05000499/LER-2013-001
| South Texas Unit 2 | |
| Event date: | 01-04-2013 |
|---|---|
| Report date: | 02-28-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4992013001R00 - NRC Website | |
No structures, systems, or components were inoperable at the start of the event that contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT
At 07:05 on 1/4/2013, Unit 2, while at 100% power, the control room commenced OPSP03-RS-0001 "MONTHLY CONTROL ROD OPERABILITY", and OPSP03-RS-0004 "CONTROL ROD OPERABILITY (6 AND 10 STEPS)". Surveillances OPSP03-RS-0004 and OPSP03-RS-0001 test the shutdown and control rods not fully inserted in the core in order to satisfy the monthly requirements of Technical Specification 4.1.3.1.2. and also provide instructions for demonstrating the shutdown and control rods not fully inserted in the core are Operable by movement of at least 10 steps in any one direction.
During testing, two rods on shutdown bank E (SBE) dropped to the bottom of the core and a manual reactor trip was required per OPOPO4-RS-0001, "CONTROL ROD MALFUNCTION". The dropped rods on SBE occurred while inserting shutdown bank C (SBC) rods in 6 steps. When SBC demand reached 255, one rod in SBE dropped (M-8) while the rest of the rods in SBE transitioned from 258 to 252/246 on Digital Rod Position Indication (DRPI). Rod motion was stopped and while validating the dropped rod, a second rod in SBE dropped (D-8). At 09:41, once both dropped rods were validated by diverse indications of power, flux, and rod positions, a manual reactor trip was performed.
This event resulted in a reactor trip from 100% power. The Unit was stabilized in Mode 3 with all systems necessary to maintain shutdown conditions, remove decay heat, control the release of radioactive material, and mitigate the consequences of an accident available.
Troubleshooting identified that the problem was isolated to the Rod Holdout Mode Selector (RHMS) switch in Rod Control Power Cabinet SCDE. This is a five-position, seven deck rotary Grayhill switch used to interface with the Rod Holdout Sequencing card and other power cabinet components to accomplish the rod lockout function for rapid refueling. Switch Pin 7 to common C2 on deck F was found to have a high resistance (-300Mohms) while in the "OFF" position.
FORM 366 (10-2010) 00 address extent of condition in Unit 2, the four other Rod Holdout Mode Selector switches in the other Rod Control Power Cabinets were tested.
E. METHOD OF DISCOVERY
The two dropped rods on shutdown bank E requiring a manual reactor trip were self- revealing.
II. EVENT-DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
All required safety systems responded as expected except that one of two Nuclear Instrument Source Range detectors, N32, did not energize automatically. This was identified and the function performed manually. In addition to failing to automatically energize, the source range audio count rate on N32 also failed to function. These conditions have been entered into the Corrective Action Program.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
N/A
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
There was no impact to radiological safety, safety of the public, or safety of station personnel during this event.
The Incremental Conditional Core Damage Probability (ICCDP) for the Reactor Trip in Unit 2 on 1/4/2013 is 1.12E-07. The resulting Incremental Conditional Large Early Release Probability (CLERP) is 5.92E-08.
III. CAUSE OF THE EVENT
The cause of this event has been determined to be high resistance circuit on the RHMS from the C (common) contact in switch position 7 on deck F due to contamination. Even though offsite failure analysis did not reproduce the high resistive contact nor did it find evidence of contamination that could have caused this condition, enough evidence exists from other failure analysis and industry experience to conclude that contamination is the cause.
The cause of this contamination could be due to:
- The low current that flows through these contacts is not enough to burn through corrosion/carbon deposits that builds up on the contacts over time.
- Solder flux on contact surfaces.
- Grease or other contaminants.
FORM 366 (10-2010) Corrective actions will be implemented in accordance with the STP Corrective Action Program. The Rod Holdout Mode Selector switch in Rod Control Power Cabinet SCDE was replaced. Corrective actions to prevent future failures will include either a design change to address the vulnerability associated with the RHMS switch contacts or replacement of existing switches with newly manufactured switches.
V. PREVIOUS SIMILAR EVENTS
There have been no similar reportable events at STP within the last three years. The Rod Holdout Mode Selector (RHMS) switches in the Rod Control Power Cabinets interface with the Rod Holdout Sequencing card and other power cabinet components to accomplish the rod lockout function for rapid refueling. There are no previous similar industry events as South Texas Project (STP) is the only plant that uses rapid refueling.
VI. ADDITIONAL INFORMATION
N/A FORM 366 (10-2010)