Management of Commercially Generated Radioactive Waste - U.S. ...

3.18
A listing of all accidents considered in this analysis and the grouping of releases to
determine source terms for environmental consequence analysis is given in Section 3.7 of
Technology for Commercial Radioactive Waste Management (DOE/ET-0028). Environmental impacts
of specific source terms are presented in the Environmental Aspects of Commercial Radio-
active Waste Management (DOE/ET-0029).
Each waste management technology was examined for potential accidents which might
result in offsite releases or significant impact on plant operations. Potential hazardous
material releases (called source terms) were developed for these accidents using successive
release fractions. The release fraction is the fraction of radionuclide inventory that is
released to the next containment barrier or to the environment. The radioactivity released
in an accident may be substantially reduced by one or more barriers, such as high-efficiency
particulate air (HEPA) filter banks. The radioactivity released to the environment was
obtained by multiplying the product of the release fraction for each release mechanism and
containment barrier (e.g., the accident, process equipment, HEPA filters, etc.) by the
radionuclide inventories involved in the operation. Where more than one waste management
technique was examined, analysis was based on the example system waste form (see figure
4.1.3 on page 4.8 for the identification of the example waste forms).
Accident frequency estimates were developed where possible. In the absence of actual
accident experience estimates are based on previous experience with similar equipment, while
others are engineering judgment based on review of the conceptual designs.
Following source term and frequency definition, the lists of representative accident
scenarios were classified into three accident severity groups:
1. Minor--Process interruptions without potential for significant release of radio-
active or other hazardous materials.
2. Moderate--Events with potential for small radioactivity release.
3. Severe--Events with a potential for significant radiation hazards.
The three accident classifications cover the spectrum of design-basis accidents. Non-
design-basis accidents (a fourth category) includes all accidents which exceed site crite-
ria(a) (e.g., meteorite impact) or involve concurrent independent failure of process and
multiple containment system barriers. By virtue of plant design and operational techniques,
the possibility of nondesign-basis accidents is extremely unlikely during the design life
of the waste treatment or storage facility and are not considered for these facilities.
However, for geologic isolation, because of the long period of required containment, sev-
eral nondesign-basis accidents (or unexpected events) are postulated (Section 5.5).
An umbrella source term concept was used to limit the number of accidents requiring
detailed impact analysis. Viewed independently of accident initiation sequences and fre-
(a) Site criteria include: 1) definition of the maximum credible earthquake, surface faulting,
floods and wind velocities based on historical evidence, local and regional
geology, and expert judgment; 2) local and regional demography; and 3) proximity and
definition of hazards caused by man.