05000499/LER-2003-002
| Docket Number | |
| Event date: | 03-09-2003 |
|---|---|
| Report date: | 05-08-2003 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4992003002R00 - NRC Website | |
DESCRIPTION OF EVENT
On March 09, 2003, Unit 2 was stable in Mode 3 with the reactor coolant system at normal operating pressure and temperature. Pressurizer pressure was being controlled in manual. The secondary plant was still in cold shutdown conditions, with the main condenser isolated due to ongoing repairs on the turbine. All Main Steam (MS) Isolation Valves, MS Isolation Bypass Valves, and, MS Isolation Valve above seat drain valves were closed. MS Power Operated Relief Valve (PORV) 2C had been in service for temperature control for approximately 3-1/2 days. At 2041 hours0.0236 days <br />0.567 hours <br />0.00337 weeks <br />7.766005e-4 months <br />, operators closed MS PORV 2C to transfer reactor coolant system (RCS) temperature control from MS PORV 2C to MS PORV 2B. MS PORV 2B was opened approximately 5 %. The expected decrease in RCS temperature was not observed so MS PORV 2B was opened to approximately 10 % to match the previous MS PORV 2C demand. At 2047 hours0.0237 days <br />0.569 hours <br />0.00338 weeks <br />7.788835e-4 months <br />, a Steam Flow/Feed Flow Mismatch alarm on Steam Generator (SG) 2B was received followed by an automatic safety injection signal from 2B Low Compensated MS Line Pressure.
All engineered safety features (ESF) systems actuated as designed. The minimum RCS pressure reached was approximately 2200 psig, greater than the shutoff head of the safety injection (SI) pumps, so no injection into the RCS occurred as a result of the actuation.
At approximately 2109 hours0.0244 days <br />0.586 hours <br />0.00349 weeks <br />8.024745e-4 months <br />, operators were transitioning to terminate the safety injection event when the reactor coolant system pressurizer PORV 0655A opened due to the integration function of the Pressurizer Master Controller in response to a pressurizer pressure of approximately 2270 psig. As a result of the SI, a phase A containment isolation actuation occurred which resulted in the isolation of instrument air. The pressurizer spray valves closed due to loss of instrument air and the pressurizer heaters remained operating in manual causing reactor coolant system pressure to increase. Instrument air was restored to the containment allowing the pressurizer spray valves to be opened and the reactor coolant system pressure to be stabilized.
An investigation into this event revealed that the horizontal run of pipe near MS PORV 2B and some of the vertical piping between the MS PORV and the SG outlet contained condensation that had settled in the MS line during the 3 days that MS PORV 2C had been in operation for temperature control. Since the MS isolation valve above seat drain valves had to remain closed due to the condenser not being available, the steam in "non-steaming" MS lines condensed and collected in the lines. When PORV 2B was initially opened at 2041 hours0.0236 days <br />0.567 hours <br />0.00337 weeks <br />7.766005e-4 months <br />, no significant change was seen in the RCS temperature because the PORV was discharging water instead of steam. A Plant Operator noted a significant amount of water running off of the Isolation Valve Cubicle building roof at the 55' level after PORV 2B was opened and observed water bubbling out of the PORV 2B discharge stack.
EVENT SIGNIFICANCE
This event resulted in no personnel injuries, radiation exposure, offsite radiological releases or damage to important safety related equipment. The event is reportable pursuant to 10CFR50.73(a)(2)(iv)(A) because it resulted in a condition that resulted in manual or automatic actuation of emergency core cooling systems.
The risk associated with Mode 3 operations is conservatively performed by using the Mode 1 PRA and eliminating certain initiating events that do not impact Mode 3 operations. In the case of an inadvertent safety injection actuation the PRA model processes this initiating event as a reactor trip. This initiator is South Texas Unit 2 05000 499 SEQUENTIAL NUMBER 2003 � 02 � 00 not valid in Mode 3 due to plant conditions therefore there is no impact to plant risk.
CAUSE OF EVENT
The safety injection signal was generated when Low Compensated Steam Pressure signals were received on all three channels of Steam Generator 2B due to anticipatory gain as a result of dynamic pressure oscillations in the 2B Main Steam line. The pressure oscillations occurred due to phase changes as steam pushed past condensed water in the Main Steam line and PORV stack.
The root cause was that STP elected to operate the plant in a non-routine configuration without properly assessing the plant conditions and consequences that it created.
CORRECTIVE ACTIONS
1. All steam lines upstream of the Main Steam isolation valves were drained as part of the secondary startup once the condenser was available.
2. Plant operating procedure, OPOP03-ZG-0001, Plant Heat-up, has been revised to add guidelines for monitoring and draining the MS lines, if appropriate. The revision includes several cautions to the operator that draining condensation in Main Steam lines may be necessary.
3. STP will develop and implement a formal process for timely identification of planned ' infrequent/non-routine plant configurations; for assessing the possible plant conditions and consequences that could be created by these plant configurations; and for specifying any required compensatory actions. This action will be completed by July 31, 2003. In the interim, the Unit 1 forced outage approved work schedule and plant configuration is being reviewed and approved by the Shutdown Risk Assessment Group using proceduralized criteria. The Unit 1 Operations Manager leads the Shutdown Risk Assessment review.
The Unit Operations Manager with the input of appropriate key stakeholders reviews specific conditions that require additional consideration on Unit 2.
4. STP will add precautions or instructions to appropriate plant procedures to ensure it is recognized that condensation can build up in the Main Steam lines if the condenser is not available and there is no draining or blowdown of the line. These procedures will be identified and revised by July 31, 2003.
ADDITIONAL INFORMATION
Consideration will be given to developing a modification that will allow the Main Steam lines upstream of the Main Steam isolation valves to be drained when the condenser is not available.
MS PORV Operability:
STP concluded that the MS PORVs remained operable during and following this event. The PORV manufacturer, CCI (Control Components International), was consulted and concurred that these valves would pass water with no adverse affects.
When this event occurred, a significant amount of water vented through the MS PORV 2B upon actuation. MS PORV 2B behaved normally during the water venting and then proceeded to vent steam. This function of venting steam, for startup purposes, is not an automatic function, but at the discretion of the plant operator. This function is therefore not time sensitive in that PORV initiation is not required to take place within seconds. The need for PORV initiation during the initial heat-up or to switch to another train is a long-term evolution therefore it is a manual operation.
Because PORV initiation is a manual function and not time sensitive, the time to vent condensation from the Main Steam line does not interfere with the ability of the MS PORV to perform its design function to vent steam and control RCS pressure/temperature. The water collection issue is therefore not an operability issue because the PORV is capable of performing its design function.
STP also considered the operability of the MS safety valves and it was concluded that they remained operable. The Technical Specification Limiting Condition for Operation 3/4.7.1.1 Action (a) that applies to the MS safety valves is only applicable if all four reactor coolant loops are in operation. Only two loops were initially in operation and the water had been drained from the MS lines prior to putting all four reactor coolant loops in operation.
Pressurizer PORV lift:
Since the start of 2003, there have been four events resulting in the lifting of pressurizer PORVs in Unit 1 and Unit 2. Although none of these events proved consequential in terms of actual impact to nuclear safety, these events have challenged nuclear safety barriers. These events were looked at for common causes due to this adverse trend.
As a result, STP identified that operator knowledge regarding the pressurizer pressure master controller was deficient. Licensed operators received training on the pressurizer pressure control system. Corrective actions planned include formal training to all licensed operators on the operational characteristics of the pressurizer pressure control system. The training will include the lessons learned from these events. The lessons learned will also be included in the appropriate lesson plans for initial Licensed Operator Training.