COMPUTATIONAL FLUID DYNAMICS ANALYSIS FOR FLOW ACCELERATED CORROSION IN CANDU6 FEEDER PIPES

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COMPUTATIONAL FLUID DYNAMICS ANALYSIS FOR FLOW ACCELERATED CORROSION IN CANDU6 FEEDER PIPES

A.CĂTANĂ, E.PĂUNA and M. IOAN

Institute for Nuclear Research Piteşti, Romania, [email protected]

ABSTRACT

CANDU6 plant management over a long time period includes various ageing and degradation mechanisms like FAC manifested mainly at first and second elbow of CANDU6 outlet feeders.

FAC take place at all CANDU6 built before 2000 year with feeders made from SA106 grade B low alloy carbon-steel (with chromium at ~ 0.02%). CFD method is used in this paper to investigate the feeder's wall thinning process taking place mainly due local flow conditions in complex 3D geometrical configurations. The 380 outlet feeders grouped in 2.5" (320) and 2.0"

feeders (60). The objective of this paper is to help, as much as possible, to focus investigation on most probable maximum thinning rate locations through 3D distribution of some TH parameters. Application of CFD methods in CANDU6 nuclear reactors implies the knowledge of real plant operating data like: long term time averaged channel power and mass flow as well as temperature, pressure, pHa etc allowing the optimization and cost reduction of wall thinning monitoring process at CANDU6 nuclear power plants.

Key words: CFD, FAC, CANDU Feeders, Corrosion

Introduction

Feeder pipes are important parts of the primary heat transport system of a CANDU reactor. Considerable wall thinning of feeder pipes (mainly outlet feeder pipes) was observed for the first time at the Point Lepreau nuclear reactor in 1995. The degradation mechanism is identified as flow-accelerated corrosion (FAC). Flow-accelerated corrosion (FAC) appears mainly in low alloy carbon steel pipes. The cause is dissolution of oxide film (which is a protection) from the pipe wall due to local water flow conditions.

CANDU6 Nuclear Reactor components with direct exposure to FAC are the outlet feeders‘ elbows that follow immediately after feeder-end-fitting junction.

Excessive thinning occurs on the inside of the feeder pipes, especially on the outlet elbows close to the exit of the pressure tubes. This phenomenon prompted generating stations administration, to implement an inspection program to assess the extent of the problem. It is important for a CANDU life extension program to predict the wall thickness and lifetime of feeders based on inspection data. FAC (Flow Accelerated Corrosion) is a process whereby the normally protective oxide layer on carbon steel dissolves into a stream of flowing water or wet steam (Dooley and Chexal 2000). Flow conditions are important factors in FAC.

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Systematic feeder inspection programs are in place at every CANDU NPP around the world. FAC produce wall pipe thickness and phenomena like rupture or leakage of pipes (feeders) are a matter of time.

Feeder inspection program implies: analysis based on dedicated FAC models, periodic feeder inspection, obtaining a feeder thickness database, calculate the wall thick and thickness rate, evaluation of feeder remaining service time, decisions to replace or not the feeder.

Periodic feeder inspection is a very expensive operation. Any possibility to prioritize feeder inspection means cost reduction. There are several prediction models (Sanchez-Calder, Lu-Luo, etc.) and codes (EPRI-CHECWORKS, EDF BRT CICERO, KWU/AREVA, WATEC/COMSY, etc). Other methods are based on both plant operational data and some laboratory experiments are proposed.

In this paper a a new approach is proposed, based on the use of advanced CFD (Computational Fluid Dynamics) thermal-hydraulic analysis, for local flow conditions in complex 3D geometry like those in outlet feeder tight first and second elbows. This can be an additional tool aiming to focus inspections on most critical locations.

CANDU6 feeder classification according to the geometry of outlet feeder lower part

In CANDU6, feeder pipes connect fuel channels and headers. High-temperature (about 310-312 ^OC) heavy water flows out of individual fuel channels via the feeder pipes into outlet headers and then goes together to steam generators. After heat exchange in the steam generators, the lower-temperature (typically 266 ^oC) heavy water flows back to inlet header and is then distributed to fuel channels through inlet feeders (Burrill and Cheluget 1999). Made of SA 106 Grade B carbon steel, the outlet feeder pipes have typical nominal outer diameter of either 2.0 or 2.5 inches at the reactor face. The flow leaves the end-fitting annulus via a right-angle turn and enters a Grayloc hub (SA 105 carbon steel), resulting in a turbulent flow at the entrance to the outlet feeder pipe, which triggers the flow-accelerated corrosion at the downstream of the hub especially at the first bend. The operating flow velocity in individual pipes varies with channel power from 8 to 18 m/s (Burrill and Cheluget 1999). The heavy water is mildly alkaline (10.2 < pHa < 10.8) and contains dissolved deuterium.

The bending process during pipe fabrication causes initial thinning at the extrados and thickening at the intrados. Depending upon the bending process and radius of the bend (bending angles), the difference in thickness between intrados and extrados can be up to 25% (Kumar 2004). Therefore, it is presumed that the extrados is most vulnerable to FAC because of the lower wall thinning allowance.

Factors affecting the rate of FAC include the fluid flow velocity, pipe geometry (e.g. bend configuration and bend angle), water temperature, water chemistry (e.g. pH value) and metallurgical variables such as chromium content in the steel (Slade and Gendron 2005). The effective factors reduce to the flow velocity and geometry configuration of bends, as the other variables are simply constant across the feeders of the reactor. Since there are only a few different geometries used in the pipes, we will discuss in detail the effects of geometry configuration in the data analysis.

The outlet (considering their lower part) feeder pipes are classified into 20 types [3, 4] according to length and bend angle of the feeder pipes, Table 1. By diameter size there are 320 of 2.5" and 60 of 2.0"

outlet feeder pipes. By direction of first bend we can group all 20 types of feeder pipes into two categories, "Out" and "In" (or "Positive" and "Negative").

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Table 1 Geometry parameters of the 1^st and 2^nd bend of outlet feeders in a CANDU6 NPP Type BT nfbt OD

"

ID

mm BA1 BA2 Dist1

[mm]

Dist2 FCH [mm]

Dist3 hub_e1

[mm]

Dist23 [mm]

Dist4 e1_e2 [mm]

Dir CFD

Group 1 1 2 2.5 59.004 32.717 32.717 101.6 82.55 16.51 99.06 38.1 Out 1 (2) 2 2 8 2.5 59.004 42.833 30.150 101.6 82.55 48.006 130.556 38.1 In

2 (8+12) 3 3 12 2.5 59.004 42.833 30.150 101.6 82.55 48.006 130.556 38.1 In

4 4A 20 2.5 59.004 73.133 70.000 101.6 82.55 15.240 97.79 24.892 In

3 (20+12) 5 5A 12 2.5 59.004 73.133 70.000 101.6 82.55 15.240 97.79 24.892 In

6 4B 18 2.5 59.004 73.133 70.000 101.6 82.55 15.240 97.79 38.35 Out

4 (18+16) 7 5B 16 2.5 59.004 73.133 70.000 101.6 82.55 15.240 97.79 38.35 Out

8 4C 16 2.5 59.004 73.133 70.000 101.6 82.55 15.240 97.79 146.304 Out

5 (16+22) 9 5C 22 2.5 59.004 73.133 70.000 101.6 82.55 15.240 97.79 146.304 Out

10 6 194 2.5 59.004 73.133 - 101.6 82.55 15.240 97.79 - Out 6 (194) 11 7 20 2.0 49.250 32.717 32.717 101.6 81.28 23.622 104.902 49.28 Out 7 (20) 12 8 8 2.0 49.250 42.833 30.150 101.6 81.28 56.896 138.176 50.8 In

8 (8+6) 13 9 6 2.0 49.250 42.833 30.150 101.6 81.28 56.896 138.176 50.8 In

14 10A 4 2.0 49.250 73.133 70.000 101.6 81.28 30.480 111.76 52.32 In

9 (4+4) 15 11A 4 2.0 49.250 73.133 70.000 101.6 81.28 30.480 111.76 52.32 In

16 10B 4 2.0 49.250 73.133 70.000 101.6 81.28 30.480 111.76 65.79 Out

10 (4+2) 17 11B 2 2.0 49.250 73.133 70.000 101.6 81.28 30.480 111.76 65.79 Out

18 10C 2 2.0 49.250 73.133 70.000 101.6 81.28 30.480 111.76 173.74 Out

11 (2+2) 19 11C 2 2.0 49.250 73.133 70.000 101.6 81.28 30.480 111.76 173.74 Out

20 12 8 2.0 49.250 73.133 - 101.6 81.28 30.480 111.76 - Out 12 (8)

The 6 and 12 BT are considered only for the first bend in CFD analysis (Group 6 and 12)

BT | Bend type []

nfbt | Number of feeders per bend type []

OD | Outer diameter of bends (inches) (63.5 mm for 2.5" and 50.8mm for 2") [mm]

BA | Bend angle (degrees) (BA1=first bend angle, BA2=second bend angle) [deg]

dis | Distance between end fitting joint and the first bend [mm]

Dir | Direction that extrados of the first bend faces (In - NEG, Out - POZ) []

Dist1 | Distance: End fitting central axis - Grey loc hub basis [mm]

Dist2 | Feeder Coupling Hub Height (FCH) (Distance Hub-basis to Hub-joint) [mm]

Dist3 | Distance: Hub (joint) - Elbow (Bend) Number 1 [mm]

Dist23 | =Dist1+Dist2 [mm]

Dist4 | Distance: Elbow nr. 1 - Elbow nr. 2 [mm]

FCH HUB TYPE I: FCH = 85.60 mm (1.5" INLET FEEDERS)

HUB TYPE II: FCH = 81.28 mm (2.0" INLET and OUTLET FEEDERS) HUB TYPE III: FCH = 82.55 mm (2.5" INLET and OUTLET FEEDERS)

For the purpose of CFD analysis in this paper we take into account the geometry of the first and second bend angles and the length of the straight pipes for each case so we can group all 380 in just 12 CFD groups:

Six CFD groups for 2.5" feeders: Six CFD groups for 2" feeders:

Group 1 including 2 Type 1 feeders, Group 7 including 20 Type 7 feeders, Group 2 including 20 Type 2 and 3 feeders Group 8 including 14 Type 8/9 feeders, Group 3 including 32 Type 4A/5A feeders Group 9 including 8 Type 10A, 11A Group 4 including 34 Type 4B/4C Group 10 including 6 Type 10B, 11B Group 5 including 38 Type 5B/5C Group 11 including 4 Type 10C, 11C Group 6 including 194 Type 6 feeders Group 12 including 8 Type 12 feeders.

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The shape of these feeder groups are depicted in Figure 1 and Figure 1bis below.

TYPE 1 TYPE 2 TYPE 3 TYPE 4 TYPE 5 TYPE 6 Figure 1 2.5 inches diameter outlet feeder pipes basic shapes

TYPE 7 TYPE 8 TYPE 9 TYPE 10 TYPE 11 TYPE 12 Figure 1bis 2.0 inches diameter outlet feeder pipes basic shapes

Table 2 The 154 feeders with inlet feeder pipe diameter equal to corresponding outlet feeder diameter

1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 A ? ? ? ? ? ?

B ? ? ? ? ? ? ? ? ? ? ? ? C ? ? ? ? ? ? ? ? ? ? ? ? ? ? D ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? E ? ? ? ? ? ? 3 3 3 3 3 3 ? ? ? ? ? ? F ? ? ? ? ? 3 3 3 ? ? 3 3 3 ? ? ? ? ? G ? ? ? ? ? ? 3 3 3 ? ? 3 3 3 ? ? ? ? ? ? H 2 ? ? ? 3 3 3 3 3 ? ? 3 3 3 3 3 ? ? ? 2 J ? ? ? ? 3 3 3 3 3 3 ? ? 3 3 3 3 3 3 ? ? ? ? K ? ? ? ? 3 3 3 3 3 3 ? ? 3 3 3 3 3 3 ? ? ? ? L ? ? ? 3 3 3 3 3 3 3 ? ? 3 3 3 3 3 3 3 ? ? ? M ? ? ? 3 3 3 3 3 3 3 ? ? 3 3 3 3 3 3 3 ? ? ? N ? ? ? 3 3 3 3 3 3 3 ? ? 3 3 3 3 3 3 3 ? ? ? O ? ? ? ? 3 3 3 3 3 3 ? ? 3 3 3 3 3 3 ? ? ? ? P ? ? ? 3 3 3 3 3 3 3 3 3 3 3 3 3 3 ? ? ? Q ? ? ? ? ? 3 3 3 3 3 3 3 3 3 3 ? ? ? ? ? R ? ? ? ? 3 3 3 3 3 3 3 3 3 3 ? ? ? ? S ? ? ? ? ? 3 3 3 3 3 3 3 3 ? ? ? ? ? T ? ? ? ? ? ? 3 3 3 3 ? ? ? ? ? ? U ? ? ? ? ? ? ? ? ? ? ? ? ? ? V ? ? ? ? ? ? ? ? ? ? ? ? W ? ? ? ? ? ?

Some geometrical data for CANDU6 outlet feeder pipes

Grouping the feeders in 12 groups according to the Table 1 we have to do fewer CFD analysis making room for parametric analyses which we can made by varying mass flow or channel power within a realistic range. Each group has the same geometry characteristics considering first and second bend. From CFD analysis this grouping is very useful, simplifying the preprocessor work. We also mention that for all outlet feeders some factors like coolant temperature and pressure, pH value, metallurgical characteristics are considered to be the same for all feeders while other factors like: mass flow, velocity, etc, are specific for each feeder.

The main goal of CFD analysis is to determine the parameters which determine the thinning rate of outlet feeders pipe wall and finally to find out the time for safe operation. Initial wall thickness is presented in Table 2bis.

Out of 380 feeders we have to focus primarily on 154 feeders for which the inlet feeder pipe diameter is equal to corresponding outlet feeder diameter. In the nearby table: "3" stand for 2.5 inch pipes;

"2" stand for 2.0 inch pipes and '?' stand for those pipes for which inlet feeder pipes have a smaller diameter than corresponding outer feeder pipes. With red characters are indicated PLGS feeders which experienced cracks.

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Table 2bis Feeder thickness (Nominal; Initial at Extrados; Minimal Allowed) Feeder

Type

Nominal straight pipe wall thickness [mm]

Initial wall thickness

at extrados [mm]

Initial minimal thickness at elbow[mm]

Minima allowed thickness [mm]

2" 5.537 5.1 - 5.2 4.86 (cot strans) 2.61 (2.69)

2.5 7.010 6.1 - 6.7 6.23 (cot strans) 3.16 (3.25)

In CFD analysis the first stage is called preprocessing in which the geometrical description of fluid flow domain is basic. The geometry of CANDU6 outlet feeders, mainly the first and second bend is the core part of flow domain in which CFD analysis take place. Adiacent geometry is also important, mainly the upstream part. From [5] we have a good decription of this flow domain segment, Figure 2.

Figure 2 Some end-fitting and a generic CANDU6 feeder data [5]

CANDU6 outlet feeder thinning rate evaluation methods

In situ measurements show us that FAC affects only outlet feeders with highest thinning rates in locations of local flow turbulence and high velocity. These conditions are meet in the tight radius bends downstream of the Gray loc hub where the highest thinning rates were measured. The coolant passed through the pressure tube is now unsaturated in dissolved iron. Long term measurements indicates the dependence of corrosion rate (mm/yr) on velocity (V), wall shear stress (τ) and friction factor (f). Friction factor can be calculated from Darcy-Weisbach equation:

f R D

f 2 log 3 . 7 2 . 51 /

_e

1

₁₀

where Re is Reynolds number, R_e V D , and ε is absolute roughness.

It is generally accepted the idea that wall shear stress resulting from velocity gradients near the metal surfaces is directly linked to dissolution phenomena in the protective oxide layer. A high wall shear stress produces a high mass transfer rate. Shear stress is a function of velocity and have the tendency

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to increase with area averaged velocity increase. For a cylindrical pipe the shear stress, τw[Pa] is calculated with formula:

2

2 1f V

W

where: f is the friction factor, is the fluid density and V is area averaged flow velocity. The above formula is based on global parameters while corrosion phenomena are highly local. It is very difficult to directly measure local wall shear stress. CFD methods can compute local thermal-hydraulic parameters and some advanced CFD tools (FLUENT) can also compute wall shear stress.

Back to empirical methods, Supa-Amornkul (2001) give a correlation between wall shear stress and corrosion rate for CANDU outlet feeders. According to this model the correlation for FAC corrosion rate of low allied carbon steel A-106 B (used in all CANDU nuclear reactors built before 2000 year) are:

RATE (mm/a) = 7.65 · (1+0.111τ^0.75) · (1+7.15E-10· e^pHA)

where: τ = fluid shear stress at wall (Pa) ; pHa = 10.4 (in analyses performed in this paper)

Starting from the above empirical formula we can evaluate the thinning rate for outlet feeders of a CANDU6 Nuclear Reactor as long as we have wall shear stress, and for that we have two main approaches:

1. The empirical approach based on experiment, measurements and global parameters, and 2. CFD approach based on advanced methods provides local 3D distribution for parameters like velocity, turbulence and even for shear stress.

1. The empirical approach

The designed thinning rate for CANDU6 feeders is 75μm/annum. Some empirical mathematical models are available regarding the thinning rate for first and second bend of CANDU6 feeders with tight bend radius R~1.5Dfider and angles >45^o. In this paper we use the above mentioned model. The application of this model requires the knowledge of some basic data: the time average channel powers and mass flow from which velocities and other parameters are derived until wall shear stress. An example of channel time averaged power map is in Table 3 and for mass flow in Table 4.

Table 3 A CANDU6 channel time averaged power map example [kW]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 A 0. 0. 0. 0. 0. 0. 0. 0. 3295. 3405. 3485. 3485. 3405. 3295. 0. 0. 0. 0. 0. 0. 0. 0.

B 0. 0. 0. 0. 0. 2934. 3505. 4015. 4306. 4506. 4546. 4546. 4496. 4306. 4015. 3505. 2934. 0. 0. 0. 0. 0.

C 0. 0. 0. 0. 3264. 3875. 4506. 4997. 5297. 5447. 5417. 5417. 5447. 5297. 4987. 4506. 3875. 3254. 0. 0. 0. 0.

D 0. 0. 0. 3395. 4086. 4777. 5367. 5788. 6018. 6108. 6018. 6018. 6108. 6018. 5778. 5367. 4767. 4076. 3385. 0. 0. 0.

E 0. 0. 3254. 4086. 4807. 5427. 5918. 6229. 6349. 6369. 6269. 6269. 6369. 6349. 6229. 5918. 5427. 4797. 4076. 3254. 0. 0.

F 0. 0. 3945. 4756. 5367. 5878. 6239. 6419. 6319. 6289. 6229. 6229. 6289. 6319. 6419. 6229. 5858. 5357. 4746. 3935. 0. 0.

G 0. 3585. 4486. 5277. 5728. 6118. 6329. 6429. 6359. 6329. 6329. 6329. 6329. 6349. 6429. 6329. 6108. 5718. 5267. 4476. 3575. 0.

H 0. 4066. 4997. 5718. 6018. 6289. 6419. 6469. 6389. 6369. 6379. 6379. 6369. 6389. 6459. 6409. 6279. 6018. 5708. 4987. 4056. 0.

J 3295. 4406. 5377. 6028. 6198. 6299. 6399. 6429. 6389. 6359. 6339. 6339. 6359. 6379. 6419. 6399. 6299. 6188. 6018. 5367. 4406. 3295.

K 3525. 4596. 5658. 6259. 6319. 6349. 6429. 6429. 6379. 6319. 6259. 6259. 6319. 6369. 6429. 6429. 6339. 6309. 6259. 5658. 4706. 3515.

L 3665. 4857. 5828. 6429. 6529. 6519. 6509. 6459. 6379. 6289. 6168. 6168. 6289. 6379. 6459. 6509. 6519. 6529. 6429. 5828. 4867. 3665.

M 3655. 4867. 5858. 6489. 6629. 6619. 6579. 6509. 6409. 6309. 6178. 6178. 6309. 6409. 6499. 6579. 6619. 6529. 6419. 5858. 4877. 3655.

N 3485. 4706. 5728. 6409. 6629. 6639. 6619. 6539. 6449. 6359. 6259. 6259. 6359. 6449. 6539. 6619. 6669. 6639. 6419. 5728. 4726. 3495.

O 3274. 4436. 5467. 6208. 6559. 6659. 6629. 6579. 6469. 6439. 6419. 6419. 6439. 6489. 6579. 6629. 6659. 6559. 6208. 5467. 4446. 3274.

P 0. 4076. 5037. 5808. 6178. 6469. 6579. 6599. 6499. 6469. 6489. 6489. 6469. 6499. 6599. 6579. 6469. 6178. 5808. 5037. 4086. 0.

Q 0. 3565. 4456. 5257. 5708. 6118. 6369. 6489. 6439. 6439. 6459. 6459. 6429. 6439. 6489. 6369. 6118. 5708. 5207. 4456. 3565. 0.

R 0. 0. 3845. 4606. 5157. 5698. 6158. 6399. 6379. 6399. 6379. 6379. 6389. 6379. 6399. 6158. 5698. 5147. 4606. 3845. 0. 0.

S 0. 0. 3114. 3875. 4516. 5177. 5788. 6168. 6359. 6419. 6319. 6319. 6419. 6359. 6158. 5788. 5177. 4516. 3875. 3114. 0. 0.

T 0. 0. 0. 3114. 3775. 4476. 5117. 5568. 5848. 5958. 5878. 5878. 5958. 5848. 5568. 5107. 4466. 3775. 3114. 0. 0. 0.

U 0. 0. 0. 0. 2904. 3535. 4186. 4686. 5017. 5187. 5157. 5157. 5187. 5017. 4676. 4176. 3535. 2904. 0. 0. 0. 0.

V 0. 0. 0. 0. 0. 2503. 3094. 3615. 3935. 4156. 4216. 4216. 4156. 3935. 3605. 3084. 2493. 0. 0. 0. 0. 0.

W 0. 0. 0. 0. 0. 0. 0. 0. 2764. 2934. 3044. 3044. 2934. 2754. 0. 0. 0. 0. 0. 0. 0. 0.

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Table 4 A CANDU6 channel time averaged mass flow map example [kg/s]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 A 0 0 0 0 0 0 0 0 11.2 12.5 12.5 12.5 12.5 11.2 0 0 0 0 0 0 0 0 B 0 0 0 0 0 11.4 12.2 15.2 16.2 16.0 17.1 17.1 16.0 16.3 15.2 11.6 11.4 0 0 0 0 0 C 0 0 0 0 12.5 13.3 15.7 17.8 19.3 20.3 20.6 20.6 20.3 19.3 17.8 15.7 13.3 12.5 0 0 0 0 D 0 0 0 12.3 15.6 16.6 18.3 20.9 22.2 22.7 22.8 22.8 22.7 22.2 20.9 18.3 16.6 14.7 12.3 0 0 0 E 0 0 12.8 14.5 18.0 19.6 21.2 22.3 23.8 25.1 24.5 24.5 24.2 23.8 22.3 21.2 19.4 18.0 14.5 12.7 0 0 F 0 0 15.0 17.8 20.4 21.4 22.3 23.7 23.4 24.0 23.1 23.1 24.0 23.4 23.7 22.3 21.4 20.4 17.8 15.0 0 0 G 0 13.5 16.6 19.8 21.6 22.5 22.5 23.5 23.6 23.7 23.1 23.1 23.7 23.6 24.4 22.6 22.5 21.6 19.8 16.8 13.5 0 H 0 15.9 18.9 21.6 22.4 22.9 22.6 23.9 23.7 23.3 22.6 22.6 23.4 23.7 23.9 22.6 22.9 22.4 21.6 19.0 15.9 0 J 12.1 15.8 20 22.4 23.9 23.7 23.8 23.9 23.9 23.6 22.6 22.6 23.6 23.9 24.0 23.8 23.8 23.9 22.4 20 15.8 12.1 K 12.8 16.5 20.5 22.8 24.5 23.7 23.8 24.3 23.7 23.4 22.5 22.5 23.5 23.7 24.1 23.9 23.7 24.6 22.8 20.5 16.6 12.8 L 13.2 17.9 20.8 23.4 24.1 24.0 23.9 24.4 23.7 23.3 22.3 22.3 23.3 23.7 24.4 23.9 24.0 24.1 23.4 20.8 17.9 13.5 M 13.1 17.8 20.6 23.0 24.6 24.0 23.7 24.1 24.7 23.2 22.6 22.6 23.2 24.7 24.1 23.7 24.0 24.6 23.0 20.6 17.8 13.5 N 13.1 17.5 20 22.5 24.4 23.4 23.5 24.0 23.6 23.2 22.1 22.1 23.2 23.6 24.1 23.4 23.4 24.4 22.5 20 17.4 13.1 O 12.3 16.4 19.5 21.6 22.9 22.6 23.0 23.6 23.5 23.1 21.7 21.7 23.1 23.5 23.6 23.0 22.6 22.9 21.6 19.5 16.4 12.3 P 0 15.7 19.0 21.0 22.4 22.8 22.6 23.5 23.4 23.2 22.3 22.3 23.2 23.4 23.5 22.6 22.8 22.4 21.0 19.0 15.7 0 Q 0 12.8 16.4 19.3 21.6 21.8 22.2 23.3 23.4 23.5 22.6 22.6 23.5 23.4 23.3 22.2 21.8 21.6 19.2 16.4 12.8 0 R 0 0 14.6 17.6 20.4 21.6 22.1 23.5 23.1 23.2 23.0 23.0 23.2 23.4 23.5 22.1 21.6 20.4 17.6 14.6 0 0 S 0 0 11.8 15.7 17.6 19.4 20.7 22.3 23.5 24.4 24.3 24.3 24.4 23.5 22.3 20.7 19.4 17.6 15.6 11.8 0 0 T 0 0 0 12.6 15.2 17.3 18.2 20.8 21.6 22.2 22.5 22.5 22.2 21.6 20.8 18.2 17.3 14.2 12.6 0 0 0 U 0 0 0 0 11.6 13.2 16.2 18.2 19.2 20.3 20.2 20.2 20.3 19.2 18.2 16.2 13.2 11.6 0 0 0 0 V 0 0 0 0 0 10.5 12.3 14.4 15.2 16.0 15.9 15.9 16.0 15.2 14.4 12.3 10.5 0 0 0 0 0 W 0 0 0 0 0 0 0 0 11.5 12.1 12.2 12.2 12.1 11.5 0 0 0 0 0 0 0 0

Based on Table 3 and Table 4 we can have the velocity feeder map like in Table 5.

Table 5 A CANDU6 feeder velocity feeder map at first and second bend [m/s]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 A 0 0 0 0 0 0 0 0 12.85 12.52 13.07 13.07 12.52 12.85 0 0 0 0 0 0 0 0 B 0 0 0 0 0 10.35 13.47 14.48 10.82 11.85 11.47 11.47 11.81 10.82 14.48 13.91 10.35 0 0 0 0 0 C 0 0 0 0 11.66 15.01 12.04 13.16 13.71 13.82 13.51 13.51 13.82 13.71 13.11 12.03 15.01 11.58 0 0 0 0 D 0 0 0 12.62 14.56 12.80 14.56 15.08 15.42 15.56 15.08 15.08 15.56 15.42 15.03 14.55 12.75 15.22 12.55 0 0 0 E 0 0 11.36 15.45 12.18 14.14 15.47 16.31 16.04 15.42 15.30 15.29 15.93 16.03 16.31 15.46 14.24 12.13 15.37 11.43 0 0 F 0 0 14.16 12.03 13.44 15.22 16.35 16.44 16.14 15.69 15.85 15.86 15.68 16.14 16.43 16.30 15.12 13.38 11.98 14.08 0 0 G 0 12.96 11.47 13.31 14.43 15.72 16.63 16.59 16.19 16.00 16.34 16.34 16.00 16.14 16.07 16.62 15.66 14.37 13.26 11.31 12.90 0 H 0 14.23 12.51 14.38 15.28 16.26 17.00 16.56 16.30 16.38 16.84 16.84 16.38 16.29 16.51 16.94 16.20 15.28 14.32 12.46 14.15 0 j 12.18 11.54 13.66 15.37 15.30 15.85 16.27 16.38 16.20 16.17 16.65 16.65 16.17 16.15 16.24 16.26 15.84 15.24 15.31 13.61 11.54 12.16 K 13.13 12.02 14.71 16.20 15.50 16.10 16.39 16.16 16.23 16.10 16.33 16.33 16.09 16.18 16.26 16.38 16.06 15.45 16.20 14.71 12.48 13.06 L 13.71 12.42 15.29 16.63 16.70 16.70 16.70 16.21 16.26 16.05 16.01 16.01 16.05 16.26 16.21 16.70 16.70 16.69 16.63 15.29 12.47 13.49 M 13.69 12.56 15.52 17.13 16.90 17.17 17.16 16.59 15.81 16.20 15.91 15.91 16.20 15.81 16.55 17.17 17.17 16.44 16.81 15.52 12.61 13.42 N 12.61 11.99 15.27 17.00 17.00 17.57 17.46 16.80 16.60 16.42 16.56 16.56 16.42 16.60 16.75 17.48 17.71 17.05 17.05 15.27 12.10 12.68 O 11.85 11.33 14.34 16.61 17.47 18.09 17.74 17.21 16.78 16.84 17.49 17.49 16.84 16.87 17.21 17.74 18.09 17.47 16.61 14.34 11.38 11.85 P 0 10.07 12.69 15.08 16.00 17.14 17.71 17.32 16.92 16.90 17.48 17.48 16.90 16.92 17.32 17.72 17.14 16.01 15.08 12.69 10.11 0 Q 0 13.41 11.45 13.49 14.31 16.08 17.02 16.96 16.65 16.62 17.18 17.18 16.57 16.65 16.96 17.02 16.08 14.31 13.30 11.45 13.41 0 R 0 0 9.64 11.46 12.45 14.25 16.10 16.43 16.57 16.61 16.64 16.64 16.57 16.42 16.43 16.10 14.25 12.40 11.46 9.63 0 0 S 0 0 11.20 9.18 11.05 13.08 15.15 16.01 16.26 16.03 15.62 15.62 16.03 16.26 15.97 15.15 13.08 11.05 9.23 11.19 0 0 T 0 0 0 10.56 8.99 11.02 13.47 14.11 14.95 15.13 14.63 14.63 15.12 14.95 14.11 13.42 10.98 9.50 10.56 0 0 0 U 0 0 0 0 1 0 12.87 10.31 11.51 12.46 12.65 12.54 12.54 12.65 12.46 11.47 10.27 12.87 1 0 0 0 0 0 V 0 0 0 0 0 8.19 10.69 8.70 9.71 10.26 10.60 10.60 10.26 9.71 8.65 10.63 8.13 0 0 0 0 0 W 0 0 0 0 0 0 0 0 9.14 9.80 7.26 7.26 9.80 9.07 0 0 0 0 0 0 0 0

From Table 3, Table 4 and Table 5 as well from thermo-physical properties in CANDU6 outlet feeders we can have the wall shear stress considering pHa=10.4 in Table 6.

Table 6 CANDU6 wall shear stress on 380 feeders [Pa]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 A 0 0 0 0 0 0 0 0 206 225 235 235 225 206 0 0 0 0 0 0 0 0 B 0 0 0 0 0 169 235 315 168 181 187 187 181 168 315 232 169 0 0 0 0 0 C 0 0 0 0 209 286 180 223 252 268 266 266 268 252 222 180 286 207 0 0 0 0 D 0 0 0 223 152 202 254 300 326 337 328 328 337 326 299 254 202 149 222 0 0 0 E 0 0 208 149 209 264 313 347 364 370 358 358 367 364 347 314 263 208 149 208 0 0 F 0 0 141 205 261 310 348 371 360 359 350 350 359 360 371 347 308 260 204 141 0 0 G 0 251 181 252 297 337 358 371 365 362 360 360 362 364 375 358 336 296 251 181 250 0 H 0 151 226 296 327 355 367 377 368 365 363 363 365 368 376 366 354 327 295 226 150 0 J 211 174 261 328 349 359 370 373 369 365 359 359 365 368 372 370 359 348 327 260 174 211 K 241 189 287 352 363 364 373 374 367 360 351 351 360 366 373 373 363 362 352 287 197 239 L 260 213 304 371 384 383 381 378 367 357 341 341 357 367 378 381 383 384 371 304 213 261 M 258 213 306 376 396 393 388 382 373 358 343 343 358 373 381 388 393 386 369 306 214 260 N 237 200 292 366 396 393 391 385 374 363 349 349 363 374 385 391 396 397 367 292 201 238 O 209 178 267 342 382 391 390 387 375 371 362 362 371 378 387 390 391 382 342 267 178 209 P 0 151 230 303 343 373 383 389 379 375 372 372 375 379 389 383 373 343 303 230 152 0 Q 0 245 179 249 295 334 360 377 372 373 371 371 372 372 377 360 334 295 244 179 245 0 R 0 0 134 193 243 294 339 369 365 367 364 364 366 366 369 339 294 242 193 134 0 0 S 0 0 190 137 186 242 300 341 364 373 363 363 373 364 340 300 242 186 137 190 0 0 T 0 0 0 191 130 182 234 280 308 321 313 313 321 308 280 233 181 129 191 0 0 0 U 0 0 0 0 166 244 159 200 228 245 242 242 245 228 199 159 244 166 0 0 0 0 V 0 0 0 0 0 124 188 257 141 157 161 161 157 141 256 187 123 0 0 0 0 0 W 0 0 0 0 0 0 0 0 151 170 183 183 170 150 0 0 0 0 0 0 0 0

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Applying the above mentioned thinning models based on wall shear stress we have the following thinning rates:

Table 8 A CANDU6 feeder corrosion rate map from wall shear stress [μm/a]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 A 0 0 0 0 0 0 0 0 77 81 83 83 81 77 0 0 0 0 0 0 0 0 B 0 0 0 0 0 67 83 101 67 71 72 72 70 67 101 83 67 0 0 0 0 0 C 0 0 0 0 77 95 70 81 87 91 90 90 91 87 80 70 95 77 0 0 0 0 D 0 0 0 81 63 76 88 98 104 106 104 104 106 104 98 88 76 62 80 0 0 0 E 0 0 77 62 77 90 101 108 112 113 110 110 112 112 108 101 90 77 62 77 0 0 F 0 0 60 76 89 100 108 113 111 110 109 109 110 111 113 108 100 89 76 60 0 0 G 0 87 71 87 97 106 110 113 112 111 111 111 111 111 114 110 106 97 87 71 87 0 H 0 63 81 97 104 110 112 114 112 112 111 111 112 112 114 112 109 104 97 81 63 0 J 78 69 89 104 108 110 113 113 112 112 110 110 112 112 113 113 110 108 104 89 69 78 K 85 72 95 109 111 111 113 114 112 111 109 109 111 112 113 113 111 111 109 95 74 84 L 89 78 99 113 116 115 115 114 112 110 107 107 110 112 114 115 115 116 113 99 78 89 M 89 78 99 114 118 117 116 115 113 110 107 107 110 113 115 116 117 116 112 99 78 89 N 84 75 96 112 118 117 117 116 114 111 108 108 111 114 116 117 118 118 112 96 75 84 O 77 70 91 107 115 117 117 116 114 113 111 111 113 114 116 117 117 115 107 91 70 77 P 0 63 82 98 107 113 115 117 114 114 113 113 114 114 117 115 113 107 98 82 63 0 Q 0 86 70 87 97 105 111 114 113 113 113 113 113 113 114 111 105 97 86 70 86 0 R 0 0 58 73 85 97 106 112 112 112 112 112 112 112 112 106 97 85 73 58 0 0 S 0 0 73 59 72 85 98 107 112 113 111 111 113 112 107 98 85 72 59 73 0 0 T 0 0 0 73 57 71 83 94 100 102 101 101 102 100 94 83 71 57 73 0 0 0 U 0 0 0 0 67 85 65 75 82 86 85 85 86 82 75 65 85 67 0 0 0 0 V 0 0 0 0 0 56 72 88 60 64 65 65 64 60 88 72 55 0 0 0 0 0 W 0 0 0 0 0 0 0 0 63 68 71 71 68 63 0 0 0 0 0 0 0 0

CFD approach for FAC at CANDU6 outlet feeders

A F.R.Greening (Ph.D.) statement[6]: " Up to February 2005, ultrasonic measurements at the extrados of the first elbow of Pickering 'A' feeders found a minimum wall thickness greater than 3 mm. At this time the extrados of the first elbow was considered to be the location most susceptible to wall thinning and it was therefore concluded that the Pickering feeders were within the fitness-for-service guideline"

and then "In April 2005 direct micrometer measurements were made at the extrados and intrados of two removed feeders from P1. These measurements gave the unexpected result that the elbow intrados was thinner than the associated extrados, and well below the fitness-for-service guideline". Further, "To conduct a full outlet feeder inspection on a large CANDU reactor costs about $10 million and involves a radiation dose to the inspection crew of about 27 man-rem. To remove and replace one feeder pipe from a CANDU reactor costs about $1 million".

Taking into account that initial wall thickness at intrados is systematically larger than initial wall thickness at extrados, due to fabrication technological process, it is obvious that finding the location of actual minimum wall thickness is not a simple task. The wall thinning process is a slow one and strongly dependent on local flow conditions. After a long period of operation we must couple local FAC rate with initial pipe wall thickness distribution, Figure 3 (taken from [2]) and Table 8.

Figure 3 Circumferential distribution of the initial CANDU6 feeder pipe wall thickness

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The process to determine feeder thinning rates by ultrasonic measurements at "in situ" critical locations require a number of repeat costly measurements. Costs of feeder inspection, removal and replacement as well as radiation exposure of personnel are huge, Table 9. That is why knowledge based wall feeder pipe thinning rate strategy must be adopted in order to reduce costs and radiation hazards. Within this strategy CFD analysis methods play a key role mainly for assessment of local flow conditions expressed in 3D distributions of some key thermal-hydraulic parameters with direct impact on FAC.

Table 9 Feeder inspection, removal and replacement costs [6]

Operation Cost [$] Rad.dose [man-rem] Duration [yr]

Full outlet feeder inspection ~10 milion 27 man-rem > 1 year (2 yr & 3 mo W1 full refurbishment) Remove & Replace of one feeder ~1 milion -

Remove & Replace all 380 feeders ~300 milion -

CFD analysis requires the knowledge of detailed geometrical configuration of flow domain and adiacent zones (upstream and downstream). In our case we need geometrical data for end-fitting, Greyloc hub and feeder geometrical data: diameters, segments, angles, curvature radii, segment length etc. like in Table 1 and Figure 2. For all 20 feeder groups we can now define the geometry of flow domain in a realistic manner, Figure 1. The next step is mesh generation for numerical simulation of flow.

Figure 4 Geometry and mesh generation for flow domain in CFD analysis of CANDU6 feeders

CFD analysis can't be accomplished without operating and boundary conditions. In this paper we used a generic operating conditions but realistic or real boundary conditions. On this basis intensive CFD analysis [7-10] were conducted for different CANDU6 outlet feeders. The results were post-processed in order to extract graphical and numerical results. A CFD analysis methodology was developed for CANDU6 outlet feeder pipes to assess the flow conditions in critical positions.

CFD results are

presented below.

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Figure 5 Feeder Group1(CFD). Croos section velocity distribution at first bend

Figure 6 Feeder Group2(CFD). Cross section velocity distrib. at first/second bend

Figure 7 Feeder Group3(CFD). Cross section velocity distrib. at first/second bend

Figure 8 Feeder Group4(CFD). Cross section velocity distrib. at second bend

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Figure 9 Feeder Group5(CFD). Figure 10 Feeder Group5(CFD) Cross section velocity distrib. at second bend Velocity distribution

From Figure 5 - Figure 10, we can see the non-uniform velocity distribution in cross section.

While the area averaged velocity is ~11.65 m/s, we can see that near the intrados of the bend (in this case the second bend) the velocity is ~15.73 M/s while at the extrados ~ 5.3 m/s. It is only obvious that the wall shear stress is accordingly. With CFD methods providing local flow conditions on the analyzed feeder in term of 3D field variable distribution we can also obtain wall shear stress, pressure and turbulence parameters.

a) b) c)

Figure 11 3D variable distribution: wall shear stress: a) 1 st bend) b) second bend; c) static pressure

Velocity Magnitude Distribution is a decisive TH parameter with consequences in CANDU6

outlet feeder wall thinning rate. In Figure 12, we have two cross section contour representation

for Velocity Magnitude at the first and second bend. In the same figure we have a longitudinal

section contour representation for this parameter.

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a) b)

Figure 12 Velocity Magnitude Distribution: Cross Section a) first bend b) second bend

Other thermal-hydraulic parameters are obtained by applying CFD methods for CANDU6 outlet feeders.

In Figure 13, we have both Dynamic and Static Pressure Distribution on a cross section at the first bend, as well as Turbulent Kinetic Energy (k) at the same spot.

Figure 13 Dynamic & Static Pressure and TKE_k Distribution: Cross Section for first bend

Also velocity magnitude and other significant parameters have some different distribution according to first bend orientation (In / Out), Figure 14.

Figure 14 In/Out variable distributions for 1st and 2nd bend Velocity Magnitude

Distribution at 1st bend

Vmax ~ 18.5 m/s Velocity Magnitude

Distribution at 2nd bend Vmax ~ 17.5 m/s

Dynamic Pressure

distribution at 1st bend Static Pressure distribution at 1st bend

Turbulent Kinetic Energy_k distribution at 1st bend

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Conclusions

A methodology for using CFD methods to CANDU6 outlet feeder FAC thinning rate problems was setup in this paper. Using CFD methods, the 3D distribution of some representative field variables have been obtained in order to characterize FAC rates. A new kind of tools are added to the existing methods and tools able to focus on most critical locations that is the location which maximum probability for FAC originated corrosion. Some CFD parameters (fluid velocity, pressure and wall shear stress) can be directly used for FAC rate evaluation

. Feeder wall thinning accelerated in some thermo-hydraulic conditions is severe ageing problem for all CANDU6 nuclear reactors built before 2000 year with SA 106 grade B steel. Among these: Cernavoda-1, Wolsong1,2,3,4, Point Lepreau, Gentilly-2.

A pessimistic conclusion [6, F. R. Greening]:

"CANDU was destined to run into difficulties due to the complexity of its design. Corrosion is a well- known concern for all nuclear plant, but when it occurs in essentially inaccessible pipe work, such as the annulus gas system and feeder pipes, it presents a problem that is next to impossible to fix. It is time to declare the CANDU experiment over, and move on to something simpler, something proven, something better".

The optimistic conclusion:

Wolsong-1 has been successfully refurbished and resumed its full capacity production. The new (refurbished) materials (ex. more Cromium, etc.) and pipe thickness (PT, CT) are due to ensure a safe operation for another (at least 30 yr) life-time period for this nuclear reactor.

References

[1] Alexandru Catana, ―Advanced Methods and Tools for Thermal-hydraulic Analysis of Nuclear Reactors (CFD and sub-channel analysis for CANDU6 Reactor Core)‖, PHD Dissertation Thesis, University POLITEHNICA Bucharest, Nov. 2010

[2] A. Catana, E. Pauna, D. Nastase, D. Ionesu, G. Gabriel, ―Analiza fezabilităţii metodelor CFD pentru reactorii nucleari prin utilizarea unor coduri OpenSource‖, RI. 9133/2011

[3] Xianxun Yuan, "Stochastic Modeling of Deterioration in Nuclear Power Plant Components", A thesis presented to the University of Waterloo in fulfilment of the thesis requirement for the degree of Doctor of Philosophy in Civil Engineering, Waterloo, Ontario, Canada, 2007

[4] Han-Sub Chung, "A review of CANDU fider wall thinning", Nuclear Engineering and Technology, vol.42, no.5, october 2010

[5] Dong Gu Kang, Jong Chull Jo, "Prediction of the local areas of CANDU fider pipes highly susceptible to wall thinning due to flow-accelerated corrosion",Korea Institute of Nuclear Safety, Proceedings of PVP200, July 22-26, 2007, San Antonio, Texas, PVP2007-26507

[6] F. R. Greening, "A Submission to the Ontario Power Authority (OPA) Concerning the Selection of New Electricity Generation Capacity for Ontario"

[7] http://www.caelinux.com/CMS/

[8] http://www.salome-platform.org/user-section/online-documentation

[9] http://research.edf.com/research-and-the-scientific-community/softwares/code- saturne/introduction-code-saturne-80058.html

[10]

http://www.paraview.org ; https://wci.llnl.gov/codes/visit/home.html