E. A. Clark
Westinghouse Savannah River Company
Aiken, SC 29808

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Spent targets (Tritium Producing Burnable Absorber Rods, TPBARs) from the Commercial Light Water Reactor-Tritium Extraction Facility (CLWR-TEF) at the Savannah River Site will be sent to waste disposal contained in a so-called "overpack". The tritium permeation rate through a welded stainless steel overpack was estimated using a finite difference computer program in a previous report [1]. This report is an evaluation of tritium permeation through three additional overpack designs: i) a stainless steel overpack sealed using a mechanical closure with a metal gasket, ii) a mild steel overpack with the same mechanical closure, and iii) an aluminum overpack sealed by welding. Assuming the total leak rate through the mechanical closure is equal to 1*10^-3 cc/s, the tritium leak rate of the mechanical seal is estimated to be 3953 Ci/year during the first year, and decreases each year to 8 Ci/year after 40 years. The average tritium leak rate is 649 Ci/year over 40 years. These results can be compared to the calculated permeation rate of tritium through a welded stainless steel overpack, which has a maximum leak rate of 1.3*10^-5 Ci/year. The tritium release from a mechanically sealed stainless steel overpack is overwhelmingly determined by the tritium leak rate through the seal. The permeability of hydrogen isotopes through carbon steel is about 1.8*10⁴ times greater than that through stainless steel at 40°C. Since the average yearly mechanical seal leak rate is 5*10⁷ times greater than the yearly welded stainless steel tritium leak rate (much greater than the 1.8*10⁴ factor increase in permeability), the tritium leak rate through the carbon steel container with a mechanical seal is also determined by the seal leak rate. This evaluation does not consider degradation of the mechanical seal. The permeability of hydrogen isotopes through aluminum is a factor of about 2*10⁷ times less than that through stainless steel at the assumed storage temperature (40°C), so the permeation rate of tritium through the welded aluminum overpack is much less than that through the welded stainless steel overpack.

The tritium permeation rate through a welded stainless steel overpack was estimated previously using assumed conditions and source terms as input to a finite difference computer program [1]. For the welded stainless steel overpack from TEF, the maximum annual offgas rate was found to be 1.3*10^-5 Curie/year through the container, and 1.3*10^-6 Curie/year through the weld (assuming the weld thickness was one-half the container wall thickness). The yearly offgas rate is initially zero for both the container and the weld, and increases with time for both until the maximum permeation rate is achieved, after 115 years for the container and after 80 years for the weld [1]. The yearly permeation rate then declines, since tritium is depleted both by permeation and by decay to helium-3. The maximum offgas rate was used as input in the preliminary performance assessment for disposal of CLWR-TEF waste [2].

Currently, three additional designs for the TEF overpack are being considered. These are i) a stainless steel overpack sealed using a mechanical closure with a metal gasket, ii) a mild steel overpack with the same mechanical closure, and iii) an aluminum overpack sealed by welding. This report describes estimates of tritium offgas rates for the three additional designs, obtained by comparison with the original detailed estimate for the welded stainless steel container.

A mechanical seal, using a bolted flange and an all-metal engineered O-ring gasket, is being considered to seal either a stainless steel or a mild steel container. The total leak rate requirement for this seal is assumed to be 10^-3 cc helium per second for the purpose of this analysis. Leak rates are measured using a total pressure differential of 1 atmosphere. The tritium leak rate through the seal is assumed to be equal to the ratio of the tritium partial pressure to the total pressure inside the container, which is also 1 atmosphere, times the assumed 10^-3 cc/s total leak rate.

The void volume was estimated to be 12.9 cubic feet or 365,287 cc from the previous report [1]. The storage temperature is assumed to be 40°C, or 313°K. The amount of tritium in the waste is assumed to be 133 Ci per TPBAR, 300 TPBARs per overpack, or a total of 39,900 Ci per overpack. This analysis assumes that all of this tritium exists in the form T₂ gas in the void volume. This is an extremely conservative assumption, and neglects the retention of tritium in the spent TPBARs that will highly likely occur. The total amount of tritium per overpack is equivalent to 15,411 standard cc of T₂ (= 39,900Ci/2.589 Ci per standard cc T₂ [3]).

A spreadsheet was created that calculates the total amount of tritium leaking through the seals on a yearly basis for a period of 40 years. Each year, the total amount of tritium inside the overpack is decreased by the amount that decays to helium. The amount of tritium that escapes through the seal is calculated use Equations 1 and 2, and this is also subtracted from the previous total amount. Using the values assumed above, and a tritium half-life of 12.32 years [3], the results are shown in Table I. In the first year, 3953 Ci and in the fortieth year 8 Ci is calculated to leak. The average yearly amount of tritium leaking through the seal over 40 years is 649 Ci. This spreadsheet was employed to estimate the tritium dose via the air pathway for CLWR-TEF waste [6].

This evaluation of tritium release through a mechanical seal does not consider degradation of the seal in any manner.

The permeability constants for hydrogen in aluminum and iron are [4]:

From the above permeability values, the permeability of iron (mild steel) is about 1.8*10⁴ times greater than that of stainless steel at 40°C. Since the average yearly mechanical seal leak rate calculated here is 5*10⁷ times greater that the yearly welded stainless steel tritium leak rate, the tritium leak rate through the carbon steel container with a mechanical seal is also determined by the seal leak rate.

The permeability of aluminum is a factor of about 2*10⁷ times slower than that of stainless steel. This means that tritium permeation through an aluminum overpack will be infinitesimal compared to the welded stainless steel overpack.

1. E.A. Clark, "Tritium Permeation Estimate from APT and CLWR-TEF Waste Packages (U)". Report WSRC-TR-98-00238, Westinghouse Savannah River Company, Aiken, South Carolina (31 July 1998).
2. L.B. Collard, J.R. Cook, R.A. Hane, T. Hang, and E.L. Wilhite, "Preliminary Performance Assessment for Disposal of APT and CLWR/TEF Wastes at SRS (U)". Report WSRC-RP-98-01055, Westinghouse Savannah River Company, Aiken, South Carolina (21 September 1998).
3. DOE Handbook: Tritium Handling and Safe Storage. Report DOE-HDBK-1129-99, U.S. Department of Energy, Washington, D.C. (March 1999), appendix A.
4. S.A. Steward, "Review of Hydrogen Isotope Permeability Through Materials". Report UCRL-53441, Lawrence Livermore National Laboratory, Livermore, California (15 August 1983).
5. G. Caskey, "Hydrogen Solution in Stainless Steels". Report DPST-83-425, E.I. DuPont de Nemours and Co., Savannah River Laboratory, Aiken, South Carolina (18 May 1983).
6. E. Wilhite, "Analysis of Tritium Dose via the Air Pathway For Tritium Extraction Facility Waste". Report WSRC-RP-2001-00402, Westinghouse Savannah River Company, Aiken, South Carolina (24 April 2001).