05000499/LER-2008-001
| South Texas Unit 2 | |
| Event date: | 10-16-2008 |
|---|---|
| Report date: | 12-11-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4992008001R00 - NRC Website | |
determined that the checkerboard storage requirements of Technical Specification T.S. 5.6.1.4, allowable configurations of spent fuel assembly categories, were not met. Consequently, STP Unit 2 was in a condition prohibited by Technical Specifications.
B. PLANT OPERATING CONDITIONS PRIOR TO EVENT
STP Unit 2 was in Mode 6 with the reactor vessel defueled.
C. STATUS OF STRUCTURES, SYSTEMS, AND COMPONENTS THAT WERE INOPERABLE
AT THE START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
No other inoperable structures, systems, or components contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT
Technical Specification 5.6.1.2 categorizes each fuel assembly by its reactivity based on several characteristics (initial enrichment, burnup, integrated fuel burnable absorber (IFBA) content, and decay time). Allowable storage configurations, or patterns, are then developed using the guidance in TS 5.6.1. Eleven fuel categories are defined by TS 5.6.1.2, from the highest to lowest reactivity (Category 1 to 11). The stored fuel assemblies must comply with the checkerboard requirements as specified in the TS 5.6 figures.
On 16 October, 2008, while planning fuel movements in the SFP during 2RE13, a Category 11 fuel assembly was discovered in a location where only Category 9 fuel is allowed. Following this discovery, the incorrectly stored fuel was removed from its location and placed in an area of the SFP with no adjacent fuel assemblies. Both Unit 1 and Unit 2 SFPs were checked for a similar condition, and no other occurrences of incorrectly stored fuel were found.
The investigation determined that the error occurred in the mapping of the SFP storage configuration, which is subsequently used to create fuel transfer forms. The storage configuration information available for planning displays only the maximum Category of each fuel assembly and the Reactor Engineer (RE) did not realize that some fuel assemblies had decayed directly from a Category 8 to a Category 11. This resulted in fuel moves that stored Category 9 fuel assemblies adjacent to Category 11 assemblies, which is not prescribed in the Technical Specifications.
The Fuel Transfer Form (FTF) with the incorrect information was approved and verified on 16 May, 2006 and implemented 17 May, 2006. This error was identified while reviews were being conducted for subsequent SFP moves on 16 October, 2008 during a U2 refueling outage.
Although not resulting in challenging the safety analysis, the requirements of T.S. 5.6.1.4, which prescribes allowable checkboard fuel assembly storage configurations, were not met. This event resulted in no personnel injuries, no offsite radiological releases, and no damage to other safety-related equipment.
E. METHOD OF DISCOVERY OF EACH COMPONENT FAILURE, SYSTEM FAILURE, OR
PROCEDURAL ERROR
This error was captured when a Reactor Engineer was verifying fuel transfer forms in preparation for B5B fuel transfers. During the review and discussion, the originator and reviewer identified the error that a Category 11 fuel assembly was in a Category 9 location.
IL EVENT-DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
No safety systems were required to respond during this event.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
This event did not cause the inoperability of any safety systems. The FTF with the incorrect information was approved on 16 May, 2006. The FTF was implemented by placing the Category 11 fuel assembly, in a Category 9 location on 17 May, 2006. The incorrect SFP configuration was discovered on. 16 October, 2008 and relocated to an appropriate location on the same day.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
Category 11 fuel is less reactive than a Category 9 fuel. Therefore, placing a Category 11 fuel in a Category 9 location did not involve a challenge to any design criteria or safety barrier. In addition, the safety analysis accounts for a misplaced fuel assembly in the SFP. The analysis assumes a fresh assembly of 4.95 weight percent (w/o) is inserted into a rodded position of the checkerboard configuration where the Category 11 assembly resided. By maintaining the SFP boron concentration above the Technical Specification required value, the design criteria for maintaining Keff unanalyzed condition.
This event resulted in no personnel injuries, no offsite radiological releases, and no damage to other safety-related equipment.
III. CAUSE OF THE EVENT
The root cause of the error was that both the fuel transfer form preparer and verifier performed inadequate self-checking and review. One contributing factor was a lack of robust guidelines for planning and verification of the SFP configuration and its compliance with Technical Specification requirements which contributed to the human performance error. Another contributing factor was determined to be the organization and format of the SFP configuration map used in planning fuel moves which only identified the highest fuel Category and did not identify any skipped Categories.
IV. CORRECTIVE ACTIONS
- Following discovery of the non-conforming condition, a fuel transfer form was prepared and the incorrectly stored fuel assembly was removed and put into a cell location that met the T.S.
requirements.
- All individuals who are trained on this Qualification (#9334) were briefed on management expectations related to preparation of Fuel Transfer Forms (FTF), peer checking, and paying attention to detail.
- Both Unit 1 and Unit 2 SFPs were checked for a similar condition. No other occurrences of incorrectly stored fuel assemblies were found.
- Existing procedural guideline will be revised to provide additional controls for developing SFP configuration maps and Fuel transfer Forms (1-114). This guidance will also include indicating skipped fuel Categories in addition to the highest fuel Category on the SFP map.
V. PREVIOUS SIMILAR EVENTS
A history review of condition reports did not identify any previous similar events at STP.
VI. ADDITIONAL INFORMATION
None.