05000315/FIN-2012007-04
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Finding | |
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| Title | Concerns with Ensuring Margin to Overfill in a Ruptured SG |
| Description | During the week of July 23, 2012, the inspectors reviewed the licensing bases and plant response to a SGTR event. During the postulated design basis event, operators prevent overfill of the ruptured SG using the SG PORVs. During a SGTR, the normal source of motive force for the SG PORVs is air supplied by the compressed air system (CAS). The CAS compressors are powered from the offsite power supply and are non-safety-related. Assuming a concurrent loss of offsite power (LOOP) to the station, the only available source of immediate and remote (from the control room) motive force is air supplied by the control air compressor (CAC) since it could be powered from an EDG. The CAC is also non-safety-related. The inspectors also noted the backup Nitrogen system could be manually aligned to provide motive force for the SG PORVs. The facilitys EOPs 1(2) OHP-4023-E-3, Steam Generator Tube Rupture, Step 7, directs the operators to perform a rapid RCS cooldown by fully opening the SG PORVs on the 3 intact SGs. The margin to overfill (MTO) analysis assumes this step occurs on demand (the SGTR PORVs open immediately as soon as the operators manipulate the valves from the control room). Once the SG PORVs are full open, the MTO analysis assumes it takes 12 minutes to complete the RCS cooldown. This is a calculated (modeled) value which takes into account plant specific characteristics. If the SG PORVs do not open, the EOPs direct operators to establish backup Nitrogen. Because aligning Nitrogen to the SG PORVs is a manual operation requiring multiple manipulations outside the control room, the inspectors were concerned the additional time to complete these actions would result in a longer time to complete the RCS cooldown than assumed in the MTO analysis and the affected SG could overfill. On Friday, July 27, 2012, the inspectors requested the licensee to provide a non-licensed operator to perform a walkthrough of the procedures for establishing backup Nitrogen to locally open the SG PORVs. Although no significant issues were discovered during performance of the evolution, the operator required 13 minutes to establish backup Nitrogen to the auxiliary building and 14 minutes to place the local control stations in service and open the SG PORVs, a total of 27 minutes. The inspectors were concerned because this additional 27 minutes was not accounted for in the MTO analysis. Specifically, the MTO analysis assumes a total of 12 minutes for RCS cooldown to occur from the moment the operators get direction to open the SG PORVs (assumed to occur immediately) until commencement of RCS depressurization. The MTO analysis calculates it will take the operators and the plant a total of 52 minutes to mitigate the SGTR accident (no more RCS flow through the ruptured SG) and thus prevent overfilling the affect SG with a calculated MTO of approximated 8ft3. The MTO analysis has no margin to accommodate the additional 27 minutes for RCS cooldown and therefore the procedure fails to potentially prevent overfilling the ruptured SG. In response to the inspectors concerns, the licensee implemented immediate compensatory actions to ensure (1) backup Nitrogen was continuously available to the auxiliary building and (2) an operator would be immediately dispatched to commence lining up backup Nitrogen to the SG PORVs if a SGTR with a concurrent LOOP occurred while the EDG or CAC was unavailable. The licensee disagreed with the inspectors assumption with respect to the initial conditions of a LOOP affecting both units. The licensee believes their licensing basis for a SGTR is a concurrent LOOP in the affected unit and the unaffected unit maintains an intact source of offsite power. In the licensees scenario, the affected units SG PORVs would still have a source of immediate remote (from the control room) motive force via the compressed air system. On the contrary, the inspectors believe the licensees design bases accident is a SGTR coincident with a LOOP to the station (both units). To resolve this issue, the inspectors requested support from NRR through a concurrence Task Interface Agreement (TIA) to determine the licensing bases for this event. In a TIA memorandum dated December 7, 2012, NRR concluded the licensing bases included a LOOP for both units (ML13011A382). Although the inspectors received a response from NRR, the issue remained open awaiting additional licensing bases information from the licensee. This issue is considered an unresolved item (URI) pending further review this information (URI 05000315/2012007-04, 05000316/2012007-04; Concerns with Ensuring Margin to Overfill in a Ruptured SG). |
| Site: | Cook  |
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| Report | IR 05000315/2012007 Section 1R21 |
| Date counted | Dec 31, 2012 (2012Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 71111.21 |
| Inspectors (proximate) | A Stone C Brown C Tilton G Skinner J Corujo _Sandin R Baker W Sherbinr Baker B Jose C Tilton |
| INPO aspect | |
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Finding - Cook - IR 05000315/2012007 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Cook) @ 2012Q4
Self-Identified List (Cook)
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