Airborne Powder

DOE-HDBK-1113-98

Radiological Safety Training for Uranium Facilities

Module 101 - Properties of Uranium

Uranium in contact or solution with water is common. The primary hazards associated with a

uranium solution are criticality (for enriched uranium) and spills. Water decreases the quantity

of enriched uranium required for criticality. This topic will be discussed in Module 105 -

Criticali ty Safety.

3. Airborne Powder

A spill of any radioactive solution is a concern. As the solution evaporates, it leaves behind a

radioactive residue, or powder, that can easily become airborne. Airborne uranium may be

inhaled and absorbed into the bloodstream through the lungs.

4. Gas

Another form of uranium that is an inhalatio n hazard is the volatile UF6, becoming a gas above

56C. However, most ura nium daughters a re not volatil e, and so can acc umulate in sto rage

cylinders. When the volatile UF 6 is extracted, the nonvolatile daughters remain in the cylinder,

resulting in the buildup of residual radioactivity. However, in the case of uranium-232 (232U),

uranium-235 (235U), and uranium-238 (238U), each of these uranium isotopes has a radon

daughter. Radon is a gas at all but very low temperatures; therefore, if the radon escapes, the

subsequent daughters can accumulate in closed or poorly ventilated areas.

In some situations, pressure from volatilized UF 6 gas can build up in small volumes such as a

sealed container or a pipe run between two valves. Line breaks and leaks will cause a release of

the UF6. As the escaping UF6 gas cools, it becomes particulate, which may have a suffocating

effect on any nearby workers.

Another reason for pressure buildup is alpha particles emitted in radioactive decay eventually

becoming inert helium gas. The amount is only significant for high specific activity forms of

uranium. For example, a sample of 99% uranium-233 (233U) with 1% 232U creates approximately

its own volume of helium gas every year. Sealed containers must include adequate gas space or

be fitted with pressure release valves. Once the pressure is relieved, the low-pressure helium gas

is harmless.