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.
Typically the use of SHELL finite elements to model nozzle/vessel interfaces will not include details of the weld at the interface. The omission of the weld details from SHELL element models is due to the difficulty in implementing such details and the assumption that additional interface stiffness due to the weld will have a negligible effect on results at locations of interest for Code evaluation. This study will demonstrate a proposed method for modeling weld details with SHELL elements and then evaluate the magnitude of the weld stiffness effect on results and Code compliance.
The method of modeling the weld details with SHELL elements used in this study will follow the guidance provided by ASME BPVC Section VIII, Division 2, Annex 5.A [2] for such interfaces. Models of nozzle/vessel interfaces will be shown comparing results of SOLID element models with and without the weld detail, and then SHELL element models both with and without the weld detail. The results from these models will be evaluated and recommendations for future modeling and evaluation of nozzle/shell interfaces with SHELL elements will be offered.
Stress analysis and qualification of a parent vessel with sizeable internal vessels, extensive internal piping and associated supporting structures is performed using a single finite element model. A single model is desirable because of the coupling and interaction between various sized components, particularly for seismic, hydrodynamic and thermal evaluations. Qualification of the various types of components (shells, nozzles and beams) requires multiple design codes. Examples of some unique features and challenges encountered in the process of qualifying such vessels are presented.
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