05000498/LER-2011-001
| South Texas Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4982011001R01 - NRC Website | |
South Texas Project (STP) Unit 1 was in Mode 5, with Reactor Coolant System loops not filled, as part of refueling outage 1RE16.
C. STATUS OF STRUCTURES, SYSTEMS, AND COMPONENTS THAT WERE INOPERABLE AT
THE START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
No structures, systems, or components were inoperable at the start of the event that contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT
On April 30, 2011, STP Unit 1 was in refueling outage 1RE16, with the unit in Mode 5, Reactor Coolant System loops not filled. In support of a planned evolution to transfer water from the Boron Recycle System (BRS) 1B Recycle Holdup Tank (RHT) to the Volume Control Tank, a Senior Reactor Operator (SRO) assigned to review atypical plant conditions during the outage identified a potential unborated water source that was not included in the plant surveillance procedure used to confirm compliance with Technical Specifications 3.4.1.4.2 regarding unborated water sources.
Technical Specification (TS) 3.4.1.4.2 (Reactor Coolant System — Cold Shutdown — Loops not Filled) requires in Mode 5 that "Each valve or mechanical joint used to isolate unborated water sources shall be secured in the closed position." Note that a similar requirement in TS 3.9.1 (Refueling Operations — Boron Concentration) applies during Mode 6. The associated surveillance procedure was focused on systems directly coupled to the Chemical and Volume Control System, such as Reactor Makeup Water (RMW), where an opened valve can reduce the RCS boron concentration by the addition of unborated water. The SRO review revealed that transferring water from the RHT to refill the RCS loops indirectly exposed the RCS to unborated water via Demineralized Water (DW) connected to the BRS. The surveillance procedure did not ensure that the connecting valves between the RHT and the DW system were secured closed. Although the valves were not secured closed as required by TS, the valves were closed and no dilution occurred.
Because the same TS, plant procedures, and administrative controls applied to both units, similar valves in Unit 2 were also not secured closed during refueling outages when required by TS 3.4.1.4.2.b and 3.9.1.
FORM 366 (10-2010} large early release frequency (LERF) data for this event (there is no PRA evaluation of boron dilution during shutdown). However, since the valves of concern in the potential unborated water path were not opened, and no dilution occurred, the potential impact to CDF and LERF from this event is negligible and the event is considered to be of low safety significance.
III. CAUSE OF THE EVENT
In 1997, STP procedures were revised to allow using water from the RHT to refill the RCS loops in addition to using the Refueling Water Storage Tank (RWST) or the Boric Acid Tanks (BATs). Prior to this, only the RWST or BATs were used for RCS refill. This change allowed clean, hydrazine-treated, borated water from the RHT to be used as a water source to refill the RCS during a refueling outage.
In 2003, Technical Specifications 3.4.1.4.2 and 3.9.1 were revised to remove references to specific valves that were required to be secured to protect against dilution and replaced with the more generic language discussed in Section I.D above. However, the potential consequences of using the RHT as a fill source was not adequately addressed with respect to compliance with the revised TS requirements, and thus the surveillance procedure used to ensure compliance did not address all potential unborated water sources.
IV. CORRECTIVE ACTIONS
The surveillance procedure used to ensure compliance with TS 3.4.1.4.2 and 3.9.1 requirements will be revised to reflect the additional valves that must be secured in the closed position to comply with the TS.
Tracking and implementation of this action will be controlled in accordance with the STP Corrective Action Program. Action Completed.
FORM 366 (10-2010) Because the same TS, plant procedures, and administrative controls applied to both units, Unit 2 was also not in compliance with TS. Therefore both Units were in non-compliance with TS 3.4.1.2.b and 3.9.1 when required from the time the TS were amended in 2003 until the plant surveillance procedure was revised.
VI. ADDITIONAL INFORMATION
N/A FORM 366 (10-2010)