Project Subjects
Pipe Wall Thinning Evaluation & Seismic Evaluation of Thinned Pipeline
Background and Objective
Pipe wall thinning due to Flow Accelerated Corrosion (FAC) and Liquid Droplet Impingement Erosion(LDI) is a one of the major degradation mechanisms in aged nuclear power plants for ﬁtness-for-service. Thoughpipe wall thinning rate and the remaining period are managed based on wall thickness measurement, more thanseveral hundreds of inspection are currently conducted in every periodic outage.
In this project, evaluation methods for FAC and LDI are developed to rationalize the number of inspection
points, where local thinning distribution can be evaluated. A conservative evaluation assuming the overall (fullcircumference) thinning is required for seismic evaluation. The pipe with local thinning will be shown to have acomparable strength with the pipe without thinning to rationalize the criterion of wall thickness.

Main results
1. Approach to performance improvement of FAC prediction
・FAC prediction model under single phase ﬂow was proposed. The corrosion of carbon steel and the diffusion(mass transfer) of iron ion were taken into consideration. Prediction performance of FAC rate is revised bymodel veriﬁcation using basic test data with PWR secondary water condition (Fig.1) [L09006].
・The corrosion potential of the carbon steel in FAC are predicted by using the theoretical concentration of chemical species at a materials/solution interface. Predicted results demonstrate that the corrosion potential increases
suddenly when the dissolved oxygen concentration approaches the FAC suppression value (Fig.2) [Q09028].
2. Rapid evaluation of wall thinning for FAC and LDI・With the proposed model, “Geometry Factors” for typical pipe elements, such as elbows and oriﬁces, arederived to evaluate the relative FAC trend. In addition, FAC susceptible locations are pointed out for each element. As a result, maximum FAC rate is able to be rapidly predicted by using simple equations without precise
3D-CFDs for each element [L09006].
・For evaluation of thinning surface shape by LDI, high-speed evaluation method without 3D calculation isdeveloped. This method enables to evaluate the thinning surface shape and position of maximum thinning ratein a short time (Fig.3) [L09004].
3. Seismic experiments of wall thinning elbow
・Hybrid tests are conducted with a wall-thinning elbow model incorporating a numerical analysis of whole
piping system. By these results, the full circumference thinning condition is found to be too conservative toevaluate seismic response of wall thinning piping system. The seismic evaluation method of wall thinningpiping system can be rationalized by the consideration of locally wall thinning (Fig.4).

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NuclearTechnology
Technology
Nuclear-0.45

Corrosionpotential
potential(V(Vvs.
vs.SHE)
SHE)
Corrosion

FAC rate (Calculation) [mm/10kh]

Test conditions
Velocity :1.5 ～10m/s
Temperature: 150degC
2 pH8.84～9.81
Revised model
Former model

1001

Test condition
condition
Testo
Temp. 140 ºC
Temp.
140 C
pH 7pH
7 wt% Cr
0.001
0.001wt% Cr

-0.50

-0.55

-0.60

-0.65
0.02

0.2

0.4

0.6

0.8

1.0

1.2

Dissolved
oxygenconcentration(relativevalue)
Dissolved
oxygenconcentration(relativevalue)

FAC rate (Experiment) [mm/10kh]
Fig. 1 Prediction of FAC rate
Prediction accuracy of FAC rate is revised by model
veriﬁcation using basic test data with PWR secondarywater condition.

Fig. 2 Effect of dissolved oxygen concentration on corrosion potential.

Normalized Thinning Rate [-]0.60-0.80
0.00-0.20

0.40-0.60
9081

Pipeelement

Sample of Geometry Factor(varied by geometric paramters)

Susceptible locationof FAC in the element

Elbow

4.8
(short elbow)

upstream to middle region ofintrados

Orifice3.8(dia. contraction ratio 0.6)

downstream region within
2-times dia. from orifice

Reducer9.0(reducing angle 20 degree)downstream region
in reducer

GlobeValve

10.2(open ratio 100%)

upper region ofvalve downstream

Tee-Union

Main:7.0, Branch:3.0(Branch/Main dia. Ratio 1.0)

branch adjacent region of
main pipe

TeeSeparation

Main: 7.8, Branch: 9.6(Branch/Main dia. Ratio 0.66)

region from junction upstream
to branch back-side

θ：0° Flow Direction: θA

72

Circumferential
Angle:φ(Dorsal Side)
θ：90°：Evaluation Region0°<θ<90°-90°<φ<90°63544536271890-90 -72 -54 -36 -18 0 18 36 54 72 90
Circumferential Direction[deg., Dorsal:0deg.]

Flow Direction[deg.，Elbow Inlet:0deg.]

0.80-1.00
0.20-0.40

Table 1 General FAC trend for pipe elements(Geometry Factor evaluated by comparison with straight pipe)

Fig. 3 Example of Thinning Shape by
High-Speed Evaluation Method

Numerical
analysis model

Loading Test of Wall-thinning elbow

Through-wall crack occurred
at flank of elbow and water
spouted out.

Fig. 4 Hybrid Test of wall-thinning piping system
Hybrid tests have been conducted incorporating a seismic response analysis of piping system as a whole model with a static
loading test of wall thinning elbow model. The through-wall crack only occurred in the case of full circumference thinningmodel, although cracks didnʼt penetrate in the non thinning model and the partially thinning model.

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