Resolution of Generic Safety Issues: Item B-50: Post-Operating Basis Earthquake Inspection (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–35 )
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DESCRIPTION
Historical Background
Appendix A of CFR 10 Part 100 specifies that the operating basis earthquake (OBE) shall be defined by response spectra and that the maximum vibratory ground acceleration of the OBE shall be at least one-half of the maximum vibratory ground acceleration of the safe shutdown earthquake (SSE). Suitable instrumentation is required at the reactor site, so that the seismic response of the nuclear power plant components that are important to safety can be determined promptly in order to permit a comparison of such response with that used as a design basis. Some detailed guidance on the nature and extent of this seismic instrumentation is provided in Regulatory Guide 1.12, Rev. 1, "Instrumentation for Earthquakes," April 1984. Shutdown of the nuclear plant is required in the event that vibratory ground motion exceeds that of the OBE. Prior to resuming operation, the licensee is required to demonstrate that no functional damage has occurred to those features necessary for continued operation without undue risk to the health and safety of the public. This item was originally identified in NUREG-0471.3
A demonstration that all of the required equipment is functional following an earthquake greater than the OBE might be done by analysis, inspection, and/or test in some appropriate combination. At present, there are no specific requirements for demonstrating the safety of the plant in the event of an earthquake that exceeds the OBE. This issue is intended to improve this matter by providing guidance on the nature of the analysis, inspection, and/or tests that would be required, thereby permitting for a more rapid restart of the plant.
Safety Significance
Even though it would be expected that a plan would tend to provide systematic guidance, there is no safety significance for this issue inasmuch as it is likely that the NRC requirements for a reactor inspection following an earthquake that exceeds the OBE would contain the same elements and cover the same plant details whether it was planned in advance or developed following the earthquake. The intention in this issue is to establish a methodical approach to the inspection that might be necessary based on a systematic inspection plan, which would not result in any change in the potential core-melt frequency but would enable the licensee to conduct a site inspection expeditiously.
Possible Solution
Establish a post-OBE inspection plan that is comprehensive and detailed enough so that the inspection can be completed in a thorough, yet expeditious manner.
PRIORITY DETERMINATION
The prioritization of this issue was formulated with the technical assistance of PNL.64
Frequency/Consequence Estimate
There is no reduction in core-melt frequency as a result of the implementation of the resolution of this issue.
Cost Estimate
An earthquake that exceeds the OBE has occurred at only one nuclear site - Humboldt Bay (1975). This earthquake occurred during a refueling outage and has been reported by the licensee, PG&E.770 For this plant, an emergency operating procedure covering earthquakes and tsunamis was already in place prior to the earthquake and covered power operation inspection procedures. The licensee decided to have an engineering inspection performed by Bechtel Power Corporation who had accumulated a large amount of detailed knowledge of the plant in connection with their ongoing seismic re-analysis of the plant. The Bechtel inspection was completed in 2 days.
For this analysis it is assumed that, without an inspection and test procedure in place before the earthquake (the base case), the inspection would require 2 weeks by the licensee using 10 of the licensee's personnel. Moreover, it is assumed that, with a comprehensive and systematic inspection procedure specified in advance (adjusted case), the inspection process could be completed more expeditiously and require only 4 days, with the same number of licensee personnel with no special seismic knowledge of the plant. On this basis the costs are estimated as follows.
(1) The incremental cost of the post-OBE inspection based on 10 workers on a 20 hrs/day shift is:
(10 workers/plant)(20 hrs/day)(14 - 4) days = 2,000 man-hrs/plant.
(2) The occupational dose increment associated with the improved inspection procedures, based on an assumed dose rate of 175 mr/hr (Reference: Virgil Summer PWR FSAR Amendment 7, August 1978, Table 12.2-22b) is calculated to be:
(-2,000 man-hrs/plant)(0.075 R/hr) = -150 man-rem/plant.
(3) Based on the reduced inspection time of 2,000 man-hrs/plant noted above, the labor cost is calculated to be:
(-2,000 man-hrs/plant)(40 man-hrs/man-wk)($2,270/man-wk) = -$114,000/plant.
(4) The cost of the plant's downtime, based on the improved inspection procedures, is estimated as follows:
(4 - 14) days ($300,000/day) = -$3M/plant (net savings).
(5) There are no operation and maintenance costs associated with this issue, nor are there any accident avoidance costs involved. The development of this inspection program is assumed to be borne by the industry, presumably through ANSI. The cost to develop these inspection procedures is assumed to require technical assistance for a total cost of approximately $100,000.
(6) The total future cost to the industry is, therefore, estimated to be:
(-$114,000) + (-$3M) = -$3.114M (net savings).
(7) It is expected that the cost to the NRC in the development of the new inspection procedure would be limited to a review of the final inspection procedure submitted by the industry. It is assumed that the NRC review would require 20 man-weeks for a total cost of approximately (20 man-weeks)($2,270/week) or $45,400.
(8) It is anticipated that the NRC staff would participate in some way at the plant site during an inspection of the facility following an OBE. It is assumed that this would require 3 full-time staff members for the 2 weeks of inspection as well as an additional 2 weeks after the completion of the inspection as a follow-up measure. It is expected that, with the improved inspection procedures, only 2 full-time NRC staff members would be required for the 4 days of the inspection and for the 2 weeks following the completion of inspection and the resumption of normal operations. The incremental cost to the NRC for the implementation of the improved procedures is estimated as follows:
($2,270/man-wk)[(112 man-hrs/man)(2 men/plant)
-(160 man-hrs/man)(3 men/plant)] = $581,000.
(9) The total future costs to the industry and the NRC for the development and implementation of the resolution of this issue (C) is estimated from the results above to be:
C = -$3,114,000 - $581,000 = -$3,695,000.
(10) The present value of the net future costs per plant (PW) is given by:
PW = (sp)(r)-1(e-rt1 - e-rt)
| where S | =$3,695,000 |
| P | =frequency of exceeding the OBE per year ti and tf are the initial and final times over which the savings are realized, with ti = O and tf = T = average plant life = 28 yrs |
| r | =the real discount rate = 5%/yr |
Based on these values the present value is calculated to be:
| PW | =($3.695M)(P)(0.05)-1(1 - e0.05t) |
| =($3.695M)(P)(15) | |
| =$55.4M(P) per plant |
The frequency of exceeding the OBE is site-dependent but, on the basis of general studies, this frequency is in the order of 10-3 to 10-2/year.771
These values represent an expected frequency range for exceedance of the OBE for a given plant. For this analysis, however, it is necessary to estimate the expected number of plants affected by any one OBE occurence. In view of the existence of multiple plant sites, for purposes of this analysis it will be assumed that a twin plant site will be affected with a frequency of OBE exceedance of Pc = 10-2/yr. The present value (PW) of the cost savings per site on this basis is estimated to be (2 plants)[($55.4M)(10-2/plant] = $1.11M.
It is to be noted that the actual number of sites that would be affected is uncertain. In the West, only one site would likely be affected by any one OBE whereas, in the East, more sites might be affected but the probability for the latter would be lower by a factor of 10. Therefore, the present worth will be assumed as the total industry cost.
CONCLUSION
It is estimated that the implementation of this issue requires a total combined cost of approximately $145,000 for the industry and NRC in order to formulate a plan and procedure for expeditious inspection of a plant site following exceedance of an OBE. The advanced planning of this inspection procedure is also estimated to provide net future savings to the NRC and the industry of approximately $3.7M at the time of the OBE, which is calculated to have a present worth of about $1.1M.
In view of these costs and benefits it would seem that this is a Regulatory Impact issue that should be completed because it appears to have merit. However, it is to be noted that a considerable amount of uncertainty exists in the results on which to base realistic estimates of inspection requirements, costs, and the time involved. Moreover, it is not entirely clear that a detailed inspection plan attempting to anticipate the effects of an OBE on a complicated facility such as a nuclear reactor site can be adequately and meaningfully established in advance to the degree that the formulated plan and procedure will result in the estimated reduction of inspection time. It is also noted that this issue primarily concerns economic benefits rather than safety and that there is no burden on the licensees concerning this matter at this time. In view of these considerations as well as the uncertainties noted above, it is concluded that this issue should be left to be developed by industry initiatives.
Therefore, the NRC should assign a low priority to this regulatory impact issue unless requested by the industry to endorse a proposal submitted to NRC for this purpose.
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