05000277/LER-2010-001
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i) |
| 2772010001R00 - NRC Website | |
Unit Conditions Prior to the Event Unit 2 power was reduced to approximately 60% rated thermal power for a planned maintenance / testing load drop. The unit power was maintained between approximately 55% to 65% rated thermal power during the Control Rod Drive (CRD) surveillance test. There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On 1/31/10, based on review of surveillance testing performed on 1/30/10 and 1/31/10, Operations personnel identified a potential condition prohibited by Technical Specification (TS) Limiting Condition for Operation (LCO) 3.1.4, Control Rod Scram Times. TS LCO 3.1.4 states that the number of slow operable control rods shall be limited to 13. Contrary to this requirement, a total of 21 control rods (EMS: AA) were identified as being slow from control rod notch position 48 to position 46. All 185 control rods were ultimately scram time tested during the surveillance test. During the surveillance test, control rods were declared inoperable for repair and therefore, there was no time during the test where the declared number of slow control rods exceeded the TS allowable number.
TS Surveillance Requirement (SR) 3.1.4.2 requires that for a representative sample, each tested control rod scram time is within the limits of TS Table 3.1.4-1 with reactor steam dome pressure > 800 psig. Four separate scram time limits are required to be met for each control rod tested. These four scram time limits meaure the timing of the control rod from the fully withdrawn position (notch 48) to notch position 46 ( seconds), to notch position 26 ( 21 slow control rods were only slow from notch position 48 to notch position 46. All other scram times for these control rods were met.
In accordance with TS, surveillance testing had begun with a 10% sample of all control rods (19 of 185). Three of the 19 control rods (16%) exhibited a slow time between control rod notch position 48 to notch position 46. As required by TS 3.1.4 bases, if more than 7% of the sample is declared slow, additional control rods are required to be tested until less than 7% of the tested control rods are slow. This resulted in testing all 185 control rods, with 21 slow rods (11% of control rods) identified.
This report is being submitted pursuant to:
10CFR 50.73(a)(2)(i)(13) — Condition Prohibited by TS — This event is reportable under this criterion since 21 slow operable control rods existed during Mode 1 operations. TS LCO 3.1.4 only allows for a maximum of 13 slow operable control rods. Additionally, of the 21 slow control rods, there were 5 pairs of adjacent slow operable control rods during Mode 1 operations. TS LCO 3.1.4 only allows for a maximum of 2 operable slow control rods being in adjacent positions.
�NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER Description of the Event, continued 10CFR 50.73 (a)(2)(vii) — Common Cause Inoperability — This occurrence is reportable under this criterion since certain slow control rods were declared inoperable during surveillance testing in order to perform maintenance and to ensure that the TS LCO 3.1.4 limit for slow control rods (i.e., 13 control rods) would not be exceeded. Therefore, there were multiple control rods declared inoperable as a result of having slow stroke times from position 48 to position 46.
Once control rods were found slow, actions were promptly taken to identify and remedy the slow control rod condition. It was identified that the Scram Solenoid Pilot Valves (SSPVs) associated with the Hydraulic Control Unit (HCU) for each affected drive exhibited signs of degradation associated with the SSPV diaphragm.
The surveillance testing of the 185 control rods was completed by 1500 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.7075e-4 months <br /> on 2/1/10. The affected HCU SSPV diaphragms were replaced and re-tested by 2/3/10.
Analysis of the Event
There were no actual safety consequences associated with this event.
The control rod drive mechanism provides control of reactor power, including the ability to provide a sufficiently rapid insertion of control rods (scram) so that no fuel damage results from any abnormal operating transient and limits fuel damage under accident conditions. The 185 control rods (EllS: ROD) are located uniformly throughout the core. The control rods are operated by CRD mechanisms. The CRD hydraulic system HCU (ENS: HCU) supplies and controls the pressure and flow requirements to the CRDs. The HCUs provide hydraulic power to be able to position Control Rods in the reactor core. HCU scram accumulators are designed with a limited nitrogen pressure and volume, which are sufficient to initiate control rod scram motion.
Operation of the HCU scram valves are initiated by SSPVs for each HCU. During a scram signal, the SSPVs (EIIS: FSV) vent the air from the scram inlet and outlet valves for each CRD HCU, allowing the control rods to rapidly move to the full in position and shutdown the reactor.
The SSPVs are supplied by the Automatic Switch Company (Model # HVA 266000-2J).
In accordance with TS 3.1.3, Control Rod Operability, SR 3.1.3.4, if a control rod has a scram time of greater than 7 seconds, then the control rod is considered inoperable. None of the 185 Unit 2 control rods had a scram time that exceeded 7 seconds.
In accordance with TS 3.1.4, Control Rod Scram Times, SR 3.1.4.2 requires that for a representative sample, each tested control rod scram time is within the limits of TS Table 3.1.4 1 with reactor steam dome pressure > 800 psig. Four separate scram time limits are required to be met for each control rod tested. These four scram time limits measure the timing of the control rod from the fully withdrawn position (notch 48) to notch position 46 ( � Analysis of the Event, continued notch position 36 ( 06 ( analyses. The scram times have a margin that allows up to 7% of the 185 control rods (13 control rods) to be slow, combined with an assumed single stuck control rod and a single failure of another control rod to scram in accordance with the single failure criterion. Based on surveillance testing performed, there were no stuck control rods and all control rods were able to be scrammed. In addition, the 21 slow control rods were only slow from notch position 48 to notch position 46. Licensing basis analyses state that a relaxation of the 5% scram insertion time requirement has a negligible impact on plant transient performance. Because all other scram times for these control rods (i.e., notch 48 to 36, notch 48 to 26 and notch 48 to 06) were met, there were no stuck control rods and all control rods were able to be scrammed, it can be concluded that there was no significant adverse impact to the control rod drive reactivity safety function.
This event is not considered risk significant.
Cause of the Event
The primary cause of this event was due to degradation of the Viton-A diaphragms of the SSPVs associated with 21 of the 185 control rod HCUs. The degradation of these diaphragms resulted in delays in control rod motion which caused the control rod notch 48 to notch 46 scram time speed to be slightly slow. The TS required time from control rod notch position 48 to notch position 46 is 0.565 seconds.
Further review of the 21 slow control rods revealed that all 21 had SSPVs that were installed between 4/18/95 and 5/10/95 and all had indicated a scram time from control rod notch position 48 to notch position 46 of greater than 0.360 seconds during the previous surveillance test.
Another cause of this event was determined to be inadequate performance monitoring associated with the SSPVs. Other contributing causes are being pursued in accordance with the Corrective Action Program.
_
Corrective Actions
All 21 slow control rods were removed from service and the SSPV diaphragms were replaced.
The 21 control rods were re-tested satisfactorily and returned to service.
Near term replacements of other SSPV diaphragms are scheduled for both Units 2 and 3 for other SSPVs with Viton-A diaphragms.
Performance monitoring and the preventive maintenance program will be upgraded.
Other actions are being pursued in accordance with the Corrective Action Program.
Previous Similar Occurrences There were no previous LERs identified involving conditions prohibited by TS involving slow control rods. Previous concerns with SSPV diaphragms occurred at PBAPS Units 2 and 3 in 1995. The resolution of these previous concerns is being factored into a root cause evaluation that is being performed for this event.
�