Resolution of Generic Safety Issues: Issue 162: Inadequate Technical Specifications for Shared Systems at Multiplant Sites When One Unit is Shut Down (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–35 )
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DESCRIPTION
Historical Background
This issue was identified1481 by NRR as a result of a diagnostic evaluation team inspection of the SWS at Zion, a multiplant site. The inspection identified the potential for some additional shared systems to have an inadequate number of available components or inadequate flow rates when one unit is shutdown and the other unit is operating. The staff concluded that these inadequacies at Zion were due both to the existing TS and to the licensee's poor understanding of the original design basis. Since the staff's concerns with the SWS were already being addressed as part of the resolution of Issues 130 and 153, no additional generic follow-up of the SWS concerns was conducted.
NRR identified the following systems as having the potential for inadequate TS when they are shared: (1) HVAC systems for shared control room areas and switchgear rooms; (2) onsite AC and DC systems; and (3) the CCW system. From the total of 39 multiplant sites in the U.S., updated FSARs for 10 sites (7 PWRs and 3 BWRs) were selected for review to determine the extent of sharing of these systems and to assess the potential impact of TS on the adequacy of this sharing. This review, along with a review of applicable Bulletins, Information Notices, and Generic Letters, resulted in the following observations:
(1) The CCW system is generally not shared. At BWRs, it is not a safety-related system and is not addressed in the TS. For most PWRs, this system is not normally shared because it is a closed system with separate surge tanks for each train and cross-connected operation can be difficult. Although the CCW system is shared at Surry 1 & 2, Zion 1 & 2, and D. C. Cook 1 & 2, any plant-specific vulnerabilities were to be identified in the IPE program. In addition, calculations in the generic shutdown study indicated that one of the above 3 PWR sites was examined and showed that the CDF contribution of CCW system failures during shutdown was relatively small (about 4 x 106 /RY). Therefore, the staff concluded that the CCW system did not warrant additional generic review to determine if TS are adequate for those shared CCW systems.
(2) The HVAC system for common control room areas is required to be operable in all modes so that the operating mode of one unit does not affect the operability of the other unit's HVAC system. Preliminary estimates4 of the CDF resulting from loss of cooling to the control room for a typical PWR and BWR would be about 7 x 107/RY and 2 x 107/RY, respectively. In addition, control room HVAC systems were reviewed under Issue 143. Consequently, it was concluded that the HVAC system for common control room areas did not warrant additional generic review.
(3) The design of HVAC systems servicing non-control room areas (e.g., switchgear rooms) is plant-specific and required examination by licensees as part of the IPE program to identify plant-specific vulnerabilities. This study indicated that, although the switchgear room is the most important non-control room area, the CDF resulting from loss of room cooling for a typical PWR or BWR would not be large (about 106/RY and 107/RY, respectively). In addition, calculations4 indicated that one of the above PWR sites was evaluated and showed that the CDF contribution of failed HVAC in switchgear rooms was negligible (about 1.2 x 107/RY). Also, HVAC systems were reviewed under Issue 143. Therefore, it was concluded that the adequacy of TS associated with HVAC systems for non-control room areas did not warrant additional generic review.
(4) Onsite AC and DC systems for multiplant sites are generally physically and electrically separated and redundant and are rarely shared between units. Special inspections for the AC and DC systems identified some plant-specific issues at some multiplant units and were addressed. The objective of these special inspections was to assess the performance capability of the electrical distribution system to adequately support the operation of the licensees' safety-related components. These systems were reviewed under the station blackout program and licensees were also expected to identify vulnerabilities associated with AC and DC systems in the IPE program. Therefore, it was concluded that the adequacy of TS associated with shared onsite AC and DC systems did not warrant further generic review.
(5) The Maintenance Rule, 10CFR 50.65(a)(3), requires that, in performing monitoring and preventive maintenance activities, an assessment of the total plant equipment that is out of service be taken into account to determine the overall effect on performance of safety functions. It was expected that this assessment would provide additional assurance that an adequate number of shared components will be available at multiplant sites.
CONCLUSION
The staff concluded that the concerns raised with the adequacy of TS associated with shared CCW systems, common control area HVAC, shared switchgear room HVAC, and onsite AC and DC systems had been or will be addressed by existing programs or activities. Therefore, this issue was DROPPED from further consideration as a new and separate issue. In an RES evaluation,1564 it was concluded that consideration of a 20-year license renewal period did not change the priority of the issue.
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