Issue 36

T. Fekete, Frattura ed Integrità Strutturale, 36 (2016) 99-111; DOI: 10.3221/IGF-ESIS.36.10

P HASE 2: PTS S TRUCTURAL I NTEGRITY C ALCULATIONS IN THE 1990 S

Objective of the study: he main objective of the study was to reassess the safety of Paks NPP using internationally accepted design- and safety -codes and guidelines. Experiences from various international PTS-related projects were used in the elaboration of the methodology [1, 5, 9, 15]. Codes and guides used during problem definition: The PNAE G-7-002–86 [8], the ASME BPVC XI, and the 10 CFR 50.61 were used during the project. Geometry definition:  The RPV Beltline Region was selected for the study, with geometrical dimensions selected from the manufacturer’s documentation, o in the beltline region, axial, semielliptical shaped through clad cracks with depths a=13, 23 and 35 mm and shape factor a/c=2/3; and o in weld N o 5/6 a circumferential, semielliptical shaped through clad crack with depths a=13, 23 and 35 mm and shape factor a/c=2/3, were defined. Description of neutron-transport calculations; The fluence calculations were based on a semi-empirical approach. During generic studies, the lead factors for the critical beltline region and for the weld N o 5/6 were determined, and data from fluence-measurement results were used to assess factual neutron fluencies for EOL conditions at the selected locations. Materials, constitutive models:  Both the cladding and the ferritic materials (15Ch2MFA forging and the weld metal) were modeled during calculations. For thermal and strength calculations, the data were derived from the manufacturer’s documentation.  In thermal calculations, the thermo-physical parameters were independent from temperature; during calculations, averaged values of the manufacturer’s documentation were used.  In strength calculations, the Neumann-Duhamel constitutive model was used with temperature-independent, averaged values of the manufacturer’s documentation.  For describing the fracture toughness of the structural materials, the equation:     0.0217 , 35 53 k T T Ic k K T T e     (5) T

was used; the ageing was modeled by using the temperature-shift ( ∆T k

) of the critical temperature of brittleness in the

following form:

T T T   

3     T A

(6)

k

k

k

k

0

where Φ is neutron fluence.  The above material parameters were based on experimental results, performed in a qualified laboratory. Thermal-hydraulics:  The selection of the overcooling sequences was based on engineering judgments integrated with conclusions of international experiences [15] as well. A broad spectrum of various overcooling sequences, various loss of coolant accident (LOCA) scenarios, e.g. rupture of various pipelines (e.g. Ø90, Ø135, Ø233 and Ø492) were selected for calculations.  The thermal-hydraulic assessments were performed by the system thermal-hydraulic codes Relap 5 – Mode 2 and Athlet – mod 1.4.. During thermal-hydraulic assessments, a simplified ‘One Loop model’ of the primary system was used. For mixing calculations, the Remix code was used.

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