Humid-air and aqueous general and pitting corrosion models (including their uncertainties) for the carbon steel outer containment barrier were developed using the corrosion data from literature for a suite of cast irons and carbon steels which have similar corrosion behaviors to the outer barrier material. The corrosion data include the potential effects of various chemical species present in the testing environments. The atmospheric corrosion data also embed any effects of cyclic wetting and drying and salts that may form on the corroding specimen ' surface. The humid-air and aqueous general corrosion models are consistent in that the predicted humid-air general corrosion rates at relative humidities between 85 and 100% RH are close to the predicted aqueous general corrosion rates. Using the expected values of the model parameters, the model predicts that aqueous pitting corrosion is the most likely failure mode for the carbon steel outer banier, and an earliest failure (or initial pit penetration) of the 100-mm thick barrier may occur as early as about 500 years if it is exposed continuously to an aqueous condition at between 60 and 70°C.The current design concept of a multi-barrier waste container for the disposal of the nation's high-level nuclear waste in the potential repository at Yucca Mountain, Nevada, calls for a thick layer (about 100 mm) of carbon steel corrosion-allowance material (CAM) as the outer barrier, and a thin layer (about 20 mm) of Alloy 825 corrosion-resistant material (0 as the inner barrier. The initial post-closure near-field environment of the potential repository will be hot and dry, and following the peak temperature period, the near-field environment would cool down gradually. Thus, the waste containers are expected to undergo humid-air corrosion initially at elevated temperatures, and their corrosion mode w i l l gradually shift to aqueous corrosion as the near-field environment continues to cool down to lower temperatures and more humid conditions. This paper discusses the general and pitting corrosion models for CAM in humid-air and aqueous conditions, which were developed for the total system performance assessment-1995 (TSPA-1995) [l]. The models will be improved (or refined) as more site-relevant corrosion data become available. HUMID-AIR CORROSION MODELS. . ata C w A total, of 166 atmospheric corrosion data (up to 16 years) of cast irons and carbon steels which have similar corrosion behaviors to the candidate CAM were collected from literature.
A detailed stochastic waste package degradation simulation model was develhe aJYrB incorporating the hdd-air and aqueous general and pitting corrosion models for the carbon steel corrosion-allowance outer barrier and aqueous pitting corrosion model for the Alloy 825 corrosion-resistant inner barrier. The uncertainties in the individual corrosion models were also incorporated to capture the variability in the corrosion degradation among waste packages and among pits in the same waste package. Within the scope of assumptions employed in the simulations, the. corrosion modes considered, and the near-field conditions fiom the drift-scale thennohydrologic model, the results of the waste package performance analyses show that the current waste package design appears to meet the 'controlled design assumption' requirement of waste package performance, which is currently defined as having'less than 1% of waste packages breached at 1,000 years [l]. It was shown that, except for the waste packages that fail early, pitting corrosion of the corrosion-resistant inner barrier has a greater control on the failure of waste packages and their subsequent degradation than the outer banier. Further improvement and substantiation of the inner barrier pitting model (currently based on an elicitation) is necessary in future waste package performance simulation model.
A two-layer waste package (carbon steel outer barrier and Alloy 825 inner barrier) is specified to dispose of high-level nuclear waste at the potential repository at Yucca Mountain. A set of improvements and more realism have been added to a stochastic waste-package degradation model which was developed for a recent total system performance assessment of the potential repository [1]. The waste-package surface is divided into “patches” to better represent the general corrosion of the carbon-steel outer barrier. The “corrosion-time” concept is developed to represent the corrosion of the carbon-steel outer barrier in changing exposure conditions with time such as those expected in the potential repository. With the patches approach and the corrosion-time concept implemented into the waste-package degradation model, sensitivity of the waste package degradation (failure and pitting degradation) to different threshold spalling thicknesses of the corrosion products from the carbon-steel outer barrier is analyzed. The results show that the waste-package pitting degradation is sensitive to the corrosion-products spalling thickness of the carbon-steel outer barrier. A greater pitting degradation of the waste packages is predicted with a smaller spalling thickness. Further understanding of the corrosion-products spalling in different exposure conditions (i.e., water chemistry, water contact mode, etc.) and its effects on carbon steel corrosion is needed to enhance the confidence in the waste-package performance modeling in the potential repository.
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