05000278/LER-2009-002

LER-2009-002, Inoperable 'A' Wide Range Neutron Monitor Results in Condition Prohibited by Technical Specifications

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2782009002R00 - NRC Website

Unit Conditions Prior to the Event Unit 3 entered Mode 2 in preparation for a planned startup from an outage to replace the 3C Main Transformer when this event occurred on 1/26/09. The 'E' Wide Range Neutron Monitor (WRNM) was considered inoperable. The reason for the inoperability of the 'E' WRNM is not known to be related to the reason that the 'A' WRNM became inoperable. There were no other structures, systems or components out of service that contributed to this event.

Description of the Event

As a result of an analysis of a cause evaluation completed on 2/25/09 involving an inoperability of the 'A' WRNM (EIIS: MON), it was identified that a condition prohibited by Technical Specifications (TS) occurred when Unit 3 entered Mode 2 operations for plant startup on 1/26/09 at 0900 hours0.0104 days <br />0.25 hours <br />0.00149 weeks <br />3.4245e-4 months <br />. Specifically, the TS 3.0.4 requirements were not met to allow for an entry into a mode of applicability for TS equipment that was inoperable. The evaluation had determined that the cause of the inoperable 'A' WRNM was a result of inadequate human performance regarding a technical decision made during the outage (prior to 1/26/09 startup) that affected the operability of the 'A' WRNM. Specifically, the technical decision allowed for entry into Mode 2 after an adjustment was made to the mean square voltage (MSV) component of the WRNM function resulting in the MSV being inaccurate for a small range of neutron flux while in Mode 2.

The formal cause evaluation was based on an Engineering evaluation completed on 2/4/09, which determined that the 'A' WRNM was inoperable during the plant startup on 1/26/09. In the evaluation, Engineering personnel determined that operability of the 'A' WRNM could not be justified over the complete range of neutron flux during Mode 2 startup operations. Although the range of concern was small when compared to the entire range of neutron fluxes experienced while in Mode 2, it was determined that the 'A' WRNM was inoperable during Mode 2.

Technical Specification Limiting Condition for Operation (LCO) 3.3.1.1, Reactor Protection System (RPS) Instrumentation requires that at least three of the four channels of WRNMs be operable for each RPS (EIIS: JC) trip system. There are 2 RPS WRNM trip systems. The 'A', 'C', 'E' and 'G' WRNMs belong to the 'A' RPS trip system. The WRNMs provide trips for a reactivity short period during startup operations. Because the 'E' WRNM was already inoperable and the 'A' WRNM became inoperable as a result of maintenance activities during the unit shutdown, only two WRNMs were operable when the unit was placed in Mode 2 on 1/26/09 at 0900 hours0.0104 days <br />0.25 hours <br />0.00149 weeks <br />3.4245e-4 months <br />.

� Description of the Event, continued This report is being submitted pursuant to:

10CFR 50.73(a)(2)(i)(B) — Condition Prohibited by TS — This occurrence is reportable under this criterion since the LCO 3.0.4 requirements were not met for allowing entry into a mode of applicability for inoperable equipment.

Unit 3 criticality was achieved on 1/26/09 at approximately 1142 hours0.0132 days <br />0.317 hours <br />0.00189 weeks <br />4.34531e-4 months <br />. A 1/2 scram was received at approximately 1200 hours0.0139 days <br />0.333 hours <br />0.00198 weeks <br />4.566e-4 months <br /> as a result of a short period sensed by the 'A' WRNM.

Subsequent analysis determined that the 'A' WRNM was operable after the 1/2 scram since it had passed its range where it was considered inoperable.

Analysis of the Event

There were no actual safety consequences associated with this event.

The safety objective of the WRNM system (EIIS: 1G) is to detect conditions in the reactor core that could potentially threaten the overall integrity of the fuel barrier. In the startup range, the most significant source of reactivity change is due to control rod withdrawal. The WRNM provides mitigation of the neutron flux excursion.� The safety analysis evaluates the consequences of control rod withdrawal events during startup that are mitigated only by the WRNM period-short trip function.

There are a total of eight WRNMs in the reactor core. The A and B RPS trip systems have independent WRNM inputs. The A, C, E, and G channels of WRNM input the A RPS trip system. The B, D, F, and H channels of WRNM input the B RPS trip system. The safety analysis assumes that one channel in each trip system is bypassed. Therefore, six channels with three channels in each trip system are required for WRNM operability to ensure that no single instrument failure will preclude a scram from this function on a valid signal.

In Mode 2 (Startup) and Mode 5 (Refueling), the WRNM system provides short-period trips to RPS.

The WRNM system provides inputs into the RPS circuitry to ensure a reactor scram occurs in the event that core reactivity increase (shortening period) exceeds a predetermined reference rate. The TS allowable value for WRNM short period is > 13 seconds. The WRNM provides diverse protection from the Rod Worth Minimizer (RWM), which monitors and controls the movement of control rods at low power. The RWM prevents the withdrawal of an out-of­ sequence Control Rod during startup that could result in an unacceptable neutron flux excursion. The RWM was not affected by this event.

� Analysis of the Event, continued In MODE 1, the Average Power Range Monitor (APRM) system and the Rod Worth Minimizer (RWM) provide protection against control rod withdrawal error events and the WRNMs are not required. The WRNMs are automatically bypassed when the mode switch is in the Run position (i.e., Mode 1 operations).

The indicated WRNM Flux is generated from a combination of two primary inputs: the counting based neutron flux and the mean square voltage (MSV) based neutron flux. At low neutron flux levels (less than 3.2E8 nV), the counting based flux is more accurate than the MSV based neutron flux, and at high neutron flux levels (greater than 1.0E9 nV) the MSV flux is more accurate. An intermediate neutron flux region exists where both flux measurement methods are accurate. This is called the transition region, and is defined based on the measured MSV neutron flux. The transition region is the flux level from 3.2E8nV to 1.0E9nV. Below the transition region ( flux. Above the transition region (>1 .0E9nV), the WRNM neutron flux is based on the MSV flux.

Within the transition region, the WRNM neutron flux is a weighted average of the counting and MSV flux values. The 1/2 scram experienced on 1/26/09 occurred as a result of the affects of the MSV flux overcoming the MSV flux offset, which was adjusted during the unit shutdown. Just prior to the MSV flux overcoming the MSV flux offset, the reactor period measurement was still calculated from the counting flux component. Consequently, when the transition from the counting flux to the MSV flux occurred, the transition was not smooth, but was seen as a step change in flux. The 'A' WRNM period calculation included this upward step change and indicated a very short period, resulting in a 1/2 scram ('A' RPS trip).

Because the RWM was operable and the inoperability of the 'A' WRNM only occurred during the transition region, the impact of the event was small. The 'C' and 'G' WRNMs were operable throughout the event as well as the WRNMs associated with the 'B' RPS trip system. The actual time that the unit operated in the transition region (i.e., 'A' WRNM inoperable) was less than 5 minutes.

The WRNM is provided by General Electric Company, NUMAC Chassis, Model # 304A3712G005.

Cause of the Event

The cause of the event was primarily due to inadequate technical human performance associated with a decision made on 1/23/09 to allow an MSV flux offset adjustment made to the 'A' WRNM during the outage to remain in place for the 1/26/09 unit startup (Mode 2).

_ Cause of the Event, continued On 1/21/09, during a unit shutdown, the 'A' WRNM was declared inoperable due to inadequate monitor response. The 'A' WRNM was later found to be reading high as a result of electrical noise associated with the MSV neutron flux component of the WRNM. This noise was cancelled during the shutdown by adjusting the MSV flux offset on 1/22/09 to 8.0E9 nV. This was acceptable for shutdown conditions and performed in accordance with a procedure.

However, on 1/23/09, inadequate technical input was provided by Engineering and Instrumentation & Controls (I&C) personnel (Utility, non-licensed) to Operations personnel (Utility, licensed) on 1/23/09 to allow this adjustment to remain in place for plant startup. This incorrect decision was based on a misinterpretation of a previous Engineering evaluation performed for a MSV flux offset adjustment. This previous Engineering evaluation was only for troubleshooting conditions when the WRNM was not required to be operable.

A contributing cause to allow this offset to not be resolved prior to plant startup was a weakness with the associated l&C procedure (IC-11-00395). This procedure controls the adjustment of the MSV flux offset value and required the flux offset to be lower than the threshold for entering the transition range (i.e., 3.0E8 nV). If the MSV flux offset was left equal to or higher than this value, l&C supervision was required to be notified. This action to notify l&C supervision was determined to be an inadequate procedural barrier for assuring appropriate MSV flux offset adjustment values.

Corrective Actions

Individuals involved with the event have been counseled regarding the importance of rigorous technical evaluations when making decisions that could affect TS equipment performance.

The lessons learned from this human performance issue will be shared with other station personnel in accordance with the site Corrective Action Program.

IC-11-00395 and the WRNM vendor manual will be upgraded to ensure that the limits of MSV offset flux are well controlled and documented.

The source of the 'A' WRNM electrical noise will be investigated and resolved.

Previous Similar Occurrences There were no previous LERs identified relating to inoperable WRNMs or inadequate technical human performance associated with operability determinations involving plant instrumentation.

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