Resolution of Generic Safety Issues: Item C-11: Assessment of Failure and Reliability of Pumps and Valves (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )
The operating experience of nuclear power plants indicates that a number of valves, valve operators, and pumps fail to operate as specified in the technical specifications either under testing conditions or when they are called upon to perform. Most of these occurrences relate to valve leakage, valve actuation, and safety/relief valve operation outside their operational bounds. The main steam isolation, safety, and solenoid valves caused the most frequent abnormal occurrences in safety-related systems. Valve malfunctions can cause forced outages of operating plants. It is noted that about 10% of all outage time can be attributed to the malfunction of the critical pumps and valves within the plant. Of primary interest are outages caused by the main steam isolation and safety/relief valves.
The principal activity under this NUREG-04713 task will be the evaluation of active pumps and valves with respect to their operability and reliability under accident loading, i.e., LOCA and SSE, and implement a corrective action program specifically directed toward improved design and fabrication of active pumps and valves.
Unreliability of active valves and pumps in nuclear plant safety systems contributes to the risk associated with postulated core-melt accident sequences.
Resolution of this issue will serve to identify active pumps and valves that need redesign and replacement. Other issues related to this issue and whose results will supplement the equipment identification and redesign process of this issue are as follows: Issue 23, "Reactor Coolant Pump Seal Failures"; Issue 54, "Valve Operator-Related Events Occurring During 1978, 1979, and 1980"; Item B-55, "Improved Reliability of Target Rock Safety-Relief Valves"; and Item II.E.6.1, "In-Situ Testing of Valves - Test Adequacy Study." The reduction in public risk will result from a decreased probability of valve and pump failure.
Information provided by PNL64 on potential risk reduction and costs was used to determine the priority of this issue.
Oconee 3 and Grand Gulf are selected as the base case PWR and BWR, respectively, with estimated base case core-melt frequencies of 8.2 x 10-5/RY and 3.7 x 10-5 /RY, respectively. It is assumed that this issue affects accident sequences and base case frequencies as follows: (1) In PWRs, all the core-melt accident sequences except three directly involve active pumps and valves and, thus, are assumed to be affected; (2) Interfacing system LOCA and loss of AC power sequences are not directly affected; and (3) In BWRs, all accident sequences involve active pumps and are thus assumed to be affected.
NUREG/CR-0848102 summarizes the LERs relating to valve failures filed during the period 1965-1978. The tabular data provided was used to estimate the reduction in number of reports due to resolution of this issue. It was assumed that administrative, installation, maintenance, and operator error would not be affected (i.e., not directly applicable to failure due to hardware malfunction) and that, due to issue resolution, design and fabrication problems resulting in valve failures would be reduced. By decreasing the number of valve failures due to design error and fabrication error by 25%, fatigue failure (assumed to be a direct result of design error) by 25%, and all inherent causes by 10%, the total number of projected reports would be reduced by 9% in BWRs and PWRs. Therefore, it was assumed that the overall probability of failure of valves for both PWRs and BWRs due to issue resolution was reduced by 9%. This assumption was also applied to pumps.
Affected parameters include all elements of dominant minimal cut sets relating to active pumps and valves. The assumed valve and pump reduced failure probability was used to calculate the reduction in core-melt frequencies for affected accident sequences. The reduced frequencies are estimated to be 10-5/RY for PWRs and 4 x 10-6/RY for BWRs.
Assuming typical midwest site meteorology, a uniform population density of 340 persons per square mile within a 50-mile radius, and the average releases from all of the WASH-140016 release categories, the risk reduction is estimated to be 19 man-rem/RY and 22 man-rem/RY for PWRs and BWRs, respectively.
This analysis assumes resolution of this issue will affect 134 plants including backfit (operating) and forward-fit PWRs and BWRs. It also assumes that it will require at least 5 years to redesign, fabricate, and install improved-design active valves and pumps. By that time, it is estimated that the affected plants and their average remaining life will be as follows:
|Plants||Remaining Lifetime (yrs)||Plant-Years (RY)|
The public risk reduction for all plants for their remaining life is:
(19 man-rem/RY)(2,270 RY) + (22 man-rem/RY)(1,040 RY)
= [(4.3 x 104) + (2.3 x 104)] man-rem
= 6.6 x 104 man-rem
Assuming 19,900 man-rem is required for accident cleanup and using the above estimated accident frequency reduction, the estimated ORE reduction would be about 500 man-rem.
For backfit plants, implementation would mean replacement of valves and pumps designated as inadequate through resolution of the study portion of this issue. For forward-fit plants, implementation is essentially eliminated because replacement valves are introduced during design and construction phases. It was assumed
that 20 valves and 10 pumps per plant might be redesigned and replaced and that the replacement work would require 40 man-hours/valve and 80 man-hours/pump in areas averaging 0.05R/hr. For the 123 backfit plants, this would total about 9,800 man-rem of exposure. Since the redesigned valves and pumps would have lower failure rates, the time interval between repairs or replacement of this equipment due to subsequent failure would increase over the remaining plant life. This effect would reduce the accumulated labor as well as the occupational dose for subsequent repair or replacement of this equipment. This ORE reduction has not been quantified in this analysis because of the large uncertainty in identifying the types of valves and pumps to be replaced and each type of valve or pump may have different failure rates. This effect, however, would partially offset the estimated 9,800 man-rem for initial equipment replacement indicated above.
Industry Cost: Assume that each plant requires 2 man-years (88 man-wks) of engineering for pump/valve evaluation and replacement planning plus the equipment replacement labor assumed above. At $2,000/man-wk, the labor cost is estimated to be:
Further, assuming an average valve cost of $30,000 and an average pump cost of $500,000, the cost of replacement equipment per plant (20 valves and 10 pumps) would be $5.6M. The estimated implementation costs for the 123 backfit plants are $720M. However, this $720M implementation cost would be partially offset by the savings in labor costs over the remaining plant life associated with the subsequent repair or replacement of the redesigned equipment as discussed above.
NRC Cost: Assume NRC will expend 3 man-years at $100,000/man-year plus $2M in technical assistance funds to generically assess pump and valve failures related to design and fabrication deficiencies and relate them to accident sequences to recommend equipment which warrants redesign and replacement. Also, assume 4 man-weeks per backfit plant to monitor replacement activities at $2,000/man-wk. The NRC costs are estimated to be $3M.
Based on a risk reduction of 6.6 x 104 man-rem, the value/impact score is given by:
An additional consideration is that plant damage is estimated to be $1.65 Billion
per plant for a core-melt. Using the estimated accident frequency reduction, the averted plant damage could be as follows:
PWR: ($1,650M)(1 x 10-5/RY)(2,270 RY) = $37M
BWR: ($1,650M)(4 x 10-6/RY)(1,040 RY) = $7M
The assumption that valve and pump failure rates due to design and fabrication causes can be reduced 25% is highly judgmental. Less improvement in failure rates would directly decrease accident frequency reduction.
Taking longer than the 5 years assumed to install improved valves and pumps would decrease public risk reduction and increase the number of backfit plants, thus raising industry replacement costs.
If more than 20 valves and 10 pumps have to be replaced to achieve the estimated public risk reduction, the equipment replacement costs and occupational exposure would increase accordingly. Conversely, if less than 20 valve or 10 pump replacements can achieve the same estimated public risk reduction, the costs and ORE would decrease.
There is a potential saving in labor costs and ORE associated with repair or replacement of improved design valves and pumps subsequent to their installation.
This saving is presently an uncertainty that could be estimated after the types and number of valves and pumps to be initially replaced are identified.
As stated above, about 10% of all plant outage time can be attributed to the malfunction of critical pumps and valves. There is a potential cost saving due to reduced outage time as a result of reduced equipment failure rates. However, as estimated above, reducing failures attributable to design and fabrication by 25% would reduce overall valve and pump failure probability by about 10%. Also, there is very little firm data showing how much forced plant outage is directly attributable to design and fabrication failure of valves and pumps in safety systems compared to systems necessary for normal power operation. Identification of the valves and pumps which warrant replacement would permit a reasonable estimate of this saving to be made. The 9,800 man-rem ORE estimated to initially install the improved design valves and pumps partially offsets the estimated public risk reduction of 66,000 man-rem resulting in a net radiation exposure reduction of about 55,000 man-rem.
Based on the value/impact score, the potential for reducing net radiation exposure by about 55,000 man-rem, and the consideration of the other uncertainties described above, this issue received a medium priority ranking.
In pursuing a resolution to this issue, NRR recognized the existence of the Nuclear Plant Aging Research (NPAR) program that was being conducted by RES. Systematic studies under this program were to be performed to: (1) identify aging and service wear effects associated with mechanical components that could impair plant safety, and (2) identify techniques that will be effective in determining aging and service wear effects, prior to loss of safety function, so that proper maintenance and timely repair or replacement can be implemented. Although the NPAR program is intended to encompass many component types, it is envisioned to include various safety-related valve types and pump components. RES intends to evaluate LWR operating experience and identify aging trends. One of the specific benefits cited is improved reliability and availability.
NRR evaluated the NPAR program and concluded that it generally encompasses the scope of the programs inferred by Items B-58 and C-11. When this program is completed, it is expected that recommendations will be made by RES for maintenance, repair, or replacement according to component type. At that time, these recommendations will be grouped into manageable tasks and considered by NRR for possible changes to regulatory requirements. Thus, this issue was RESOLVED and no new requirements were established.863