STRENGTH AND EMBRITTLEMENT UNDER HTGR OPERATING CONDITIONS-

WORK IS PLANNED TO FORMULATE A PHYSICALLY-BASED CON- CEPTUAL MODEL FOR THE TIME-DEPENDENT METALLURGICAL EMBRITTLEMENT AND STRENGTHENING OF TYPICAL ALLOYS CONSIDERED TO BE POTENTIAL CANDIDATES IN THE HIGH- TEMPERATURE, GAS-COOLED REACTOR (HTGR) APPLICATIONS, PARTICULARLY IN THE TEMPERATURES OF 750 DEGREES TO 950 DEGREES CELSIUS. PHASE I ALSO IS DIRECTED TOWARD ESTABLISHING THE SCOPE AND FEASIBILITY OF INTEGRATING THE FORMULATED MODEL INTO A BROADER PROGRAM CONTRIBUTING TO THE NEEDED BASIS FOR THE STRUCTURAL DESIGN AND ANALYSIS OF THE RELEVANT METALLIC COMPONENTS. THE MODEL IS TO BE DEVELOPED ESSENTIALLY TO BETTER QUANTIFY THE INTERNAL MATERIAL CHANGES, PARTICULARLY RELATED TO CARBURIZATION, AND RELATE THESE TO THE RESULTING MECHANICAL DEFORMATION AND FRACTURE RESPONSE OF THE MATERIAL. THE WORK PLAN CALLS FOR (1) AN ASSESSMENT OF CURRENTLY AVAILABLE TEST DATA AND FIELD OBSERVATIONS TO IDENTIFY THE MACROSCOPIC PHENOMENA AND FAILURE MODES OF IMPORTANCE, (2) IDENTIFICATION OF AND SUGGESTIONS FOR THE UNDERLYING PHYSICAL PROCESSES AND MECHANISMS, THE RESULTING INTERNAL CHANGES AND VARIABLES TO CHARACTERIZE THE CHANGES, (3) EXAMINATION OF THE RELEVANT THERMODYNAMIC AND KINETIC ASPECTS OF THE KEY PROCESSES, (4) MODEL DEVELOPMENT RELATING THE RESULTS OF (1) THROUGH (3) TO THE EMBRITTLEMENT AND STRENGTH CHARACTERISTICS OF THE MATERIAL, AND (5) PRELIMINARY ASSESSMENT AND INTERPRETATION OF THE MODEL WITH THE SCOPE DETERMINATION FOR THE SUBSEQUENT PHASES OF THE R&D EFFORT.