05000410/LER-2013-004

LER-2013-004, Manual Reactor Protection System Actuation due to Loss of Reactor Recirculation Flow

Nine Mile Point Unit 2
Event date:	12-2-2013
Report date:	1-31-2014
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 49593	10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation
4102013004R00 - NRC Website

A. PRE-EVENT PLANT CONDITIONS:

Prior to the event, Nine Mile Point Unit 2 (NMP2) was lowering reactor power level to remove the main turbine from service to support maintenance. The plant was operating at approximately 25% power when the manual reactor scram was inserted.

B. EVENT:

On December 2, 2013, at 0900, Unit 2 Operations was lowering reactor power level to remove the main turbine from service to support maintenance on a 5th point Feedwater heater level control valve. At approximately 41% reactor power, Operations attempted to shift reactor recirculation pumps 2RCS-P1A and 2RCS-P1B from fast speed to slow speed in support of the plant downpower, in accordance with the operating procedure. During the downshift, the high speed power breakers for both recirculation pumps (CB5A/B) tripped as expected, neither LFMG supply breaker (CB1A/B) closed, resulting in a failure of the LFMGs to start. As a result, the LFMG output breakers (CB2A/B) did not close due to the required permissives not being met. This caused a loss of reactor recirculation flow and a reduction in reactor power to 25.5%.

Operations was able to successfully close both LFMG supply breakers (CB1A/B) to manually start the motor generator sets, but the speed of the recirculation pumps had slowed to a value that prevented the LFMG output breakers (CB2A/B) breakers from closing. It was at this point that Operations manually initiated a reactor scram in accordance with the special operating procedure N2-SOP-29, "Sudden Reduction in Core Flow.

This automatic start feature of LFMGs does not affect Nine Mile Point Unit 1 because the LFMGs are not used.

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 December 2, 2013 at 1103 (Event Number 49593).

EVENT:

There were no other inoperable structures, components, or systems that contributed to the event.

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:

December 2, 2013 0508 NMP2 commenced lowering power to remove the Main Turbine from service to support maintenance.

0903 Both recirculation pumps failed to start automatically in slow speed when down shifting.

Operators attempted to start the recirculation pumps manually.

0904 A manual reactor scram is initiated when the recirculation pumps did not restart and the loss of recirculation flow condition was not corrected.

1046 One recirculation pump restarted in slow speed.

1048 Second recirculation pump restarted in slow speed.

E. OTHER SYSTEMS OR SECONDARY FUNCTIONS AFFECTED:

There were no other systems or secondary functions affected.

F. METHOD OF DISCOVERY:

This event was discovered when operators observed the loss of recirculation flow condition in the control room.

G. MAJOR OPERATOR ACTION:

Upon discovery of the loss of recirculation flow condition, operators attempted to restart the LFMGs manually. The LFMGs were able to be started manually, but the output breakers did not close and the recirculation system pumps did not start in slow speed due to the required speed of the recirculation system pumps being too low. 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:

Both reactor recirculation pumps failed to downshift due to high resistance connection(s) in the auto transfer circuits, associated with switches SIO1A and SIO1B. This caused the auto transfer relays K121A and K121B to not actuate during the reactor recirculation high to low speed transfer.

The industry guidance is that hand switches are exempt from the AP-913 process as defects could be found during routine surveillance and rotation of equipment activities and the equipment repaired without generating scrams or adversely impacting plant operations. Oxidation of contacts that occurs naturally over time would be addressed by the periodic cycling of the switches during surveillance and equipment rotation activities as well. This assumption should not have been applied in the case of reactor recirculation switches SIO1A and SIO1B as they are only cycled during plant startup, power ascension, and down-power activities to less than 40% which may only occur once every two years.

The infrequent operation of these switches led to buildup of oxidation or other non-conductive material on the contacts resulting in a latent failure of the switches. Preventive maintenance on control switches SlOIA and SIO1B, be it regularly cycling the switch or cleaning the contacts, is not possible due to the plant impact and physical location of the switch.

The root cause of this event is a failure to identify that the control switches for the reactor recirculation pump 1A and pump 1B motor breaker control (switches SIOIA and S101B respectively) are single point vulnerable (SPV) components because they were exempted from the AP-913 classification process. Since the switches were not classified as SPV components no mitigation strategies were developed. The complex design in place for the reactor recirculation pumps control circuitry combined with the application of industry guidance associated with hand switches resulted in a failure to identify the switches as SPV components.

This event has been entered into the Nine Mile Point Nuclear Station corrective action program as condition report number CR-2013-009735.

III. ANALYSIS OF THE EVENT:

There were no actual nuclear safety consequences associated with this event. This event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as a condition that resulted in manual or automatic actuation of any of the systems listed in 10 CFR 50.73(a)(2)(iv)(B). The Reactor Protection System is listed in 10 CFR 50.73(a)(2)(iv)(B).

The plant transient that occurred is similar to the Trip of Two Reactor Recirculation Pumps transient described in Updated Safety Analysis Report (USAR), Chapter 15, Section 15.3, "Decrease in Reactor Coolant System Flow Rate." The December 2, 2013 recirculation pump transient was bounded by the USAR described transient assumed to occur at more limiting high power conditions. The transient described in the USAR is terminated by the Reactor Pressure Vessel (RPV) level 8 (L8) trip and subsequent turbine trip resulting in a reactor scram. In the actual transient, operator action was taken prior to any adverse trends in reactor water level being noted. During this event the Mode Switch was placed in shutdown in accordance with procedure N2-SOP-29, "Sudden Reduction in Core Flow." The USAR described transient describes reactor vessel water level control through the use of High Pressure Core Spray (HPCS) or Reactor Core Isolation Cooling (RCIC) systems. Reactor vessel water level was controlled by the operators by lowering pressure and using the condensate booster pumps. The use of IIPCS or RCIC was not required. During the transient, a subsequent reactor scram signal was received on low reactor water level (L3). This is an expected signal due to water level shrink. One recirculation pump was restarted in slow speed at 1046, 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and 43 minutes after the loss of recirculation flow, and the second recirculation pump was restarted at 1048. These actions are more conservative than those covered in the USAR; therefore this transient is fully bounded by the event analysis per USAR section 15.3.

Based on the above 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.

This event affects the NRC Regulatory Oversight Process (ROP) Index for Unplanned Scrams. Due to this scram, the Unplanned Scrams Index value will be 0.8 compared to the Green-to-White threshold value of greater than 3. This reduction will not result in entry into the Increased Regulatory (White) Response Band.

IV. CORRECTIVE ACTIONS:

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

1. Cleaned the contacts on the control switch.

B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:

1. Revised the operating procedure to start the LFMG sets using their respective control switches prior to initiating the reactor recirculation downshift logic, and to install a temporary jumper to defeat the automatic transfer relay from preventing the high-speed to low-speed transfer.

2. Revise the operating procedure, to include guidance to cycle the S101A and/or SIOIB switches if LFMG transfer does not immediately occur.

A. FAILED COMPONENTS:

There were no other failed components that contributed to this event.

B. PREVIOUS LERs ON SIMILAR EVENTS:

There have been no previous similar LERs for NMP2.

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

COMPONENT

IEEE 803

IDENTIFIER

IEEE 805 SYSTEM

IDENTIFICATION

SBM Type Control Switch HS AD Low Frequency Motor Generator MG AD Reactor Recirculation Pump P AD Automatic Transfer Relay 83 AD Reactor Recirculation System NA AD High Pressure Core Spray System NA BG Reactor Core Isolation Cooling System NA BN

D. SPECIAL COMMENTS:

None