Resolution of Generic Safety Issues: Issue 153: Loss of Essential Service Water in LWRs (Rev. 2) ( NUREG-0933, Main Report with Supplements 1–35 )
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DESCRIPTION
Historical Background
The reliability of essential service water (ESW) systems and related problems have been an ongoing staff concern which has been documented in NUREG/CR-2797,1334 IE Bulletins 80-24201 and 81-03,207 Generic Letter No. 89-13,1259 and Issues 51, 65, and 130. In a comprehensive NRC review and evaluation of operating experience related to service water systems (NUREG-1275, Volume 3),1079 a total of 980 operational events involving the ESW system were identified, of which, 12 resulted in complete loss of the ESW system. The causes of failure and degradation included: (1) various fouling mechanisms (sediment deposition, biofouling, corrosion and erosion, foreign material and debris intrusion); (2) ice effects; (3) single failures and other design deficiencies; (4) flooding; (5) multiple equipment failures; and (6) personnel and procedural errors.
In the resolution of Issue 130, the staff surveyed seven multiplant sites and found that loss of the ESW system could be a significant contributor to CDF. The generic safety insights gained from this study supported previous perceptions that ESW system configurations at other multiplant and single plant sites may also be significant contributors to plant risk and should also be evaluated. As a result, this issue was identified1280 by DSIR/RES to address all potential causes of ESW system unavailability, except those that had been resolved by implementation of the requirements stated in Generic Letter No. 89-13.1259
Safety Significance
At each plant, the ESW system supplies cooling water to transfer heat from various safety-related and non-safety-related systems and equipment to the ultimate heat sink. The ESW system is needed in every phase of plant operations and, under accident conditions, supplies adequate cooling water to systems and components that are important to safe plant shutdown or to mitigate the consequences of the accident. Under normal operating conditions, the ESW system provides component and room cooling (mainly via the component cooling water system). During shutdowns, it also ensures that the residual heat is removed from the reactor core. The ESW system may also supply makeup water to fire protection systems, cooling towers, and water treatment systems at a plant.
The design and operational characteristics of the ESW system are different for PWRs and BWRs and also differ significantly from plant to plant within each of these reactor types. The success criteria associated with the functions of an ESW system are also plant-specific. A complete loss of the ESW system could potentially lead to a core-melt accident, posing a significant risk to the public. This issue affected all plants not covered in the resolution of Issue 130 and included consideration of Issue B-32.
Possible Solutions
The design of the ESW system varies substantially from plant to plant and the ESW system is highly dependent on the NSSS. As a result, generic solutions (if needed) are likely to be different for PWRs and BWRs. The possible solutions are: (1) installation of a redundant intake structure including a service water pump; (2) hardware changes of the ESW system; (3) installation of a dedicated RCP seal cooling system; or (4) changes to TS or operational procedures. These potential improvements were considered for the seven multiplant sites covered in the scope of Issue 130; however, these options will now be evaluated for the remaining LWRs (65 PWRs and 39 BWRs).
PRIORITY DETERMINATION
Frequency Estimate
The CDF resulting from the loss of service water system (LOSW) has been estimated in a number of PRAs and is listed in Table 3.153-1.
| Plant | Frequency (RY(1)) |
|---|---|
| TABLE 3.153-1 | |
| Estimated CDF Contribution from LOSW | |
| Plant A(Old PWR),1333 3 SWP/unit, CT | 1.2 x 10(6) |
| Plant B(New PWR),1333 3 SWP, CT | 1.6 x 10(5) |
| Plant C(Old BWR),1333 Multiple SWS | 2.7 x 10(4) |
| Plant D(Old PWR),1333 2-3 SWP, CT | 6.7 x 10(5) |
| Plant E(New PWR),1333 Unique SWS | 9.0 x 10(6) |
| Plant F(New BWR),1333 Multiple SWS | 3.0 x 10(5) |
| Plant G(Old PWR)1081 | 1.2 x 10(4) |
| Plant N-T(Old and New PWRs, Mean CDF)1408 | 1.5 x 10(4) |
CT = cross-tie SWP = service water pump SWS = service water systemThe mean value of the above frequencies was calculated to be 8.3 x 10(5)/RY.
Consequence Estimate
Dose consequence was estimated on the basis of the 15 release categories defined in WASH-1400.16 Based on Issues 65 and 130, the release categories of PWR-2 and BWR-2 are dominant for LOSW and these were estimated64 to be 4.8 x 106 man-rem and 7.1 x 106 man-rem, respectively. Assuming an average remaining lifetime of 30 years for the affected plants and using the calculated mean CDF of 8.3 x 10-5/RY, the public risk for the base case was calculated to be:
| (1) PWRs: | W = (30)(4.8 x 106)(8.3 x 10-5) man-rem/reactor |
| = 12,000 man-rem/reactor | |
| (2) BWRs: | W = (30)(7.1 x 106)(8.3 x 10(5)) man-rem/reactor |
| = 18,000 man-rem/reactor |
The consequence estimate of 12,000 man-rem/reactor was used for this analysis and compared favorably with the estimate of 9,700 man-rem/reactor calculated for Issue 130.
Cost Estimate
Industry Cost: The cost of installing a redundant intake structure, including a pump, was estimated in Issue 130 and showed a range from $12M to $72M, with a best estimate of approximately $43M.
The cost estimate for the solution involving hardware changes could include: additional crosstie, additional valving and piping, or additional water source (fire water). The cost for these hardware changes was expected to be less than that for redundant intake structures, but higher than that for TS or procedure changes. The least expensive solution was estimated to cost $50,000/plant to change requirements for TS or procedures.
NRC Cost: The NRC costs were negligible in comparison to the industry costs.
Total Cost: The estimated total NRC and industry cost of the possible solution was $50,000/reactor.
Value/Impact Assessment
Separate value/impact scores (S) were calculated for the four possible solutions:
| Possible Solution | Mean-Value Public Risk (Man-rem/R) | Estimated Reduction Coefficient | Risk Reduction (Man-rem/R) | Estimated Cost/Reactor ($M) | S (Man-rem/$M) |
|---|---|---|---|---|---|
| 1 | 12,000 | 0.8 | 9,600 | 43.0 | 220 |
| 2 or 3 | 12,000 | 0.5 | 6,000 | <43.0 | >220 |
| 4 | 12,000 | 0.1 | 1,200 | 0.05 | 24,000 |
The reduction coefficient was defined as the estimated effectiveness of the possible solution after implementation and was based on operational experience and engineering judgment.
Other Considerations
(1) The mean CDF was derived from PRAs and studies of 20 operating plants some of which have multiple units. However, since the ESW system is highly plant-dependent and the key contributor to CDF varies, the uncertainty of the mean CDF could be a factor of 10; this did not affect the priority ranking.
(2) The possible TS changes should be applied to PWRs only because the ESW systems for BWRs are already required in the cold shutdown or refueling mode. For BWRs, possible changes to operational procedures to cope with a complete loss of service water systems would apply.
(3) Issue 23 identified the LOSW as one of the events that could cause failure of RCP seals. Among the options considered by the staff in the resolution of Issue 23 was the installation of an alternate AC source to provide seal cooling and performance of plant modifications to allow backup cooling from an existing plant water system other than the ESW system. The reduction in CDF for this proposed resolution could be substantial (10-5). Should this proposed solution be implemented, the CDF resulting from a LOSW event could be reduced as much as 50%.1408 This reduced CDF, however, would still place Issue 153 in the high priority category as discussed in the value/impact assessment above. However, in resolving Issue 153, the staff was expected to consider the proposed resolution of Issue 23.
(4) Issue 51 addressed service water system fouling and was considered to be resolved with the implementation of the baseline fouling program required by Generic Letter No. 89-13.1259 Implementation of this program could result in a CDF reduction1258 of approximately 2.6 x 10-6. As indicated earlier, biofouling was to be excluded from the resolution of Issue 153.
(5) Issue 65 was integrated into Issue 23 and its impact on Issue 153 was discussed above.
(6) Issue 130 addressed the limited scope of multiplant configurations with 2 ESW pumps per plant; the possible solution was limited to 7 PWRs with a total of 14 units. The resolution of Issue 153 included all plants not covered in the resolution of Issue 130.
CONCLUSION
Based on the potential public risk reduction, this issue was given a high priority ranking (See Appendix C). In resolving the issue, the staff found that the concerns involving ESW system reliability were being addressed on a plant-specific basis in various ongoing NRC and industry initiatives such as the Service Water System Operational Performance Inspection Program, Generic Letter 89-13,1259 the IPE Program, and EPRI research programs. In addition, ESW system reliability concerns were to be addressed by the Maintenance Rule and in the resolution of Issue 23. The staff's technical findings were documented in NUREG/CR-59101514 and SEASF-LR-92-022, Revision 11519; the regulatory analysis was documented in NUREG-1461.1512 In the resolution of the issue, a license renewal period of 20 years was considered.1564 Thus, this issue was RESOLVED and no new requirements were established.1513
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