Erfahrungs- und Forschungsbericht 2012 - Ensi

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Erfahrungs- und Forschungsbericht 2012 - Ensi

3.3.3. SIF distributions along the crack front

In addition to the analysis for the deepest point

(Φ=π/2) of the crack front, the SIF and T-stress

around the crack front at different angles (Fig. 2)

are also analyzed. Figure 5 shows the SIF distributions

around the crack tip during two transients.

During the MLOCA transient, the SIF generally

decreases with crack angle and then increases to

its maximum value at the deepest point. During

the SLOCA transient, SIF displays a similar increasing

and decreasing trend with the crack front

angle. However, it is noted that before a transient

time of 160 second, the SIF at surface point (Φ=0)

is always higher than that at the deepest point

(Φ=π/2). It is because the circumferential stress at

the surface point is higher than that at the deepest

point. This implies that cracks may initiate first

at the surface point and later at the deepest point.

Thus, in the integrity analysis of RPVs subjected to

PTS transients, attention should be paid to both

the surface and deepest points of a crack tip.

4. National Cooperation

On the national level the cooperation and technology

transfer takes place within the nuclear

community. The regulatory authority ENSI and the

Swiss utilities are regularly informed about the

results of the project. Exchange of information

with the representatives of the utilities also takes

place during the annual meetings of the steering

committee «Begleitgruppe Material» of the swissnuclear

Plant Life Management (PLiM) project.

5. International Cooperation

In the frame of the PISA project we are represented

in the Network of Excellence NUGENIA (NUclear

GENeration II & III Association) which is an international

non-profit organisation, according to Belgian

law.

6. Assessment of 2012 and

Perspectives for 2013

The project goals for the first project year of PISA-II

are achieved. The integrity analysis of an example

reference case RPV subjected to two PTS transients

is performed by using the FAVOR and ABAQUS

codes. The K-T method is used to consider the

crack tip constraint effect. Based on this study, the

following conclusions are drawn:

(1) Considering the WPS effect reduces the failure

probability and increases the safety margin of the

RPV. However, since K I > K IC occurs both in the falling

and rising part of SIF-temperature curve, WPS

effect is not able to completely exclude crack initiation

and failure for this RPV. (2) By quantifying

the constraint effect with the T-stress, the safety

margin of the RPV is increased and at the same

time the corresponding conservatism of the result

is decreased. (3) The variation of the SIF along the

crack front shows that in the integrity analysis of

RPVs, both the surface point and the deepest point

of the crack tip should be considered, in order to

get outright results.

Note that the K-T method is only valid for the

elastic analysis. For the constraint effect analysis

in the elastic-plastic calculation, a two parameter

J-Q method will be used. In 2013, a 3D model

of the RPV will be used for considering the nonuniform

temperature profile which is generated

by cold water plumes. Local approach to fracture

will be used for the micromechanical analysis of

the RPV based on the weakest link principle and

cleavage fracture.

It is planned that future stress calculations will be

based on non-uniform temperature fields, calculated

with CFD codes.

7. Publications

G. Qian, M. Niffenegger, S. Li, Probabilistic analysis

of pipelines with corrosion defects by using

FITNET FFS procedure, Corrosion Science, Vol.

53 (2011) 855–61.

G. Qian, M. Niffenegger, Probabilistic fracture

assessment of piping systems based on FITNET

FFS procedure, Nuclear Engineering and Design,

Vol. 241 (2011) 714–22.

M. Niffenegger, K. Reichlin, The proper use of

thermal expansion coefficients in finite element

calculations, Nuclear Engineering and Design

243 (2012) 356–359.

G. Qian, M. Niffenegger, D. Karanki, S. Li, Probabilistic

leak-before-break analysis with correlated

input parameters, Nuclear Engineering and

Design, Vol 254 (2013) 266–271.

ENSI Erfahrungs- und Forschungsbericht 2012 155

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