ML101660585
| ML101660585 | |
| Person / Time | |
|---|---|
| Issue date: | 04/30/2011 |
| From: | Office of Nuclear Regulatory Research |
| To: | |
| R. A. Jervey, RES/DE/RGDB, 301-251-7404 | |
| Shared Package | |
| ML101650671 | List: |
| References | |
| DG-1217 RG-1.115, Rev 2 | |
| Download: ML101660585 (6) | |
Text
NRC Responses to Comments Comments on DG-1217 (Draft RG 1.115 Rev. 2) ADAMS document ML092250316 Page 1 Comments were received from four sources:
Comment numbers Organization Date Address Representative 1-8 GE Hitachi Nuclear Energy 1/12/10 1299 Pennsylvania Ave., NW Washington, DC Patricia L. Campbell Comment and response are tabulated below Comment Section/Page Text Comment NRC Response 1
General Comment on DG-1217 All Recommended Changes to the Related NUREG-0800, Standard Review Plan.
GEH recommends that related NRC guidance in NUREG-0800, "Standard Review Plan," also be revised to conform to the final revisions for RG 1.115 by addressing:
reductions in the frequencies of turbine missiles for large forging integral rotors verses assembled rotors electronic speed control designs and other types of redundant and diverse protection systems less frequent isolation valve testing based upon actual failure rates and accounting for human interactions with testing programs acceptance of remote indication of valve stroking for purposes of maintaining occupational exposure No Change The NRC considers these design features too specific to be addressed in DG-1217. However, they may be incorporated when revising NUREG-0800, Section 10.2.3, Turbine Rotor Integrity.
2 Section C Regulatory Positions 2 and 3 Position 2 For favorably oriented turbinesthe NRC considers the protection of essential Consideration of Pathways for High Trajectory Missiles The NRC should consider including additional guidance regarding potential turbine missile pathways for high-trajectory missiles. Otherwise, Change made to clarify regulatory position.
The staff accepts the approach of using barriers and using P1, P2, and P3. By
NRC Responses to Comments Comments on DG-1217 (Draft RG 1.115 Rev. 2) ADAMS document ML092250316 Page 2 Comment Section/Page Text Comment NRC Response Page 7 structures, systems, and components against high-trajectory missiles acceptable if either the frequency of high-trajectory missiles is less than 1x10-4 per year or appropriate barriers provide protection against high-trajectory missiles, consistent with Regulatory Position 5 below.
Position 3 For unfavorably oriented turbinesThe evaluation for high-trajectory turbine missiles is not needed because the turbine missile generation frequency for low-trajectory missiles is bounding.
an analysis could result in identification of overly conservative, unrealistic protection features, such as shielding roofs of reactor buildings, control buildings, fuel buildings, independent spent fuel storage buildings or casks, for extremely low probability scenarios. An alternative is to include guidance that would indicate acceptance of a P4 cut off value of 10-7 where further evaluation is not required for either low-trajectory or high-trajectory turbine missiles.
definition, high-trajectory missile pathway will be factored in the P2 calculation. Unfortunately, this position which is listed in Table 1 of the DG was not included along with other regulatory positions in Section C of the DG. Staff added new Regulatory Position 5 for clarity: For unfavorably oriented plants, the NRC will consider approaches considering P2 and P3 for both high and low trajectory missile analyses. This approach, which applies to a site with single or multiple units, is acceptable if P1 is less than 1x10-4 per year and P4 is less than 1x10-7 per year.
P1 of less than 1x10-4 per year is more generous than the current criterion for high-trajectory missiles and is kept to ensure that the calculated turbine failure rate wont exceed the historical turbine failure rate.
3 Section C Regulatory Position Page 7 Position 4 In addition to material and fracture toughness properties, the P1 calculation should consider initial crack depth, crack branching effects, and crack growth caused by degradation mechanisms of comparable significance such as high-cycle fatigue, low-cycle fatigue, and stress-corrosion cracking. Furthermore, the calculated P1 values for normal (or rated) speed, design overspeed, and destructive Consideration of Robust Rotor Designs Revised guidance should allow credit for improved mono-block or welded disk/rotor designs in evaluating the probability of turbine missiles. In addition, analyses by turbine manufacturers conclude that no turbine blades/disk parts would exit turbine casings. Guidance should be included as to how an applicant might credit robust designs (with associated confirming analysis) to be exempt from regulatory guidance for protection of essential equipment from turbine missiles.
No Change - As described in Regulatory Position 4, the P1 calculation should consider material and fracture toughness properties, initial crack depth, crack branching effects, and crack growth caused by degradation mechanisms of comparable significance such as high-cycle fatigue, low-cycle fatigue, and stress-corrosion cracking.
A different rotor design will use a set of different values for the input parameters discussed above or use a modified set of input parameters to reflect its design features. Hence, credit is taken automatically in the turbine missile analysis for a rotor of a robust design.
NRC Responses to Comments Comments on DG-1217 (Draft RG 1.115 Rev. 2) ADAMS document ML092250316 Page 3 Comment Section/Page Text Comment NRC Response overspeed failures should be considered. Selection of these parameters for the turbine missile analysis depends on the turbine rotor construction and should be justified and documented properly.
Consideration for very robust designs will be undertaken on a case by case basis, However, sufficient time to accumulate operating experience for any new rotor design would be helpful to support the analysis results.
4 General Comment on DG-1217 All Regulatory Process for Approving New Rotor Designs The NRC should consider addressing in the guidance credit for improved designs in new nuclear power plants. For example, credit for new designs could be a two-step process: (1) a bounding analyses are used for licensing the new design, and (2) a confirmatory report is generated at the conclusion of the manufacturing program to provide assurance the materials of construction, inspections and nondestructive examinations demonstrate conformance with the assumed bounding values in the original analyses.
No Change - This can be handled through existing regulatory framework and processes. The bounding, or generic, analyses for the new design can be submitted as a topical report, and the confirmatory report can be submitted on a plant-specific basis.
5 Section C Regulatory Position Page 7 Position 5 (Position 6 in the revision of this DG)
When barriers are used to protect essential systems, dimensioned plan and elevation layout drawings should include information on wall or slab thicknesses and materials of pertinent structures. The protection is considered acceptable if no missile can compromise the final barrier protecting an essential system.
Steel barriers should be thick enough to prevent perforation.
BWR Turbine Radiation Shielding Enclosures as Barriers The NRC should consider including guidance that would allow credit for certain BWR design features that are inherent barriers in potential pathways of ejected turbine missiles. The specific design features are the turbine radiation shielding enclosures. These sky-shine shields and radiation shield walls provide mitigating barriers against the remote possibility of an ejected blade penetrating through the turbine casing. The guidance could address how to credit these inherent BWR design features in the analysis of high-trajectory turbine missiles and most low-trajectory potential turbine missiles (excluding those low-trajectory missiles No Change - As discussed in the staffs response to Comment 2, DG-1217 now accepts the approach of using barriers and using P1, P2, and P3. The BWR turbine radiation shielding enclosure can be considered in the P2 and P3 analyses, barrier analyses, or their combination.
DG-1217 is not designed to be prescriptive. Therefore, the NRC will consider rigorous analyses with adequate assumptions in addressing additional barriers. DG-1217 is not the appropriate place to address how to consider a specific design feature in the analysis of turbine missiles.
NRC Responses to Comments Comments on DG-1217 (Draft RG 1.115 Rev. 2) ADAMS document ML092250316 Page 4 Comment Section/Page Text Comment NRC Response Concrete barriers should be thick enough to prevent backface scabbing.
assumed to be ejected inside the shielding enclosures).
6 Section B Discussion Page 3 The paragraph preceding the title Scope of Essential Systems In 1995 (Ref. 4), the NRC evaluated the incidence of turbine overspeed protection system failure events in the nuclear industry before that year. Currently, the turbine overspeed failure (160 percent of the turbine rated speed) that occurred at the Salem Generating Station, Unit 2 (Salem 2), in 1991 remains the only destructive turbine overspeed failure among U.S.
nuclear plants. Reference 4 provides a point estimate of 1x10-3 per turbine year for the turbine overspeed failure rate based on the single event that occurred at Salem 2 and on approximately 1,000 years of accumulated nuclear turbine operation as of 1995. Although the Salem 2 event did not produce the large turbine disk or rotor missiles that could have damaged safety-related systems, the study of the Salem 2 event and other overspeed events described in Reference 4 exposed Changing the Probability for Low-Trajectory Turbine Missiles GEH suggests that the NRC should consider whether industry data since 1986 supports changing the probability for low-trajectory turbine missiles of unfavorable orientations.
No Change -The quoted text in Column 3 of this Table gives the primary reason behind NRCs decision of not changing the probability for low-trajectory turbine missiles of unfavorable orientations after considering industry data since 1986.
However, the NRC agrees with the comment in principle and will consider new industry data presented in topical reports and plant-specific reports regarding the turbine missile issue, especially for turbines of unconventional design features.
NRC Responses to Comments Comments on DG-1217 (Draft RG 1.115 Rev. 2) ADAMS document ML092250316 Page 5 Comment Section/Page Text Comment NRC Response deficiencies in testing, valve maintenance, control system fluid quality, and human factors.
Operating experience since that time demonstrates that the industrys corrective actions addressing the findings described in Reference 4 are effective in reducing the potential of such failures.
Therefore, the NRC considers the overall turbine failure rate of 1x10-4 per turbine year still valid.
7 Section B Discussion Page 6 The paragraph preceding the subtitle, Protection Provided by Missile Barriers.
A recent industry report (Ref.
- 8) proposed the management of turbine missiles by focusing on their contribution to core damage frequency in lieu of their probability of damaging essential systems. Adoption of this approach would require a revision of both General Design Criterion 4 and the NRCs underlying regulatory philosophy on missile protection. Without a significantly improved turbine missile operating record and a comprehensive study of the approach based on core damage frequency versus the conventional approach, the staff is not prepared to endorse an Risk-Informed Approaches Alternatively, the guidance could address how risk-informed approaches could be used in an evaluation. While there may be insufficient turbine operation time to employ a full risk-informed evaluation on the turbine missile generation, there should be an acceptable approach for event or consequence evaluations using risk-informed methods.
No change - At the beginning of the effort for DG-1217, the NRC considered performing a comprehensive study of the risk-informed approach versus the conventional approach on the turbine missile issue to prepare for a possible fundamental change of the regulation regarding turbine missiles. This approach would take a long time and require significant resources in technical and regulatory areas because a revision of General Design Criterion 4 may be needed. After considering the above and the fact that RG 1.115 is no longer self-contained and needs a fix, the NRC decided to revise RG 1.115 not using the risk-informed approach but following the traditional approach to make the revision self-contained. This is not a purely technical decision. However, to account for the progress made in the past 20 years in analyses beyond P1, DG-1217 no longer emphasizes the P1 approach. Approaches beyond P1 consideration will be accepted for
NRC Responses to Comments Comments on DG-1217 (Draft RG 1.115 Rev. 2) ADAMS document ML092250316 Page 6 Comment Section/Page Text Comment NRC Response approach that would permit licensees to rely solely on the final defense (i.e., prevention of core damage).
review.
8 Section C Regulatory Position Page 7 Position 6 (Position 7 in the revision of this DG)
The strike zones associated with the turbines of all present and future nuclear and nonnuclear units at the site should be considered. The NRC will review turbine designs that are significantly different from the current 1,800-rpm machines on a case-by-case basis to determine the applicability of the strike zone.
Sites with Multiple Units For example, the very small chance a turbine missile could strike an essential SSC, divided by the chance it could actually damage an SSC, divided by the number of redundant equipment available could produce a statistically acceptable result for the probability of a turbine missile. This approach could be very useful for future evaluations of sites with multiple units. In addition, a bounding value of less than 10-7 should be considered low enough to negate the need for further evaluation of unit-to-unit interactions for sites with multiple units.
Partially Accepted - Again, DG-1217 is not designed to be prescriptive and will not rule out any fundamentally sound approach. However, Comment 8 helped the NRC to identify the need to add more guidance to Regulatory Position 6 (Position 7 in the revision of this DG) regarding sites with multiple units. For these multiple units, Regulatory Position 6 is revised to: For favorable orientation determination and the subsequent P1 and P2 calculations for both low-trajectory and high-trajectory missiles for a unit in a site of multiple units, the strike zones associated with the turbines of all present and future nuclear and nonnuclear units at the site should be considered. Likewise, all units essential systems should be considered. The NRC will review turbine designs that are significantly different from the current 1,800-rpm machines on a case-by-case basis to determine the acceptability of any proposed new approach, including the determination of the strike zones and essential systems.