Pilgrim's Scram On MSIV Closure-Damaged Tubing Unistrut

Two recent events shows problems during refueling drywell/primary containment maintenance. This pulled out unistrut and Hope Creeks misaligned SRV and discharge line.    

So what about the extent of cause/condition on all relays?  

During the post trip review of the event, an anomaly was identified in the reactor protection system (RPS) channel A2. Reactor pressure exceeded the value for generating a reactor pressure trip signal. However, the channel A2 reactor high pressure trip was not recorded in the plant process computer log of the event.  The most probable cause was oxidation on the relay contacts that provide the signal to the plant process computer. The relay contacts were burnished.

All I can say is, think about all the tremendous energy that is going on here and just seconds for relays to do they protective functions.

Can they get away from relays today... 

Licensee Event Report 201 5-007-00, Single Main Steam Isolation Valve Closure Resulting in Automatic Reactor Scram 

On Saturday, August 22, 2015, at 1628 [EDT], with the reactor at 100% core thermal power the Pilgrim Nuclear Power Station experienced an automatic reactor scram due to Average Power Range Monitor High Flux signal from the rapid closure of one main steam isolation valve (MSIV). All plant systems responded as designed.

The Root Cause of this event is inadequate worker practices resulting in an excessive load being applied to the unistrut enclosed pneumatic line (during or prior to 2001) causing it to lay on a Main Steam Line subject to vibration. The excessive load caused an initiating crack in the nipple threads that over time resulted in a shear of the line.

The pneumatic supply line nipple was replaced and the line was re-supported. 

This event posed no threat to public health and safety.

BACKGROUND

There are four Main Steam lines and each line contains two Main Steam Isolation Valves (MSIVs), one inboard valve located in the Drywell and one outboard valve located in the Steam Tunnel. The MSIVs are normally open during normal plant operation. Each MSIV is a 20" diameter Y-body globe valve with a pneumatic actuator to open and pneumatic & spring to close. The valve is a fail-safe closed design. A hydraulic cylinder sharing a common piston rod is used to control the closure speed via a needle valve. The safety function of these valves is to automatically close on a Primary Containment Isolation System Group 1 isolation signal.

EVENT DESCRIPTION:

On Saturday, August 22, 2015, at 1628 [EDT], with the reactor at 100% core thermal power the Pilgrim Nuclear Power Station (PNPS) experienced an automatic reactor scram signal due to the rapid closure of one MSIV. Inboard MSIV AO-203-1 C had closed.

Primary Containment Isolation System Group II - Sampling Systems, Group VI - Reactor Water Cleanup System and Reactor Building Isolation System isolations occurred as expected due to the reactor scram at 100 percent power.

Reactor water level was restored to normal and maintained by the feedwater system. Reactor pressure control was maintained using the main turbine bypass valves. The plant was brought to a cold shutdown condition using the normal systems. All plant systems responded as designed to perform the safety functions.

The primary containment was de-inerted. A drywell entry was made to investigate the cause of the inboard MSIV AO-203-1 C closure. The pneumatic supply to the inboard MSIV AO-203-1 C valve control manifold was found sheared.

During the post trip review of the event, an anomaly was identified in the reactor protection system (RPS) channel A2. Reactor pressure exceeded the value for generating a reactor pressure trip signal. However, the channel A2 reactor high pressure trip was not recorded in the plant process computer log of the event. Due to the de-energize-to-actuate circuit design, once a scram trip signal is generated within the channel (in this case a reactor high flux signal), actuation of another signal has to be determined by other means, usually by review of the plant process computer log. Investigation of the circuit did not determine a definitive cause. The instrumentation loop was calibrated and a relay replaced prior to return to service. The most probable cause was oxidation on the relay contacts that provide the signal to the plant process computer. The relay contacts were burnished.

CAUSE OF THE EVENT

Direct Cause: The cause of the event is that the pneumatic supply to the inboard MSIV AO-203-1C valve control manifold was found sheared at a threaded nipple location.

Root Cause: The Root Cause of this event is inadequate worker practices resulting in an excessive load being applied to the unistrut enclosed pneumatic line (during or prior to 2001) causing it to lay on a Main Steam Line subjecting the line to vibration. The excessive load caused an initiating crack in the nipple threads that over time resulted in a shear of the line.

Contributing Cause: Inadequate identification and repair of adverse conditions. When the support failure was identified in 2001, an inadequate repair was performed.

CORRECTIVE ACTIONS

Completed:

1. The pneumatic supply nipple was replaced and the piping was re-supported.

2. A leak check of all fittings to the inboard MSIVs was performed.

Planned: 

Additional corrective actions are captured in the corrective action program by Condition Report CR-PNP-2015- 07285.

SAFETY CONSEQUENCES

UFSAR Appendix R.2.1 .3.2 provides a summary of the initial core analysis for "Closure of One Main Steam Isolation Valve." A single MSIV closure with initial power is expected to cause a high neutron flux scram, increased reactor pressure, and increased fuel temperature. UFSAR Figure R.2-5 illustrates the predicted transient response including flux above the high neutron flux scram setpoint which causes the reactor scram.

No fuel damage or challenge to safety limits occurs.

Abnormal Operational Transients (AOT) are evaluated in the Updated Final Safety Analysis Report (UFSAR) to ensure that fuel cladding Minimum Critical Power Ratio (MCPR) Safety Limits are satisfied given transients caused by a single operator error or equipment malfunction. The spectrum of postulated initiating transient event categories is identified in U FSAR Section 14.3.2. The transient analysis contained in UFSAR Section 14 is supplemented by the original transient analysis contained in UFSAR Appendix R.

Closure of a single MSIV is an evaluated transient that is not required to be evaluated for each core reload because the transient is non-limiting. Only the most limiting AOT events affecting MCPR are addressed for each core reload to validate safety system settings and establish reactor fuel thermal limits.

This event is bounded by the analysis.