05000275/LER-2013-005, Both Trains of Residual Heat Removal Inoperable Due to Circumferential Crack on a Socket Weld

Both Trains of Residual Heat Removal Inoperable Due to Circumferential Crack on a Socket Weld

ML13235A101
Person / Time
Site:	Diablo Canyon
Issue date:	08/22/2013
From:	Allen B Pacific Gas & Electric Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
DCL-13-085, OL-DPR-80 LER 13-005-00
Download: ML13235A101 (5)

LER-2013-005, Both Trains of Residual Heat Removal Inoperable Due to Circumferential Crack on a Socket Weld

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2752013005R00 - NRC Website

text

Pacific Gas and Electric Company August22, 2013 PG&E Letter DCL-13-085 Barry S. Allen Site Vice President Diablo Canyon Power Plant Mail Code 104/6 P. O. Box 56 Avila Beach, CA 93424 805.545.4888 Internal: 691. 4888 Fax: 805.545.6445 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 10 CFR 50.73 Docket No. 50-275, OL-DPR-80 Diablo Canyon Unit 1 Licensee Event Report 1-2013-005-00, Both Trains of Residual Heat Removal Inoperable Due to Circumferential Crack on a Socket Weld Dear Commissioners and Staff; Pacific Gas and Electric Company (PG&E) submits the enclosed Licensee Event Report (LER) regarding completion of a shutdown of Diablo Canyon Power Plant, Unit 1, as required by the technical specifications when both trains of the residual heat removal system were declared inoperable due to a circumferential crack on a socket weld. PG&E is submitting this LER in accordance with 10 CFR 50.73(a)(2)(i)(A), 50.73(a)(2)(ii)(B), and 50.73(a)(2)(v)(D). All systems operated as designed with no problems observed.

PG&E makes no new or revised regulatory commitments (as defined by NEI 99-04) in this report.

This event did not adversely affect the health and safety of the public.

Sincerely, v5~

Barry S. Afien d ngd/4955/50570623 Enclosure cc: Thomas R. Hipschman, NRC Senior Resident Inspector Jennivine K. Rankin, NRR Project Manager Steven A. Reynolds, Acting NRC Region IV Diablo Distribution INPO A member of the STARS (Strategic Teaming and Resource Shari ng) Alliance Callaway. Comanche Peak. Diablo Canyon. Palo Verde. San Onofre. South Te x as Project. Wolf Creek

NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31/2013 (10-2010)

, the NRC may digits/characters for each block) not conduct or sponsor, and a person is not required to respond to, the information collection.

roPAGEl Diablo Canyon Power Plant, Unit 1 50-275 OF 4

4. TITLE Both Trains of Residual Heat Removal Inoperable Due to Circumferential Crack on a Socket Weld

5. EVENT DATE

6. LER NUMBER

7. REPORT DATE

8. OTHER FACILITIES INVOLVED YEAR I SEQUENTIAL I REV FACILITY NAME DOCKET NUMBER MONTH DAY YEAR NUMBER NO.

MONTH DAY YEAR FACILITY NAME DOCKET NUMBER 06 25 2013 2013. 005. 00 08 22 2013

9. OPERATING MODE

11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply)

D 20.2201 (b)

D 20.2203(a)(3)(i)

D 50.73(a)(2)(i)(C)

D 50.73(a)(2)(vii) 1 D 20.2201(d)

D 20.2203(a)(3)(ii)

D 50.73(a)(2)(ii)(A)

D 50.73(a)(2)(viii)(A)

D 20.2203(a)(1)

D 20.2203(a)(4)

[(] 50.73(a)(2)(ii)(B)

D 50.73(a)(2)(viii)(B)

D 20.2203(a)(2)(i)

D 50.36(c)(1)(i)(A)

D 50.73(a)(2)(iii)

D 50.73(a)(2)(ix)(A)

10. POWER LEVEL D 20.2203(a)(2)(ii)

D 50.36(c)(1)(ii)(A)

D 50.73(a)(2)(iv)(A)

D 50.73(a)(2)(x)

D 20.2203(a)(2)(iii)

B 50.36(c)(2)

D 50.73(a)(2)(v)(A)

D 73.71(a)(4) 100 D 20.2203(a)(2)(iv) 50.46(a)(3)(ii)

D 50.73(a)(2)(v)(B)

D 73.71 (a)(5)

D 20.2203(a)(2)(v)

[(] 50.73(a)(2)(i)(A)

D 50.73(a)(2)(v)(C)

D OTHER D 20.2203(a)(2)(vi)

D 50.73(a)(2)(i)(B)

[(] 50.73(a)(2)(v)(D)

Specify in Abstract below or in C. Status of Inoperable Structure, Systems, or Components That Contributed to the Event None.

D. Other Systems or Secondary Functions Affected

None.

E. Method of Discovery

REV NO.

00 3

3. PAGE OF 4

During a walk down of the Unit 1 containment, maintenance personnel noted an accumulation of boric acid on the inlet pipe to relief valve RHR-I-RV-S70S. Subsequent cleanup of the boric acid accumulation revealed an active leak of 3 dpm. A visual inspection identified that the source of the leak was a circumferential crack on the socket weld.

F. Operator Actions

Operators declared both RHR trains inoperable.

G. Safety System Responses None.

III. Cause of the Problem DCPP determined that the socket weld on the inlet piping to relief valve RHR-I-RV-S70S failed due to low-stress, high-cycle fatigue (from operation-induced system vibration) and a lack of fusion in the weld root. The crack initiated at a stress riser and propagated due to cyclical loading.

IV. Assessment of Safety Consequences

DCPP assessed the Unit 1 risk significance of a leak in the vent line of the RHR supply line to the Hot Leg 1 and 2 supply lines. This assessment concluded that the incremental conditional core damage probability was less than 1.OE~06/year. Therefore, the event is not considered risk significant and did not adversely affect the health and safety of the pUblic.

V. Corrective Actions

A. Immediate Corrective Actions DCPP replaced the socket weld on June 27, 2013. The new weld uses a 2:1-leg ratio fillet weld, which has improved fatigue resistance over the original equal-leg socket weld. This weld configuration has been recommended by the Electric Power Research Institute and is used by the industry and DCPP where enhanced fatigue resistance is required.

B. Other Corrective Actions DCPP will install new, rigid-lateral support(s) on the discharge line from relief valve RHR-1-RV-8708. The new support( s) will reduce the chances of piping vibration amplification by increasing its natural frequencies to significantly higher than the current resonant frequencies. DCPP will also assess extent of cause for additional socket welds in susceptible locations.

VI. Additional Information

A. Failed Components DCPP discovered a circumferential crack on the socket weld on the inlet to relief valve RHR-1-RV-870S.

B. Previous Similar Events

DCPP - On February 8, 1999, an operator observed a leak on the weld for chemical and volume control system valve CVCS-2-839 while on routine rounds. The cause of the leak was determined to be high-cycle fatigue of the socket-welded connection due to degraded positive displacement pump operation causing excessive vibration of the piping near the pumps, which led to the growth of through-wall cracks. The corrective action was to remove the valve/pipe assembly and install a plug in the fitting.