The liquid-salt-cooled very high-temperature reactor (LS-VHTR), also called the Advanced High-Temperature Reactor (AHTR), is a new reactor concept that combines in a novel way four established technologies: (1) coated-particle graphite-matrix nuclear fuels, (2) Brayton power cycles, (3) passive safety systems and plant designs previously developed for liquid-metal-cooled fast reactors, and (4) low-pressure liquid-salt coolants. Depending upon goals, the peak coolant operating temperatures are between 700 and 1000°C, with reactor outputs between 2400 and 4000 MW(t). Several fluoride salt coolants that are being evaluated have melting points between 350 and 500°C, values that imply minimum refueling temperatures between 400 and 550°C. At operating conditions, the liquid salts are transparent and have physical properties similar to those of water. A series of refueling studies have been initiated to (1) confirm the viability of refueling, (2) define methods for safe rapid refueling, and (3) aid the selection of the preferred AHTR design. Three reactor cores with different fuel element designs (prismatic, pebble bed, and pin-type fuel assembly) are being evaluated. Each is a liquid-salt-cooled variant of a graphite-moderated high-temperature reactor. The refueling studies examined applicable refueling experience from high-temperature reactors (similar fuel element designs) and sodium-cooled fast reactors (similar plant design with liquid coolant, high temperatures, and low pressures). The findings indicate that refueling is viable, and several approaches have been identified. The study results are described in this paper.
In temper bead welding, the heat input from welding is purposefully utilized to temper the hard microstructure for improving toughness. An optimal temper bead welding requires careful control of heat input and bead placement. In this study, the effect of linear heat input on heat-affected zone (HAZ) tempering was studied by a combination of experimental testing and numerical modeling. Temper bead welding experiments were performed on SA-533 steel using three different heat inputs while keeping the power ratio constant. The extent of tempering in the HAZ was quantified using micro-hardness mapping. A 2-D weld heat transfer model, using the double-ellipsoidal heat flux equation, was developed to calculate the temperature evolution. The peak temperatures experienced in the substrate’s HAZ was correlated to the hardness distribution. The results indicate that the linear heat input can have a significant influence on the extent of tempering in temper bead welding.
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