ML040840465
| ML040840465 | |
| Person / Time | |
|---|---|
| Site: | Quad Cities  |
| Issue date: | 03/25/2004 |
| From: | NRC/NRR/DLPM/LPD3 |
| To: | |
| Shared Package | |
| ML040840459 | List: |
| References | |
| Download: ML040840465 (2) | |
Text
SUMMARY
OF TELEPHONE CONFERENCE WITH EXELON ON QUAD CITIES UNIT 2 STEAM DRYER CRACKING On March 18, 2004, the NRC staff held an extended telephone conference with Exelon to discuss the repair of the cracks identified in the steam dryer at Quad Cities Unit 2 during the current refueling outage following approximately 8 months of operation at extended power uprate (EPU) conditions.
During the telephone conference, the licensee described the repairs to the Quad Cities Unit 2 steam dryer, uncertainties in its analysis of the steam dryer, its testing plan following plant restart, and applicability of Quad Cities Unit 2 steam dryer damage to Quad Cities Unit 1 and Dresden Units 2 and 3. As a result of the VT-1 inspection during this outage, the licensee identified numerous indications ranging from minor scratches to more significant cracks in the steam dryer at Quad Cities Unit 2. The licensee considered many of those indications to have been missed by the less detailed VT-3 inspection that had been conducted following the failure of the steam dryer in 2003. The licensee dispositioned most indications through minor drilling or acceptance of the indication in the as-found condition. The licensee discussed repairs associated with four sets of the most significant indications: (1) cracks at the gussets installed as part of the steam dryer repairs in 2003; (2) weld cracks in tie bars internal to the steam dryer; (3) cracking of a stitch weld on a dryer stiffener plate; and (4) a 1-inch crack at the intersection of a vertical and horizontal plate near the cover plate. The licensee described its test plans for Quad Cities Unit 2 to establish the steam dryer loads with respect to flow rate and to identify any operating limitations for the as-repaired dryer. The licensee plans to continue to operate Quad Cities Unit 1 at pre-EPU levels until the results of the tests at Quad Cities Unit 2 are evaluated. The licensee considered that sufficient basis existed to justify operation of Dresden Units 2 and 3 at EPU levels as a result of less observed steam dryer damage at those units following longer EPU operation, apparent smaller loads on the Dresden steam dryers, and structural improvements. The licensee plans to inspect the steam dryers in the Quad Cities and Dresden units at their next refueling outages.
At the end of the telephone conference, the NRC staff summarized several concerns that were identified in the course of the discussion on the licensees repair and test plan for Quad Cities Unit 2. These concerns are as follows:
(1) The licensees evaluation of its steam dryer repairs in Quad Cities Unit 2 applied only static loading conditions rather than a more representative dynamic loading to verify the adequacy of those repairs.
(2) The repaired Quad Cities Unit 2 steam dryer will continue to have discontinuities (such as weld joints between 0.5 to 1 inch plates) that might represent high stress locations susceptible to cracking in the future.
(3) The Finite Element Model (FEM) of the steam dryer applied by the licensee does not appear to fully address the steam dryer structure, such as weld connections or tie bars.
(4) The licensees presentation focused only on the four most significant sets of steam dryer indications in Quad Cities Unit 2, and did not provide details
regarding the basis for disposition of the steam dryer indications considered to be less significant.
(5) The reverse-engineering approach applied by the licensee in evaluating the steam dryer repair provided loading values that caused the cracks to occur in Quad Cities Unit 2, but did not provide a bounding value for those loads.
(6) The licensee only provided viewgraphs for NRC staff review, and had not completed its evaluation of the Quad Cities Unit 2 steam dryer repairs, including its independent review effort.
(7) The licensee in its test plan does not indicate the collection of specific steam dryer data at Quad Cities Unit 2, but rather only plans to obtain pressure time history data at the main steam line venturi and turbine instrumentation locations.
(8) The licensees more detailed acoustic circuit analysis to be used in evaluating the structural integrity of the steam dryer only addresses differential pressure across the dryer without consideration of dynamic loading or potential fluid-structure interactions.
(9) Although the licensee believes that the stresses on the steam dryer will be lower at the Dresden units than at the Quad Cities units, the licensee continues to operate Dresden Unit 2 at approximately EPU conditions without near-term plans to correct the gusset design in its steam dryer which failed prematurely at Quad Cities Unit 2.
(10) The licensees presentation did not provide details regarding the adequacy of the computational fluid dynamics (CFD) model used in determining the structural integrity of the steam dryer.
In summarizing the discussions, the NRC staff noted that the licensees resolution of the potential adverse flow effects from EPU operation at Quad Cities and Dresden continues to rely primarily on questionable analyses. Further, the licensees plans for operating Quad Cities and Dresden did not include any commitment to perform periodic inspections of the steam dryer or other potentially impacted components from EPU operation (such as the electromatic relief valves) except during future refueling outages at 2 year intervals. A follow-up telephone conference is planned to discuss the regulatory aspects of operation of the Quad Cities and Dresden units at EPU power levels.