ENS 43886
ENS Event | |
|---|---|
17:42 Jan 8, 2008 | |
| Title | Potential Loca Injection Flow Rate Issue |
| Event Description | At 11:42 CST, a condition report was initiated that questioned the specified flow path for simultaneous hot and cold leg injection following a large break loss of coolant accident (LOCA). When an unisolated LOCA event exists, simultaneous hot leg and cold leg injection should be implemented if the plant cannot be placed on shutdown cooling within six hours of the LOCA initiation and RCS pressure is less than 120 psia. The procedure is implemented at five and one-half hours to provide adequate time to align simultaneous hot/cold leg injection before the six hour time limit. Injecting to each side of the reactor vessel at an injection rate greater than 165 gpm, ensures that fluid from the reactor vessel (where the boric acid is being concentrated) flows out of the break regardless of the break location and is replenished with a dilute solution of borated water from the other side of the reactor vessel. The action is taken between 5.5 and 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> after the LOCA in order to ensure that the buildup of boric acid is terminated well before the potential for boric acid precipitation occurs which could restrict coolant flow through the core. Once the RCS is refilled, the boric acid is dispersed throughout the RCS via natural circulation. If entry into shutdown cooling system operation is anticipated before the 5.5 hour5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> limit, then the realignment to hot/cold leg injection is unnecessary.
The potential concern is associated with a charging line thermal relief valve CH-202 bypassing flow from hot leg injection and preventing the required flow rate needed to prevent boron precipitation from occurring. A minimum injection rate of 147 gpm to the cold legs and 159 gpm to the hot legs is required to prevent boric acid precipitation. Total hot leg injection flow is measured at FIA-236. Cold leg injection flow is the total of the four HPSI flow instruments, FI-313, FI-316, FI-319, and FI-322 with 50 gpm the minimum flow indication. A total cold leg injection flow of at least 200 gpm ensures at least 150 pm flow into the core, assuming 25% spillage out the break. This meets the required minimum of 147 gpm. It could not be determined through a review of the design basis documents and associated calculations what, if any, bypass flow is assumed through CH-202. Current procedural guidance in the emergency operating procedures is to align a high pressure safety injection pump to the charging header and provide hot leg injection from auxiliary pressurizer spray valves attached to the charging headers through the pressurizer and into the hot leg. The current procedural guidance does not isolate CH-202 and due to the location of flow instrument FIA-236, it cannot be guaranteed that all the flow through the charging system is being injected into the hot leg or being diverted through the normal charging line. As a result the potential existed which could have prevented the fulfillment of the safety function of a system needed to remove residual heat. Therefore this report is being made in reference to 10 CFR 50.72 (b) (3) (v) (B). Efforts are continuing to review design basis documents and calculations to determine if bypass flow was assumed past CH-202 when determining the minimum hot leg injection rate. As a compensatory measure, Operations management has restricted the use of hot and cold leg injection via the charging header until the design basis review confirms the adequacy of the current procedural guidance or the procedural guidance is revised. Pre-approved alternative methods will be utilized via the emergency operating procedures to perform simultaneous hot and cold leg injection if required. No LCO condition exists. The licensee notified the NRC Resident Inspector.
On January 8, 2008, (Event Number 43886) Fort Calhoun Station reported that there could be a potential reduction of injection flow to the hot leg during Long Term Core Cooling (LTCC) simultaneous hot and cold leg injection. The charging line thermal relief valve/check valve CH-202 could potentially divert flow from hot leg injection and reduce hot leg flow below the required flow rate needed to prevent boron precipitation from occurring. On January 8, 2008 it could not be determined through a review of design basis documents and associated calculations if bypass flow has been assumed through CH-202. Divergence of flow through CH-202 would result if a valve failure occurred. Assuming flow is diverted through CH-202, the operators would not realize that flow was going through the wrong flow path (cold leg) as their flow indication (FE-326) is located upstream of where the flow path to the hot leg and cold legs branch off. Therefore, there was nothing to alert the operator to isolate CH-202 or go to alternate hot leg injection. Previous procedural guidance was not adequate to address this condition. Current procedural guidance is adequate to address this condition as the procedures now require isolating CH-202 for LTCC. A reanalysis was performed to evaluate the required flow rate needed to prevent boron precipitation and ensure adequate LTCC. Calculations performed assumed full flow (failure) through CH-202. Under postulated design scenarios it was determined that adequate flow would have been provided to the hot legs during simultaneous hot and cold leg injection during LTCC. The calculations determined that under the evaluated scenarios, divergence of flow through CH-202 was acceptable, and that the requirements to maintain adequate flow to the core for LTCC decay heat removal and boron flushing would have been met. As a result of the analysis that were performed, it has been determined that the system was capable of performing its design function even under bypass flow conditions through CH-202. Therefore, this event is NOT reportable under 10 CFR 50.72( b) (3) (v) (B) as previously reported. The licensee notified the NRC Resident Inspector. Notified R4 DO (Miller) |
| Where | |
|---|---|
Ft Calhoun  Nebraska (NRC Region 4) | |
| Reporting | |
| 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat | |
| Time - Person (Reporting Time:+2.85 h0.119 days <br />0.017 weeks <br />0.0039 months <br />) | |
| Opened: | Scott Lindquist 20:33 Jan 8, 2008 |
| NRC Officer: | Jeff Rotton |
| Last Updated: | Feb 20, 2008 |
| 43886 - NRC Website | |
Unit 1 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (100 %) |
| After | Power Operation (100 %) |