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This is a new concept in the modeling of dispersion in fractured rocks; the only additional information required in this model, over and above that in standard dispersion models, is the size-frequency distribution of the fractures. One pre- diction from the stratified-flow model, i. Other models can be used to calculate dispersion effects in fractured media, for example, a hydrodynamic dispersion model in which the dispersion coefficient is made to increase with spatial scale thereby mimicking the observed behavior at FinnsjSn and Studsvik. This can be done by requiring that the Peclet number, rather than the dispersion coefficient, remain constant.

Reason- able agreement with experimental results can be obtained with this model, using a wide range of values for the Peclet number.

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Neretnieks is aware, of course, that much additional work is needed to perfect the dispersion models for frac- tured rocks, but a good start has been made toward under- standing the complex phenomena involved. The extent to which sorption can be expected to retard the movement of radionuclides dissolved in groundwater is particularly difficult to estimate.

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 criti- cism 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.

Despite these cogent objections, laboratory experiments seem the only feasible way to get rough estimates of the role of sorption in nature, and much effort has been concentrated on improving the experiments so as to counter the objections.

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In this effort the KBS scientists have played a major part, especially in the last few years. Basic laboratory data on sorption coefficients Kd's have been generated in abundance by Allard, Rydberg, and their colleagues at Chalmers University e. A substantial part of the tabulated data now used worldwide comes from this laboratory, and of course the Swedish workers are continually in touch with similar work in progress elsewhere.

In the KBS work, laboratory methods for determining sorption coefficients were refined long ago in the more obvious ways of ensuring similarity with natural environ- ments: 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 that sorption of a given nuclide is a maximum in the pH range where neutral hydroxide complexes of the nuclide are dominant in solu- tion, 2 that the sorption of actinides in their lower oxidation states is independent of ionic strength over much of the pH range, and 3 that the amount of sorption on granitic rock can be approximated by a formula using Kd's for individual minerals TR A literature survey of work on organic complexes leads KBS scientists to conclude that increased mobility of radionuclides from this cause is not a serious problem in Sweden, because the content of organic matter in the groundwater is low and because many organic complexes are markedly sorbed on mineral surfaces TR The possible flooding of sorption sites by lead ions dissolved from the canister filling was considered in the review of KBS-2; if KBS-3 adopts the alternative of an all-copper canister, as seems likely from conversations with Swedish scientists, a flood of lead ions and also possible galvanic reaction with copper is no longer a concern.

In addition to the continuing work on sorption coef- ficients and complexes of various sorts, much recent effort has been devoted to the question of the role of colloids, especially for the actinide elements TR Experiments in the presence of air showed, for example, that Am and Pu form colloidal particles as the pH is raised from low values; but Np and U do not, presumably because they are complexed by carbonate.

Precipitation of the Am colloid increased, as would be expected, with increasing ionic strength and increasing temperature. Comparison of solutions of colloidal and noncolloidal Am passed through a column of crushed granite showed little increased mobility for the colloid; evidently these colloidal particles were sorbed as readily as the dis- solved ions. Certainly not all the nagging questions about colloids and complexes are answered by the new experimental work, but a good start has been made.

Three nuclides were used: Eu, Npv, and Puiv. As would be expected from batch experiments, Np showed little retardation, Pu a great deal, and Eu intermediate values. Clearly, it would be desirable to check laboratory results with sorption measurements made in the field, and in this difficult enterprise the KBS scientists have been especially active. These are unusually low values for Sr, but the check is satisfying. A similar experiment with Sr at FinnsjBn showed less satisfactory agreement with laboratory values, probably because frac- tures here are lined with calcite, which has poor sorptive properties TR In the experimental facility at Stripa, a study is under way to follow details of the diffusion of solutes into the rock matrix as solutions move along a small fissure.

Preliminary results show satisfactory agreement with diffusivities previously obtained in the laboratory TR , TR The above discussion is a brief and incomplete sampling of recent Swedish work on retardation. The KBS scientists, nevertheless, have succeeded. In the panel's opinion, recent work has added greatly to this understanding and created increased confidence that the Swedish estimates of rates of radionuclide transport are soundly based.

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 con- servative 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 scien- tific 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.

When contaminated water moving along a fissure zone from the repository comes in contact with the biosphere, for example when it flows into a well or lake, it will be greatly diluted by uncontaminated water from near-surface sources. The radiation dose to users of the water depends on the amount of dilution.

In KBS-3 the contaminated water is assumed to be diluted 10, times before it is consumed by humans, but the basis for this assumption is questionable. According to Tbnis Papp KBS, personal communication, , was chosen as a "reasonably conservative" value after review of available information.

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Quantitative estimates of the dilution factor for wells range very widely, from to The lower part of this range to is advocated by Thunvik TR , who uses a finite-element model to calculate water flux through the repository and into a well for several combinations of boundary conditions and well-repository configurations. The model rests on the assumption that flow in fractured granite can be treated as porous-medium flow.

The lowest figures from the model are about for a well m deep much deeper than most domestic wells in Sweden and for a well 60 m deep. Other KBS authors T6nis Papp, KBS, personal communication, think that Thunvik's assumptions for deriving the lowest factors high withdrawal rate and little infiltra- tion from the land surface are unrealistic. Carlsson TR , using a different model, calculates factors. The chosen figure, , is a compromise among these widely diverse estimates. The panel has little basis for judging the suitability of as a dilution factor for the well scenario.

It is indeed "reasonably conservative," but perhaps not conservative enough for a safety analysis that claims to be based on pessimistic choices throughout. There seems no reason to dismiss out-of-hand the assumptions behind Thunvik's lower estimates.

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If a factor of were used instead of , calculated radiation doses would still be acceptably low except for the most extreme scenarios of early canister failure. The panel suggests that model-dependent uncertainties in estimating the factor may be reduced by incorporating fracture-flow considerations into the calculation of dilution at various receptor locations.

In the case of a lake, dilution is determined by the lake volume and the turnover rate, with the assumption that all of the contaminated water from the repository enters the lake. 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.

Methods of filling and sealing described in KBS-3 are similar to those in KBS boreholes will be plugged with bentonite, and tunnels and shafts with a bentonite-sand mixture; to prevent movement of fluids through the disturbed zone adjacent to tunnels and shafts produced by blasting, seals will be constructed at intervals by sawing slots through the disturbed zone into sound rock and filling the slots with compacted bentonite blocks. To some extent this deficiency has been remedied by recent work.

Regarding emplacement of backfill, Pusch TR and personal communication, accompanied by recent photographs reported that he and his colleagues, using an experimental tunnel in the granite at Stripa, have shown the feasibility of emplacing backfill by mech- anically compacting layers of sand-plus-bentonite on the floor and then filling the upper part of the tunnels by use of shotcrete equipment.

The shotcreted material remained homogeneous and had a density only slightly less than that of the compacted layers below. An experiment is under way at Stripa to demonstrate the sealing of a tunnel by filling a slot sawed through the disturbed zone with compacted bentonite blocks and concrete Roland Pusch, University of Lulea, personal communication, To ensure the complete filling of boreholes with bentonite, Pusch TR has devised an ingenious method of placing cylinders of compacted bentonite in perforated copper tubing that is then inserted into the borehole.

As the bentonite absorbs water, it swells through the perforations and ultimately fills the entire hole. The method has proved successful in both labora- tory and field experiments. In the field experiment at Stripa , overcoring of the filled borehole and slicing of the extracted blocks for examination showed that the bentonite had expanded uniformly and formed a tight seal against the rock walls.

The experiments are impressive, but whether they are sufficient to silence all doubts remains unclear.

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A recent American technical report Meyer and Howard, 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. Pusch and his colleagues at Lulea have made a good start at this kind of research, but addi- tional in-situ experiments would make the demonstration more convincing.

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In the panel's opinion, the KBS conclusion that open- ings into a repository can be adequately sealed has much better support than it did five years ago. Continuing research may take care of remaining uncertainties long before the actual sealing of a filled repository is.

It should be kept in mind also that the adequacy of seals in a wet-rock repository is important only in the unlikely event of early failure of a large number of canisters. Evaluation of the feasibility of safe disposal of spent nuclear fuel depends ultimately on the calculated radiation doses to this and future generations from the radionuclides that may escape from a repository.

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  • The dose calculations for KBS-3 TR are made using a compartment model to simulate the various pathways by which radionuclides can move to human beings from assumed contaminated water sources. Concentrations in the water sources are estimated by using some of the models des- cribed 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 ground- water approaches the places where water is obtained for human use.

    Even such unlikely radionuclide transport conditions produce radiation exposures that are at least fold less than International Commission on Radiological Protection ICRP limits. If a single canister somehow fails completely within the first century after repository closure Scenario B ,. 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. How secure is the basis for these calculations? Reliability of the calculations depends on 1 the validity of the models and 2 the assumptions made as to the values of parameters used in the models.

    The effort by the KBS-3 group in the development of models espe- cially the fracture flow work of Neretnieks is impres- sive. A complex box compartment model has been used to describe the dynamic processes that distribute both radioactive and nonradioactive substances throughout the biosphere, with simplifications tailored to the variables important to a specific site. Thus, the accumulation of radiation doses by an individual has been evaluated from sources local e. Click here to subscribe. If you would like to write a letter to the editor, please forward it to letters globeandmail.

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    Customer help. Contact us. Log in. Log out. Article text size A. To view your reading history, you must be logged in. Log in Register. Brent Jang. Published March 13, Updated March 14, Published March 13, This article was published more than 6 months ago. Please log in to bookmark this story. Log In Create Free Account.

    Find your bookmarks by selecting your profile name. The Boeing Max incorporates the Maneuvering Characteristics Augmentation System MCAS — an anti-stall feature introduced to compensate for the heavier engines, which changed the aerodynamics of the jet, tending to push the nose of the aircraft up. AOA sensor. Winglet aligns itself with airflow. Level flight: Normal angle of attack AOA — angle at which airflow hits aircraft. Aircraft trajectory. Haphazard survey effort means that many species can go undetected in an area for decades, not because they are absent necessarily, but because no effort is made to survey for them Boakes et al.

    When resurvey efforts are later conducted and the species is not detected, we struggle to know when or why the species disappeared, and even whether they are truly extirpated or we have just not tried hard enough to find them Boakes et al.