05000410/LER-2012-004

LER-2012-004, Manual Reactor Scram due to a Loss of Main Turbine Gland Sealing Steam Resulting in Lowering Condenser Vacuum

Docket Number
Event date:	07-12-2012
Report date:	09-10-2012
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 48097	10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation
4102012004R00 - NRC Website

A. PRE-EVENT PLANT CONDITIONS:

Prior to the event, Nine Mile Point Unit 2 (NMP2) was operating at approximately 96 percent of rated thermal power with no inoperable systems affecting this event.

B. EVENT:

On July 12, 2012 at 02:13, with NMP2 operating at approximately 96 percent power (3,811 MWt), plant operators identified that the indicated offgas system inlet pressure was rising and main condenser vacuum was lowering. In accordance with plant procedures, the operators lowered reactor power to approximately 85 percent by adjusting the reactor recirculation system flow. Offgas system inlet pressure continued to increase, and condenser vacuum continued to degrade, resulting in the operators initiating a manual reactor scram at 02:20. All control rods fully inserted and all systems functioned as expected following the scram. There was no impact on Nine Mile Point Unit 1 from this event.

The immediate cause of the increasing offgas system inlet pressure and flow and, the degrading condenser vacuum was the failure of the turbine gland sealing (TME) system to provide an adequate supply of sealing steam to the main turbine gland seals. This occurred because the steam supply to the inservice 'EV clean steam reboiler isolated due to a low water level condition, and the backup sealing steam supply (from the main steam system) was subsequently unable to provide steam at the pressure needed to effectively perform the gland sealing function.

This event involved the manual actuation of the Reactor Protection System, which resulted in a reactor scram. The NRC notification per 10 CFR 50.72(b)(2)(iv)(B) was completed on July 12, 2012, at 04:49 (Event Number 48097).

THE EVENT:

There were no inoperable structures, systems, or components at the time of the scram that contributed to this event.

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:

July 12, 2012 01:570 The steam supply to the inservice 'B' clean steam reboiler isolates due to a low water level condition.

02:130 Plarit operators reduce reactor power to approximately 85 percent by adjusting reactor recirculation system flow, after identifying that indicated offgas system inlet pressure is rising and main condenser vacuum is lowering.

02:200 A manual reactor scram is initiated when offgas system inlet pressure exceeds the procedural limit, and with main condenser vacuum continuing to degrade.

E. OTHER SYSTEMS OR SECONDARY FUNCTIONS AFFECTED:

No other systems or functions were affected.

F. METHOD OF DISCOVERY:

This event was discovered by the operators when indications of rising offgas system inlet pressure and lowering condenser vacuum were observed in the control room.

G. MAJOR OPERATOR ACTION:

Upon discovery of the rising offgas system inlet pressure and degrading condenser vacuum, the operators lowered reactor power to approximately 85 percent by adjusting the reactor recirculation system flow. When indicated offgas system inlet pressure exceeded 19 psia, the reactor was manually scrammed in accordance with plant procedures.

H. SAFETY SYSTEM RESPONSES:

Following initiation of the manual scram, all control rods fully inserted. No other operational conditions requiring the response of safety systems occurred as a result of this event.

II. CAUSE OF EVENT:

The turbine gland sealing (TME) system is designed to provide clean sealing steam from the clean steam reboilers. Backup sealing steam from the main steam system is provided in the event that the normal sealing steam source fails. The sealing steam prevents steam leakage out through the high-pressure turbine shaft and turbine steam control valves, and prevents air in-leakage through the low-pressure turbine shaft. Normal sealing steam operating pressure is approximately 4 psi.

The immediate cause of the increasing offgas system inlet pressure and flow and the degrading condenser vacuum, leading to the manual reactor scram, was the failure of the turbine gland sealing system to provide an adequate supply of sealing steam. The event was initiated by isolation of the steam supply to the in-service 'B' clean steam reboiler due to a low water level condition. The low water condition resulted from excess steam demand on the 'B' clean steam reboiler due to leakage past relief valve 2TME-RV135 located on the sealing steam piping downstream of the reboiler. A gag that had been incorrectly installed on 2TME-RV135 during the 2012 refueling outage as part of a permanent design change allowed leakage past the relief valve, resulting in a steam demand that exceeded the capacity of the makeup water supply line to the reboiler.

Following the trip of the 'B' clean steam reboiler, the backup sealing steam supply failed to maintain the turbine seals due to a low setpoint on the controller for initiation of the backup supply. The backup sealing steam supply initiated; however, the supply pressure was too low to maintain proper low-pressure turbine sealing. It was determined that, although the normal sealing steam system was being operated at higher than normal operating pressure (approximately 6 psi) to account for degraded 'A' low-pressure turbine seals, the controller for the backup sealing steam supply was set at 2.7 psi. The cause for this deficiency was inadequate guidance provided in the TME system operating procedure off-normal section. The procedure failed to address the impact of raising TME system pressure to account for the degraded low-pressure turbine seals.

The cause of the event is attributed to inadequate management guidance to ensure that changes in scope during engineering change package development are adequately reviewed and assessed.

The original scope of the design change regarding relief valve 2TME-RV135 was to perform a review to determine if a setpoint change would be necessary to support operation at extended power uprate conditions; however, the design change scope was subsequently altered to install the gag instead. The revised design change package did not contain an adequate level of detail in the installation instructions for gagging the relief valve, and did not contain adequate testing provisions to verify that the gag was installed properly. These deficiencies resulted in the loss of gland sealing steam to the turbine generator and the subsequent manual reactor scram. This event was entered into the Nine Mile Point Nuclear Station corrective action program as condition report number CR­ 2012-006615.

III. ANALYSIS OF THE EVENT:

This event involved a valid actuation of the Reactor Protection System which resulted in a reactor scram, and is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A).

There were no actual nuclear safety consequences associated with this event. All control rods fully inserted following initiation of the manual reactor scram. There were no other automatic initiations of safety systems, and immediate actions performed by the operators were adequate and appropriate in placing and maintaining the reactor in a safe shutdown condition. The loss of condenser vacuum is modeled in the probabilistic risk assessment (PRA) and contributes approximately 2 percent to the base core damage frequency. The manual reactor scram was without complications and was not risk significant. Based on this discussion, it is concluded that the safety significance of this event is low and the event did not pose a threat to the health and safety of the public or plant personnel.

The NRC performance indicator for Unplanned Scrams per 7,000 Critical Hours is projected to rise to approximately 1.96 and remains green. No other NRC performance indicators were impacted by this event.

IV. CORRECTIVE ACTIONS:

A. ACTION TAKEN TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL STATUS:

The gag installed in relief valve 2TME-RV135 was adjusted to secure the valve disk from opening, and a design change was implemented to increase the initiation setpoint for the backup sealing steam supply to address operation at the higher turbine gland sealing system pressure. Analyses have determined that sufficient margin exists within the turbine gland sealing system to continue operation at the higher operating pressure until repairs can be made to the 'A' low-pressure turbine seals. The turbine gland sealing system was returned to service, and the plant was subsequently returned to power operation (Mode 1) on July 15, 2012.

B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:

1. Revise the engineering change package management procedure to address required reviews when scope changes are identified. The revised procedure will also require that implementing departments review the installation and testing sections of design changes to ensure an adequate level of detail.

2. Revise the THE system operating procedure to require the following if the system is to be operated at pressures above the normal operating pressure: (1) prepare and issue an engineering change package (ECP) to reflect revisions to backup sealing steam supply initiation pressure; and (2) develop and implement a plan to monitor system margin and capacity to assure no adverse consequences.

3. A work order has been prepared to inspect/refurbish the 'A' low-pressure turbine seals during the 2014 refueling outage.

A. FAILED COMPONENTS:

There were no failed components that contributed to this event.

B. PREVIOUS LERs ON SIMILAR EVENTS:

submitted on May 5, 2006, describes an event in which NMP2 automatically scrammed from 86 percent power. The scram was caused by a main turbine trip due to low condenser vacuum that resulted from a failure in the main turbine gland sealing system. A high level condition caused isolation of the steam outlet valve from the inservice 'A' clean steam reboiler, and the backup sealing steam supply (from the main steam system) failed to function due to a disconnected mechanical linkage for the backup system pressure indicating controller. The actions taken following this event would not have prevented the July 12, 2012 event from occurring.

C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EllS) COMPONENT FUNCTION

IN THIS LER:

IEEE 803 FUNCTION IEEE 805 SYSTEM

COMPONENT IDENTIFIER IDENTIFICATION

Reactor Protection System N/A JC Condenser COND SG Main Turbine TRB TA Main Turbine Gland Sealing System N/A TC Clean Steam Reboiler RBLR TC Relief Valve RV TC

D. SPECIAL COMMENTS:

None