Plutonium decays into uranium in most cases. Nearly every common plutonium isotope is an alpha emitter, meaning it releases a small particle (two protons and two neutrons, identical to a helium nucleus) and transforms into a uranium isotope with an atomic number two steps lower. The one major exception is plutonium-241, which takes a different path and becomes americium instead.
How Alpha Decay Works
When a plutonium atom undergoes alpha decay, it ejects a cluster of two protons and two neutrons from its nucleus. That ejection drops the atom’s mass by four units and its atomic number by two, converting it from element 94 (plutonium) to element 92 (uranium). This is the decay mode for plutonium-238, 239, 240, and 242. The ejected alpha particle carries significant energy, which is why plutonium samples generate heat.
Decay Products by Isotope
Each plutonium isotope produces a specific uranium daughter:
- Pu-238 decays into uranium-234, with a half-life of about 88 years.
- Pu-239 decays into uranium-235, with a half-life of roughly 24,119 years.
- Pu-240 decays into uranium-236, with a half-life of 6,561 years.
- Pu-242 decays into uranium-238, with a half-life of about 375,000 years.
- Pu-244 decays into uranium-240 (99.88% of the time), with a half-life of roughly 80 million years. A tiny fraction, about 0.12%, undergoes spontaneous fission instead, splitting the nucleus into two smaller atoms.
The daughter uranium isotopes are themselves radioactive and continue decaying through long chains that eventually end at stable lead or bismuth. Uranium-235, for example, passes through a series of over a dozen intermediate steps before reaching stable lead-207.
The Exception: Plutonium-241 Becomes Americium
Plutonium-241 is the odd one out. Instead of emitting an alpha particle, it emits a beta particle, which is a high-speed electron released when a neutron in the nucleus converts into a proton. This bumps the atomic number up by one rather than down by two, turning plutonium-241 into americium-241.
This matters because americium-241 is an alpha emitter with a 432-year half-life, and it’s considerably more radiotoxic than its plutonium parent. After the Chernobyl disaster, researchers tracked how the released plutonium-241 gradually converted into americium-241 over the following decades, actually increasing the alpha radioactivity in contaminated areas for a period of time. Models predicted the ratio of americium to other plutonium isotopes would peak around the end of this century before slowly declining.
Americium-241 then continues its own decay chain, becoming neptunium-237 (half-life of 2.1 million years), which eventually decays to uranium-233.
Why Pu-238 Decay Is Useful
The heat generated by alpha decay is not just a byproduct. It powers spacecraft. When plutonium-238 decays, each alpha particle slams into the surrounding material and converts its kinetic energy into heat. Pu-238 produces about 0.57 watts of thermal power per gram, and because its half-life is 88 years, that power output drops very slowly. After nearly a century, a sample still generates half its original heat. NASA uses this property in radioisotope power systems that have kept missions like Voyager, Curiosity, and Perseverance running far from the sun, where solar panels would be impractical.
Pu-238 is preferred over other radioactive heat sources not only for its energy density and long half-life but also because alpha particles are easy to shield. They can be stopped by a sheet of paper or a few centimeters of air, making the devices safer to handle compared to sources that emit more penetrating radiation.
How Long the Decay Process Takes
Half-life tells you how long it takes for half of a sample to decay, but the full picture spans much longer. After 10 half-lives, less than 0.1% of the original material remains. For plutonium-239, with its 24,119-year half-life, that means a sample stays significantly radioactive for over 200,000 years. For plutonium-238, the timeline is shorter but still substantial: roughly 880 years to drop below that 0.1% threshold.
The daughter products then start their own clocks. Uranium-234 (from Pu-238 decay) has a half-life of about 245,000 years. Uranium-235 (from Pu-239) lasts 704 million years. So while the plutonium itself eventually disappears, the radioactivity doesn’t stop. It simply shifts to a different element and, in most cases, becomes less intense over time as the decay chain works its way toward stable atoms.