Resolution of Generic Safety Issues: Item A-18: Pipe Rupture Design Criteria (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–35 )
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DESCRIPTION
Historical Background
A major objective of this NUREG-03712 item was the development of consistent criteria for application in licensing processes. Additional research programs to implement licensing positions were to be conducted under separate issues. The problems specific to this issue were as follows:
(1) Existing design criteria for the postulation of pipe breaks and protection therefrom had been developed over a period of time and lacked consistency when applied inside and outside containment. Regulatory Guide 1.46,18 issued in 1973, which addressed pipe breaks inside containment, was based on the concept of a limited number of design basis breaks. Section 3.6 of the SRP,11 issued in 1975, which addressed pipe breaks outside containment, combined limited design basis breaks for mechanistic protection and unlimited breaks for non-mechanistic protection. At the time this issue was identified in 1978, staff efforts toward documentation of the rationale and engineering justification for existing pipe break criteria was ongoing. These efforts were expected to assist in focusing on areas requiring first attention and providing a valuable document for both public and staff use as bases for testimony before the ACRS and hearing boards. Work in this area was completed prior to 1983.
(2) An evaluation of the pipe break exclusion concept in the containment penetration area of both PWRs and BWRs was required. The need to specify the extent of break exclusion regions, criteria for the use of guard pipes, and adequacy of design requirements for piping systems in break exclusion regions were topics for which improved guidance were to be developed.
(3) The development of postulated pipe rupture criteria and the trend towards more conservative seismic criteria placed increased emphasis on piping system design to withstand these dynamic events. However, these criteria had also resulted in systems which were significantly more rigid. These more rigidly designed systems in the plants that were not in operating in 1978 had resulted in calculated stresses for normal operation which, although still within code limits, were significantly higher than in earlier plants. In addition, dynamic event devices, such as snubbers and pipe-whip restraints which had been added in increased numbers, had the potential for deleterious interaction with the piping system during its normal operation. It was believed that a balance in piping system design for both normal and abnormal situations should be achieved to ensure that consideration is given to the effects of abnormal situation design criteria on normal operation.
The evaluation of this issue included consideration of Item B-16. 2
Possible Solutions
At the time of the evaluation of this issue in 1983, a study of the effects of abnormal loading scenario design criteria on normal operation had been completed. Determining licensing positions and the consequences of implementing the results of this issue were not considered in this evaluation. Item B-62 more directly addressed: (1) the safety consequences of combining unusual dynamic events and normal plant operating conditions; and (2) the option of limiting the number of dynamic event devices.
The criteria used for designing and constructing containment penetrations were to be evaluated in this issue. Guidelines for limiting the extent of break exclusion areas, criteria for the use of guard pipes, and the adequacy of design requirements for piping systems in break exclusion areas were of concern. The consequences of implementing the resultant guidelines was expected to differ for various plant types and piping systems. It was assumed that the resolution would, in general, limit the number of break exclusion areas. It was further assumed that this limitation would affect only 60% of all forward-fit PWRs and BWRs.
Priority determination
Frequency/Consequence Estimate
The reduction in public risk was determined to be negligible (100 man-rem)64 and limiting the extent of break exclusion areas did not increase or decrease the probability of a pipe rupture.
Cost Estimate
Industry Cost: It was estimated that only 60% of all forward-fit plants (43 PWRs and 20 BWRs) would be affected by limitations on break exclusion areas. Thus, the total number of affected plants was 38. The average remaining life of these affected plants was (38)(30) RY or 1,140 RY. Labor included: (1) implementation of criteria for defining pipe break and crack locations and configurations; (2) implementation of criteria dealing with special features, such as augmented ISI or use of postulated event devices; and (3) the review of analysis results, including jet-thrust and impingement forcing functions, pipe-whip dynamic effects, and design adequacy of systems to ensure that function is not impaired as a result of pipe-whip or jet impingement loadings.
It was assumed that labor included the time required to analyze lines located outside the break exclusion regions and that analysis procedures, computer codes, applicable transient data, etc., were readily available. It was also assumed that only 50% of the 12 welds under investigation needed analysis (i.e., those excluded either already fell into an analyzed line or did not fall into a high energy/high stress area which required analysis). The total industry cost for implementing the possible solution was estimated to be $2.07M.64
Industry operation and maintenance costs associated with the solution would result in cost savings to the industry due to fewer ISI periods when weld design locations are shifted from a break exclusion area. Based on a labor decrease of 2.2 man-hours/RY at a cost of $2,270/man-week, this cost saving was $125/RY. The total industry cost savings that would result from reduced operation and maintenance at all affected plants were ($125/RY x 1,140 RY) or $143,000.
NRC Cost: It was assumed that NRC would provide the criteria to limit the extent of break exclusion regions for plant types and piping systems. Independent plant reviews with respect to new SRP11 regulations would then be conducted. At the time this issue was evaluated in 1983, the resolution had been completed. Therefore, based on an implementation estimate of 3 man-weeks/plant, the total NRC cost was estimated to be ($6,810/plant)(38 plants) or $259,000.
NRC costs for reviewing piping systems were not expected to change. However, a review of the consequences of imposing limitations on break exclusion areas would result in NRC costs of approximately $191/RY. Thus, the total NRC cost to support operation and maintenance was estimated to be (1,140 RY x $191/RY) or $220,000.
Total Cost: Summing all costs outlined above, the total cost associated with the possible solution was estimated to be $[2.07 + 0.143 + 0.259 + 0.22]M or approximately $2.7M.
Value/Impact Assessment
Based on an estimated public risk reduction of less than 100 man-rem and a cost of $2.7M for a possible solution, the value/impact score was given by:
Other Considerations
(1) Implementation Occupational Risk Increase
Implementation of the solution was estimated to occur during plant design stages. Therefore, any alterations made in break exclusion areas would occur before plant operation and startup. Thus, there was no occupational risk increase from implementation of the solution in the affected plants.
(2) Operation and Maintenance Occupational Risk Decrease
When a line is excluded from a break exclusion area, associated welds would no longer require a 100% volumetric inspection every 10 years. Instead, ISI of these welds would be scheduled once during the lifetime of a plant (i.e., 25% of welds would be inspected every 10 years).
Implementation of the possible solution was estimated to reduce operation and maintenance time in radiation zones by 2.2 man-hours/RY. Based on an average expected dose rate of 0.1 rem/hour for ISI, the total occupational risk reduction was estimated to be:
(3) Accident Avoidance Occupational Risk Decrease
Implementation of the solution would not change the frequency of a core-melt accident. Thus, there was no occupational risk reduction associated with the solution.
Summing up the above three factors, the total occupational risk decrease was 251 man-rem. Inclusion of this factor in the value/impact score calculation would produce a value/impact score of S 130 man-rem/$M.
CONCLUSION
Based on the estimated public risk reduction and the value/impact score, this issue was DROPPED from further consideration. Consideration of occupational risk decrease did not affect this conclusion.
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