PSI - Issue 48

Tamás Fekete / Procedia Structural Integrity 48 (2023) 302–309 Fekete / Structural Integrity Procedia 00 (2023) 000 – 000

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An industrial PTS case study The accuracy of PTS calculations is becoming an increasingly important issue in SIC s for NPP s, during second phase extended OL projects. A significant issue will be whether SIC s will be able more precisely assess CTDF s resulting from complex loads in inhomogeneous material systems of an equipment. The example illustrates capabilities of the new theory, via the calculation of a PTS transient case that has been shown to be a relevant TAL limiting PTS event in previous SIC projects. For the analysis, a customized form of the above framework theory was used, which was easy to implement into the MSc.Marc/Mentat Finite Element ( FE ) system used for computations. The problem to be solved was a coupled thermal – mechanical – fracture-mechanical one. The computational model was implemented with large strain – large displacement kinematic model , elastic – plastic material models and the corresponding J-integral version. The simulations were performed by the software using a staggered coupling scheme. System Geometry The investigated system is a VVER 440 V – 213 type Reactor Pressure Vessel ( RPV ) with an assumed semi-elliptic, underclad crack with depth a=0.1 * wall_thickness , a / c = 1 / 3 , at the safety critical region of weld 5/6. A section of the relevant part of the RPV with the sketch of the crack model is shown in Fig. 4 . On the crack model, the crack front points at which J -integral was evaluated, are marked ( 1 … 6 ). On the RPV wall, a system of regular FE meshes with sufficient density were applied. A hierarchy of locally refined FE meshes around the crack were used. The finest mesh around the crack front has elements with linear dimensions ≈ 25 μm . The model is based on 20 nodes brick elements.

Fig. 4. A relevant section of the investigated VVER 440 RPV geometry with a sketch of the crack model geometry

The PTS event The selected PTS transient was ‘ Steam Line Break in the Containment, Initiated from Shutdown Mode ’ . Time evolution of coolant temperature ( T ) and pressure ( p ) over the transient are shown in Fig. 5. ( T on left, p on right).

Fig. 5. Time evolution of T and p over ‘Steam Line Break in the Containment, Initiated from Shutdown Mode’ PTS event.