Characterizing the Hazards - doe-std-1136-20040192

DOE-STD-1136-2004

Guide of Good Practices for Occupational Radiation Protection in Uranium Facilities

If there is potential for accidental nuclear criticality, the consequences of that event should be

analyzed as discussed later in this section. However, the quantity and enrichment of uranium needed to

achieve criticality are sufficiently large that for practical purposes, radiological or chemical toxicity will

always serve as the basis for determining whether a given inventory does or does not need to be analyzed.

9.2.1.3 Characterizing the Hazards

The objective of this step is to describe the hazardous materials in sufficient detail to allow accurate

modeling of releases and calculation of consequences. The following properties of uranium and its

compounds strongly influence the release potential and consequences.

Chemical form. The chemical toxicity and reactive properties of any uranium compound

must be weighed against the inherent toxicity of the compound or the uranium alone. For

example, gaseous uranium hexafluoride (UF6) reacts with atmospheric moisture and

undergoes hydrolysis, producing uranyl fluoride (UO2F2) and hydrogen fluoride (HF), a

highly corrosive and toxic gas. Depending on the temperature, humidity, and uranium

enrichment, the HF may be a more serious health and safety concern than either the UF6

or the contained uranium. Some uranium compounds ignite violently on contact with air,

water, or hydrocarbons.

Physical form. Physical form influences the release potential and toxicity of uranium and

its compounds in numerous ways. Large, monolithic pieces of uranium metal may be

relatively benign; however, they can develop a pyrophoric surface due to effects of air and

moisture. Finely divided metallic uranium can react violently with numerous other

materials or self-ignite in air, yielding respirable particles of uranium compounds. UF6 is a

solid at ambient temperature but goes directly to a gaseous state above ~270 F at

atmospheric pressure.

Solubility. For air exposure, permissible exposure levels for soluble uranium

compounds are based on the chemical toxicity (particularly to the kidney), while for

insoluble compounds, radiotoxicity (radiation dose to the lung) is limiting.

Particle size. Particle size and the range of sizes have a large effect on the radiotoxicity

of inhaled materials. Larger particles will be cleared rapidly from the upper respiratory

regions, delivering little radiation dose to the lung tissues. Small particles are deposited

deeper in the lung and are cleared very slowly, producing a much larger dose per unit

activity inhaled.

Enrichment. Enrichment, or specific activity of the isotope mixture, often determines the

relative importance of radiological and chemical toxicity for more soluble materials.

9.2.1.4 Developing Event Scenarios

Properties of the hazardous material do not significantly affect the manner in which this step of

the hazards assessment is performed.

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