Nuclear Wastes Technologies for Separations and Transmutation (1996) / Chapter Skim
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G EFFECTS ON REPOSITORY
G EFFECTS ON REPOSITORY
Pages 315-354
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From page 315... ...
~_ ~ ~ lo, ^^-an ~ ,~. An underground configuration for the proposed Yucca Mountain repository, shown in Figure G-1 (reference: Yucca Mountain Site Characterization Plan)
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316 z z _ ~ a: c c IL - IL c:: DJ 1 1 0 Z Z <~ ~5~ 111 4$ cat I ~ in 5 ~[L _ ~m 0 Oo LL is cr: Zen 2 _ ~ _ hi, cS in in or IIJ Z > LL z I I, fir is A N ; 11 CC z o m lo "r _ ~e ~0 ~cn .5: m .
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At least 10,000 MTU will have an average age greater than 30 years. The conceptual designs for repositories determine a reference value for thermal loading density by evaluating temperatures expected for the waste packages, the excavation drifts, and surrounding rock units.
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The U.S. Congress decided In 1992 that the current EPA standard shall not apply to the proposed repository at Yucca Mountain.
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APPENDIX G - EFFECTS ON REPOSITORY 319 FIGURE G-2 Contributions to total radioactivity content of spent fuel (Ci/MTU)
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The discussion in this section emphasizes the Yucca Mountain study site as the potential location for the first repository because of the wealth of information already available on this site. However' the consequences of thermal elects on a saturated repository site (such as granite)
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APPENDIX G EFFF,CTS ONRF,POSITORY 321 FIGURE G-3 Contributions of individual radionuclides to releases calculated using compositeporosity flow model. / 1-1 29 / 1 9.7% SO - CE: Barnard et al.
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322 TECHNOLOGIES FOR SF,PARA TIONSA ND TRANSMUTATION FIGURE G-4 Contribution of indi~dua] radionuclides to releases on the surface Tom human · .
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The latter decay with a thermal-power half-life ranging from 400 to 500 years. The contribution of the actin~des to thermal power equals that of the fission products abler about 70 years out of the reactor and exceeds that of the fission products thereafter.
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324 TECHNOLOGIES FOR SF,PARA TIONS AND TRANSAdUTA TION FIGURE G-S Thermal power profiles of spent EWR fifed and its major constituents groups.
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. This occurs because the thermal power declines much more slowly with actinides present, and other temperature limits specified in the site characterization plan of the Yucca Mountain site are soon encountered.
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In this option, 90Sr, Tics, and the actinides are removed and the thermal power of the waste is reduced to very' low, levels as compared with spent Mel. This means that the waste form temperature limits of the waste become essentially irrelevant which yields the following benefits if the waste continues to be emplaced according to the concept specified in the site characterization plan (e.g., in vertical boreholes)
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Temperature maxima within waste packages (very near field) are expected within a few to several years after closure, based on heat conduction to surrounding rock, although this time might be extended if boiling occurred during repressurization.
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Long Term Performance Effects IMPORTANT RADIONUCLIDES OF THE REFERENCE ONCE-THROUGH URANIUM FUEL CYCLE IMPACTING WATER PATHWAYS AND INDIVIDUAL DOSE To evaluate possible benefits from transmuting various radionuclides, it is necessary to first identifier those species that are calculated to be the main contributors to long-term individual doses Dom geologic disposal. For this purpose it is sufficient to focus on the relative values of maximum annual dose to individuals Dom each radionuclide.
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If the effective solubility of neptunium within the waste package does turn out to be near the high end of that assumed range, then either transmutation of 237Np or improved waste forms containing separated 237Np could improve the performance of a tuffrepository. The high individual doses appearing in the recent studies of a repository in unsaturated tun are a consequence, In part, ofthe s~mpli:Qing assumption that a failed waste container would present no barrier to the release of radionuclides from a waste package.
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03 3xlo-5 6xlo-2 4Xl0-l2 36cl 3.01 x 105 2x 10-3 2X 10-9 79Se 6 5X104 3xlo-3 lxlo-2 2xl0-1l 126sn 1.0X105 lxlo-2 6xlo-4 2xlo-l4 226Ra ~.6x 103 1 x 10-3 2x lo-2 231pa 3.28XlO4 2xlo-2 3xlo-2 4xlO-~ 234u 2.45X105 lxlo-8 6xlo-5 238u 4.47xlO9 1X10-8 lxlo-5 237Np 2.14X106 3x 10-5 4x10-4 239pu 2.41X104 3x10-5 2xlo-3 24opu 6.56X103 1X10-8 242pu 3.73X105 3x10-5 4xio-2 243Am 7.38X 103 4x 10-4 aMaximum dose rate from the listed radionuclide divided by the maximum dose rate Dom the radionuclide that contributes the largest dose. The maximum values occur at different times.
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eTW is the assumed time for ground water to flow from the buried waste to the environment. fThe maximum dose rate from any of these radionuclides is less than INTO of the maximum dose rate from 99Tc.
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The calculations by Sweden and Finland take into account the limited solubility oftechnetium and predict that technetium doses will be several orders of magnitude below the iodine dose from granitic groundwater. To reduce the potential individual doses that result from dissolution and transport in a geologic repository, either the 99Tc must be transmuted or the reprocessing operations must produce waste forms that release technetium far more slowly than is calculated here for unreprocessed spent fuel.
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The data in Tables G-2 and G-3 show that there would be relatively little reduction in the doses to individuals by transmuting only the actinides. However, if the more significant problems of the long-lived soluble fission products are solved, as discussed above, reducing the potential individual doses from dissolution and transport of the actirudes would become important.
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If that is to be the ultimate fate of the uranium, the long-term individual doses resulting from growth of 23 iPa and its ultimate dissolution in surface water could surely be greater than the doses calculated for that same amount of 23 Spa that would form in deep geologic disposal in the reference once-through fuel cycle. In this sense, implementing the transmutation proposals that would leave uranium from spent fuel on the earth's surface indefinitely could reduce the overall safety of waste disposal.
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IMPORTANT RADIONUCLIDES CONTRIBUTING TO LONG-TERM INDIVIDUAL DOSES FROM INTRUSION SCENARIOS FOR THE REFERENCE ONCE-THROUGH FUEL CYCLE Intrusion scenarios, such as inadvertent exploratory drilling, magmatic intrusion from volcanic activity, etc., could bring portions of the buried waste packages to the surface. Also, intrusion by drilling into a repository in unsaturated rock could result in waste cuttings falling through an empty borehole to a lower aquifer, with a correspondingly shorter time of transport of dissolved species to the environment.
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THE REFERENCE ONCE-THROUGH URANIUM FEEL CYCLE: PRELIMINARY ESTIMATES OF CONFORMANCE WITH TECHNICAL CONTAINMENT LIMITS OF THE EPA STANDARD 40CFR191 Even though the present EPA standard 40CFR191 no longer applies as such to the proposed Yucca Mountain repository, it is useful to examine how a conceptual repository in unsaturated tuff containing unreprocessed spent fuel would perform under the technical criteria used in the standard. Barnard et al.
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(yT-l) 2,11x105 5.0X10-1 1.5X10-14 1.8X 1o-l 5.4x 10-16 7.0X 10-15 1.8x 10-7 Dose Rate Risk (mrem/yr)
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Here hydrogeologic transport of dissolved radionuclides is predicted to be a more important contributor to the overall CCDF than human intrusion. Determining which of the flow models best represents unsaturated tuffat Yucca Mountain must await data from further experiments and Dom site characterization.
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APPENDIX G - EFFECTS ON REPOSITORY 339 FIGURE G-6 Overall CCDF for releases assuming the composite model for aqueous transport, spent fuel in unsaturated luff.
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340 TEC'HNOLOGIF,S FOR SEPARATIONS AND TRANSMUTATION FIGURE G-7 Overall CCDF for releases assuming the conductive-fracture model for aqueous transport, spent fuel in unsaturated turf.
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A reprocessing filet cycle using aqueous reprocessing could benefit disposal in unsaturated tuffbecause the borosilicateglass waste is expected to release soluble fission products more slowly than spent fuel. Also, radioiod~ne is recovered separately and could be incorporated in a special waste package as AgI.
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conclude that dissolution rates from reprocessing waste packages are approximately the same as from LWR spent fuel. Even though TRUs have been reduced in inventory by over two orders of magnitude, solubilities rather than inventories control the predicted release rates of low-sol-ubility species from a given waste packaged Thus, for the amount of waste that has resulted from a given generation of thermal energy, the predicted doses and risks from dissolution and transport of solubility-lim~ted species to the environment would not be significantly lower for the ALMR wastes that contain lower inventory of TRUs, as compared to spent EWR fuel waste.
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The relative peak dose from 237Np in unsaturated luff would increase and could exceed those frothy I if the more conservative solubility data for neptunium, discussed earlier, were used. These calculations show that there could be a penalty in increased individual dose if transmutation were employed so that waste from a greater amount of electrical energy generation could be loaded in the same emplacement area of the proposed Yucca Mountain repository.
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has calculated the effect of reprocessing and actinide transmutation on the probabilistic distribution of curie releases by aqueous pathways for a conceptual repository in unsaturated tuff. The results are shown in Figure G-8, expressed as the complementary cumulative probability as a Unction of the EPA sum.
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Comparing Figures G-S and G-9, surface releases from human intrusion would be greater, for a Even probability, than aqueous-pathway releases for unreprocessed spent fuel' but less for waste from reprocessing and transmutation. For probabilities greater than about l o-4, the EPA sums for surface releases from human intrusion would be lower for transmutation wastes, but the EPA sums would then become dominated by releases from the aqueous pathways.
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· .............. I \~\ EPA limit LWR Spent Fuel Pyro-reprocessed Waste Aaueous-reDrocessed Waste 1 ~ \ .\ )
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E 1 0-3 . :::: :.:.:::.:.: : EPA limit LWR Spent Fuel -- - Pyro LWR + Pyro ALMR ~Aqueous LWR + Pyro ALMR 10~ 107 106 10-5 10" 10-3 10-2 EPA Sum Source: Barnard and Lee (1992~.
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if a "cool" repository concept should be deemed desirable. However, it would also eliminate the ability to establish a "hot" repository that might be employed to keep a repository constructed in unsaturated rock dry for an extended time, as has been proposed for Yucca Mountain.
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. The committee recommendations pursuant to the above conclusions are as follows: · The Department of Energy should consider the removal of actinides as one option in its broader systemic evaluation of the thermal strategy for Yucca Mountain.
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The magnitude of the benefit depends on the specific radionuclide, with more soluble species being benefitted more. a The relative peak doses Mom 237Np in unsaturated tud would increase and could exceed those hom i29I if the more conservative solubility data for neptunium, discussed earlier, were used.
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Nuclear Waste Repository at Yucca Mountain.
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~ 989. A conceptual design for a nuclear waste repository at the Yucca Mountain site.
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1991. Projected Waste Packages Resulting from Alternative Spent-Fue!
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