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.
(c) 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 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
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.