RS-03-101, Additional Information Regarding Inservice Inspection Program Relief Request I4R-02
| ML031560623 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 05/29/2003 |
| From: | Simpson P Exelon Generation Co, Exelon Nuclear |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| RS-03-101 | |
| Download: ML031560623 (5) | |
Text
ExelenSM Exelon Generation www.exeloncorp.com Nuclear 4300 Winfield Road Warrenville, IL 60555 RS-03-1 01 May 29, 2003 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
Subject:
Additional Information Regarding Inservice Inspection Program Relief Request 14R-02
References:
(1)
Letter from R. J. Hovey (Exelon Generation Company, LLC) to U. S.
NRC, "Dresden Nuclear Power Station, Units 2 and 3, Fourth Interval Inservice Inspection Program Plan," dated September 6, 2002 (2)
Letter from U. S. NRC to J. L. Skolds (Exelon Generation Company, LLC), "Dresden Nuclear Power Station, Units 2 and 3 - Request for Additional Information Regarding Inservice Inspection Program Relief Request 14R-02," dated April 29, 2003 In Reference 1, Exelon Generation Company, LLC requested relief from certain requirements of 10 CFR 50.55a, "Codes and standards," for the fourth interval inservice inspection program plan for Dresden Nuclear Power Station, Units 2 and 3. In Reference 2, the NRC requested additional information regarding proposed relief request 14R-02. The attachment to this letter provides the requested information.
Should you have any questions concerning this letter, please contact Mr. Allan R. Haeger at (630) 657-2807.
Respectfully, Patrick R. Simpson Manager - Licensing Mid-West Regional Operating Group Attachment cc:
NRC Regional Administrator - Region Il NRC Senior Resident Inspector - Dresden Nuclear Power Station Office of Nuclear Facility Safety - Illinois Department of Nuclear Safety p< l)-Il
Dresden Nuclear Power Station (DNPS), Units 2 and 3 Additional Information Regarding Inservice Inspection Program Relief Request 14R-02 Question #1 Have any welds that were selected for inspection in the Risk-Informed Inservice Inspection (Rl-ISI) program that was approved by the staff in Reference 1 been removed from the population of welds that will be inspected during the fourth ten-year interval? If so, why was [were] the weld[s]
removed from the population of welds to be inspected.
Response
Changes in inspection populations are addressed in the response to Question #2 below.
Question #2 Have any welds that were not selected for inspection in the RI-ISI program that was approved by the staff in Reference 1 been selected for inspection during the fourth ten-year interval? If so, why was [were] the weld[s] added to the population of welds to be inspected.
Response
DNPS has made changes to the weld selections in the RI-ISI program that was approved by the NRC in Reference 1 for the following reasons.
Substitutions were made when an alternate weld would provide significantly better access or increase examination coverage. These substitutions were made within pipe segments which are subject to the same degradation mechanisms and consequences. Therefore, the number of welds within each risk category was not impacted.
A portion of the main steam drain line (1W2' nominal pipe size (NPS)) had been exempted from examination during the third inspection interval in accordance with American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section Xl, "Rules for Inservice Inspection of Nuclear Power Plant Components," Subparagraph IWB-1220(a).
During fourth interval program development, the flow make up calculation that is utilized to exempt components, based upon the capacity of make up systems in accordance with IWB-1220(a) was revised, and, as a result, the 1/2" NPS main steam drain line welds were incorporated into the RI-ISI program. The addition of these welds resulted in selection of additional high risk category welds for both Units 2 and 3.
An updated probabilistic risk assessment (PRA) model was incorporated into the RI-ISI program resulting in the recategorization of piping elements. Weld selections were adjusted, prorated by system, so that 25% of the welds in each high risk category and 10% of the welds in each medium risk category are selected for inspection during the interval.
The following table summarizes the changes in inspection populations (additions and removals) between the RI-ISI program that was approved by the NRC in Reference 1 and the population of welds that will be inspected during the fourth ten-year interval.
Page 1 of 4
Dresden Nuclear Power Station (DNPS), Units 2 and 3 Additional Information Regarding Inservice Inspection Program Relief Request 14R-02 DNPS Unit 2 Number of
,,Number of Risk Welds -
Welds -
Category Initial RI-ISI 4 th Interval Rl-Description of Weld Selection Changes Program ISI Program High 42 49
- Eight welds were substituted within the high risk category.
- Nine welds were added due to incorporation of the main steam drain line.
Medium 53 47
- Five welds were substituted within the medium risk category.
DNPS Unit 3 Number of Number of Risk Welds -
Welds -
Category Initial RI-ISI 4th Interval RI-Description of Weld Selection Changes Program ISI Program High 36 41 Ten welds were substituted within the high risk category.
- Eight welds were added due to incorporation of the main steam drain line.
Medium 58 50
- Three welds were substituted within the medium risk category.
Page 2 of 4
Dresden Nuclear Power Station (DNPS), Units 2 and 3 Additional Information Regarding Inservice Inspectiori Program Relief Request 14R-02 Question #3 The relief request includes the following paragraph:
"The Risk Impact Assessment completed as part of the original baseline RI-ISI Program was an implementationtransition check on the initial impact of converting from a traditional ASME Section Xl program to the new RI-ISI methodology. For the Fourth Interval ISI update, there is no transition occuring between two different methodologies, but rather, the currently approved RI-ISI methodology and evaluation will be maintained for the new interval. As such, the initial screening of the risk impact assessment is not a part of the living program process and is not required to be continually updated."
The staff does not concur with the implication that, if there is no change in methodology, the change in risk assessment in not part of the living process. RG 1.178, SRP 3.9.8, and the EPRI Topical report (Refs. 2, 3, and 4) require an evaluation of the change in isk arising from the, proposed change in the ISI program. Please provide a discussion on the potential change in risk between the RI-ISI program proposed for implementation in the fourth interval and the ASME Section XI requirements from which relief was granted in Reference 1. If inspections were discontinued or relocated between the third and the fourth intervals' RI-ISI programs, please provide an estimate of the change in risk.
Response
The DNPS response to NRC Questions I and 2 summarizes the changes in weld selections that would impact the risk assessment. An updated PRA model for the fourth ten-year interval, which has been incorporated into the RI-ISI program, has also impacted the risk assessment. The following table provides the results of the changes in core damage frequency (CDF) and large early release frequency (LERF) from the initial risk assessment for the RI-ISI program implemented in the third interval and the risk assessment for the RI-ISI program proposed for implementation in the fourth interval. These risk assessments evaluate the change in risk between the RI-ISI programs and the ASME Section Xl requirements from which relief was granted in Reference 1.
DNPS Unit 2 Initial RI-ISI Program 4th Interval RI-ISI Program A CDF/yr A LERF/yr A CDF/yr A LERF/yr 3.14E-09 7.57E-10 2.50E-09 2.23E-09 DNPS Unit 3 Initial RI-ISI Program 4th Interval RI-ISI Program A CDFlyr l
A LERFlyr A CDFlyr l
A LERF/yr 2.89E-09 1.32E-09 1.57E-09 1.42E-09 The change in risk between the RI-ISI program proposed for implementation in the fourth interval and the ASME Section XI requirements from which relief was granted in Reference 1 is well within the guidelines of Regulatory Guide 1.174, An Approach for Using Probabilistic Risk Page 3 of 4
Dresden Nuclear Power station (DNPS), Units 2 and 3 Additional Information Regarding Inservice Inspection Program Relief Request 14R-02 Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis," and Reference 4 (1.OE-06/yr and 1.OE-07yr for CDF and LERF, respectively). In addition, each system was found to meet the acceptance criteria for changes in CDF and LERF (1.OE-07/yr and 1.OE-08/yr for CDF and LERF respectively).
As shown by the comparison in the above table the change in risk between the third and the fourth intervals, which takes into account inspections which were discontinued or relocated as well as an updated PRA model, the RI-ISI program proposed for implementation in the fourth interval does not represent a significant change in risk.
DNPS will continue to maintain the delta risk assessment between the current RI-ISI fourth interval program and the ASME Section Xl requirements.from which relief was granted in Reference 1 consistent with the methodology described in Reference 4.
References
- 1. Letter from U. S. NRC to 0. D. Kingsley (Exelon Generation Company, LLC), "Safety Evaluation of Third Interval Risk-Informed Inservice Inspection Program Relief Request,"
dated September 5, 2001
- 2. U.S. Nuclear Regulatory Commission, "An Approach for Plant-Specific Risk-Informed Decision Making: Inservice Inspection of Piping," Regulatory Guide 1.178, September 1998
- 3. U.S. Nuclear Regulatory Commission, "Standard Review Plan for Trial Use for the Review of Risk-Informed Inservice Inspection of Piping," NUREG-0800, SRP Chapter 3.9.8, May 1998
- 4. Electric Power Research Institute, Revised Risk-informed Inservice Inspection Evaluation Procedure, EPRI TR-1 12657," Revision B-A, January 2000 Page 4 of 4