COFRENTES UNDERVESSEL PROJECT IN RFO 16 (2007) [ Replacement of all the CRDH undervessel piping]

Transcription

1 COFRENTES UNDERVESSEL PROJECT IN RFO 16 (2007) [ Replacement of all the CRDH undervessel piping] IRPA th International Congress of the International Radiation Protection Association Buenos Aires, Argentina October Eduardo Sollet (eduardo.sollet@iberdrola.es) COFRENTES NPP (SPAIN)

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3 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) 3

4 2.- WORK SCOPE - CRDH Pipe : Sectors 1, 3 & 4 in DW through bioshield Sectors 1, 2, 3 & 4 inside UV Supports modifications - Cables: Replacement nuclear instrumentation cables Material improvements - Control Rod Drives (CRDs): Remove & installation of all 145 CRDs Decontamination & reuse of internal filters - Nuclear Instrumentation IRMs, SRMs: Removal & Inspection 4

5 WORKING ENVIRONMENT 5

6 3.- ALARA PROGRAM Main ALARA techniques implemented in the project: 1. Preparation and planning all activities with the contract 2. Reduction of most radiation fields (shielding, cleaning and decontamination) 3. Design of automatic tools for pipe cutting, machining and welding 4. Mock-up training 5. Continuous radiological monitoring by RPpersonnel 6. Selection of a high qualified and experienced contractor in the nuclear field 6

7 3.- ALARA PROGRAM 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 s Replacement of 290 CRDH piping. 7

8 3.- ALARA PROGRAM 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 reduce time 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: h Nuclear safety: All fuel unloaded from vessel prior starting of activities 8

9 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 9

10 3.- ALARA PROGRAM CONTINUOUS MONITORING OF INDIVIDUAL AND COLLECTIVE DOSES OF THE PROJECT: DRDs (electronic dosemeters) Tele dosemeters Continuous radiological surveillance of the workplace CCTV 10

11 3.- ALARA PROGRAM MOCK-UP TRAINING AND QUALIFICATION 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 11

12 3.- ALARA PROGRAM MASSIVE SHIELDING INSTALLATION DRY WELL: TEMPORARY SHIELDS 12

13 3.- ALARA PROGRAM MASSIVE SHIELDING INSTALLATION DRY WELL: PERMANENT SHIELDS 13

14 3.- ALARA PROGRAM SOURCE TERM REDUCTION Shielding and cutting of the reactor bottom drain line and thermometric collector BEFORE MODIFICATION HOT SPOT 10 msv/h REACTOR VESSEL BOTTOM DRAIN LINE TERMOMETRIC COLECTOR 0,3 Sv/h 0,5 Sv/h HOT SPOTS IN ELBOWS AND PIPE 14

15 3.- ALARA PROGRAM SOURCE TERM REDUCTION Shielding and cutting of the reactor bottom drain line and thermometric collector AFTER MODIFICATION NEW VALVE General area dose rate 1 msv/h THERMOMETRIC COLECTOR, DRAIN LINE ELBOWS AND HOT SPOTS ELIMINATED NEW FLANGE 15

16 3.- ALARA PROGRAM SOURCE TERM REDUCTION Removal of all nuclear instrumentation cables prior CRDH pipe replacement TIP PIPES LPRMS CABLES 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 16

17 3.- ALARA PROGRAM EXTRACTION OF 145 CRDS 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 LEAD CRD FILTHER SHIELD 17

18 3.- ALARA PROGRAM EXTRACTION OF 145 CRDS Chemical decontamination + cleaning of all CRD filters before reuse 18

19 3.- ALARA PROGRAM CRDH PIPING REPLACEMENT Use of special tools for removing the CRDH pipe from the CRD flange DRILLING EQUIPMENT FOR PIPE WELDING REDUCTION HYDRAULIC TOOL FOR OLD CRDH PIPE EXTRACTION 19

20 3.- ALARA PROGRAM CRDH PIPING SUBSTITUTION Use of special semiautomatic machines for welding NEW WELDING EQUIPMENT 20

21 4.- RADIOLOGICAL CONDITIONS DRYWELL CONDITIONS (Chemical decon performed in the 15 th outage) High recontamination of the clean-up system (carbon steel) Low recontamination of the Recirculation Loops (stainless steal) BRAC POINT HISTORICAL DATA 16,00 14,00 Dose Rate (msv/h) 12,00 10,00 8,00 6,00 4,00 2,00 0,00 Descon Loop A DZO jun 96 H2 mar 97 6ª REC. 7ª REC. 8ª REC 10ª REC DIC-99 13ª REC (d.d.) Partial Decon A B Full Decon 14ª REC 15ª REC (d.d.) 16ª REC LAZO A LAZO B 21

22 4.- RADIOLOGICAL CONDITIONS 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 22

23 4.- RADIOLOGICAL CONDITIONS PEDESTAL CONDITIONS WORKS ON THE PEDESTAL PLATFORM WORKS ABOVE THE CRDs&PRMs FLANGES (2M HIGH FROM THE PEDESTAL PLATFORM) WORKING AREAS POINTS 1 & 2: Pedestal Platform Area Before shielding and cleaning PRMS 0,9-1,8 DOSE RATE (msv/h) After shielding and cleaning PRMS 0,7-1,2 POINTS 3 & 4: Pipe working areas in sectors 1,2,3&4 0,8-2,5 0,7 1,6 23

24 4.- RADIOLOGICAL CONDITIONS DRYWELL CONDITIONS MAP 1 2 DRYWELL WORKING AREA AREA OF CRDH PIPE SUBSTITUTION DOSE RATE (msv/h) 1.- Before Shielding 2.- After Shielding 0.6 1,5 0,3 0,8 24

25 5.- MAIN PROBLEMS FOUND HIGHER 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%) 25

26 5.- MAIN PROBLEMS FOUND HIGHER 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) 26

27 5.- MAIN PROBLEMS FOUND HIGHER THAN EXPECTED UNDERVESSEL AREA DOSE RATE 3 rd 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 27

28 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 28

29 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 29

30 5.- MAIN PROBLEMS FOUND CRDH PIPE WELD IN CRD FLANGE THICKER THAN EXPECTED WHAT HAPPENED? Original design CDR pipe CDR flange Extent of the original weld more than expected 30

31 5.- MAIN PROBLEMS FOUND CRDH PIPE WELD IN CRD FLANGE THICKER THAN EXPECTED WHAT HAPPENED? Machining the old weld Incomplete weld removal 31

32 5.- MAIN PROBLEMS FOUND CRDH 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 32

33 5.- MAIN PROBLEMS FOUND CRDH PIPE WELD IN CRD FLANGE THICKER THAN EXPECTED WHAT HAPPENED? PT after welding did not discover cracks New weld 33

34 5.- MAIN PROBLEMS FOUND IMMEDIATE CORRECTIVE ACTIONS IMPLEMENTED Diagnostic of the failure (root-cause analysis): Visual inspection; machining of the old welds; PT; pressure test at 20 kg/cm 2 ; 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). 34

35 5.- MAIN PROBLEMS FOUND 2 nd 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. More Dose 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. 35

36 6.- DOSE ESTIMATES AND FINAL RESULTS CRDH Piping substitution Project ESTIMATED COLECTIVE DOSES (man.msv) INITIAL DOSE ESTIMATE REESTIMATED DOSES DURING RFO16 FINAL DOSE ESTIMATE ACTUAL DOSES (man.msv) CRDs works: Removal/Installation of all (145) CRDs and rebuild (50) CRDs, including shoot-out steel disassembly/assembly LPRMs works: New LPRMs installed, replacement of all cables (132). New ALARA techniques to shield and clean all of the PRM tubes SRMs-IRMs works: Cable change and inspections Pedestal platform modification (not necessary) CRDH piping substitution works: 290 CRDH pipes were replaced (includes auxiliary works such as cleaning, shielding) , , , , ,85 RPIS works: 290 PIPs cables change ,5 Welding repair works in CRD Flanges (reworking activity) ,13 TOTAL DOSE (man.msv) ,6 36

37 6.- DOSE ESTIMATES AND FINAL RESULTS Days: Outage expected duration 60 man.msv ACTUAL DOSE 4,26 Sv INITIAL DOSE ESTIM. 2,6 Sv Nº WORKERS > 20 msv (initial estimation was 0) Nº PERSONS Days: Final DAYS outage duration 0 37

38 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 38