K.3.0 UNCERTAINTY IN SOURCE TERMS
Source terms refer to the waste inventory, which is the total quantity of the hazardous material within the tanks, and to the release term, which is the amount released to environmental media such as soil, groundwater, surface water, and soil under normal or accident conditions. The following sections describe the uncertainty associated with inventory and release terms.
K.3.1 WASTE INVENTORY DATA
There is considerable uncertainty associated with the inventory data used in the EIS. Tank waste data are available on a tank-by-tank basis, but the accuracy of these data are suspect because they primarily are based on historical records of transfers between tanks rather than statistically based sampling and analyses programs. However, while the inventory of any specific tank may be suspect, the overall inventory for all of the tanks combined is considered more accurate. The lack of accepted tank-by-tank inventory data would affect the Ex Situ/In Situ Combination 1 and 2 alternatives more than other alternatives because the tank selection criteria and the impact assessment of waste disposed of in situ are dependent on tank-by-tank data.
The waste inventory data used in developing the alternatives and their associated impacts are derived from model predictions and sample analysis. While the waste is currently undergoing additional characterization and the inventory may be revised as a result of ongoing analyses, the inventory used in the EIS is not expected to result in discrimination for or against any of the alternatives analyzed. DOE has identified the key radionuclides for tracking in development of a "best basis inventory" for Hanford tank waste. These include the radionuclides that dominate the risk estimates in this EIS: C-14, I-129, neptunium-237 (Np-237), protactinium-231 (Pa-231), selenium-79 (Se-79), Tc-99, and uranium (U) isotopes. This information will be incorporated into National Environmental Policy Act (NEPA) analysis of closure alternatives. For additional information on tank inventory data accuracy and its effect on the EIS see Volume Two, Section A.3.
K.3.2 RELEASE TERMS
Releases to the environment for both routine releases during remediation and acute releases during an accident are a function of the waste inventory. The inventory used for developing routine emissions is based on a nominal waste stream based on overall tank waste inventory. Acute releases were developed using both a nominal and bounding inventory.
K.3.2.1 Chronic Releases (Routine)
Chronic releases were developed for each alternative using average inventory data. The No Action and Long-Term Management tank waste alternatives include routine emissions from the tank farms. The In Situ Fill and Cap alternative includes routine tank farm emissions as well as tank emissions during tank filling. The In Situ Vitrification alternative includes routine tank farm emissions plus releases from the evaporator and vitrification processes. The ex situ alternatives include routine tank farm emissions, releases during waste retrieval, and releases during waste treatment.
The routine releases developed for the ex situ alternatives are based on material balance calculations and waste processing rates. Conservative assumptions were made for the release of certain volatile radionuclides. It was assumed for each alternative that included vitrification that the entire inventory of I-129 and C-14 would be released to the atmosphere during waste treatment. Some capture of the I-129 in the off-gas treatment system could be expected and would result in lower I-129 releases.
Uncertainties associated with the chronic releases are based on the available inventory data. Increased inventory of any constituent would be expected to result in some increase in chronic releases. This especially would be true for the volatile contaminants.
K.3.2.2 Acute Releases (Accident)
The respirable fraction of inventory released from an accident, from which the receptor dose is calculated, is referred to as the source term. The source term depends on a variety of release fractions associated with the mechanics of the accident scenario. Uncertainties associated with each of these release fractions are based on available data, and in some cases may depend on engineering judgement. For specific scenarios, nominal and bounding values were estimated for the applicable release fractions as follows:
- Damage ratio (DR) - The fraction of the material at risk impacted by the event;
- Leak path factor (LPF) - The fraction that escapes the confinement boundary by design, natural causes, or degradation caused by the event;
- Airborne release fraction (ARF) - The fraction of released material made airborne by the event;
- Airborne release rate (ARR) - The fractional airborne release rate of material from the accident. ARR is converted to ARF by integrating over the time available for release; and
- Respirable fraction (RF) - The fraction of airborne droplets or particulate matter with individual particle aerodynamic equivalent diameter less than or equal to 10 microns (µm).
For the spray release scenario, the nominal and bounding applicable release fractions are presented in Table K.3.2.1. When a particular release fraction is well understood, the uncertainty diminishes, decreasing the difference between the bounding and nominal values. For purposes of this analysis, the DR is the only parameter with uncertainty. Setting LPF, ARF, and RF equal to 1.0 maintains conservatism even for the nominal case. The difference between the bounding and nominal cases is a factor of 7.
Table K.3.2.1 Bounding and Nominal Release Fractions Used in the Spray Release Scenario
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