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6 CONCLUDING REMARKS The KBS plan for waste disposal differs from most plans involving mined geologic repositories in that primary reliance is placed on the long-term integrity of copper canisters. The bedrock and the surrounding buffer of compacted bentonite in which the canisters will be placed are expected only to maintain a stable and benign environ- ment; the bedrock does not, as in most geologic-disposal schemes, play an important role in slowing the migration of radionuclides to the biosphere. It could play that role in the event of widespread early canister failure, but such failure is so unlikely that the retarding effect of the rock is only secondary insurance against a very low-probability accident. (The point deserves stressing, because so much of the technical back-up for KBS-3 is devoted to this secondary insuranceâto estimates of migration rates through bedrock and the resulting radia- tion releases.) Although the bentonite buffer provides some retardation, no credit is taken for it by the KBS authors in their safety analysis. The essential question in judging the technical background of the KBS plan is whether a good case has been made for the durability of copper in a stable repository environment; all else is incidental, necessary for insurance purposes but probably almost irrelevant to the central issue of human safety. The engineered barriers called for in the KBS plan are the important ones, whereas in most geologic-disposal plans the engineered barriers are assumed to fail within short times (i.e., compared with radionuclide half-lives), and the natural geologic barriers are then depended on for slowing long-term radionuclide release. The technical work on the canisters seems beyond reproach. Methods of fabrication have been demonstrated in the plants that would be called on to produce the 66
67 ultimate 4400 canisters, at a scale large enough that extension to full scale should be no problem; sealing of the canistersâeither by electron-beam welding if lead is used as a filler, or by hot isostatic pressing if copper powder is usedâhas also been demonstrated; rates of corrosion have been tested at extreme values of the con- trolling variables; and stress corrosion cracking has been eliminated as a possible threat to canister integ- rity. Much of the work was completed by the time of KBS-2, and further studies in the past five years have abundantly confirmed earlier results and earlier con- jectures about canister performance. Assurance that bentonite will perform all the functions assigned to it in the KBS plan is somewhat less complete. No entirely satisfactory source of bentonite has yet been identified; and even though properties of bentonite can be in some measure tailored to specifications, there is as yet no guarantee that a uniform product with all the desired properties can be obtained in the necessary large quantities. The compacted bentonite to be placed in the deposition holes around the canisters has been exhaus- tively tested, and the feasibility of emplacing it has been demonstrated; there seems little doubt that it can perform its expected functions of keeping groundwater stationary or nearly so at canister surfaces, of slowing the motion of corrodents to canister surfaces, and of helping to maintain pH in the range of 8 to 9.5. Some question remains about its ability to protect canisters from sudden mechanical disturbance. Emplacement of bentonite-sand mixtures as backfill in tunnels and shafts has been demonstrated in field tests, and permeabilities have been shown to be comparable to those in adjacent rock. A method of sealing boreholes with bentonite enclosed in perforated metal sleeves has been tested in both laboratory and field. Whether bentonite can be used to make effective seals in more permeable parts of shafts and tunnels is not certain, but field tests are under way. The bentonite used in experiments has lived up to expectations, and there is good reason to think that it will fulfill its various roles at least well enough to preserve canister integrity. Even if its performance is to some extent deficient, estimates of groundwater amounts and flow rates are sufficiently low and sufficiently conservative to ensure that local movement at unantici- pated rates will not seriously affect the canisters. The tectonic stability of large areas of Swedish bedrock seems amply confirmed. Demonstration that sites
68 can be found in the bedrock where groundwater is suf- ficiently small in quantity, slow in movement, and bland in composition to pose no threat to copper canisters seems convincing to the panel, even though no site com- pletely satisfactory in all details has yet been located. Three sites explored recently look promising, and better ones will probably be found in the planned continuing investigation. The motion of groundwater in fractures at depth in granitic bedrock cannot be accurately deduced from measurements at the surface and in boreholes, especially if there is much channeling along fractures; but conservative estimates for the studied sites are a source of confidence that sizable rock volumes exist where groundwater would not adversely affect a repository. The most troubling uncertainties remaining in the KBS proposal relate to the part least likely ever to be relevant to actual repository performance, i.e., the migration of radionuclides through buffer and bedrock in the improbable event of early canister rupture. Uncer- tainties here are great simply because of the number of variables that must be considered in any model used to describe the dissolution of radionuclides from fuel rods and their movement through a variety of materials to the point where they may become hazardous to humans. Solu- bilities, effects of local oxidation due to radiolysis, retardation by sorption of some radionuclides as they migrate through buffer and rock, effect on retardation of the formation of complexes and colloids, dispersion of nuclides into rock along fractures where flow is con- centratedâall these must enter into a system of models for radionuclide behavior. The panel could point to uncertainties in some of the data and assumptions, but since the assumptions are mostly conservative and since the calculated releases are within acceptable limits even for the most radical disruption scenariosâand since, in addition, there is little probability of early canister disruptionâthe panel concludes that the technical basis for the calculations of low release rates is sound. The KBS team of scientists and engineers has accom- plished a great deal of pertinent research, especially in the accelerated work done for KBS-3. The research program has included geologic field studies, in-situ testing at the experimental facilities at Stripa, Studsvik, and Finnsjbn, laboratory work at Swedish universities, tech- nical institutes, and consulting agencies, and work per- formed at laboratories in England and the United States. The program has covered all aspects of the disposal prob-
69 lem so thoroughly that a critic has difficulty finding weak points that have not already been considered. Where gaps in knowledge still exist, they mostly concern points where the complexities of natural phenomena defeat efforts to arrive at definitive answers. The KBS research has not only achieved its purpose of showing that radioactive waste can be disposed of in Sweden with reasonable assur- ance of safety for at least one million years, but it has provided the world with a wealth of basic data on the corrosion of copper, on the movement of groundwater in fractured rock, on the properties of bentonite, and on the many factors that influence radionuclide migration. In compiling these data, the KBS authors have performed a valuable service both for their own country and for any country or group considering the disposal of nuclear waste in crystalline bedrock.