Principal Research Results

Investigation of the Effect on Flow Accelerated Corrosion (FAC)
in Plant Power Uprate
Background
In USA, power uprate up to 20%, which can decrease energy cost with no nuclear safety loss, has already been conducted inmany nuclear power plants in the world, and it is now under review in Japan. By uprating operation, plant flow conditions such as
flow velocity and the temperature of the system are varied, and those change will lead to (or expose) flow induced problems like wall
thinning, FIV (Flow Induced Vibration) and so on. In fact, some problems have actually been reported in US power plants.
In those problems, FAC (Flow Accelerated Corrosion), a kind of wall thinning, especially needs to be investigated because
FAC cause rapture of piping. Therefore, it is necessary to examine the rate change of FAC before and after the plant power uprate.

Objectives
The objective of this study is to investigate the effect on changing rate of FAC and management planning of wall thinning
for Japanese BWR plants on 5% /15% uprate.

Principal Results
BWR5 is selected as model plant because 58% of Japanese NPP is BWR5, and changing rate of FAC in subsystem chosen
by the FAC ranking (Table 1) defined in the JSME* technical rules of the pipe-wall-thinning management for BWR (JSME S NH12006) are investigated. FAC rate is calculated by Kastner's model (W. Kastner et al., Nuclear Engineering and Design, 119 (1990)
431) which is reported in the open literature, and it shows a similar tendency to the inspection data (Fig.1). The results are as follows;(1) 5% power uprate (Table 2)
There is no subsystem where the shortest remaining period for fit for service becomes less than 10 years, which means that thechange in the remaining period for fit for service is not so large. The remaining period for fit for service becomes short in subsystems 10-2 and 12-2, which are in the extraction systems categorized in FAC-2 from the low-pressure turbine to the third and thefifth feed water heater, respectively. In these systems, the shortest remaining period for fit for service became 80% and 88%,respectively. In other systems, there are a few systems where the remaining period for fit for service becomes slightly shorter, andin some systems, the remaining period for fit for service becomes longer for the high-temperature subsystem because the FAC
wall-thinning rate decreases with increasing temperature above 150℃.
(2) 15% power uprate (Table 2)
The remaining period for fit for service becomes 2/3 for subsystems 10-2 and 12-2, which are in the extraction systems categorizedin FAC-2 from low-pressure turbine to the third and the fifth feed water heater, respectively. In these subsystems, though we musttake care of wall thinning for carbon steel piping, the shortening of the remaining period for fit for service is greatly reduced in theplant where the piping was replaced by a FAC-resistant material such as low-alloy steel and stainless steel including chromium.
Excluding subsystems 9-2 (the extraction system from low pressure turbine to second feed water heater), 10-2, and 12-2, theremaining period for fit for service was 15 years or longer, and the shortening of it was quite small and it even became longer inmany systems, which means the effect of the power uprate was not so large.
As a result, we can say the effect on FAC management by power uprate is not so large.
Main Researcher: Fumio Inada Ph. D.,*1 and Ryo Morita Ph. D.,*2*1 Senior Research Scientist, Unit Leader, Pipe Wall Thinning Research Unit, PLM (Plant Life Management) Project*2 Research Scientist, Pipe Wall Thinning Research Unit, PLM (Plant Life Management) ProjectNote: This research is collaborated with Hitachi-GE Nuclear Energy Ltd.,90

5. Nuclear

FAC-1-Downstream of oxygen injection and small
effects of turbulence where oxygen injection
decreases the corrosion of the carbon steel.
-Part of condensate system and feed watersystem etc.
-Periodic measurement (within 10 years ) ofwall thickness at representative points

FAC-2-Upstream of oxygen injection-Part of main steam system and heater ventsystems-Remaining period for fit for service to becontrolled by wall thickness measurement

FAC-S-Downstream of oxygen injection & large
effects of turbulence (extraction ofparticularly high wall-thinning-rate points in
FAC-1 subsystems (of which wear rate ismore than 0.2mm/ 104hr))-Elbow downstream of feed water pump, andother components.
-Remaining period for fit for service to becontrolled by wall thickness measurement atparticular points.

Wear Rate [mm/yr]

Table 1 FAC Rank for management in the technicalrules of the pipe-wall-thinning management
for BWR

Temperature [℃]
Fig.1 FAC thinning Rate for Carbon Steelvs. Temperature and Velocity (pH9.05), Ref. : G. J. Bignold, K.
Garbett and I. S. Woolsey, in Ph.
Berge and F. Kahn, eds.,
Corrosion-Erosion of Steels in High
Temperature Water and Wet Steam(France: EdF, Les Renardieres,1982) Paper No. 12.

Table 2 Changing rate R L of the remaining period before and after the power uprate in piping
system where the remaining period is relatively short(Kastner’s FAC model is applied, pH=7, chromium concentration=0.01%)

Before poweruprating

5% power uprating

15% power uprating

SystemRank of

AssumedRemaining

Remaining

Changing rate,

Remaining

Changing rate,

No.

management

DO (ppb)

period (yr)period (yr)RLperiod (yr)

RL

2
7
8-2
9-2
10-2
12-2
25-1
105-1
105-2
106
551

FAC1
FAC2
FAC2
FAC2
FAC2
FAC2
FAC1/FACS
FAC2
FAC2
FAC2
FAC11511111151111517.9519.32
17.19
10.64
14.42
15.84
16.92
18.55
15.13
18.88
15.02

18.2
25.0
20.1
11.6
11.6
13.9
16.7
18.7
15.3
20.4
15.0

1.01
1.29
1.17
1.09
0.80
0.88
0.99

18.1
29.5
23.6
13.8
9.5
10.7
16.3

1.01
1.01
1.08
1.00

19.1
15.6
22.9
15.0

1.01
1.53
1.37
1.30
0.66
0.68
0.96
1.03
1.03
1.21
1.00

*1

: Remaining period is extended.
: Though remaining period is shorten, it is more than 10 years*1 : Dissolved Oxygen
Note : Line No. in table 2 corresponds to the Line No. noted in JSME technical rules of the pipe-wall-thinning
management for BWR (JSME S NH1-2006)

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