Decay Chains

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

2.1.2 Decay Chains

The natural uranium isotopes decay by alpha emission. The decay products are also radioactive and

form "decay chains " that ultimately lead to a stable isotope of lead. Tables 2-3 and 2-4 present the

decay chains of 238U and 235U (234U is a member of the 238U decay chain), along with the half- lives and

characteristic radiations of each nuclide.

Uranium-processing steps (milling or refining) separate the decay products and other impurities in

the ore from the uranium. It takes months after processing before the first few decay products build up

and come to equilibrium with the parents. In depleted uranium, the beta radia tion from the decay of

234

Th and 234mPa amounts to nearly twice the alpha radiation from 238U and 234U. In commercially

enriched uranium, the beta radiation from 231Th, 234Th, and 234mPA nearly equals the alpha radiation

from 238U, 234U, and 235U. In natural ore, the later decay products (especially 230Th and 226Ra) are present

and add significant gamma radiation to the emitted radiation. In processed uranium (natural, enriched,

or depleted) all decay products below 234U and 235U are removed. Because of the long half- lives of 234U

and 231Pa the radionuclides that follow these two nuclides are generally ignored.

The mining and milling stages are usually conducted by commercial enterprises. DOE facilities do

not routinely process uranium ore concentrates and, as a result, the decay products formed during DOE

processing operations of virgin feed are limited. However, radium and its progeny may be present in

waste water streams of certain facilities, so it is prudent to consider those nuclides in effluent and

environmental monitoring programs.

For workplace radiological controls, 234Th, 234mPa, 231Th and the uranium isotopes are those

requiring primary consideration; however, if there are large quantities of aged highly enriched uranium,

there may be a need to also consider 231Pa in establishing radiological controls. In addition, elevated

radon concentrations can occur in poorly ventilated uranium storage areas from the small amounts of

226

Ra that grow in and carry over as contaminants in the chemical separation processes.

2-4