Mode of Uranium Entry into the Body

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

photon radiation will not be indicative of neutron exposures. This is because the higher photon penetrating

radiation exposures tend to be associated with used but empty containers, where decay products have plated

out on the sides, while the maximum neutron exposures are associated with full containers. There is a small

additional neutron flux from spontaneous fission associated with full containers. Neutron sensitive

personnel monitoring badges are recommended for operations dealing with uranium fluoride compounds.

2.3.2 Mode of Uranium Entry into the Body

Work practices are designed to control radiation exposure to levels that are as low as is reasonably

achievable (ALARA). Reductions in exposure time and increases in shielding help reduce external doses.

Effective contamination control techniques and ventilation/filtering systems help reduce airborne

radioactive material concentrations and resulting internal doses. Where complete contamination control is

not reasonable, internal exposure of uranium compounds as aerosols or deposited particulates may occur.

The effects of uranium exposure on the body depend on the mode of exposure. External exposure concerns

are limited to beta and gamma emissions, of which the gamma field is quite low and the beta field may be

mitigated using protective clothing including safety glasses with side shields. Internal exposure and its

potential effects through radiological or chemical toxicity depend on the route of entry, and its distribution

depends on the solubility of the material. Solubility is complicated by the wide variety of stoichiometric and

crystalline uranium compounds. Inhalation and ingestion are most commonly assessed as routes of entry.

Although not covered here, entry of uranium into wounds is also a concern, and its distribution depends on

its solubility (See sections 5.9 and 5.10 for further discussion). Absorption through intact skin is unlikely.

The type of radiation to which the body is exposed and the length of the exposure determine the biological

effect of the radiation exposure.

2.3.2.1 Inhalation

Inhalation hazards from uranium result primarily from the alpha emissions. Inhalation of uranium

particles and deposition into the respiratory system are dependent on particle size. The nasal-pharynx

system filters out most large particles that are still small enough to be inhaled. Larger particles can be

inhaled--a common convention is to assume inhalation possible for all particles 10- m or less aerodynamic

equivalent diameter (AED)--but most particles that penetrate to the lower respiratory tract are less than 3- or

4- m AED. Uranium in the lungs has been shown to exhibit a wide range of retention values. Clearance

may occur through physical processes removing particles that are not embedded into the lung by cilia

motion to the esophagus. Uranium particles that are soluble in lung fluid are chemically dissolved, and the

ions are transported into the bloodstream where they are further distributed. Uranium particles remaining in

the lung constitute a potential radiological hazard as they impart their alpha emission energy into the

surrounding absorbing tissue, potentially causing significant damage within a small sphere around each

particle. Particles removed from the lung to the bloodstream primarily represent a potential chemical

hazard.

The significance of these hazards is evaluated using models of uptake and removal recommended by

national and international scientific radiation protection organizations. The lung model described in ICRP

Publication 66 (ICRP 1994) uses solubility Types of F (fast), M (moderate), and S (slow). In comparison to

previous models, this model better describes deposition, retention, and clearance data and decouples

physical and chemical clearance processes.

2-20