PSI - Issue 54

Oleksii Ishchenko et al. / Procedia Structural Integrity 54 (2024) 241–249 Yaroslav Dubyk et al./ Structural Integrity Procedia 00 (2023) 000 – 000

247

7

Fig. 7 shows the stress components in the CB at the point opposite the broken nozzle. In both models, the maximum stresses were ( ) m b   + for the first 0.1 s and then decreased to a minimum. Further CB assessment is performed using FE model result since it has a more comprehensive character.

Fig. 7. Stresses in opposite broken nozzle (a) approximate shell theory; (b) FEM Ansys Transient.

3.2. Core barrel brittle strength and plastic collapse results The bottom part of CB opposite to active zone is under irradiation influence, affecting material characteristics such as fracture toughness, yield and ultimate strength. Thus, it is logical to evaluate the permissible cracks in CB using FAD considering the material degradation. For CB irradiated parts, new material characteristics ( IC K , y  and ult  ) are obtained using VERLIFE (2013) equations (see Table 1). It should be mentioned that CB's inner and outer surface characteristics differ due to the fluence attenuation through the CB thickness.

Table 1. Boundary values of CB material. Material steel 321 (X6CrNiTi18-10S)

IC K , MPa·√m

y  , MPa

ult  , MPa

Surface Fluence, d.p.a.

Non-irradiate part

Internal

0

242

221.98

384.1

External After 60 years working Internal

12.9 7.74

82.47

778.94 804.63

804.63 749.78

External

101.32

The maximum stress values from FEM (corresponding to each other membrane and bending stress) were obtained and used for next analysis. In the maximum loaded areas (danger zones), semi-elliptical cracks with dimensions a / c = 0.3, a = 0.25 h , h = 60 mm were postulated in inner and outer surfaces in both irradiated and non-irradiated CB parts. The assessment procedures, SIF, and reference stress equations are taken from API (2016). According to API (2016) recommendations the FAD is buit in dimensionless coordinates. The results of brittle strength and plastic collapse calculation are shown in Table 2, we can see the most conservative calculation results, corresponding crack direction and regime number. In non-irradiation, the most dangerous crack is located in CB outside surface, and in irradiation part - on the inside surface. The FAD graphs are shown in Fig. 8, we can see that CB structural integrity is maintained (n>1). From FAD graphs, we may conclude that the most probable failure mechanism for non-irradiatied part is plastic collapse and brittle failure mechanism for the irradiated part. We have also estimate the limit crack sizes for the most dangerous crack (axial crack on the outside surface), a = 0.65 h (40 mm) . Crack of such size will be easily found during regular in-service inspections. Thus, we may conclude that the failure of CB during the LB LOCA will not occur.