05000335/LER-2012-002
| Docketsequentia Revmonth Day Year Year Month Day Year Na Numberl Number No. | |
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3352012002R01 - NRC Website | |
Description of the Event
On March 18, 2012 at 2336 EDT, while St. Lucie Unit 1 was performing low power physics testing while in Mode 2 and subcritical at less than 1% power, the reactor was manually tripped due to a control element assembly (CEA) [ZC] exhibiting unexpected movement. The reactor trip was uncomplicated and all CEAs fully inserted. No automatic safety system actuations were required and none occurred.
Reactor coolant system (RCS) heat removal was maintained with auxiliary feedwater and atmospheric dump valves. The Off site power grid was available and stable.
Cause
The root cause of this event was determined to be inadequate design control of a modification which installed new automatic control timing modules (ACTM) with deficient 10 volt regulators that introduced signal noise that resulted in system malfunction.
Analysis of the Event
The design modification to the rod control system was the fourth phase of a five phase project to improve the reliability of the aging rod control system and related circuit cards. The option to replace ACTMs was selected considering sub-components on the circuit boards have become either obsolete or the replacements have been increasingly difficult to obtain.
The original equipment manufacturer provided a technical evaluation that concluded the noise in the rod control system will not have any short-term or long-term adverse impact on the function of the ACTM. During the event investigation, additional testing of the new ACTM revealed that the noise could interrupt or create false control signals as well as voltage or current overstress issues that could degrade system performance.
Integrated testing of the entire system was not performed that may have been able to detect unacceptable component / system performance.
This licensee event report is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as "an event or condition that resulted in manual or automatic actuation of the Reactor Protection System including reactor scram or reactor trip.
Analysis of Safety Significance At the time of the event the plant was in Mode 2. The trip was uncomplicated and all CEAs fully inserted when the reactor was manually tripped while subcritical. No automatic safety system actuations were required and none occurred. Reactor coolant system (RCS) heat removal was maintained with auxiliary feedwater and atmospheric dump valves and the offsite power grid was available and stable.
The conditional core damage probability (CCDP) and conditional large early release probability (CLERP) values were evaluated and found to be significantly below the thresholds required by RG-1.74 for a non risk-significant event. This event had no effect on the health and safety of the public.
Prompt Corrective Action Degraded power supplies and ACTM cards have been replaced.
Corrective Actions
The corrective actions listed below are entered into the site corrective action program.� be managed under the corrective action Any changes to the actions will program.
1.Revise the design control process to require a risk based review for the proper sequencing of PMT for modifications that are risk significant.
2.Design improvements to the voltage regulators on the ACTMs to reduce noise.
Similar Events A search of the corrective action database for three years was performed and identified no issues that were related to the faults identified with this Condition Report.
Failed Component(s) ACTM Input/Output Board - Part No. E-00000-431-015-2 Manufacture Westinghouse