Integrated Approach to Modeling Long-Term Durability of Concrete Engineered Barriers in LLRW Disposal Facility
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INTEGRATED APPROACH TO MODELING LONG-TERM DURABILITY OF CONCRETE ENGINEERED BARRIERS IN LLRW DISPOSAL FACILITY J.H. Lee*t, D.M. Roy*, B. Mann**, and D. Stahl*** * Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802. ** INTERA, 101 Contention Center Dr., Suite P-110, Las Vegas, NV 89109.
*** B&W Fuel Co., 101 Convention Center Dr., Suite P- 110, Las Vegas, NV 89109. ABSTRACT This paper describes an integrated approach to developing a predictive computer model for long-term performance of concrete engineered barriers utilized in LLRW and ILRW disposal facilities. The model development concept consists of three major modeling schemes: hydration modeling of the binder phase, pore solution speciation, and transport modeling in the concrete barrier and service environment. Although still in its inception, the model development approach demonstrated that the chemical and physical properties of complex cementitious materials and their interactions with service environments can be described quantitatively. Applying the integrated model development approach to modeling alkali (Na and K) leaching from a concrete pad barrier in an above-grade tumulus disposal unit, it is predicted that, in a nearsurface land disposal facility where water infiltration through the facility is normally minimal, the alkalis control the pore solution pH of the concrete barriers for much longer than most previous concrete barrier degradation studies assumed. The results also imply that a highly alkaline condition created by the alkali leaching will result in alteration of the soil mineralogy in the vicinity of the disposal facility. INTRODUCTION
In the design of low-level radioactive waste (LLRW) and intermediate-level radioactive waste (ILRW) disposal facilities, concrete and other cement-based materials are increasingly utilized as major structural components that, at the same time, function as engineered barriers against migration of radionuclides and other hazardous species. Therefore, assurance that the engineered barriers will perform adequately during an expected service lifetime has been one of the major concerns regarding the successful operation of the facilities. Numerous models that have been proposed for long-term performance prediction of such concrete barriers are, however, empirical in nature, developed from short-term accelerated laboratory tests. Therefore, extrapolation of the
performance models beyond the laboratory time-scale always risks inherent unknown uncertainties. Additionally, concrete degradation processes, having once reached a certain degree, often proceed synergistically, i.e. one degradation process accelerates the others; hence, empirical models are hardly capable of simulating such synergistic processes.
There are pressing needs to develop scientifically sound and technically defensible tools to model long-term performance of concrete barriers. The modeling tools should be flexible enough to account for variations in concrete materials and mix designs, and for different s
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