05000413/LER-2002-002
| Catawba Nuclear Station, Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| 4132002002R00 - NRC Website | |
BACKGROUND
Catawba Nuclear Station Units 1 and 2 are Westinghouse Pressurized Water Reactors [EIIS: RCT]. Catawba has installed a Boron Dilution Mitigation System (BDMS) [EIIS: IG]. The primary purpose of BDMS is to mitigate the consequences of the inadvertent addition of unborated primary grade water into the reactor coolant system (RCS) [EIIS: AB]. The shutdown margin monitor portion of BDMS measures the count rate from a neutron counting instrument. It also provides an alarm output to indicate a decrease in reactor shutdown margin when the count rate increases by a calculated amount. The shutdown monitor alarm setpoint is continuously recalculated and automatically reduced as the reactor is shutdown and the neutron flux is reduced. An alarm will occur when the time averaged neutron count rate increases due to a reactivity addition to a value determined by the BDMS. There are two redundant channels.
In addition to providing an alarm on the main control boards, an alarm in either channel will automatically shutoff both reactor makeup water pumps [EIIS: P], align the suction of the charging pump [EIIS: P] to highly borated water from the refueling water storage tank (RWST) [EIIS: DA], and isolate flow to the charging pumps from the volume control tank [EIIS: TK].
Limiting Condition for Operation (LCO) 3.9.2 requires that when the unit is in mode 6, that two BDMS trains be operable to ensure that appropriate monitoring capability is available to detect changes in core reactivity.
With only one or no BDMS trains available core alterations and positive reactivity additions must be suspended immediately. In addition, the unborated water source must be isolated within one hour and RCS boron concentration must be verified once per 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. An option to isolate the unborated water source is provided to allow alternate methods of monitoring core reactivity conditions and controlling boron dilution incidents. This includes the utilization of the two Source Range Neutron Flux Monitors [EIIS:
IG]. These monitors must be verified to operate with alarm setpoints less than or equal to one-half decade above the steady- state count, each with continuous visual indication in the control room. In addition, the combined flow rate from both Reactor Makeup Water Pumps must be verified to be within the limits specified in the Core Operating Limits Report (COLR) in 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />. Once these options are verified, core alterations and positive reactivity changes can continue.
On April 30, 2002, at approximately 0100 with Unit 1 in Mode 6, vital AC instrument [EIIS: EF] panel board lERPD [EIIS: BD] was removed from service for scheduled maintenance. Removing 1ERPD from service rendered train B of BDMS inoperable and it also disabled the Source Range Nuclear Instrument N-32 High Flux at Shutdown alarm function. Operations did not recognize that these functions were inoperable until approximately 0530. Therefore, the required actions of TS 3.9.2 were not completed within the time allowed. This is a condition prohibited by TS.
This event is being reported under 10 CFR 50.73(a)(2)(1)(B) (any operation or condition prohibited by the plant's Technical Specifications (TS)), and 10CFR50.36(c)(2)(i) (Limiting Condition for Operation (LCO) not met).
At the time this condition was identified, Unit 1 was operating in Mode 6, "Refueling." The reactor head was still in place and no core alterations were in progress. No structures, systems, or components were out of service at the time of this event that contributed to the event.
EVENT DESCRIPTION
(Dates and times are approximate) Date/Time � Event Description 04/2002 Several weeks prior to 1EOC13, work requiring isolation of Vital AC Panel board 1ERPD was scheduled to be performed in Mode 5 with the RCS System at 100% level. Tech Spec 3.3.9 BDMS will allow one train of BDMS to be inoperable during Mode 5 for 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> prior to requiring action. The information provided for the schedule review did not include clear information stating that both B-Train BDMS and the Source Range High Flux at Shutdown alarm for N-32 function would be inoperable.
4/17/02 4/22/02 Removal and Restoration (Block Tag Out (BTO) BTO 12-470) was reviewed by the Electrical Assistant Outage Manager (AOM) group. It was recognized that B train BDMS would be affected, and that this would require both Source Range detectors (N-31 and N-32) to be operable.
A conflict was identified with removing lERPD from service during Mode 5 when low temperature overpressure protection (LTOP) is required.
RCS Wide Range Pressure indication would be deenergized which would affect the LTOP input to RCS pressurizer power operated relief valve 1NC-32B. A significant schedule change request was initiated per procedure to move the lERPD work in the outage. It was recognized at this time that B train of BDMS would be inoperable.
This information was not put in the pre-job brief for this evolution.
4/27/02/ 1243 Unit 1 entered Mode 5.
4/29/02/ 0942 Unit 1 entered Mode 6.
4/29/02/ 2337 The execution package for the lERPD evolution was taken to the outage control center (OCC).
The OCC SRO made an entry into the Technical Specification Action Item Log (TSAIL) for the isolation of vital AC panel lERPD. Neither BDMS nor the Source Range High Flux at Shutdown Alarm for N-32 was entered into TSAIL. The package provided did contain information that if reviewed could have led to identification B train BDMS and the source range high flux at shutdown alarm for N-32.
04/30/02 0015 The Electrical AOM conducted a pre-job brief with the control room team for this evolution.
Several items that would be lost were discussed. BDMS and the Source Range High Flux at Shutdown Alarm were not discussed.
Vital AC panel 1ERPD was removed from service per operating procedure OP/1/A/6350/008, 125VDC/120VAC Vital Instrument and Control Power System, for scheduled maintenance.
Removing 1ERPD from service rendered B train of BDMS and the Source Range High Flux at Shutdown Alarm for N-32 inoperable.
The reactor makeup water pumps (RMWP) failed to start in support of a Pressurizer Relief Tank (PRT) [EIIS: TK] flush. The control room contacted the Electrical AOM to assist in troubleshooting. This resulted in determining that B train of BDMS was inoperable and the control room began actions to comply with the required actions of TS 3.9.2.
The maintenance on lERPD was completed and the maintenance technicians returned the BTO tags to the control room to allow for restoration of the vital AC instrument panel.
Vital Instrument Panel lERPD was returned to service and B train of BDMS was declared operable.
4/30/02-0100 4/30/02-0500 4/30/02-0530 4/30/02--0630
CAUSAL FACTORS
The root cause was less than adequate use of available information to properly identify the effects of isolating lERPD. There were 2 specific areas where this inappropriate action occurred.
The first occurred when the lERPD work was scheduled in Mode 5, where Catawba Nuclear Station Directive (CNSD) CNSD 3.1.30, "Unit Shutdown Configuration Control" recommends "two operable trains of Source range detectors or Boron Dilution Mitigation". The information provided for the schedule review did not include clear information stating that both B-Train BDMS and the Source Range High Flux at Shutdown alarm function for N-32 would be inoperable. If this information had been provided, the schedule review may have been successful in identifying that this was in conflict with CNSD 3.1.30 requirements.
The second occurred in that the guidance provided to the shifts did not include clear information stating that both B-Train BDMS and the Source Range High Flux at Shutdown alarm function for N-32 would be inoperable. If this information had been provided, the Outage Control Center (OCC) SROs may have been successful in addressing the applicable TS and applying the appropriate required action.
A contributing cause to this event was unclear management expectation for the required level of review performed by the OCC SROs. The current management expectation is that an independent review of the load lists would not be expected of the SRO making the TSAIL entry for complex evolutions. The Electrical AOM would be expected to provide the detailed review. Moving the SRO's responsible for making TSAIL entries from the Work Control Center to the OCC may have reduced the level of review provided by the SRO's making the TSAIL entry.
CORRECTIVE ACTIONS
Immediate:
1. The maintenance performed on Vital AC panel board lERPD was completed, 1ERPD was returned to service, and BDMS train B was declared operable.
Planned:
1. This event will be discussed with the personnel involved to ensure they understand the root cause and corrective actions.
2. The wording in the low voltage breaker list (CNLT-1705-01.02-04) will be revised to clarify how the source range high flux at shutdown alarm is affected by this panel. This revision will also include the inputs to the other Source Range detectors for both Catawba Units 1 and 2.
3. Operations management will develop guidelines for the expected level of review for the execution SRO in the OCC for removal and restorations and TSAIL entries during outages.
4. The Operations Pre-Job briefings for removing the Vital AC panel boards that power BDMS and the source range detectors will be revised to include information stating that both BDMS and source range detectors are affected. This corrective action will be done for both Units 1 and 2.
5. This event will be discussed during Licensed Operator Training via Operations Proficiency review.
The planned corrective actions as well as any future corrective actions will be addressed via the Catawba Corrective Action Program. There are no NRC commitment items contained in this LER.
SAFETY ANALYSIS
At the time this event occurred, Unit 1 was in Mode 6 with the reactor vessel head in place and no core alterations or positive reactivity additions on going. Train A of BDMS was operable through out this event. De-energizing vital panel board lERPD also disables the RMWPs from being started. During this event the unborated water source isolation valve(s) were not closed and secured, nor was the flow rates of the RMWPs verified to be within the COLR limits. However, since the sources of unborated water require the RMWPs for delivery, the net effect was to prevent the addition of unborated water to the reactor coolant system.
UFSAR Chapter 15.4.6, "Chemical and Volume Control System Malfunction that Results in a Decrease in Boron Concentration in the Reactor Coolant," provides an analysis of a boron dilution event for the Catawba Nuclear Station. For Modes 3-6 with the BDMS inoperable, the results presented in UFSAR Table 15-23 show that, with limitations on flow rates from potential sources of unborated water, there is adequate time for the operator to determine the cause of the dilution, isolate the source of unborated water, and initiate reboration before the shutdown margin is exhausted. In accordance with USNRC Standard Review Plan, Section 15.4.8, NUREG- 0800, Rev. 2, July 1981, adequate time is judged to be at least 15 minutes for Modes 3-5 and at least 30 minutes for Mode 6.
In conclusion, the overall safety significance of this event was determined to be minimal and there was no actual impact on the health and safety of the public.
ADDITIONAL INFORMATION
A review of LERs from the last three (3) years found no LERs written due to outage schedule change review process. Therefore, this event was determined to be non-recurring in nature.
Energy Industry Identification System (EIIS) codes are identified in the text as [EIIS: XX].
This event did not include a Safety System Functional Failure.
There were no releases of radioactive materials, radiation exposures or personnel injuries associated with this event.