PSI - Issue 13

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

2087

5

T k trend curves for weld metal (No. 5/6) of Unit 1 based on surveillance data

180

Fl. Calculated Fl. Measured Fitted T k Fitted T k +16 o C

160

140

120

100

Data: CV1A_B Model: user31

T k [°C]

80

Chi^2/DoF = 9.11428 R^2 = 0.98323

60

a b

62.36351 0.39205

±2.59772 ±0.03846

o C

40

T k0

25

20

0

1

2

3

4

5

Fluence [*10 20 n/cm 2 E>0.5 MeV]

Fig. 2. Trend curve of RPV 1 weld metal.

.

5. Fracture mechanics analysis

All the transients, selected by PTS event screening criterion were the subject of fracture mechanics analysis. Taking the loads (pressure, temperature and heat transfer coefficient) resulting from thermal-hydraulic analyses for the particular PTS events into account as boundary conditions, the global temperature, strain and stress fields were firstly evaluated using 3D finite element (FE) models of the RPV without crack, assuming linear elastic material behavior. After having the stress field, a set of underclad cracks of semi-elliptic type were postulated for calculation of the stress intensity factors K I with the maximum depth of a = 0.1t and with an aspect ratio of a/c = 1/3 . In the base metal the postulated crack orientation was normal to the principal stress and in the circumferential weld it was circumferential. The stress intensity factor values were calculated at the deepest point of the crack and at the near interface location of the cladding and the base / weld metal using an engineering evaluation method (Marie and Chapuliot 2008). T k allow values for selected locations of flaws were then evaluated. K Ic was computed using T k and the calculated through wall temperature distribution for each flaw depth and evaluation point on the flaw boundary as a function of time during the transient. Then, K I and K Ic was plotted as a function of temperature for each flaw depth and evaluation point on the flaw boundary for each time during the transient. Each K Ic vs temperature curve was moved along the K I axis until it was just tangent to the associated curve. The T k value at which the K Ic vs temperature curve was just tangent to the associated K I vs temperature curve was T k allow for the selected flaw depth and evaluation point. T k allow value for a designed location was defined as the minimum of T k allow values computed for all flaw depths and all evaluation points defined for that location. For demonstration of the results, one of the most severe transients is shown here: Ø492 mm hot leg line, double ended guillotine brake (large break loss-of-coolant accident, LB LOCA). Fig. 3 shows K I (as a function of local temperature) at the deepest point of the crack front and at the closest point to the cladding-base metal interface. The same figure shows the K Ic reference curve belonging to T k allow value determined for loads appearing at these points.