The Forming Die Design and Experimental Research of CFETR Vacuum Vessel Shells
Vacuum vessel (VV) is the main component of the China fusion engineering test reactor. Shell forming is relatively in the earliest phase, one of the most important processes during VV manufacturing. The profile errors caused by spring back of shells at room temperature will directly influence the subsequent manufacturing and experimental studies, so a set of dies, whose profiles can be easily revised, has been designed based on the spring-back theory. The empirical formula has been used to calculate the theoretic die profile value, which is mostly determined by the mechanical properties of 316LN stainless steel. Then, choosing four relevant values to simulate the forming process in which the optimal profile errors of shell must be controlled within 1.5 mm. The optimal dimensional value of the dies has been confirmed to guide the forming experimental study. After the experiment, the actual profile errors, thickness reduction, maximum spring back, maximum deformation rate, and the surface residual stress have been measured, and those data are an accurate indicator from the finite element analysis, also verifying the reasonability of the manufacturing process in turn.
The evaluation on the effect of shell thickness reduction for ITER vacuum vessel sector 1 and 6
Abstract The structural analyses of vacuum vessel have been performed to investigate the effect of shell thickness reduction on structural integrity. The finite element models of vacuum vessel considering original design and thickness reduction have been developed. The expected maximum thickness reduction possibly caused by forming and bending processes during fabrication was applied to the curved region of the analysis models. The linear elastic and nonlinear limit analyses have been performed. The structural integrity of main vessel including lower port stub extension has been verified in accordance with the requirements of RCC-MR. It is concluded that the inner and outer shells of main vessel still have enough strength margins under pressure and VDE (Vertical Displacement Event) load conditions in spite of thickness reduction. These results have been reviewed and approved by ITER Organization and ANB (Agreed Notified Body).
Experimental investigation of the geotechnical properties and microstructure of lime-stabilized saline soils under freeze-thaw cycling
Abstract The Song-Nen Plain in northeastern China holds one of the three largest expanses of soda saline-alkali soil in the world. Due to its disadvantageous geotechnical properties, the saline soil in this area is unable to meet the needs of construction projects. This study evaluates the stabilizing effect of adding lime to such soils. Specifically, it investigates how the physicochemical and mechanical properties and microstructure of soda saline-alkali soil are altered by lime and how these changes are affected by both freeze-thaw cycling and the duration of curing. The results reveal that adding lime to saline soil transforms clay particles into sand and silt particles and causes the pore size distribution (PSD) to become bimodal, with small-pore and macropore populations. An increase in lime content leads to an increase in the unconfined compressive strength (UCS) but a gradual decrease in both the cation exchange capacity (CEC) and plastic index. Freeze-thaw cycles break up coarse particles and aggregate finer particles, increasing the diameter of macropores and decreasing the USC; however, the effect on specimens with >9% lime is negligible. It is further shown that the geotechnical properties of the soil change immediately after the addition of lime, indicating that the cation exchange reaction takes place very quickly. Furthermore, the pozzolanic reaction leads to an increase in the percentage of sand-sized particles and a decrease in porosity after 28 days of curing. Overall, these results indicate that it is feasible to use lime to improve the engineering properties of soda saline-alkali soil in this area.