Resolution of Generic Safety Issues: Issue 152: Design Basis for Valves That Might Be Subjected to Significant Blowdown Loads (Rev. 3) ( NUREG-0933, Main Report with Supplements 1–35 )
In order to export or print, this web site requires you to enter text displayed in the distorted image on the left, which is known as Captcha, into the text field below the image.
DESCRIPTION
Historical Background
This issue was identified1416 by DSIR/RES following ACRS concerns raised during the review of the resolution of Issue 87, "Failure of HPCI Steam Line Without Isolation," which addressed the design bases for those MOVs that isolate the HPCI, RCIC, and RWCU systems in BWRs. These design bases required that the MOVs close against loads imposed by a double-ended pipe break at design basis flow conditions.
In resolving Issue 87, the staff issued Generic Letter No. 89-101217 which required licensees to identify safety-related valves that might not perform adequately under design basis conditions. However, the ACRS believed that the design basis for the HPCI steam line valves and other valves in some plants might not specify the type of heavy duty. Thus, it was possible that heavy duty loads might not be considered for these valves by licensees in response to Generic Letter No. 89-10.1217 The ACRS recommended that the staff amend the generic letter to require licensees to examine their design bases to determine if safety-related valves, including but not limited to MOVs, were capable of operating against blowdown loads that might not have been considered (by licensees) in their original designs.
Safety Significance
The inability of valves that might be subjected to significant blowdown loads to meet their design bases is a compliance concern. Therefore, the safety significance of this issue lies in the environmental conditions that could result from the inability of containment isolation valves to close under accident conditions. The resulting environmental conditions could cause the malfunction of equipment required to cool the reactor. This issue affects all operating and future plants.
Possible Solution
A possible solution to this issue would include the following: (1) amendment of Generic Letter No. 89-101217 to ensure complete compliance with the original design bases; (2) licensee review of design bases for compliance; (3) licensee analyses to assess operability of valves; and (4) hardware modification of isolation valves and additional licensee analyses to bring the valves into compliance with the original design bases.
PRIORITY DETERMINATION
Assumptions
It was assumed that 50% of all 112 operating plants will find that they are in compliance with the amended generic letter. Of the remaining 50% that will have to perform analyses, 80% will demonstrate compliance. Thus, only 10% of all operating plants will make hardware modifications and perform additional analyses to comply with the amended generic letter. Therefore, the potential exists for a reduction in public risk and occupational dose at approximately 11 plants: 7 PWRs and 4 BWRs. Future plants would not require any modifications since their design would be based on the requirements of the amended generic letter. Oconee-3 and Grand Gulf-1 were selected as the representative PWR and BWR, respectively.
Frequency Estimate
For PWRs, a steam line break was assumed to correspond to an S3 LOCA. If this LOCA is not isolated, the potential exists for introducing a harsh environment into the containment which may affect the operation of certain components needed to mitigate the LOCA. These components were assumed to be MOVs and pumps, specifically for failure modes designated as hardware or control circuitry, found in accident sequences initiated by an S3 LOCA.
It was assumed64 that the potential for increased failure under harsh environmental conditions was not factored into the failure probabilities of the affected parameters in the original plant evaluations. Therefore, the base case failure probabilities were assumed to be 10% higher than their original values. For Oconee 3, this resulted in a base case core-melt frequency of 1.18 x 10-6 /RY.64
For BWRs, a steam line break was assumed to correspond to an S LOCA. Assuming the same accident scenario and resultant effects described above for PWRs, the base case core-melt frequency for Grand Gulf 1 was estimated64 to be 2.48 x 10-7/RY.
It was assumed that resolution of the issue would return the failure probabilities to their original values in both PWRs and BWRs; this represented a 10% reduction in the base case values. Thus, the adjusted case core-melt frequencies were estimated to be 1.01 x 10-6/RY and 2.09 x 10-7 for Oconee-3 and Grand Gulf-1, respectively. The potential core-melt frequency reduction associated with the possible solution was calculated to be 1.7 x 10-7/RY and 3.9 x 10-8/RY for the affected PWRs and BWRs, respectively.
Consequence Estimate
The affected release categories for Oconee-3 were PWR-2,-3,-4,-5,-6, and -7, and the base case and adjusted case public risk were estimated to be 3.14 man-rem/RY and 2.68 man-rem/RY, respectively, with a potential reduction of 0.46 man-rem/RY. For the 7 affected PWRS with an average remaining life of 25.8 years, the public risk reduction was estimated to be (0.46)(7)(25.8) man-rem or 83 man-rem.
Affected release categories for Grand Gulf 1 were BWR-1 and -2 and the base case and adjusted case public risk were estimated to be 1.76 man-rem/RY and 1.48 man-rem/RY, respectively, with a potential reduction of 0.28 man-rem/RY. For the 4 affected BWRs with an average remaining life of 24.1 years, the estimated public risk reduction was (0.28)(4)(24.1) man-rem or 27 man-rem. Therefore, the total public risk reduction associated with the possible solution was estimated to be 110 man-rem.64
Cost Estimate
Industry Cost: The review of design bases was estimated to require 6 man-weeks/plant at all 112 operating plants affected by the amended generic letter. At $2,270/man-week, this cost was estimated to be $1.525M.
Additional analyses at 56 plants (50% of all affected plants) were estimated to require 12 man-weeks/plant for a total cost of $1.525M. Equipment costs were estimated to be $20,000/plant (10% of all affected plants) that will have to make valve modifications. These modifications were estimated to require 8 man-weeks of skilled labor and 16 man-weeks for additional engineering analyses. Thus, the total estimated cost for 11 plants that require modifications was $0.82M and the total industry cost associated with the possible solution was $3.87M.
NRC Cost: It was estimated that 8 man-weeks would be required to amend Generic Letter No. 89-101217 at a cost of $18,000. Review of licensee responses from all 112 plants was estimated to require 2 man-weeks/plant. Responding to the half of these plants that would have to submit analyses was estimated to require 6 man-weeks/plant. For the 11 plants that would have to be modified, NRC review of the additional analyses was estimated to require 12 man-weeks/plant. Thus, the total NRC review time was estimated to be 692 man-weeks. At $2,270 man-week, this translated to a cost of $1.57M.
Total Cost: The total industry and NRC cost associated with the possible solution was $(3.87 + 1.57)M or $5.44M.
Value/Impact Assessment
Based on a potential public risk reduction of 110 man-rem and an estimated cost of $5.44M for a possible solution, the value/impact score was given by:
S = 110 man-rem $5.44M
~ 20 man-rem/$M
CONCLUSION
Based on the potential public risk reduction, the issue was given a low priority ranking (see Appendix C) in January 1993. Additional concerns raised1509 by the ACRS on the ability of safety-related MOVs to close under pipe break conditions were addressed1510 by the staff but did not affect the priority ranking of the issue. Consideration of a 20-year license renewal period did not change the priority of the issue.1564 Further prioritization, using the conversion factor of $2,000/man-rem approved1689 by the Commission in September 1995, resulted in an impact/value ratio (R) of $50,000/man-rem which placed the issue in the DROP category.
Following a periodic review of LOW-priority issues, new information was provided1699 by NRR that required a reevaluation of the issue. However, consideration of this new information did not result in any change in the priority of the issue.1775
REFERENCES
|
