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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The transition temperature shifts due to fast neutron irradiation ( ΔT F ) were determined by tangent hyperbolic curve fitting to Charpy impact test results of the given set applying the 41 J criterion. The transition temperature shift curves depending on the neutron fluence were derived from a two-parameter fitting as follows: ∆ ( ) = { 0 ⁄ } , in °C, (2) where: F n is the fast neutron fluence ( E>0.5 MeV) calculated for the surveillance specimens by the neutron fluence calculations, F=10 20 n/cm 2 ; A F and n are the parameters searched during fitting. Based on experimental temperature measures, e.g. (Ballesteros et al 2003), specimen overheating in capsules of irradiation chains was not considered. During research reported in (IAEA 2010), it was established that in the case if copper content of the structural material is low, and the lead factor is greater than that recommended in relevant standard (ASTM E 185) and the irradiation temperature does not exceed 290 °C, flux effect should not be considered. The copper content of the RPV materials at Paks NPP is low, the lead factor is much higher than the range recommended in ASTM E 185 (~12 vs 1 to 3) and the irradiation temperature is not higher than 270 °C; however, based upon current understanding and limited data, the effect of neutron flux cannot be determined and is assumed to be negligible. The transition temperature shifts of the RPV critical elements due to thermal embrittlement ΔT T were determined on the basis of the results of analyses performed on unit specific surveillance specimens where thermally aged specimens were included into the chains. The transition temperature shift for thermal embrittlement is as follows: (3) where: T kt is the transition temperature of the material after thermal embrittlement. Sensitivity to thermal ageing of materials with low copper content such as 15Ch2MFA and Sv-10ChMFT is negligible in the range of operating temperature (~270 °C). The transition temperature shifts of the RPV critical elements due to fatigue ΔT N could be neglected for the belt line region of the RPVs. This is because there are no stress concentrations here, and the numbers and slope of heating up and cooling down cycles are limited. The critical temperature of brittleness T k of the RPV critical elements was determined according the following formula: = + ∆ + ∆ + ∆ + , in °C, (4) The uncertainty range of 1∙σ (10 °C for forgings, and 16 °C for welds). As an example, the equation of the ( ) k n T F trend curve developed for RPV 1 weld metal is as follows: T k = 25.0 + 62.364( F n /10 20 )0.392+16, in °C. (5) The T k values of RPV 1 weld metal (core weld) as a function of fast neutron fluence, i.e. the trend curve, pertaining to service life up to the 50th year is shown in Fig 2. As part of the in-service inspection (ISI) program, standardized non-destructive testing (NDT) methods such as visual testing (VT), ultrasonic testing (UT), eddy current testing (ET), liquid penetration testing (PT) and acoustic emission testing (AET) have been applied to the parts of the RPV that are important from the point of view of integrity or nuclear safety, and performed manually or by remotely controlled manipulator according to the accessibility and radiation conditions of the given location. UT is implemented from both the inside diameter (ID) and outside diameter (OD) of the RPV with 10 years periodicity. ID inspection has always been done by vendor whilst examination from OD is performed by the plant staff using the upgraded USK-213 equipment (originally Russian design). The ID UT system (equipment, procedure) applied for examination of circumferential welds, base metal in beltline region, nozzle inner radii and safe end-to-pipe dissimilar welds, and the ET system (equipment, procedure) applied for examination of cladding are qualified in accordance with the European methodology (ENIQ 2007). The ISI results showed that the integrity of the RPV with regard to the areas within the scope of ISI NDT of the circumferential welds, the belt line material and the cladding is proven k0 kt T T T T  = − , in °C, 4. Evaluation of RPV in-service inspection results