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The Savannah River National Laboratory (SRNL), in response to a request from Solid Waste Management (SWM), conducted a Special Analysis (SA) to evaluate the performance of nineteen heat exchangers that are to be disposed in the E-Area low level waste facility Slit Trench 9 (ST 9). Although these nineteen heat exchangers were never decontaminated, the majority of the radionuclides in the heat exchanger inventory list were determined to be acceptable for burial because they are less than the 'generic' waste form inventory limits given in the 2008 Performance Assessment (PA) (WSRC, 2008). However, as generic waste, the H-3 and C-14 inventories resulted in unacceptable sum-of-fractions (SOFs). Initial scoping analyses performed by SRNL indicated that if alterations were made to certain external nozzles to mitigate various potential leak paths, acceptable SOFs could be achieved through the use of a 'Special' waste form. This SA provides the technical basis for this new 'Special' waste form and provides the inventory limits for H-3 and C-14 for these nineteen heat exchangers such that the nineteen heat exchangers can be disposed in ST 9. This 'Special' waste form is limited to these nineteen heat exchangers in ST 9 and applies for H-3 and C-14, which are designated as H-3X and C-14X, respectively. The SA follows the same methodology used in the 2008 PA and the 2008 SA except for the modeling enhancements noted below. Infiltration rates above the heat exchangers are identical to those used in the 2008 PA; however, flow through the heat exchangers is unique. Because it is unknown exactly how sealed heat exchanger openings will perform and how surface and embedded contaminants will be released, multiple base cases or scenarios were established to investigate a set of performances. Each scenario consists of flow options (based on the performance of sealed openings) and a near-field release of contaminants (based on corrosion and diffusion performance). Two disposal configurations were analyzed where heat exchangers were assumed to be disposed four across and five lengthwise (the 4x5 configuration, with one empty) and three across and seven lengthwise (the 3x7 configuration, with two empty). A large range of conditions was considered. For example, peak well concentrations at the 100-m boundary for H-3 are shown in Figure ES-1 for a wide range of configurations (i.e. release mechanism and degree of sealing options). The maximum contaminant level (MCL) and a 10% SOF goal for H-3 are also shown. The 10% goal was based on an estimated volume fraction that these nineteen heat exchangers would consume in ST 9 and was solely used for scoping purposes to assess disposal feasibility and sealing requirements. Because various line breaks and poor sealing greatly exceeded that 10% goal, the determination was made that mitigating activities were needed, such as protection from line breaks and better sealing. An initial set of scenarios was run to assess the requirements for sealing the heat exchanger openings and the need to ensure that the sealed heat exchangers stayed sealed during transit and disposal operations. After discovering that such mitigating activities were required, additional scenarios were run that included the mitigating activities. Scenarios deemed to have a very low probability of occurrence were excluded from consideration for calculating inventory limits (for example, those scenarios that assumed an instantaneous release of contaminants along with poor sealing). The SA used the most recent K{sub d} values for the C-14 analyses and the most recent Dose Conversion Factors for H-3 and C-14 which have been updated since the 2008 PA was issued. This SA took into account the location and the disposal timing of these heat exchangers. The disposal location is within a small area of the overall Slit Trench unit (about 6% of the total) and is behind a line that is 200 ft from the down-gradient edge of ST 9. The disposal timing is assumed to be after July 1, 2012 (because disposals cannot occur until this document is approved and mitigating activities are completed) which means that the disposal occurs after the first time period for the 2008 PA beta-gamma pathway (that time period is from December 1995 until December 2007), thus that pathway time period is not considered. Table ES-1 provides new 'Special' waste form groundwater pathway inventory limits for C-14X and H-3X in the heat exchangers. Inventory limits for generic C-14 and H-3 in the West Slit Trenches are included for comparison. The lowest limit for generic C-14 is 1.9E-1 Ci, while for C-14X it is 2.7E0, an increase of more than 14 times. Because time windows are employed, at later times C-14X exhibits lower limits than those for generic C-14 because with its smaller K{sub d} the C-14 moves much faster. The lowest limit for generic H-3 is 3.6E0 Ci, while for H-3X it is 1.7E3, an increase of almost 500 times

SPECIAL ANALYSIS FOR SLIT TRENCH DISPOSAL OF THE REACTOR PROCESS HEAT EXCHANGERS