05000456/FIN-2014007-01
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Finding | |
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Title | Incorporation of Westinghouse NSAL 9905, Reactor Coolant Pump Operation During Loss of Seal Injection, Into the Current Licensing Basis and Corrective Actions For 2B RCP Degraded Thermal Barrier |
Description | In 1999, Westinghouse notified Braidwood Station through NSAL 9905, Reactor Coolant Pump Operation During Loss of Seal Injection, of a potential safety issue that had not been previously identified as part of the original design. Specifically, the NSAL described concerns that during a postulated loss of seal injection (LOSI) event, the RCP seal package and/or lower bearing temperatures may rise more rapidly than previously assumed in the original design. The original design considered the thermal barrier and associated heat exchanger as a fully functional backup to seal injection during a LOSI event. Consequently, following a loss of seal injection, the RCP thermal barrier and associated TBHX would cool the reactor coolant fluid that would flow up the RCP shaft and through the seals to maintain the lower bearing and the RCP seal temperatures within their normal temperature range for an extended period of time (i.e., 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />). The NSAL notified the licensee of a non-conformance to this original design. Specifically, during a postulated LOSI for RCPs with less than 2.5 gpm seal leakoff rate, it was determined that the RCP TBHX system would not be capable of maintaining the RCP seals within their nominal temperature range for the previously assumed extended period of time. Instead, the NSAL concluded that the RCP seal temperatures would rise above acceptable operating temperatures within 1 to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> if the RCP had less than a 2.5 gpm seal leakoff rate. With low seal leakoff rates, even though the reactor coolant system (RCS) water is cooled in the TBHX, the water will be heated after it flows through the TBHX. Westinghouse postulated that there may not be enough mass flow up along the shaft to absorb the heat transferred from the RCS, potentially raising the seal and bearing temperatures above their operating limits. In addition to NSAL 9905 applicability to all Braidwood RCPs, the Braidwood Unit 2, 2B RCP TBHX system was particularly adversely affected since the 2B RCP thermal barrier had also been identified to have degraded insulating properties. The licensee estimated that for a bounding set of plant conditions, Operations personnel would have approximately 27 minutes to respond after losing seal injection before the 2B RCP trip criteria would be reached. This condition had existed since 1999 and as of the end of the inspection the licensee had not taken any action to correct the degraded thermal barrier The inspectors discussed the potential consequences that a LOSI event may have on the 2B RCP after reaching the RCP trip criteria. One consequence would be the need for operators to insert a manual reactor trip prior to the manual 2B RCP pump trip in accordance with plant procedures, training, and associated expectations. Also, the licensee informed the inspectors that the event was bounded by the total loss of RCP seal cooling analysis that concluded 21 gpm of controlled leakage could occur. The licensee entered the original NSAL 9905 operating experience issue into their CAP in 1999 and determined that the Updated Final Safety Analysis Report (UFSAR) was not required to be updated. Additionally, during the inspection the licensee affirmed that the decision to not update the UFSAR was correct because the UFSAR was still correct and the level of detail in NSAL 9905 was not required to be discussed in the UFSAR. Although a loss of RCP seal injection and/or RCP TBHX function was discussed in numerous instances in the UFSAR, the following excerpt in the UFSAR generally described the CLB discussed in other UFSAR sections. (REF: original Safety Analysis Report (SAR) and UFSAR Section 5.4.1.2). High-Pressure seal injection water is introduced through a connection on the thermal barrier flange. A portion of this water flows through the radial bearing and the seals; the remainder flows down the shaft through the thermal barrier where it acts as a buffer to prevent system water from entering the radial bearing and seal section of the unit. The thermal barrier heat exchanger provides a means of cooling system water to an acceptable level in the event seal injection flow is lost. The licensee informed the inspectors that the UFSAR was still correct because the RCP would be tripped upon reaching the pre-established temperature limits upon a LOSI event and that the worst case leakage through each RCP would be 21 gpm for a total of 84 gpm from the four RCPs. This amount of controlled leakage was within the capacity of a single high head injection charging pump. However, the inspectors questioned this response because the conditions described in NSAL 9905 had not been identified during the time frame that the original SAR was approved. Consequently the inspectors questioned whether or not the assumptions in the original SAR that established an acceptable level of safety and the licensing basis for the RCP TBHX system were adversely affected. Additionally, Westinghouse NSAL 9905 recommended that all plants review their SAR relative to the loss of seal injection and ensure that the SAR was consistent with the NSAL, indicating a limited time frame for operation without seal injection (Ref: Westinghouse NSAL 9905, Recommended Actions #1). At the conclusion of the inspection, a detailed review of the CLB was in progress. This URI will remain open until that review is completed and the inspectors determine whether NSAL 9905 was adequately incorporated into the Braidwood CLB and whether the licensee should have implemented additional corrective actions to address the degraded 2B RCP thermal barrier. |
Site: | Braidwood ![]() |
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Report | IR 05000456/2014007 Section 4OA2 |
Date counted | Sep 30, 2014 (2014Q3) |
Type: | URI: |
cornerstone | Mitigating Systems |
Identified by: | NRC identified |
Inspection Procedure: | IP 71152 |
Inspectors (proximate) | C Hunt D Betancourt E Duncan G Hausman J Benjamin M Doyle N Feliz-Adorno T Go D Sargism Perryr Winter C Brown E Duncan J Benjamin J Lennartz |
INPO aspect | |
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Finding - Braidwood - IR 05000456/2014007 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Braidwood) @ 2014Q3
Self-Identified List (Braidwood)
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