Characterizing the Hazards - doe-std-1128-98

DOE-STD-1128-98

Several possible sources of screening threshold values are suggested by the EMG.

For radioactive materials, the primary source of screening threshold values is 10

CFR 30.72 Schedule C, Quantities of Radioactive Materials Requiring

Consideration of the Need for an Emergency Plan for Responding to a Release

(CFR, 1992a). For plutonium, the most applicable number from that source is the 2

Curie value for "all other alpha emitters". However, facilities may select lower

screening values based on the properties of the material. A screening threshold can

be determined by modeling a unit release of the material to the atmosphere at

ground level and determining the consequences at some reference distance under

conservative dispersion conditions. The screening threshold value is typically a

quantity which, if released, would produce consequences on the order of one-tenth

the threshold for protective action at the facility boundary.

The quantity of plutonium needed to achieve criticality, even under optimum

conditions of moderation and reflection, is sufficiently large that the radiotoxicity

of the plutonium itself will always serve as the basis for determining whether a

given inventory does or does not need to be analyzed.

hazardous materials in sufficient detail to allow accurate modeling of releases

and calculation of consequences.

The following properties of plutonium and its compounds influence the release

potential and consequences.

Chemical and physical form. The chemical toxicity of plutonium and its

compounds is of much less concern than the radiotoxicity of the plutonium.

However, the chemical and physical form may strongly influence the release

potential. Plutonium metal oxidizes readily in humid air at elevated

temperatures to form loosely-attached oxide particles, a source of readily

dispersible airborne and surface contamination. Plutonium metal fines and

turnings can ignite spontaneously in the presence of air, creating aerosol-size

oxide particles and providing energy to disperse them. Also, some plutonium

compounds may ignite violently on contact with air, water or hydrocarbons

(Benedict, et al., 1981).

Solubility. The committed effective dose equivalent (CEDE) per unit activity

inhaled is about three times greater for plutonium of solubility class W than for

class Y. No plutonium compounds of solubility class D are generally

recognized.

Particle size. Particle size distribution has a large effect on the radiotoxicity of

inhaled materials. Larger particles tend to be cleared rapidly from the upper

respiratory regions and swallowed, thereby delivering little radiation dose to

the lung tissues. Because plutonium is poorly absorbed in the gut, very little

dose is attributed to the larger particles that are cleared from the body by this

process. Small particles are deposited deeper in the lung and are cleared very

slowly, producing a much larger dose per unit activity inhaled. Extremely small

particles tend to be exhaled and not deposited.

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