There are two NPPs operated in the Czech Republic: NPP Dukovany of WWER 440/213 type and NPP Temelin of WWER 1000/320 type. Four units of NPP Dukovany have started their operation during 1985 – 1987, two units of NPP Temelin have been put into operation during 2002 – 2003. Pressurised-thermal shock evaluation, i.e. assessment of resistance of reactor pressure vessel (RPV) against fast fracture, was for both NPPs performed during the plant operation. For NPP Dukovany the PTS evaluation started in 1996 and the project was completed in 2004. For NPP Temelin the PTS project was conducted during 2001 – 2004. At the beginning of the project for NPP Dukovany the PTS evaluation was performed according to the IAEA “Guidelines on PTS Analysis for WWER NPP” [1]. At later phases of this project and for the whole project of NPP Temelin, the appropriate PTS evaluations were performed according to the IAEA PTS Guidelines, Revision 1 [2] and the “Unified Procedure VERLIFE” [3]. The PTS analyses consisted of two main parts: 1) Thermal hydraulic (TH) analyses: The system TH analyses were performed mainly with using the RELAP5 computer code. Reactor coolant system, secondary circuit, and emergency core cooling system were modelled in a very detailed way according to the actual NPP configuration, signals specific to the NPP were built into the models and operator actions were taken into account for some PTS transients. For relevant cases, the behaviour of media in such components as hermetic confinement (NPP Dukovany), containment (NPP Temelin), sumps and ECCS coolers, was investigated by MELCOR or RELAP5 code with primary objective — to determine the exact temperature of water from safety injection system. The detailed TH mixing analyses were performed for all non-symmetric cases using CATHARE (with 2-D model of reactor downcomer) or REMIX/NEWMIX codes. 2) Structural Analyses: All computations were focused on circumferential welds in the beltline zone. The overall approach to the analyses (for the final stages of the projects) was as follows: Temperature and stress fields were computed by finite element method (FEM) on RPV model with crack included in the mesh. SYSTUS FEM code was used for this purpose, with elastic-plastic modelling of material properties. After performing the stress analysis, the energy release rate G was calculated using the postprocessor of SYSTUS code. Finally, the stress intensity factors KI were computed based on values of G. The defects were finally postulated as semielliptical underclad cracks, both axial and circumferential, with aspect ratios a/c = 0.3 and 0.7 and with the depth 1/10 of the wall thickness. Maximum allowable critical temperature of brittleness Tka was determined by comparison of KI and [KIC]3 (the allowable value of stress intensity factor) curves. Some examples of results are presented in the paper.

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