Function Transporting heavy water coolant to and from the fuel channels and the inlet and

outlet headers

Layout Sheer number (up to 308 ~ 480 inlet and 308 ~ 480 outlet)

Congested layout at the reactor face and in the feeder cabinet

One or two tight-radius bends and several long radius bends

Flow orifice for controlling flow, flow element for measuring flow, and swaged reducer

Several thousands of welds throughout the feeder piping

Material specification SA-106 Grade B carbon steel (Grade C in CANDU plants beginning with refurbished

plants in 2008)

Flow devices fabricated from alloy 600

Design specification Nuclear Class 1 piping(ASME Sec. III NB)

Size : 1.5 ~ 3.5 NPS 80 (mostly 2” and 2.5” for feeder outlets)

Thickness : 5 ~ 8 mm

Elbow : Bend or Fitting

Pressure : 1475 ~ 1550 Psig(10.0 ~ 11.3MPa)

Temperature : 515 ~ 585 ℉(299 ~ 318℃)

Maximum mass flow rates : 24 ~ 29 kg/s

Feeder S ize(in )

BendType

No . ofFeeders

1st BendAngle (deg)

2nd BendAngle (deg)

Hub to1st Bend

Length (in )

1st to2nd BendLength (in )

Tw istingAngle(deg)

2 .5 1 2 32.717 32.717 0.65 1 .50 180

2 8 42.833 30.150R 1.89 1 .50 128.8

3 12 42.833 30.150L 1 .89 1 .50 128.8

4A 20 73.133 70.000R 0.6 0 .98 146.38

4B 18 73.133 70.000R 0.6 1 .51 146.38

4C 16 73.133 70.000R 0.6 5 .76 146.38

5A 12 73.133 70.000L 0 .6 0 .98 146.38

5B 16 73.133 70.000L 0 .6 1 .51 146.38

5C 22 73.133 70.000L 0 .6 5 .76 146.38

6 194 73.133 70.000 0.6 ** **

Sub-Total 320

2.0 7 20 32.717 32.717 0.93 1 .94 180

8 8 42.833 30.150R 2.24 2 .00 128.8

9 6 42.833 30.150L 2 .24 2 .00 128.8

10A 4 73.133 70.000R 1.2 2 .06 146.38

10B 4 73.133 70.000R 1.2 2 .59 146.38

10C 2 73.133 70.000R 1.2 6 .84 146.38

11A 2 73.133 70.000L 1 .2 2 .06 146.38

11B 2 73.133 70.000L 1 .2 2 .59 146.38

11C 4 73.133 70.000L 1 .2 6 .84 146.38

12 8 73.133 ** 1 .2 ** **

Sub-Total 60

Total 380

Wall thinning Beginning in the mid 1990s, higher than expected rates of wall loss of outlet feeders

were first reported at the PLGS (Point Lepreau Generating Station) and eventually confirmed at all CANDU stations

Wall thickness inspections performed on the tight radius bends near the reactor face, first at PLGS in 1995 and subsequently at other CANDU stations

Cracking In 1997, a through-wall crack was discovered in the tight-radius bend of the S08 outlet

feeder at the PLGS. Bend cracking has not been observed at any other CANDU station

In 2003 at the Gentilly-2 Nuclear Generating Station, a crack was found in the G09 feeder field weld, which had been repaired during construction

Argentina- FAC of CNA/ANO/Embalse Feeder pipe & Cracking of CNA inlet feeder pipe bend(2011)

Re-tubing/En-messe refurbishments

- The existing feeders will be replaced by new ones with a different material (Chrome,

phosphor and sulphur content will be lowered) from the one provided by the original

design, in order to improve structural resistance as well as the resistance to corrosion

by minimizing flow-assisted corrosion (FAC)

- UT(cracking) for bend regions and gralyroc welds of feeder pipes

1994 2005 2009

Visual inspection All feeder pipes and all supports

13.3.2.2 Periodic inspectionThe pre-service baseline inspection

NA

Wall thickness measurements

A minimum of 20 feeder pipes

A minimum of 20 inlet and 20 outlet feeder pipes

A minimum of 20 inlet and 20 outlet feeder pipes

1994 2005 2009

Wall thickness measurements

A minimum of 10 feeder pipes -Second unit : 7-Third unit : 4-Fourth unit : 3

A minimum of 10 inlet and 10 outlet feeder pipesA minimum of 50% of the feeder pipes successively inspected

Same to 2005

Acceptance criteria

- Recording & reporting : 20%- Submission to regulatory authority : 40% - For visual inspection : Clause 8.2.2

Less than the design minimum wall thickness at the end of the next periodic inspection interval Fitness-for-service assessment

Same to 2005

Calibration specimen

For 2009 edition only- A length of feeder pipe of the same material specification and grade and the same nominal diameter and thickness- Wall reductions covering the range of thicknesses to be measured

13.4 Volumetric inspection of feeder piping of CSA N285.4(2014)

As a minimum, the assessment report on feeder cracking potential shall be reviewed by the licensee at least once every 6 years to confirm continued report validity or to determine that the report requires an update. The review shall be documented and the regulatory authority shall be notified of the review findings. The inspection intervals defined in Clause 13.2.3.1 may be used to define intervals for review of the feeder cracking potential assessment report.

Note: As the feeder cracking potential assessment report can be used to limit volumetric inspection of feeder piping, it is considered good practice to review the validity of the assessment interval on a regular basis to include new operational experience and changes in conditions affecting cracking potential.

Inspection history of feeder pipes withdrawn for Wolsong X

Wall thickness reduction (over 35%) for outlet feeder pipe of Wolsong X

NPS # Over 50% 40% < t < 50% 35% < t < 40% Cumulative ratio

2.0 inch 59 0 12 7 32.2%

2.5 inch 321 29 116 72 67.6%

Sum 380 29 128 79 62.1%

Cumulative ratio 7.6% 35.6% 62.1%

Feeder pipes withdrawn for Wolsong X

- Average wall reduction loss for all feeder pipes withdrawn after 30 years operation (2009)

- Consideration of average wall reduction rate

For feeder pipes of Wolsong Xs

- Increase of average chromium content : 0.02 0.036 ~ 0.051

For refurnished feeder pipes of Wolsong X

- Material specification change : SA 106 Gr. B SA 106 Gr. C

- A minimum chromium content was increased rather than that of Woslong Xs

- Increase of nominal wall thickness for 2” feeder pipes : 5.537mm 6.350mm

Feeder pipes for Wolsong Xs(max. up to ~ 16 EFPY)- Thickness measurement(44) during periodic inspection+ residual stress evaluation(3)

during fabrication

Units Year Classification 1.5” Inlet

2” Inlet

2.5”Inlet

2”Outlet

2.5”outlet

Wolsong X

2016

Total amount 3 7 2 8 24> 10% 2 6 1 2 7> 20% - - - 6 16> 30% - - - - 1

Wolsong X

2017

Total amount 3 7 2 8 24> 10% 2 5 2 6 11> 20% - - - 2 13> 30% - - - - -

Wolsong X

2015

Total amount 3 7 2 8 24> 10% 0 0 0 7 13> 20% - - - 1 10> 30% - - - - -

- Point Lepreau : ~ 400 Mpa ↔ Woslong Xs : ~ 150Mpa(

- Stress relief heat treatment for Woslong Xs : 0.5 hour at 608~635℃

- High potential micro cracks during cold worked bending process

- Relatively low residual stress at bends for feeders during manufacturing stage and

tempering heat treatment

- Low potential cracking at gralyoc welds, fluid device DWs, and swage welds

- UT for feeders subject to wall thinning over 30% of Wolsong X as a lead plant

- Lesson to be learned from micro examinations of previous removed feeders subject to

wall thinning over 55% for Wolsong X

- Assessment on feeder cracking potential every 6 years in accordance with CSA 285.4

(2014)

▣ Short-term planRe-establishment of operating velocity(v.s. design velocity) and chromium content for each feeder

Selection and measurement of weakness point due to wall thinining through survey of CFD results(shear stress), mechanical properties change by heat treatment of residual stress relief, limit load distribution of cold worked(bend angle) and wall thinned bend.

Application of conventional UT for a feeder subject to wall thinning over 30% of Wolsong X as a lead plant

Determination of the rate of change over the last interval for at least 50% of the minimum inspection sample size

Proceduralization of wall thinning rate evaluation & no cracking defects, residual lifetime determination and acceptance criteria for feeder pipes

▣ Long-term planPreparation and review of assessment report on feeder cracking potential at least once every 6 years

Evaluation of feeder cracking potential for first bend + second bend, grayloc welding, flow device DWM(Dissimilar weld metal), orifice welding, etc. in parallel with CANADA feeders LBB(Leak before break)

Introduction of performance demonstration for ultrasonic thickness measurement & volumetric inspection

Submission of feeder pipe life-cycle management program to regulatory body in accordance with regulatory document 2.6.3 & CSA 285.4

Submission of fitness for service assessment to regulatory body in accordance with FFSG(Feeder fitness for service Guidelines) and CSA 285.4