Inservice property degradation is known to occur in fuel clads of reactors systems currently in operation and under design_ Irradiation, corrosion, diffusion induced mass transfer, and a host of mechanical influences constitute the complex environmental variables responsible for the degradation. The degradation could occur in the bulk of the clad or through its thickness depending on the component of the environment and the reactor operating history. Synergistic influences of more than one component of the environment must also be considered.
From the mechanics viewpoint, the degraded alloy is a material with nonuniform properties. Thus, the basic stress-strain relations require modifications which are functions of the reactor operating history. The nature of the balance equations of stress equilibrium changes, especially if the degradation occurs through the thickness of the material. Prescription of the complete stress-strain relationship is required for an elastic-plastic type of analysis at the beginning of each new time step in modeling the performance of the material. Knowledge of the metallurgical and mechanical nature of the degraded alloy and the spatial variation of the properties which result is a prerequisite for the modeling.
Evidence from available literature is presented to illustrate this problem. The study involves the degradation of the 316 type stainless steel considered for use in Liquid Metal Fast Breeder Reactors where sodium is used as the coolant. Nonuniform changes in properties of the steel have been found to occur due to the thermal, thermochemical, and irradiation environment to which it is exposed. Variations in imparity concentrations (such as carbon in the steel) of several orders of magnitude compared to the original values have been observed under controlled sodium exposures. At temperatures relevant to the reactor system migration of impurities by diffusion, compound formations and carburization/decarburization behavior have been observed to occur. Mechanical property measurements such as tensile and yield strengths made under these conditions indicate that thermal and thermochemical influences can result in variations in the above properties quite different from the original material.
Modified formulations of the elastic and elastic-plastic analysis of the degraded fuel-clad are presented in two dimensions. The elastic and plastic parameters relating to the properties of the degraded material are represented by spatially varying functions as opposed to being treated as constants which is the conventional case. The changes in the mathematical nature of the constitutive equations are demonstrated by sample illustrations and solutions involving continuous changes in the elastic moduli through-the-thickness of the clad. Recommendations for the establishment of improved Reactor Research Development Standards are made based on the studies. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/38718 |
| Date | 08 July 2010 |
| Creators | Subbaraman, Ganesan |
| Contributors | Nuclear Science and Engineering |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | 112 leaves, BTD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 08041054, LD5655.V856_1977.S83.pdf |
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