Answer to Question #8820 Submitted to "Ask the Experts"
Category: Radiation Basics — Radionuclides
The following question was answered by an expert in the appropriate field:
You are correct that radioactive materials, by definition, do decay. However, some radionuclides decay to species that are themselves radioactive. Indeed, a parent radionuclide may decay through numerous radioactive species, often referred to as radioactive progeny, that comprise a long chain that finally terminates in a stable end product. Depleted uranium is mostly made up of a long-lived radioisotope of uranium, 238U. The 238U has a half-life of about 4.5 billion years. As it decays by alpha particle emission it yields a second radionuclide, 234Th, which has a 24-day half-life and decays by emitting beta radiation to produce a radionuclide referred to as 234mPa, which has a short half-life of a bit more than one minute, and decays by beta emission to the 234U, a radionuclide with a half-life of 2.47 x 105 years.
When depleted uranium (DU) is newly prepared, the progeny through 234mPa grow into the DU fairly quickly, and within four or five months the activity (decay rate) of each of these progeny is equal to the decay rate of the 238U. Because the 234U has quite a long half-life, it acts rather like a time stop in the chain so that apparent total activity of the DU does not appear to change noticeably after four or five months have elapsed since preparation. So if you assess the DU 5 or 10 years after it has been prepared, the radioactive constituents do not appear significantly different from what they were at five months. If one is concerned about much longer intervals of time, however—e.g., many thousands of years—the picture changes as sufficient time passes that significant decays of 234U have occurred, and these decays lead to other shorter-lived progeny in the long chain. There are a total of 10 major plus three minor radionuclides in the chain beyond 234U. These include some well-known nuclides such as radium, 226Ra, and radon, 222Rn, in addition to isotopes of thorium, polonium, lead, and bismuth. It would require about 1.25 million years following initial preparation of DU for all of the radionuclides in the chain to reach a state of equilibrium in which each of the progeny has the same activity as the original 238U. Over this 1.25 million years, however, the original activity of the 238U would have decreased by only 0.0192 percent. Thus while the 238U decreased by a tiny amount, the total activity that includes all 13 of the major progeny would have increased by 1,300 percent.
I hope this clarifies some of the statements to which you have referred.
George Chabot, PhD, CHP