COFRENTES UNDERVESSEL PROJECT IN RFO 16 (2007)

[ Replacement of all the CRDH undervessel piping]

COFRENTES UNDERVESSEL COFRENTES UNDERVESSEL PROJECT IN RFO 16 (2007)PROJECT IN RFO 16 (2007)

[ Replacement of all the CRDH [ Replacement of all the CRDH undervessel piping]undervessel piping]

IRPA 1212th International Congress of the International Radiation Protection Association Buenos Aires , Argentina – October 19-24Eduardo Sollet (eduardo.sollet@iberdrola.es)COFRENTES NPP (SPAIN)

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1.- BACKGROUND AND ROOT CAUSE

• Background- Events in RFO 15: – Leaks detected in 8 CRDH small pipes (2”) in sector 2 within the reactor

bioshield in the drywell– No access available for repair so it was necessary to replace all sector 2

CRDH pipes inside the drywell bioshield

• Root cause analysis conclusions: – Leaks were produced by Transgranular

Stress Corrosion Cracking (TGSCC) induced by external agents acting from the outside of the pipes

– Three necessary factors are involved: Residual stress (curves pipes), Humidity (normal working conditions) and a Chemical Agent (chlorides on the pipes from dust + small deposits of carbon steel)

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- CRDH Pipe :• Sectors 1, 3 & 4 in DW through

bioshield• Sectors 1, 2, 3 & 4 inside UV• Supports modifications

- Cables:• Replacement nuclear instrumentation

cablesMaterial improvements

- Control Rod Drives (CRDs):• Remove & installation of all 145 CRDs• Decontamination & reuse of internal

filters- Nuclear Instrumentation IRMs, SRMs:

• Removal & Inspection

2.- WORK SCOPE

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WORKING ENVIRONMENT

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Main “ALARA” techniques implemented in the project:

1. Preparation and planning all activities with the contract2. Reduction of most radiation fields (shielding, cleaning

and decontamination)3. Design of automatic tools for pipe cutting, machining and

welding4. Mock-up training5. Continuous radiological monitoring by RPpersonnel6. Selection of a high qualified and experienced contractor

in the nuclear field

3.- ALARA PROGRAM

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The ALARA Plan covered the following major tasks with dedicated ALARA techniques:

Removal and installation of 145 CRDs (50 CRD’s maintained & rebuilt)Removal and substitution of 145 LPRM & 290 RPIS cables (all)Inspection and removal of SRM’s & IRM’sReplacement of 290 CRDH piping.

3.- ALARA PROGRAM

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PLANNING AND PREPARATION • Creation of a working group (Mant., Operations, QC, Eng.,

RP, Purchases, Chemistry & main Contractor) with periodic meetings for planning and work preparation.

• Detailed working planning by activity in order to reducetime and dose

• Setup of a low radiation waiting area outside the Drywell• Total scope of the project planned in 45 days (in critical

path)• Total number of workers involved in the field: 490• Total time in RCA during project execution: 27.970 h• Nuclear safety: All fuel unloaded from vessel prior

starting of activities

3.- ALARA PROGRAM

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3.- ALARA PROGRAM

DOSE CONTROL LEVELS:

Individual Dose Control Levels: – 4 mSv / day–18 mSv / outage

Individual Dose Legal Limits:– 50 mSv / year– 100 mSv / 5 years

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CONTINUOUS MONITORING OF INDIVIDUAL AND COLLECTIVE DOSES OF THE PROJECT:

• DRDs (electronic dosemeters)• Tele dosemeters • Continuous radiological surveillance of the workplace • CCTV

3.- ALARA PROGRAM

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Two 1:1 mock-ups (one quarter in plant and one eighth in contractor facility) of the undervessel area were constructed for training and qualification of both workers and the new tools

CRDH PIPES

CRDS TUBES

SISMIC SUPPORTS

SRMs-IRM’s TUBES

3.- ALARA PROGRAMMOCK-UP TRAINING AND QUALIFICATION

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MASSIVE SHIELDING INSTALLATION•DRY WELL:

3.- ALARA PROGRAM

TEMPORARY SHIELDS

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MASSIVE SHIELDING INSTALLATION•DRY WELL:

3.- ALARA PROGRAM

PERMANENT SHIELDS

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SOURCE TERM REDUCTIONShielding and cutting of the reactor bottom drain line and thermometric collector

3.- ALARA PROGRAM

REACTOR VESSELBOTTOM DRAIN LINE

HOT SPOTS IN ELBOWS AND

PIPE

TERMOMETRICCOLECTOR

HOT SPOT

BEFORE MODIFICATION

10 mSv/h

0,5 Sv/h

0,3 Sv/h

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SOURCE TERM REDUCTIONShielding and cutting of the reactor bottom drain line and thermometric collector

3.- ALARA PROGRAM

AFTER MODIFICATION

THERMOMETRIC COLECTOR, DRAIN LINE ELBOWS AND

HOT SPOTS ELIMINATED

NEW VALVE

NEW FLANGE

General area dose rate ≈ 1 mSv/h

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Removal of all nuclear instrumentation cables prior CRDH pipe replacement

TIP PIPES

3.- ALARA PROGRAMSOURCE TERM REDUCTION

RPIS PIPES

• All the nuclear instrumentation cables in the pedestal were removed to reduce interferences, general area dose rate and contamination

• New better cables were installed for RPIS and LPRM without a metallic cover protection to avoid contamination

LPRMS CABLES

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EXTRACTION OF 145 CRDS3.- ALARA PROGRAM

LEAD CRD FILTHER SHIELD

• Uncuopling CRD’s from Refueling Floor• Continuous radiological surveillance by RP

during CRD extraction (telemetry)• Shielding of each CRD filter after its

removal• Use of “mururuoa suits” for CRD

extraction

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• Chemical decontamination + cleaning of all CRD filters before reuse

3.- ALARA PROGRAM

EXTRACTION OF 145 CRDS

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CRDH PIPING REPLACEMENT• Use of special tools for removing the CRDH pipe from the CRD

flange

3.- ALARA PROGRAM

HYDRAULIC TOOL FOR OLD CRDH PIPE EXTRACTION

DRILLING EQUIPMENT FOR PIPE WELDING REDUCTION

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3.- ALARA PROGRAMCRDH PIPING SUBSTITUTION• Use of special semiautomatic machines for welding

NEW WELDING EQUIPMENT

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4.- RADIOLOGICAL CONDITIONSDRYWELL CONDITIONS (Chemical decon performed in the 15th outage)•High recontamination of the clean-up system (carbon steel)• Low recontamination of the Recirculation Loops (stainless steal)

BRAC POINT HISTORICAL DATA

0,00

2,00

4,00

6,00

8,00

10,00

12,00

14,00

16,00

6ª REC. 7ª REC. 8ª REC 10ª REC DIC-99 13ª REC(d.d.)

14ª REC 15ª REC(d.d.)

16ª REC

Dos

e Rat

e (m

Sv/h

)

LAZO A LAZO B

Partial Decon A B

DZO jun 96 H2 mar 97

Descon Loop A

Full Decon

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UNDERVESSEL (PEDESTAL) CONDITIONS•Expected a dose rate decrease after removal all CRDs and

Nuclear Instrumentation cables•Complete shoot-out steel removal needed but this decreased

the shielding inside the area•Both facts cause the opposite effect: Moderate dose rate

increase in the area due to PRM tube dose rate (hot spots in the flange area >100 mSv/h contact). This was an unknown effect with not prior experience in the plant

4.- RADIOLOGICAL CONDITIONS

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4.- RADIOLOGICAL CONDITIONS

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14

2

WORKING AREAS

0,7 - 1,20,9 - 1,8POINTS 1 & 2:Pedestal Platform Area

Before shielding and cleaning PRMS

0,7 – 1,60,8 - 2,5POINTS 3 & 4:Pipe working areas in sectors 1,2,3&4

DOSE RATE (mSv/h)

WORKS ON THE PEDESTAL PLATFORM WORKS ABOVE THE CRDs&PRMs FLANGES (2M HIGH FROM THE PEDESTAL PLATFORM)

PEDESTAL CONDITIONS

After shielding and cleaning PRMS

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4.- RADIOLOGICAL CONDITIONS

DRYWELL MAP

1 2

DRYWELL CONDITIONS

0,3 – 0,80.6 – 1,5AREA OF CRDH PIPE SUBSTITUTION

2.- After Shielding1.- Before Shielding

DOSE RATE (mSv/h)DRYWELL WORKING AREA

0,3 – 0,80.6 – 1,5AREA OF CRDH PIPE SUBSTITUTION

2.- After Shielding1.- Before Shielding

DOSE RATE (mSv/h)DRYWELL WORKING AREA

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5.- MAIN PROBLEMS FOUNDHIGHER THAN EXPECTED UNDERVESSEL AREA DOSE RATE

1st ALARA technique

• The 33 PRMs flanges were first shielded with a lead cylinders of 1 cm thick and 10 cm High

Result: Local contact dose rate over the flange PRM area decreased to 10 mSv/h (DRF =10). In the UV platform the dose reduction factor was light (-30%)

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5.- MAIN PROBLEMS FOUNDHIGHER THAN EXPECTED UNDERVESSEL AREA DOSE RATE

2nd ALARA technique

• The 33 PRMs were cleaned and flushed with demin. water to remove hotspots

Result: Contact dose rate over all the PRM tubes decreased to 10 mSv/h (DRF =10)

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5.- MAIN PROBLEMS FOUNDHIGHER THAN EXPECTED UNDERVESSEL AREA DOSE RATE

3rd ALARA technique

• The peripherical PRMs were reshielded to 1 m height with lead cylinders of 1 cm thick

Benefit: Ambient dose rate in CRDH sectors decreased by a factor of 2

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5.- MAIN PROBLEMS FOUND

CRDH PIPE WELD IN CRD FLANGE THICKER THAN EXPECTED

• More time expended when machining the original weld. After all new tubes were inserted ready for welding, it was discovered an air gap higher than expected from the front end surface of the tube and the sitting area within the flange. This made impossible the welding causing the re-extraction of all new tubes for a better machining of the old weld. Almost 0,6 additional Sv were expended in this reworking activity. Then the tubes were repositioned again and welded semi automatically.

• But during the final pressure tests after all CRD were inserted ready for core load, leaks were detected in several CRD flanges

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5.- MAIN PROBLEMS FOUND

CRDH PIPE WELD IN CRD FLANGE THICKER THAN EXPECTED

• The plant decided to repair all the 145 CRDs new flanges welds of all the new CRDH pipes. This meant a new extraction of all CRD’s again.

• But this required a new authorization from the Spanish Regulatory Body (CSN) to account for an extra 1,4 man.Sv necessary to finalize the work

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5.- MAIN PROBLEMS FOUNDCRDH PIPE WELD IN CRD FLANGE THICKER THAN EXPECTED

WHAT HAPPENED?Original designCDR pipe

CDR flangeExtent of the original weld

more than expected

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5.- MAIN PROBLEMS FOUNDCRDH PIPE WELD IN CRD FLANGE THICKER THAN EXPECTED

WHAT HAPPENED?

Machining the old weld

Incomplete weld removal

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5.- MAIN PROBLEMS FOUNDCRDH PIPE WELD IN CRD FLANGE THICKER THAN EXPECTED

WHAT HAPPENED?

Mechanical extraction of the old tube

This push-out creates a crack in the old weld

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5.- MAIN PROBLEMS FOUNDCRDH PIPE WELD IN CRD FLANGE THICKER THAN EXPECTED

WHAT HAPPENED?

New weld

PT after welding did not discover cracks

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5.- MAIN PROBLEMS FOUNDIMMEDIATE CORRECTIVE ACTIONS IMPLEMENTED

• Diagnostic of the failure (root-cause analysis): – Visual inspection; machining of the old welds; PT; pressure test

at 20 kg/cm2; inspection of the extracted tubes; fabrication of

flange dummies; mock-up tests.

• Criteria to finalize the project :– Final solution.

– Complete scope: (all 145 CRD´s).

– Guaranty that the flanges will not be affected.

– To avoid new failures: (Tests after each phase).

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5.- MAIN PROBLEMS FOUND2nd PHASE OD THE CRD TUBE-FLANGE WELDINGS

• Machining and welding procedures tested in mock-up before approval.

• Complete flange machining.• Superficial tests (visual and/or PT) on

machined flanges prior welding.• Manual welding at 26 mm lower end.• Visual inspections and PT after each weld.• Pressure test of all tubes before CRD insertion.• Flange flatness check before CRD insertion.• Individual scram test before core load.

More Dose

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6.- DOSE ESTIMATES AND FINAL RESULTS

CRDH Piping substitution Project

ESTIMATED COLECTIVE DOSES (man.mSv)ACTUAL DOSES

(man.mSv)INITIAL DOSE

ESTIMATE

REESTIMATED DOSES DURING

RFO16

FINAL DOSE ESTIMATE

CRDs works: Removal/Installation of all (145) CRDs and rebuild (50) CRDs, including shoot-out steel disassembly/assembly

470 + 80 550 564,83

LPRMs works: New LPRMs installed, replacement of all cables (132). New ALARA techniques to shield and clean all of the PRM tubes

151 + 70 221 217,57

SRMs-IRMs works: Cable change and inspections 90 90 83,39

Pedestal platform modification (not necessary) 34 34 0,33

CRDH piping substitution works: 290 CRDH pipes were replaced (includes auxiliary works such as cleaning, shielding)

1473 + 600 2073 2125,85

RPIS works: 290 PIPs cables change 382 382 347,5

Welding repair works in CRD Flanges(reworking activity) + 1405 1405 920,13

TOTAL DOSE (man.mSv) 2600 + 2155 4755 4259,6

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6.- DOSE ESTIMATES AND FINAL RESULTS

0

500

1000

1500

2000

2500

3000

3500

4000

4500

1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86

DAYS

man

.mSv

0

10

20

30

40

50

60

70

Nº P

ERSO

NS

Nº WORKERS > 20 mSv (initial estimation was 0)

INITIAL DOSE ESTIM. 2,6 Sv

ACTUAL DOSE 4,26 Sv

56 Days: Outage expected duration

92 Days: Final outage duration

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7.- CONCLUSIONS (I)

• The project was planned and prepared with the best available ALARA techniques

• The lack of previous industry experience could not anticipate some of the problems

• New ALARA techniques were implemented as the problem arises (shielding, cleaning and decontamination of PRMs, so their final impact in dose was less than 5% of the additional collective dose)

• Welding machining problems (original welds not fully documented) had the greatest impact in project dose outage duration