PSI - Issue 37

M. Annor-Nyarko et al. / Procedia Structural Integrity 37 (2022) 225–232 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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4. Results and Discussion 4.1. Thermo-mechanical analysis

The highlighted position at the intersection of the inlet nozzle and inner RPV wall (path), as depicted in Fig. 4(a), experienced the maximum stress value during the direct thermo-mechanical coupling analysis. The maximum stress was due to that joint experiencing the largest temperature gradient. Hence, a PTS induced defect is most likely to be initiated from that intersection. The temperature, axial and hoop stresses extracted, starting from the maximal stress node of interest through the thickness of the vessel (path), for the duration of the AOO event are shown in Figs. 5-7. The temperature gradient observed in the course of transient time decreased steadily from the inlet nozzle-inner wall RPV intersection to the outer part of RPV wall. The temperature dropped as a result of the sudden cooling and large thermal capacity of RPV. The hoop and axial stress distributions also decreased from the same maximal location to compressive stress on the outer part of the RPV wall. This phenomenon was largely due to the combined action of the thermal and internal pressure variations during the transient event. Also, the maximum hoop and axial stresses estimated were below the yield stress of the referenced RPV material.

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Fig. 5. Temperature distribution through the inlet nozzle- inner RPV wall intersection thickness at different times

Fig. 6. Axial stress distribution through the inlet nozzle- inner RPV wall intersection thickness at different times

4.2. Fracture mechanics analysis The node-based submodel with crack insertion shown in Fig.4(b) was used to analyze the integrity parameter, SIF (K I ) following the co-simulation procedure shown in Fig.3. An axial semi-elliptical surface crack with a depth of a = 0.042 m and an aspect ratio of ⁄ = 0.3 was assumed in the calculation of K I. The assumed surface crack was inserted at the maximal stress location in the RPV submodel created from the results of the preceding thermo mechanical analysis (section 4.1). The K I values at the crack deepest point were computed using M-integral approach implemented in Abaqus-FRANC3D co-simulation. K I was also evaluated using the VCCT method. The fracture toughness of the vessel material (K IC ) was accurately estimated following the ASME approach. The comparison of K I histories along the postulated crack front vs. crack tip temperature calculated using M-integral and VCCT methods with K IC is presented in Fig. 8. The results show both methods are in good agreement irrespective of the varied assumptions and calculation methods. Also, the small K 1 values observed at the deepest crack tip was due to low compressive stresses. From the comparison of K I and K IC shown in Fig. 8, the maximum K I of 38.6 MPam 0.5 was less than K IC, therefore, the PTS induced by inadvertent operation of the safety injection system and the surface crack