Uranium Isotopes, Half-Lives and the U-238 Decay Chain

Reference article · Updated 2026-11-06

Key facts

Natural uranium is about 99.27% U-238, 0.72% U-235 and 0.0054% U-234 by mass.
Half-lives: U-238 about 4.47 billion years, U-235 about 704 million years, U-234 about 245,500 years.
One gram of natural uranium has an activity of roughly 25,000 becquerels, mostly from U-238 and U-234 in equal parts.
The U-238 chain contains 8 alpha and 6 beta decays before stable Pb-206, and includes the gas radon-222.

The uranium-238 decay series with commonly published half-lives. Original schematic.

The three natural uranium isotopes

All isotopes of uranium are radioactive. Three occur in nature: U-238, U-235 and U-234. By mass, natural uranium is about 99.27% U-238, about 0.72% U-235 and only about 0.0054% U-234. U-234 is present because it is a decay product of U-238 and is kept in balance with it.

Isotope	Half-life	Share of natural U (mass)	Main decay
U-238	4.47 × 10⁹ years	about 99.27%	alpha
U-235	7.04 × 10⁸ years	about 0.72%	alpha
U-234	2.455 × 10⁵ years	about 0.0054%	alpha

Half-life values vary slightly between evaluations and in rounded summaries; for example, the US EPA's summary page rounds U-234 to 244,000 years, while the evaluated nuclear data give about 245,500. U-235 is the isotope that sustains fission chain reactions, which is why enrichment raises its share.

Specific activity: why uranium is only weakly radioactive per gram

Specific activity is the number of decays per second per gram, in becquerels per gram (Bq/g). A long half-life means a low specific activity. Approximate values are:

U-238: about 12.4 kBq/g
U-235: about 80 kBq/g
U-234: about 230 MBq/g (2.3 × 10⁸ Bq/g)

In natural uranium, U-238 and U-234 are in activity equilibrium, each contributing about 12.4 kBq/g of the mix, and U-235 adds a few per cent. The total is therefore about 25 kBq per gram of natural uranium (roughly 0.7 microcurie). When uranium is enriched, U-234 is concentrated along with U-235, and the specific activity of the product rises well above natural uranium, mostly because of U-234. Depleted uranium has a specific activity that is lower, about 60% of that of natural uranium as commonly cited. Units are explained on the unit converter page.

The U-238 decay chain, step by step

U-238 does not decay directly to a stable nucleus. It starts a series in which each product is itself radioactive until stable lead-206 is reached. In total the series has 8 alpha decays and 6 beta decays. The diagram above shows the sequence; the table below gives the half-lives and the main radiation type for each member.

Nuclide	Half-life	Decay
U-238	4.47 billion years	alpha
Th-234	24.1 days	beta
Pa-234m	about 1.2 minutes	beta
U-234	245,500 years	alpha
Th-230	75,380 years	alpha
Ra-226	1600 years	alpha
Rn-222	3.82 days	alpha
Po-218	3.1 minutes	alpha
Pb-214	26.8 minutes	beta
Bi-214	19.9 minutes	beta
Po-214	164 microseconds	alpha
Pb-210	22.2 years	beta
Bi-210	5.0 days	beta
Po-210	138.4 days	alpha
Pb-206	stable	none

Pa-234m is a short-lived nuclear isomer of protactinium-234; a small fraction of its decays go through Pa-234 instead, with little effect on the overall chain.

Alpha, beta and gamma emissions in the chain

Alpha particles are helium nuclei. They carry high energy but are stopped by a sheet of paper or the outer layer of skin, so they matter mainly when the emitter is inside the body. Beta particles are electrons and penetrate further, but are stopped by a few millimetres of aluminium. Gamma rays are electromagnetic radiation and are the main contributor to external exposure.

U-238 itself emits very little gamma radiation, so pure, freshly purified uranium produces low external dose rates. Most of the gamma radiation from uranium ore and from tailings comes later in the chain from lead-214 and bismuth-214, which follow radon-222. Pa-234m and Th-234 contribute some beta and low-energy gamma radiation. This is why a gamma survey of ore reflects radium and its progeny rather than uranium itself.

Radium and radon: why this part of the chain gets the most attention

Ra-226, with its 1600-year half-life, is the longest-lived member after thorium-230 and is the source of radon-222. Radon is a noble gas, so unlike its parents it can leave a solid and enter air. It has a half-life of 3.82 days, long enough to migrate through soil and buildings. Its short-lived progeny, Po-218 and Po-214 among them, are solids that attach to dust and, if inhaled, deposit alpha energy in the lung. Radon and its progeny dominate the natural radiation dose to the average person; see radiation dose and risk. Releases from waste are discussed on the mill tailings page.

The chain continues through Pb-210, Bi-210 and Po-210 on a timescale of decades, and Pb-210 is a useful tracer in sediments and in air for that reason.

Secular equilibrium and what happens when uranium is separated

In an undisturbed ore body, the long-lived parent keeps the chain in secular equilibrium: after enough time, every member has the same activity as U-238 (assuming no members have escaped). Roughly, equilibrium takes several half-lives of the daughter, so short-lived members such as Th-234 reach it within months, while Ra-226 would take thousands of years after it is first separated from thorium-230.

Chemical processing breaks the balance. Milling takes out uranium, leaving thorium-230 and radium-226 behind; purified uranium then slowly regrows Th-234 and Pa-234m within a few months but not the later members in any meaningful quantity. Groundwater can also move radium, uranium and radon at different rates, so measured activities of chain members often differ from each other. Where the U-235 series is relevant, it follows a similar logic and ends at stable lead-207.

Why these properties matter in practice

The properties above determine how uranium is handled. The weak gamma emission and long half-lives of U-238 and U-235 mean that purified uranium compounds are handled with chemical-toxicity precautions as well as radiological ones, whereas ore and tailings need radon control and gamma shielding considerations because of radium and its progeny. The half-life of U-235 and the ratio of U-238 to U-235 allow geological dating and isotopic fingerprinting: U-Pb dating is among the best-established methods in geochronology, and the U-234/U-238 ratio is used in groundwater and carbonate dating studies. In fuel-cycle work, the isotopic composition drives enrichment requirements; see the nuclear fuel production chain.

Frequently asked questions

What are the half-lives of the natural uranium isotopes?

About 4.47 billion years for U-238, about 704 million years for U-235 and about 245,500 years for U-234.

Is uranium itself a strong gamma emitter?

No. U-238 and U-235 are mainly alpha emitters with weak gamma emission. Most of the gamma radiation from ore comes from lead-214 and bismuth-214 further along the decay chain.

How many decays take U-238 to stable lead?

Fourteen decays: 8 alpha and 6 beta, ending at stable lead-206.

Why is radon-222 important in the U-238 chain?

It is a gas, so it can escape from rock, soil and waste into air. Its short-lived progeny deposit in the lung when inhaled.

How radioactive is natural uranium per gram?

About 25 kBq per gram, with roughly equal contributions from U-238 and U-234 and a small contribution from U-235.