ENS 40380
ENS Event | |
|---|---|
21:38 Dec 9, 2003 | |
| Title | Unable to Achieve Cold Shutdown Using Natural Circulation at a Cooldown Rate of 50 Degrees Per Hour |
| Event Description | At 1538 on 12/09/03, it was determined that the plant could not achieve cold shutdown using Natural Circulation with the available volume of the Auxiliary Feed Water Storage Tank [AFWST] and at an allowed plant cooldown rate of 50 degrees F/hour. The plant has a functional objective to achieve Plant Shutdown from Operating Condition to Cold Shutdown Condition with the Natural Circulation Cooldown process with the volume of water available in the AFWST. Natural Circulation cooldown is required during Loss of Offsite Power (LOOP), long term cooling, Safety Grade Cold Shutdown and Appendix R type fire with LOOP. This condition also applies to Unit 2.
STP [South Texas Project] changed from a Reactor Vessel Head (RVH) T-Hot Plant to a RVH T-Cold Plant after replacement of the plant's steam generators. As a result, the plant cooldown rate under natural circulation conditions was changed from 25 degrees F/hour to 50 degrees F/hour based on a Westinghouse evaluation. While performing confirmatory calculations to address a fire safe shutdown issue, it was determined that the plant can be cooled down with two steam generators using natural circulation. However, at a cooldown rate of 50 degrees F/hour, the two steam generators not being used for cooldown would stagnate with hot water in the primary side tubes. During the depressurization process, the hot water flashes to steam such that depressurization below approximately 1000 psig is significantly challenged. This results in filling the pressurizer and a loss of RCS pressure control. The fluid in the RCS [Reactor Coolant System] loops not receiving AFW will remain at saturated conditions until cooling is achieved by other means. Cooldown and depressurization of the RCS in this configuration will take substantially longer than assumed in the current analyses. Although there is adequate core cooling, the plant will be in a condition that is outside the existing design basis and not specifically addressed by the Emergency Operating Procedures. Therefore, this condition resulted in the plant being in an unanalyzed condition that significantly degraded plant safety. Thermal Hydraulic Analyses at a cooldown rate of 25 degrees F/hr demonstrated that natural circulation will occur in the two steam generators that are not being used for cooldown such that the stagnated hot water conditions would not occur in these steam generators. This would allow for depressurization of the reactor coolant system and successfully meet the functional objective. A compensatory action is in place to limit natural circulation cooldown rate to no greater than 25 degrees F/hr. The licensee has notified the NRC Resident Inspector. |
| Where | |
|---|---|
South Texas  Texas (NRC Region 4) | |
| Reporting | |
| 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition | |
| Time - Person (Reporting Time:+-4.1 h-0.171 days <br />-0.0244 weeks <br />-0.00562 months <br />) | |
| Opened: | Ron Gibbs 17:32 Dec 9, 2003 |
| NRC Officer: | Gerry Waig |
| Last Updated: | Dec 9, 2003 |
| 40380 - NRC Website
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Unit 1 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (100 %) |
| After | Power Operation (100 %) |
Unit 2 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (100 %) |
| After | Power Operation (100 %) |