## 3 Results and Discussion  
### Deformation and Equivalent Stress under Axial Compression  
Based on an axisymmetric FEA analysis, the sealing behavior of the UMS during assembly and the loading conditions are investigated. During assembly, the UMS housed in its surrounding parts was subjected to axial compression by the displacement of the pipe pups. Figure 4 shows the von Mises equivalent stress distributions with the deformed shape of the UMS due to the applied axial displacement up to 8 mm from the left and right pipe pups, which corresponds to a compression ratio of 40%. The deformation scale was set to 1:1 for a true description of the shape. Because of its near incompressibility, the UMS was deformed by an amount that was equal to the amount of compressed volume by the imposed axial displacement during assembly, and a gradual filling of the space that was empty before compression could be confirmed in general. However, the deformation of the UMS showed different trends according to type and TR. In type 1, the open grooves on both sides, which were empty before compression, were filled first and then the empty space in the leg was subsequently filled. In type 3, which had no open groove, the empty space of the leg was filled the most and it showed the largest deformation among three types. In type 2, the leg with an open groove and the leg with no open groove showed trends similar to the deformations of types 1 and 3, respectively, resulting in asymmetrical deformations. Within the same type, a greater deformation was generated as the TR and the UMS volume increased. The greatest deformation was observed at TR = 1.50. Together with the deformation of the UMS, the change in the von Mises equivalent stress of the UMS due to axial compression during assembly was examined. In general, the larger the UMS deformation, the greater the distribution of von Mises stress. Type 1 showed the lowest stress distribution, whereas type 3 showed the highest stress distribution. Within the same type, the deformation and stress distributions were symmetrical when the geometric restraints were symmetrical under the same TR conditions, as was the case for types 1 and 3. However, asymmetric deformation and stress distribution occurred in the asymmetric geometric restraint of type 2. In the case in which the stress distribution acted on the UMS leg region, the same stress concentration was observed at the end where the inner surface of the UMS met the insulating spacer (see circles a and b in Figure 4), regardless of the type. In type 1 with open grooves on both sides, stress concentration was also observed at the corner where the outer surface of the UMS met the open groove (see circles c and d in Figure 4). In type 3 with no open groove, a stress concentration was observed in the inner corner where the inner surface of the UMS met the insulating spacer (see circles e and f in Figure 4). In type 2, the characteristics of types 1 and 3 were observed asymmetrically in each leg. Regardless of the type, as the TR increased, the stress distribution acting on the UMS leg and body regions gradually increased. The greater the TR in axial compression, the greater the compressive force  
Figure 4: UMS deformation and equivalent stress distribution at 40% of axial compression during assembly. SOP: symmetric open groove; ASOP: asymmetric open groove; SNO: symmetric no groove. in the UMS. This was because the increased compressive force could not contribute to the volume compression of the nearly incompressible UMS, and most of the imposed forces affected the distortion of the UMS. As the von Mises' equivalent stress in the UMS gradually increased, the stress was transmitted to the insulating spacer that was in contact with the inner surface of the UMS, causing an increased stress in the insulating spacer. This increase in stress was large enough to cause deformation of the insulating spacer (TR = 1.50 for type 2, and TR = 1.00 and 1.50 for type 3).