Daniel I. Kaplan, Leonard B. Collard, Elmer L. Wilhite, and James R. Cook
Westinghouse Savannah River Company
Aiken, SC 29808

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The amount of low-level radioactive waste (LLW) that can be emplaced at a disposal site is dependent on several factors. Key among these is the interactions of the radionuclide with the waste, engineered barriers, and geological media at the disposal site. Sensitivity analyses of performance assessments of LLW disposal sites commonly conclude that the distribution coefficient (K_d, mL/g), the ratio of a radionuclide concentration in the solid phase (Ci/g) to the concentration in the liquid phase (Ci/mL), is the single most important input parameter (1). There are few actual measurements of waste form K_d values because such tests are expensive and because there are often several different types of waste forms disposed at a given LLW site (2). Consequently, modelers are required to make conservative estimates of K_d values, typically based on measurements made in geological materials. Such conservatism leads to under-estimating the true amount of radioactivity that can be safely managed at a disposal site.

The objective of this study was two fold: 1) to make laboratory measurements of waste-specific K_d values, and 2) to use the laboratory measured K_d values in calculations to estimate the inventory disposal limit at the Savannah River Site’s E-Area LLW Facility. A key risk driver at this facility is I-129. Among the waste forms evaluated in this study was activated carbon, used specifically for removing I-129 from a waste stream. Consequently, it was anticipated that such waste would have appreciably larger I-129 K_d values than the generic waste K_d value, 0.6 mL/g, previously used in a performance assessment of the site (3).

The I-129 K_d values were measured by two methods, a batch desorption test and a flow-through continuous flow test (2). The waste forms evaluated were generated from the F-Area and H-Area Ground Water Treatment Units (GWTU) and the Effluent Treatment Facility (ETF): activated carbon, CG-8 cation resin, and filtercake (a predominant Fe-oxide sludge material). K_d values were measured using two types of leachates: a mildly acidic-rain simulant (pH 5; for a trench disposal scenario) and a cement simulant (pH 12; for a cement vault disposal scenario).

The PORFLOW^© contaminant transport model was then used to determine the influence of waste K_d values on I-129 concentrations at a hypothetical well 100-m down-gradient from the E-Area LLW Facility (4). Allowable I-129 inventory limits were calculated by comparing estimated groundwater concentrations with the Maximum Contaminant Level (MCL, presently 1 pCi/L) and by comparing off-site and intruder doses with DOE Order limits on doses.

Seventy-seven I-129 K_d values were measured from seven waste forms (Table 1). The most important conclusion that can be made from this data is that the measured K_d values are appreciably greater than that assumed for generic waste. Also important to note is that the K_d values measured using the acidic rain simulant are appreciably greater than those measured using the cement simulant. This can be attributed to the greater anion concentration in the cement simulant, promoting anion exchange with I-129. K_d values for a given waste form varied depending on the originating facility. For example, activated carbon K_d values in an acidic rain environment ranged from 7,400 to 132,500 mL/g. The large standard deviations associated with the average K_d values can be attributed primarily to increases in K_d values as more leachate passed through the column experiments (data not shown).

The influence of waste form K_d values on the disposal inventory limit at the Savannah River Site E-Area Disposal Facility is presented in Figure 1. The generic waste used in a previous performance assessment had a waste inventory limit of 5.3 x 10^-4 Ci of I-129. The disposal inventory limit of a waste with a K_d of 132,500 mL/g, such as the activated carbon, had an inventory limit of 4.9 Ci of I-129. The strong correlation existing between the K_d values and the allowable inventory limit provides a useful equation for predicting inventory limits without having to rerun these arduous calculations.

Table 1. I-129 K_d Values (mL/g) of Various Waste Forms Using
a Cement Leachate (for Simulated Vault Disposal) and
an Acidic Rain Leachate (for Simulated Trench Disposal)

		Cement Kd		Acidic Rain Kd
Waste Form	Waste Generating Facility	Minimum	Average ± Std. Dev.	Minimum	Average ± Std. Dev.
Activated Carbon	F-GWTU	880	11,500 ± 15,200	132500	798,000 ± 800,000
Activated Carbon	H-GWTU	320	5,400 ± 5,600	58100	270,000 ± 208,000
Activated Carbon	ETF	600	9,710 ± 14,300	7400	65,100 ± 101,000
CG-8	F-GWTU	3	1,490 ± 1,600	50	1,360 ± 1,320
CG-8	H-GWTU	100	24,500 ± 27,300	380	56,400 ± 87,000
Filtercake	F-GWTU	13	14 ± 1	50	57 ± 9
Filtercake	H-GWTU	630	9,230 ± 7,260	650	4,020 ± 3,300
Non-specified Generic Waste a		0.6		0.6

Figure 1 Influence of Waste Form K_d Values and
Disposal Inventory Limit at the E-Area Disposal Site on the SRS.

1. Piepho, M. G., R. J. Serne, D. I. Kaplan, and D. W. Langford. 1994. WHC-8H210-MGP-18, Westinghouse Hanford Company, Richland, WA.
2. Kaplan, D. I., S. M. Serkiz, and N. C. Bell. 1999. I-129 Desorption From SRS Water Treatment Media from the Effluent Treatment Facility and the F-Area Groundwater Treatment Facility. WSRC-TR-99-270, Westinghouse Savannah River Company, Aiken, SC.
3. McDowell-Boyer, L., A. D. Yu, J. R. Cook, D. C. Kocher, E. L. Wilhite, H. Holmes-Burns, and K. E. Young. 2000. Radiological Performance Assessment for the E-Area Low-Level Waste Facility. WSRC-RP-94-218, Rev. 1. Westinghouse Savannah River Company, Aiken, SC.
4. Collard, L. B. 2001. Special Analysis for Disposal of High-Concentration I-129 Waste in Slit Trenches at the E-Area Low-Level Waste Facility. WSRC-TR-2001-00021. Westinghouse Savannah River Company.