saturn.

The United States ceased producing plutonium-238 in 1988 and since 1993, all plutonium-238 used to power spacecraft for deep space exploration were purchased from Russia whose own supply is already running low. Production of plutonium-238 needs to be restarted soon in order to produce sufficient quantities to support future deep space exploration missions. In the past, plutonium-238 is produced from neptunium-237 extracted from spent nuclear fuel taken out from uranium-fuelled light water reactors (LWRs). When neptunium-237 is extracted, it is bombarded with neutrons to get neptunium-238 which beta decays into plutonium-238. Plutonium-238 cannot be directly extracted from spent nuclear fuel because the presence of uranium-238 in LWRs also leads to the production of other isotopes of plutonium from neutron absorptions. Spent nuclear fuel from LWRs typically contains slightly over 1 percent of plutonium-238 out of the total amount of plutonium produced. Since isotopes are chemically identical, it is almost impossible to separate out plutonium-238 and this makes it necessary to extract neptunium-237 out of the spent fuel to produce plutonium-238.

Figure 3: The series of neutron absorptions and beta decays leading to the production of plutonium-238 in a LFTR.

In a previous article titled “NuclearPower for Lunar Settlements”, a radically different type of nuclear reactor is described. This type of reactor is known as a liquid fluoride thorium reactor (LFTR) and it is basically a reactor where the nuclear fuel is in the form of a fluoride-based molten salt mixture. The operation of a LFTR is attractive for the production of plutonium-238 because almost all of the plutonium it produces is plutonium-238 and allows for the direct chemical extraction of plutonium-238. This is because uranium-238 is not present to produce other isotopes of plutonium. Since the nuclear fuel in a LFTR is fluid in nature, the plutonium-238 can be extracted using a small adjacent chemical plant while the LFTR continues running with no downtime incurred. In a LFTR, fissile uranium-232 that is bred from thorium-232 is used to generate energy through the fission process. For every 1000 kg of naturally occurring thorium that is fed into a LFTR, 15 kg of plutonium-238 is produced as the end product. In comparison, NASA’s Curiosity rover uses 4.8 kg of plutonium-238 while the Cassini spacecraft uses 33 kg of plutonium-238.