RS-04-055, Additional Information Regarding Request for Extended Power Uprate NRC Safety Evaluation
| ML041070392 | |
| Person / Time | |
|---|---|
| Site: | Dresden, Quad Cities  |
| Issue date: | 04/09/2004 |
| From: | Simpson P Exelon Generation Co, Exelon Nuclear |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| RS-04-055 | |
| Download: ML041070392 (6) | |
Text
Exelknm, Exelon Generation www.exeloncorp.com Nuclear 4300 Winfield Road Warrenville, IL 60555 RS-04-055 April 9, 2004 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Dresden Nuclear Power Station, Units 2 and 3 Facility Operating License Nos. DPR-19 and DPR-25 NRC Docket Nos. 50-237 and 50-249 Quad Cities Nuclear Power Station, Units 1 and 2 Facility Operating License Nos. DPR-29 and DPR-30 NRC Docket Nos. 50-254 and 50-265
Subject:
Additional Information Regarding Request for Extended Power Uprate NRC Safety Evaluation
References:
(1)
Letter from R. M. Krich (Exelon Generation Company, LLC) to U. S. NRC, "Request for License Amendment for Power Uprate Operation," dated December 27, 2000 (2)
Letter from U. S. NRC to 0. D. Kingsley (Exelon Generation Company, LLC), "Dresden Nuclear Power Station, Units 2 and 3 - Issuance of Amendments for Extended Power Uprate," dated December 21, 2001 (3)
Letter from U. S. NRC to 0. D. Kingsley (Exelon Generation Company, LLC), "Quad Cities Nuclear Power Station, Units 1 and 2 - Issuance of Amendments for Extended Power Uprate," dated December 21, 2001 (4)
Letter from Keith R. Jury (Exelon Generation Company, LLC) to U. S.
NRC, "Commitments for Resolution of Steam Dryer Degradation Issue,"
dated June 27, 2003 (5)
Letter form Keith R. Jury (Exelon Generation Company, LLC) to U. S.
NRC, "Schedule for Completing Committed Actions Regarding Resolution of Steam Dryer Degradation Issue, dated July 10, 2003 (6)
Letter from Patrick R. Simpson (Exelon Generation Company, LLC) to U. S. NRC, "Update to Information Supporting the License Amendment Request to Permit Uprated Power Operation at Dresden Nuclear Power Station and Quad Cities Nuclear Power Station," dated August 15, 2003 In the Reference 1, Exelon Generation Company, LLC (EGC) requested a change to the Facility Operating Licenses and Technical Specifications (TS) for Dresden Nuclear Power Station (DNPS), Units 2 and 3, and Quad Cities Nuclear Power Station (QCNPS), Units 1 and 2, to
April 9, 2004 U. S. Nuclear Regulatory Commission Page 2 operate at an uprated power level of 2957 megawatts thermal (MWt). In Reference 2, the NRC approved Amendment Nos. 191 and 185 to the Facility Operating Licenses for DNPS, Units 2 and 3, respectively, authorizing Extended Power Uprate (EPU) operations. In Reference 3, the NRC approved Amendment Nos. 202 and 198 to the Facility Operating Licenses for QCNPS, Units 1 and 2, respectively, permitting EPU operations.
In References 4 and 5, EGC committed to evaluate degradation of the QCNPS Unit 2 steam dryer and, if necessary, to submit supplemental information to the NRC regarding previous licensing correspondence in support of EPU for QCNPS, Units 1 and 2, and DNPS, Units 2 and
- 3. EGC provided the supplemental information in Reference 6.
On October 15, 2003, the NRC requested additional information to support a revision of the EPU Safety Evaluation. The attachment to this letter provides the requested information.
Should you have any questions concerning this letter, please contact Mr. Thomas G. Roddey at (630) 657-2811.
Rectfullp Patrick R. Simpson Manager - Licensing
Attachment:
Additional Information Regarding NRC Safety Evaluation Revision for Dresden and Quad Cities Extended Power Uprate cc:
Regional Administrator-NRC Region IlIl NRC Senior Resident Inspector-Dresden Nuclear Power Station NRC Senior Resident Inspector - Quad Cities Nuclear Power Station Illinois Emergency Management Agency - Division of Nuclear Safety
ATTACHMENT Additional Information Regarding NRC Safety Evaluation Revision for Dresden and Quad Cities Extended Power Uprate NRC RAI I Based on your design basis calculation using the ISCOR computer code, the pressure drop estimate across the dryer was found to be underestimated when a revised analysis was performed using the CFD computer code. Your modification of the dryer at QCNPS 2 after the 2003 event appears to resolve the issue of overstress in the outer hood plates.
Since the design of the dryers in the other three units of Dresden and Quad Cities was based on the ISCOR computer code, which was determined to be unconservative, discuss your intended action to address the strong likelihood of overstress in the dryers in these three units.
Response
The steam dryers for Dresden Nuclear Power Station (DNPS) and Quad Cities Nuclear Power Station (QCNPS) were designed to ensure that loose parts were not generated under the pressure loading conditions for the main steam line break outside containment faulted condition.
The faulted condition loading bounds the steady state flow pressure drop loads during normal operation. The original dryer design calculations did not address the oscillating pressure loads that occur during normal operation. Oscillating pressure loads are believed to have caused the fatigue failures observed at QCNPS, Units 1 and 2, and the fatigue cracking observed at DNPS, Unit 2.
The ISCOR code does not calculate the pressure drop across the steam dryer. The ISCOR code is used to calculate the reactor heat balance conditions. The resulting steam flow rate from the core is then used to calculate the pressure drop through the dryer vane banks. The steam flow calculated by ISCOR is used in the dryer vane bank pressure drop calculation, whether the calculation is performed using empirical correlation methodology (as was done for Extended Power Uprate (EPU)) or as input to the dryer Computer Fluid Design (CFD) model.
Both the empirical correlation method and the CFD model calculate approximately the same pressure drop for the flow through the dryer vane banks. The CFD model also calculates the additional pressure drop across the outer hood panels caused by the fluid velocity and acceleration from the dome region to the Yessel steam nozzle.
The CFD model and the empirical correlation method do not predict the oscillating pressure loads on the dryer. These approaches calculate the steady state flow pressure drop loads, which are constant (not oscillating) and do not induce fatigue failure in the dryer components.
The additional pressure drop across the outer hoods was also included in the dryer design assessment calculations for BWR/4-6 plants. However, this calculation was not retrofit to the BWR/3 vintage dryer designs because, as described above, the design basis for the BWR/3 steam dryers was to maintain structural integrity for the faulted condition. As stated in the response to NRC RAI 2 below, the LAMB code used for calculating the design basis faulted pressure differences does include the pressure drop associated with the flow across the outer hood. Therefore, the EPU dryer evaluations for the main steam line break faulted condition are still acceptable for BWR/3 plants.
As a result of the QCNPS Unit 2 dryer degradation found in 2003, Exelon Generation Company, LLC (EGC) made a commitment, in a letter dated June 27, 2003, to monitor key reactor and plant parameters at DNPS and QCNPS to provide early indication of potential dryer structural integrity issues. These actions included the following initiatives.
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ATTACHMENT Additional Information Regarding NRC Safety Evaluation Revision for Dresden and Quad Cities Extended Power Uprate
- 1. Daily monitoring of moisture carryover and other key reactor and plant parameters A monitoring program was implemented on QCNPS, Units 1 and 2, and DNPS, Units 2 and 3, to provide early indication of potential dryer damage or structural integrity problems. If indications of steam dryer damage or structural integrity concerns are identified, power will be reduced on the affected unit to pre-EPU levels and further corrective actions will be taken. As discussed with the NRC, elevated moisture carryover is considered a positive indication of dryer damage.
- 2. Additional inspections Additional inspections will be performed during the next refueling outage on all four units.
In addition, detailed finite element modeling of the steam dryer for each unit will be performed to determine the susceptible areas, both inside and outside of the dryer hood.
QCNPS Unit I used these methods to identify dryer degradation and failure. On October 26, 2003, QCNPS Unit 1 experienced main steam line (MSL) flow perturbations identified during dryer monitoring. Flow increased in MSL "D" by 0.5 million pounds mass per hour (Mlb/hr), with corresponding flow reductions in the other MSLs. On October 27, moisture carryover (MCO) was observed to increase to 0.068%. On October 31, MCO trend results exceeded the predetermined notification threshold of 0.1%. QCNPS increased the MCO sampling frequency to twice daily. With MCO continuing to trend upward for several days, on November 3, 2003, EGC reduced power on QCNPS Unit 1 to pre-EPU levels. On November 12, 2003, QCNPS Unit 1 shutdown to inspect and repair the dryer.
In addition to these measures, EGC reduced power on DNPS Unit 3 to pre-EPU levels on November 15, 2003, pending a detailed review of the QCNPS Unit 1 dryer failure and revision of the existing operability determination. On December 6, DNPS Unit 3 shutdown to inspect and repair the dryer. The previous operability determination concluded that a dryer failure does not adversely impact operation of any equipment important to safety during all normal and accident modes. Analysis of the QCNPS Unit 1 failure has not changed this finding.
The fatigue cracking found on the 2700 side of the QCNPS Unit 2 dryer during the June 2002 inspection is representative of the fatigue cracking that occurred in the other three units. In November 2003, fatigue cracking was observed in the outer hoods in the DNPS Unit 2 dryer following a full two-year operating cycle at EPU power. Operation with a failed lower cover plate contributed to the accelerated crack growth and subsequent damage observed in the second failure at QCNPS Unit 2. These lower cover plates have been replaced on all DNPS and QCNPS units.
EGC continues to address the potential for overstress in the steam dryers at DNPS and QCNPS by continuing to modify the steam dryers to reduce stresses, and utilizing increased parameter monitoring with trigger points. The Boiling Water Reactor Owners Group (BWROG) and General Electric (GE) representatives recently outlined several key industry actions to address the dryer failure issue generically, including creation of a steam dryer committee to develop an action plan to address dryer failures. General Electric Company (GE) Service Information Letter Page 2 of 4
ATTACHMENT Additional Information Regarding NRC Safety Evaluation Revision for Dresden and Quad Cities Extended Power Uprate (SIL) No. 644, Supplement 1, "BWR steam dryer integrity," dated September 5, 2003, provides recommendations for monitoring during operation and short-term inspection guidance. EGC implemented the recommendations of GE SIL No. 644, Supplement 1.
During a Commission briefing on October 15, 2003, the NRC's review of steam dryer cracking determined that loose parts resulting from dryer degradation and failure pose no immediate safety concerns and are not expected to affect operability of safety-related systems, structures, and components. In addition, NRC Information Notice 2002-26, "Additional Flow-Induced Vibration Failures After a Recent Power Uprate," Supplement 2, dated January 9, 2004, states that it is very unlikely that loose parts would adversely affect the safe shutdown of a plant, though it is important to understand the extent of damage that might be caused by steam dryer failures.
NRC RAI 2 Explain your rationale for not evaluating the adequacy of the dryer design when subjected to main steam line break faulted condition considering that the original design was based on results from the ISCOR computer code analysis, which was determined to be unconservative as comparing to the CFD analysis.
Response
The LAMB code is used to calculate the pressure difference across the dryer hood for the main steam line break faulted condition for EPU dryer structural evaluations. The LAMB code does include the pressure drop associated with the flow across the outer hood that is similar to the CFD model. Therefore, the EPU dryer evaluations for the main steam line break faulted condition are still acceptable.
NRC RAI 3 Justify the adequacy of the design of the dryers in the other three units of Dresden and Quad Cities considering that they were not modified as was Quad Cities 2.
Response
EGC is continuing to refine dryer modifications on QCNPS, Units 1 and 2, and DNPS, Units 2 and 3. The modifications to date on QCNPS Unit 2 include installation of a thicker cover plate, replacing half-inch outer hood plates with one-inch plates, and removing internal brackets that attached the internal braces to the outer hood. In addition, the modifications removed diagonal internal bracing, added gussets at the outer vertical hood plate and cover plate junction, and added stiffeners to the vertical and horizontal welds on the outer hood. Further, the gussets on QCNPS Unit 2 and DNPS Unit 3 have been extended vertically onto the one-inch plate to reduce localized stresses at the tips of the gussets. With the exception of the vertical and horizontal weld stiffeners and the extended gussets, identical modifications were made on DNPS Unit 2 and QCNPS Unit 1. The modifications increase the natural frequency of the outer Page 3 of 4
ATTACHMENT Additional Information Regarding NRC Safety Evaluation Revision for Dresden and Quad Cities Extended Power Uprate hood, reduce the maximum stress by at least a factor of two, and reduce the pressure loading by reducing the magnitude of steam flow vortices near the main steam lines. The latest repairs to QCNPS Unit 2 and DNPS Unit 3 included stop drilling measures at drain channels and support bracket channels to reduce stress intensity factors in these areas.
DNPS, Units 2 and 3, and QCNPS Unit 1 experienced dryer degradation in a manner similar to that found on QCNPS Unit 2 in 2003. EGC performed inspections and repairs on the DNPS dryers. Inspection results show that dryer cracking indications are consistent with GE modeling and predictions. Unrelated to the QCNPS Unit 2 dryer failure mechanisms identified in 2003, inspections identified similar cracking and indications on tie bars, vertical struts, vertical seam welds, and dryer drain channels on both DNPS units. The inspections also identified similar cracked diagonal internal bracing, though less extensive than that found on QCNPS Unit 2.
DNPS, Units 2 and 3, and QCNPS Unit 1 inspections confirm the results of GE modeling performed prior to their last respective outages. During a meeting on July 25, 2003, EGC presented these results to the NRC and discussed the potential for future dryer failures.
During inspections performed on the QCNPS Unit 2 steam dryer during the Spring 2004 refueling outage, EGC found additional dryer indications attributed to operation at EPU conditions. Based on these indications and subsequent discussions with the NRC regarding dryer degradation, EGC submitted a letter dated April 2, 2004, which contained our commitments relative to future actions to address, in part, the steam dryer issue related to DNPS and QCNPS.
EGC continues to perform analyses of the QCNPS and DNPS steam dryers in an effort to better understand the mechanisms contributing to dryer fatigue failures at DNPS and QCNPS. Design modifications are aimed at further reductions in localized stress intensifiers, and to support operation of each unit at EPU power levels throughout a full two-year operating cycle.
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