05000389/LER-2011-002

LER-2011-002, Unplanned Automatic Reactor Trip During Reactor Protection System Testing

St. Lucie Unit 2
Event date:	06-06-2011
Report date:	11-21-2011
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3892011002R01 - NRC Website

Description of the Event

On June 6, 2011 while performing RPS [EIIS:JC] Logic Matrix testing per plant procedure 2-OPS-63.01, step 4.10.3.K directed that the matrix relay trip select switch be placed at mid-position between position 2 and position 3 to facilitate resetting and closing of two trip circuit breakers when the operator attempted to operate the switch his hand slipped off the switch. A second attempt to position the switch was made which resulted in the switch moving from position 2, past the mid- position and on to position 3, causing a reactor trip. Upon reactor trip all CEAs fully inserted into the core, auxiliary feedwater actuation system (AFAS) initiated as designed on low steam generator water level and was restored using auxiliary feedwater. All safety related systems functioned as designed.

Cause

A root cause determined the event was "human error vulnerability" resulting from a previous 1998 procedure change in the test methodology which required depressing the matrix relay hold pushbutton during the performance of the entire test, placing the circuit in a ready-to-actuate state.

A contributing cause was human error on the part of the operator during performance of the two-out-of-four trip functional test.

Inadequate "problem resolution" was also identified as a contributing cause. In 1998 a problem during the test was corrected by adding a caution note to the test procedure to maintain the matrix relay hold pushbutton depressed during the entire test. Additionally, use of a mechanical aid to keep the pushbutton depressed was approved without adequate benchmarking of Combustion Engineering plants with a similar RPS design.

Analysis of the Event

This event is reportable under 10 CFR 50.73(a)(2)(iv)(A), as any event or condition that resulted in a manual or automatic activation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section. The systems listed under (a)(2)(iv)(B) are (1) reactor trip and (6) auxiliary feedwater system.

Analysis of Safety Significance This event is considered an uncomplicated event, and although not specifically analyzed in the Chapter 15 accident analyses it is bounded by Chapter 15 analyzed events. Analyzed events are categorized into four categories. These categories are defined as Condition I, Normal operation and operational transients, Condition II, Faults of moderate frequency, Condition III, Infrequent faults and Condition IV Limiting faults. The basic principle applied in relating design requirements to each of the conditions is that the most probable occurrences should yield the least radiological risk to the public and those extreme situations having the potential for the greatest risk to the public shall be those least likely to occur. This event would correlate to a Condition II event. ANS Condition II occurrences are faults that may occur with moderate frequency during the life of the plant. They are accommodated with, at most, a reactor shutdown with the plant being capable of returning to operation after a corrective action. In addition, no ANS Condition II occurrence shall cause consequential loss of function of fuel cladding and reactor coolant system barriers.

Since all safety related systems functioned as designed there is no impact on the health and safety of the public.

The conditional core damage probability (CCDP) and conditional large early release probability (CLERP) associated with having an automatic reactor trip due to human action over 24 hrs is 1.17E-09 and 1.0E-12, respectively. Based on these values, the safety significance due to this event is minimal (Green).

Corrective Actions

The corrective actions and supporting actions listed below are entered into the site corrective action program. Any changes to the actions will be managed under the corrective action program.

1. Revise procedure 1/2-0SP-63.01 to remove the requirement to hold the Matrix relay hold pushbutton pressed in during the entire two-out-of-four trip functional testing sections.

2. Implement Engineering Change to replace the matrix relay hold pushbuttons for RPS and AFAS with rotary switches for both Unit 1 and 2.

3. Change the monthly functional test procedure for AFAS to reposition the system select switch to the off position prior to depressing the Matrix Relay Hold Pushbutton.

Similar Events A search of the correction action (CR) database for the last five years revealed no automatic reactor trips due to human error.

Industry operating experience from other Combustion Engineering Plants could have possibly avoided this event.