PSI - Issue 13

Peter Trampus / Procedia Structural Integrity 13 (2018) 2083–2088 P. Trampus / Structural Integrity Procedia 00 (2018) 000 – 000

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• temperature and strength calculations, • fracture mechanics evaluation, • determination of allowed service life for RPVs. 2.1. PTS transient identification

The identification of PTS transients has been performed in a comprehensive way taking into account various accident sequences including impact of component malfunctions, different operator actions, internal and external hazards. Eleven specified transients were chosen and analyzed on deterministic basis, i.e. engineering judgment. Complementary to this, selection of additional transients on probabilistic basis was also performed. As for the latter those events were selected which showed a higher probability of occurrence than 10 -5 /year . 2.2. Thermal-hydraulic analyses of PTS transients The overall progression of accidents was calculated with an advanced thermal-hydraulic system codes (RELAP5/mod3.2 and ATHLET). The adequate margin of the calculation results was ensured by the use of best estimate computer codes with conservative input data and with conservative assumptions regarding the availabilities of influencing systems. In flow stagnation cases the role of the system code calculations was to estimate the onset of the stagnation, and give the initial temperature conditions, emergency core cooling injection flow rates and the so called well-mixed temperatures in the RPV down-comer segments while the actual temperature curves for the so called colder plumes in these cases were calculated with a separate code. 2.3. Neutron fluence calculations Based on core configurations implemented until current analysis and planned to be implemented in the future, calculations using the core design code and Monte Carlo transport code were performed, and end-of-life fluence F EOL for 50 operating years, were calculated for the RPV wall as well as for the surveillance position. Neutron dosimetry surveillance results were used to verify the calculations. The calculations were made in the azimuthal and radial position where the fluence has its maximal value. Table 1 shows the results of calculations for the end-of-life fluence for RPVs 1 to 4 for the targeted operating lifetime (50 years).

Table 1. End-of-life fast neutron fluence [n/cm 2 ] (E>0.5MeV) RPV Core weld Core center 1 1.94E+20 2.81E+20 2 1.97E+20 2.83E+20 3 1.89E+20 2.72E+20 4 1.95E+20 2.79E+20

3. Irradiation effects on RPV materials

The RPV shells were made of forgings from Cr-Mo-V alloyed steel 15Ch2MFA. The welding of separate shells to one another was carried out with the use of submerged method with Sv-10ChMFT wire and the flux AN-42. The anticorrosive cladding of RPV inner surface has two layers: the first layer is Sv-07Cr25Ni13; the second layer, first bead is Sv-08Cr19Ni10Mn2Nb and second bead is Sv-04Cr20Ni10MnNb. The fracture toughness reference curve of RPV materials 15Ch2MFA and Sv-10ChMFT is as follows: = {26 + 36 [0.02( − )], 200} , in MPa√m , (1) where: T is the material’s temperature and T k is the critical temperature of brittleness. During evaluation of the shift of ΔT k the effects of irradiation, thermal ageing and fatigue were taken into account. T k and ΔT k were determined during evaluation of Charpy impact test results of surveillance specimens for every RPV. As for the initial value of the critical temperature of brittleness T k0 the more conservative value of measurements performed by the manufacturer and performed during surveillance programs has been considered. The zero level measurements of surveillance programs are evaluated using the 41 J impact energy criterion, similarly to the evaluation of tests performed on irradiated specimens.