6.2　Prefabrication of Bottom Plates

6.2.1　The plan of bottom plates shall be as shown on the design cutting plan（Figure 6.2.1）.

Figure 6.2.1　Typical arrangement diagrams for bottom plates

6.2.2　The plan of bottom plates shall be as specified below if cutting plan is provided in the design drawing.

1　For an absorber that is greater than 12m in diameter，arched margin plates should be used for bottom plates（Figure 6.2.1），and the minimum size from the inside of margin plate to the internal wall along the direction of radius shall be no less than 700mm.

2　For an absorber that is not greater than 12m in diameter，strip margin plates may be used for bottom plates（Figure 6.2.1），and the minimum size of the straight edge of the margin plate shall be no less than 700mm.

3　The width of the center plate（Figure 6.2.1）shall not be less than 1000mm，and the length shall not be less than 2000mm.

6.2.3　Unequal gaps shall be employed for all butt joints of the arched margin plates.The external gap（e1）should be 6mm-7mm，and the internal gap（e2）should be 8mm-12mm（Figure 6.2.3）.

Figure 6.2.3　Butt joint gaps between arched margin plates

6.2.4　The permissible dimensional deviation of the bottom arched margin plates shall be as indicated in Table 6.2.4.See Figure 6.2.4 for dimensional measuring positions.

Table 6.2.4　Permissible dimensional deviation of arched margin plates（mm）

Figure6.2.4　Measuring positions of arched margin plates

6.2.5　The dimensional deviation of center plates shall be as indicated in Table 6.2.5.See Figure 6.2.5 for dimensional measuring positions.

Table 6.2.5　Permissible dimensional deviation of wall plates and bottom center plates（mm）

Figure 6.2.5　Dimensional measuring positions of wall plates and center plates

6.2.6　Arched margin plates with a thickness no less than 12mm shall be ultrasonically tested within 100mm on both sides（AC and BD as shown in Figure 6.2.4）.The results will be acceptable if the results meet the criteria for class III as specified in JB 4730.

6.2.7　The number of butt welds on foundation ring plates shall be reduced reasonably during prefabrication to prevent excessive deformations after welding.