A Review of the Swedish KBS-3 Plan for Final Storage of Spent Nuclear Fuel (1984) / Chapter Skim
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'Adequacy of Treatment of Radionuclide Movement'
'Adequacy of Treatment of Radionuclide Movement'
Pages 42-60
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From page 42... ...
But some unexpected, very low probability event is always possible, and the KBS authors devote much effort to showing that the kind of rock they plan to use for a repository, at a depth of 500 m, aided by the bentonite buffer, will constitute an adequate secondary barrier for controlling radionuclide release even if something goes radically wrong. Suppose that a canister is breached so effectively that all of its contained fuel is at once in contact with groundwater.
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From page 43... ...
"~ Dissolution. In estimating the rate of dissolution, the KBS-3 authors note that the radionuclides formed from uranium atoms by fission or neutron capture will in large part remain dispersed within the uranium oxide pellets, and hence will dissolve only as fast as the crystal structure of UOj is destroyed by the dissolving of uranium.
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From page 44... ...
As a general confirmation of the low solubility of uranium in reducing groundwater (and hence of the low leachability of nuclides dispersed through UO2 pellets) , uranium concentrations were measured in many samples of water obtained from the recently studied field areas.
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From page 45... ...
. Even in the absence of any bentonite buffer, radionuclide concentrations in the geosphere and biosphere would increase only twentyfivefold for the most pessimistic water flow rates and at most threefold for more representative flows (Ivars Neretnieks, Royal Institute of Technology, private communication, 1983)
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From page 46... ...
General. Movement of radionuclides in the far-field of a Swedish repository -- that is, movement through granitic or metamorphic rock between the immediate vicinity of breached canisters and the biosphere -- is one aspect of the general problem of how effectively natural rock environments can be expected to control nuclide migration.
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From page 47... ...
The more conservative KBS-3 model pictures a repository shielded from major fractures zones on all sides by at least 100 m of relatively impermeable rock, and the only effects of dispersion and sorption considered are those that occur during movement through this 100-m thickness. Once in a fracture zone, radionuclides are assumed to have free access to the biosphere, where radionuclide concentrations are affected only by dilution or by chemical processes at the receptor location.
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From page 48... ...
A common procedure is to measure sorption in the laboratory, either by letting a radionuclide solution stand in contact with crushed rock or by arranging a system of constant flow, and then to assume that the laboratory results will apply to rocks in-situ. The method is subject to criticism on several grounds: laboratory conditions may differ markedly from those in nature; the radionuclides in a natural environment may be in a different oxidation state or in the form of complexes with very different sorption properties; and the nuclides may be carried in groundwater as colloids, and for that reason may be more mobile than laboratory results would indicate.
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From page 49... ...
In the KBS work, laboratory methods for determining sorption coefficients were refined long ago in the more obvious ways of ensuring similarity with natural environments: use of groundwater samples in the experiments; excluding oxygen and carbon dioxide; control of Eh, pH, and temperature; and exposure of both fresh and altered mineral surfaces. Recent results from the continuing laboratory effort have established (1)
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From page 50... ...
Despite these efforts, the proof is not complete -- and probably never will be -- that laboratory data on retardation by sorption are adequate for completely reliable predictions of radionuclide movement from a breached canister to the biosphere. The number of variables in laboratory work, plus the much larger number in natural environments, is simply too great for complete control to be achieved -- as is witnessed by the wide disparity in values often reported for experiments conducted under seemingly identical conditions.
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From page 51... ...
The estimates will be even better when some of the research in progress is completed, but the additional refinements are hardly needed as long as conservative values are used in the calculations. Despite the reasons for skepticism alluded to above, the panel thinks that the KBS authors have developed a sound scientific basis for their conclusion that the barrier to radionuclide movement provided, to some extent, by the bentonite buffer, but mainly by the bedrock will be adequate insurance against unacceptable releases to the biosphere even in the very unlikely event of large-scale canister failure.
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From page 52... ...
TUNNEL SHAFT, AND BOREHOLE SEALING Can tunnels, shafts, and boreholes be sealed effectively enough to keep movement of water no faster than through adjacent undisturbed rock? Shafts and tunnels used in constructing a repository, as well as boreholes that may have been drilled during preliminary exploration, must be filled and sealed when the repository is ready for closure, to prevent their becoming channels of easy groundwater flow that could bypass the normal slow movement through relatively impermeable rock to major fissure zones.
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From page 53... ...
A recent American technical report (Meyer and Howard, 1983) on the use of clays for repository sealing emphasized the need for research into the long-term stability and even solubility of clays, their possible reactions with adjacent rock, and methods of emplacement so as to ensure a tight seal.
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From page 54... ...
The KBS-3 authors make dose assessments primarily for two pessimistic receptor locations in water -- a well drawing from a contaminated fracture zone in bedrock and a lake with very slow turnover -- as well as for a special scenario where a lake eutrophies to become a peat bog that is used 10,000 years later as a soil conditioner. Concentrations in the water sources are estimated by using some of the models described in preceding sections, starting with dissolution of spent fuel pellets, tracing the movement of leached nuclides through the near-field and far-field, and finally postulating substantial dilution as contaminated groundwater approaches the places where water is obtained for human use.
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From page 55... ...
. This level of exposure is about 4 percent of the ICRP limit for an average individual in a nearby population and 1 percent of the ICRP limit for the maximally exposed individual.
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From page 56... ...
Compartment models of this type have been used extensively to evaluate biospheric transport of radionuclides produced in nuclear weapons tests and by natural processes. The parameters used in the BIOPATH calculations include nuclide-independent and nuclide-dependent transfer coefficients, retention and weathering fractions, terrestrial and aquatic yield values (productivity)
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From page 57... ...
to obtain an estimate of the equivalent water flow past each canister, Qeg. Although the Canister Corrosion calculations indicate canister breaching will not occur before 106 years and perhaps not until 108 years, the KBS authors pessimistically assume as input to the calculation of Fuel Dissolution that canister failure will begin at 105 years with the last canister failing at 106 years.
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From page 58... ...
in combination with output from Far-Field Calculations. The safety analysis for a lake recipient is performed in an analogous manner, except for substitution of turnover rates in Morpa Lake (TR 83-52)
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From page 59... ...
For each of the above examples, the KBS-3 authors have provided to the panel persuasive arguments in support of the parameters selected (Tonis Papp, KBS; Ivars Neretnieks, Royal Institute of Technology; Leif Carlsson, Swedish Geological; and B Allard, Chalmers University of Technology; private communications, 1983)
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From page 60... ...
The panel notes that the KBS-3 plan did not address the matter of canister recovery in the event of extremely unlikely catastrophic events. Despite these questions concerning the calculations, it appears that only under the most unlikely and extreme combination of worst-case conditions (e.g., improbably early canister failure, rapid fuel dissolution, poor radionuclide retardation, and a complete absence of surface water infiltration to a well)
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