05000263/LER-2010-005

LER-2010-005, Average Power Range Monitor Indication Outside Allowable Range

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability
2632010005R00 - NRC Website

The site conducted a downpower to 50% Rated Thermal Power (RTP) on November 4, 2010, to allow isolation of the 15B Feedwater Heater for repairs. Calculated Core Thermal Power (CTP)(JI) and the Average Power Range Monitors (APRM)(JI) readings were consistent at the end of the downpower.

Upon isolation of the 15B Feedwater Heater(HX), calculated CTP rose approximately 50MWth (or approximately 3% RTP) while the APRM reading rose approximately 1/2% RTP. Surveillance Requirement (SR) 3.3.1.1.2 requires verifying that the absolute difference between the APRM channels and the calculated power is 5. 2% RTP while operating at 25% RTP every 7 days. Since the readings between the two indications were now >2%, the requirements of SR 3.0.1 (Failure to meet a Surveillance, whether such failure is experienced during the performance of the Surveillance or between performances of the Surveillance, shall be failure to meet the LCO) were invoked. All four APRMs were declared inoperable and Technical Specification (TS) Limiting Condition for Operation (LCO) 3.3.1.1 declared not met. LCO actions A and C were entered. The APRM gains were adjusted to raise the APRM readings until within the 2% requirement and the TS LCO was met.

On 6 November, operators noted that Turbine (TG) electrical power output was not as high as it should be for the calculated CTP. An investigation determined that calculated CTP was indicating high based on an erroneous input in the calculation algorithm which indicated there was still feed flow from the isolated 15B Feedwater Heater. This erroneous input resulted from a combination of small instrument noise effects, isolated loop calibration errors, and an improper compensation for low flow level. This condition normally impacts operation only during plant startup with normal feedwater system lineup, a time during which the heat balance is unreliable and calculated CTP is not utilized.

Engineering performed a hand validation of thermal power, verifying that the actual power of the plant was approximately 50 MWth lower than calculated CTP. The calculation algorithm was corrected, LCO 3.3.1.1 was declared not met, and the APRM gains were adjusted to lower the APRM readings to within the 2% requirement. The LCO was then declared met.

Based upon the series of actions taken, the APRM system readings were initially correct, and reasonably assured to have been within 2% of actual RTP as required by SR 3.3.1.1.1 as demonstrated by successful past surveillances. When the APRM gains were increased to match CTP, the APRMs were adjusted high outside the allowed band, a condition that existed for approximately 60 hours6.944444e-4 days <br />0.0167 hours <br />9.920635e-5 weeks <br />2.283e-5 months <br />. The effects of this condition on reactor safety and design functions were evaluated, including the effects on high flux rod blocks and scrams, operation of the, Oscillation Power Range Monitors(OPRM) and operation of the Rod Block Monitor(RBM): In all cases, it was determined that the calibration errors resulted. in changes in the conservative direction. The safety function of the APRM system is to provide a high flux scram in response to a Control Rod Drop Accident. The scram setpoint is established by an Allowable Value of 5. 122% rated thermal power. This scram setpoint was not impacted by adjusting the gains of the APRM system upward, and the action would have resulted in an earlier actuation by the system than required. The APRM system Safety Function was maintained throughout this event.

This event is reportable to the NRC under 10 CFR 50.73(a)(2)(i)(B), Operation or Condition Prohibited by Technical Specifications due to being outside of the TS required band of ±2% for longer than allowed, and under 10 CFR 50.73(a)(2)(vii), Common-cause Inoperability of Independent Trains or Channels due to all four channels being set outside the band for the same reason. It is also reportable under 10 CFR 50.73(a)(2)(v)(A and D) — Event or Condition that could have Prevented Fulfillment of a Safety Function due to being Inoperable per Technical Specifications. However, an engineering evaluation has determined that at no time was there a condition that could have prevented the fulfillment of the safety function of the APRM system.

SAFETY SIGNIFICANCE

There were no nuclear, radiological or industrial safety significant consequences related to this event.

The Monticello risk assessment group reviewed the event for risk impact. Risk of a core damage or large early release from this event was not increased due to the APRMs being calibrated high since there was no loss of safety function or degradation in the ability of the APRMs to send a trip signal.

Also, the condition would have resulted in trip signal at a lower actual power level than required. The safety significance in terms of reactor safety and radiological release to the environment from all four APRMs being declared inoperable in this event is considered not to be significant.

CAUSE

The difference in APRM readings and calculated CTP was caused by a noise-induced input in the calculation algorithm which indicated there was still feed flow from the isolated 15B Feedwater Heater.

A failure to understand this issue resulted in the gains for the APRM system being adjusted upward to a value >2% higher than actual thermal power to match calculated CTP, which was reading high. This was a cognitive error by site personnel, but was not contrary to procedures.

CORRECTIVE ACTION

The following actions were taken or are planned and will be tracked in the Monticello Corrective Action Program:

• Corrected the calculation algorithm for CTP

• Adjusted APRM gains down to match calculated CTP ±2%

• A Root Cause Evaluation

• An evaluation of calculated CTP model inputs to identify applicability and limitations

• A review of procedures impacted by calculated CTP to identify needed revisions

PREVIOUS SIMILAR EVENTS

There were no similar events over the last three years.