E. L. Wilhite
Westinghouse Savannah River Company
Aiken, SC 29808

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Proposed changes in the design of the overpack for low-level waste arising from the tritium extraction facility are being assessed as to their potential impact on the radiological performance assessment for the SRS E-Area Low-Level Waste Facility. A method to account for the emplacement of waste over time has been developed to reduce the conservatism in the performance assessment for these waste forms. Accounting for the waste emplacement over time gives acceptable impacts via the air pathway for overpack leak rates up to 1.4x10^-3 cc/second. However, the results suggest that there may be adverse near-term impacts that should be considered from an operational safety perspective.

Keywords: Tritium Extraction Facility, Low-Level Waste Disposal

The Department of Energy is developing a means of producing tritium by irradiating tritium producing burnable absorber rods (TPBARs) in a commercial light water reactor (CLWR) and extracting the tritium in a tritium extraction facility (TEF) at the Savannah River Site (SRS). Following the extraction of tritium, the TPBARs will be placed in an overpack container for disposal in the SRS E-Area Low-Level Waste Facility (LLWF). In 1998, a preliminary performance assessment (PA) for the disposal of TEF wastes as well as wastes potentially arising from a proposed accelerator facility was completed¹. At that time, the TEF waste was expected to be contained in a welded stainless steel overpack. The PA results for the TEF waste were that all of the waste could be disposed in the intermediate level vaults (ILV) at the LLWF. The preliminary performance assessment conservatively assumed that all of the waste expected to be produced over a 40-year period was emplaced in the E-Area LLWF at the same time.

At this time, the CLWR/TEF project is proposing changes to the waste overpack. An analysis of the rate of tritium release from the overpack designs² showed that the overpacks utilizing a mechanical, rather than a welded seal released too much tritium to permit assessment using the conservative assumption employed in the original preliminary PA.

This document provides results of a method to account for the emplacement of waste over time.

The assessment of the release of tritium from the proposed overpack designs² used a spreadsheet calculation for the mechanically sealed overpacks. The spreadsheet used the fractional volume of tritium (assuming each of the 300 TPBARs in an overpack contains 133 curies and all of the tritium has escaped the TPBARs) in the interior space of the overpack to represent the fraction of tritium contained in the gas leaked at the stated leak rate. In the release calculation, the reduction of the tritium inventory in the overpack from radioactive decay and release through the seal was accounted for. This gave a tritium release rate as a function of time.

To account for the disposal of the overpacks over time (i.e., 14 overpacks disposed per year for 40 years), I simply extended the calculations from reference 2 to portray release of tritium from fourteen overpacks over a span of 150 years because the assessment of dose at the 100-m compliance point is not performed until 100 years following closure of the disposal facility¹. I replicated the annual release of tritium from 14 overpacks 40 times, offsetting the initial tritium release from each set of 14 overpacks to account for the relative time of disposal. I then summed the tritium releases across the 40 disposal years for each year following the initial disposal. The total tritium release for each year following disposal was then multiplied by the appropriate dose factor (i.e., 8.5x10^-3 mrem/year per Ci released for the dose at the 100-m location and 2.4x10^-6 mrem/year per Ci released for the dose at the SRS boundary)¹.

To account for the potential disposal of other tritium containing waste in the ILV, a performance measure of 1 mrem/year from the air pathway was established for the TEF waste (i.e., one-tenth of the 10 mrem/year performance objective for the air pathway¹). This performance measure applies at all times at the SRS boundary but does not apply at the 100-m compliance point until after the assumed 100 years of institutional control.

Table 1 shows the spreadsheet for an assumed overpack leak rate of 0.001 cc/second. Several of the spreadsheet rows and columns have been hidden so that the sheet could be printed on one page. The summed annual tritium release increases to a value of about 3.7x10⁵ curies after 37 years following the first disposal. The release is constant at this value until the forty-first year after the first disposal. The dose calculated at the 100-m compliance point increases to a value of about 3,200 mrem/year in the fortieth year after the first disposal. This dose value is not pertinent for the performance assessment because the PA presumes a minimum institutional control period of 100 years following closure of the disposal facility. However, this result suggests that there may be adverse impacts from an operational safety perspective that should be considered. The dose calculated at the 100-m compliance point, after 100-years of institutional control, is about 3.4x10^-4 mrem/year, which is well below the performance measure of 1 mrem/year. The dose calculated at the SRS boundary increases to a maximum of about 0.9 mrem/year in the fortieth year after the first disposal. This value is just under the performance measure of 1 mrem/year.

Table 2 shows the results of the calculation for a number of different leak rates. The dose calculated at the 100-m compliance point, after 100 years of institutional control, increases with increasing leak rate until the leak rate is about 1x10^-4 cc/sec; the 100-m dose at this leak rate is about 0.9 mrem/year. Further increase in the leak rate results in a reduction in the dose because, since the 100-m dose is not applicable until after the 100 years of institutional control, high leak rates deplete the tritium source prior to the end of institutional control. The dose calculated at the SRS boundary increases steadily with increasing leak rate. The dose equivalent to the performance measure of 1 mrem/year is achieved at a leak rate of 1.4x10^-3 cc/sec. Thus, the maximum leak rate that can be tolerated without exceeding the 1 mrem/year performance measure is 1.4x10^-3 cc/second.

This analysis has reduced the conservatism in evaluating the air pathway dose from tritium released from two of the proposed TEF overpack designs, the steel (i.e., stainless steel and carbon steel) overpack with mechanical seal by accounting for the time sequence of waste disposal. The results show that, for the disposal of 14 overpacks per year over 40 years, with each overpack containing 300 TPBARS and each TPBAR containing 133 curies of tritium, the overpack leak rate can be no more than 1.4x10^-3 cc/second. The results also suggest that there may be adverse impacts in the vicinity of the E-Area LLWF during the operational time-frame. This should be considered from an operational safety perspective.

Although this analysis method reduces the conservatism by not assuming all of the 40-year production of overpacks are disposed at once, it retains the conservative assumption that all of the residual tritium contained in the TPBARS is immediately available for release in the interior air space of the overpack. This is undoubtedly a gross over conservatism, given the extreme conditions of elevated temperature and vacuum employed to extract the tritium product from the TPBARS. If data on the rate of tritium release from spent TPBARS under ambient temperature and pressure were available, the overpack leak rate could likely be greatly increased and the potential operational concern noted above would also likely be mitigated.

1. L. B. Collard, et al, Preliminary Performance Assessment for Disposal of APT and CLWR/TEF Wastes at SRS (U), WSRC-RP-98-01055, 9/21/98.
2. E. A. Clark, Preliminary Tritium Release Estimate from CLWR-Tritium Extraction Facility Waste Overpack, WSRC-TR-2001-00095, 4/16/2001.

* After 100 years of institutional control (i.e., year 140 and following)