05000456/LER-2010-005

LER-2010-005, Incorrect Methodology Used in Calculations in 1999 Resulted in Non-Conservative Control Room Outside Air Intake Monitor Alarm Setpoints

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
4562010005R00 - NRC Website

A. Plant Operating Conditions Before The Event:

Event Date: November 12, 2010� Event Time: 0818 CDT Unit: 1 MODE: 1 Reactor Power: 99 percent Unit: 2 MODE: 1 Reactor Power: 100 percent Unit 1 Reactor Coolant System [AB]:� Normal operating temperature and pressure Unit 2 Reactor Coolant System:� Normal operating temperature and pressure

B. Description of Event:

No structures, systems, or components were inoperable at the start of this event that contributed to the event.

On November 12, 2010, during a review of setpoint calculations for process monitors (PR) [IL], an error was identified in the methodology used in a historic (December 1999) calculation of the setpoints for the control room outside air intake noble gas channels (OPR31B, OPR32B, OPR33B and OPR34B), making the setpoints non­ conservative.

At 0818 on November 12, 2010, the four channels were declared inoperable, and Technical Specification (TS) Limiting Condition for Operation (LCO) 3.3.7, "Control Room Ventilation (VC) Filtration System Actuation Instrumentation," Conditions A and B were entered, which required placement of the control room ventilation [VI] in Emergency mode within one hour.

The setpoint calculations and setpoints were revised for the gas channels.

At 1725 on November 12, 2010, LCO 3.3.7 Conditions A and B were exited when the four PR channel setpoints were revised and were declared operable.

This event is being reported pursuant to 10 CFR 50.73(a)(2)(i)(B), any operation or condition which was prohibited by Technical Specifications. This event was originally reported via ENS pursuant to 10 CFR 50.72(b)(3)(v), any event or condition that at the time of discovery could have prevented the fulfillment of the safety function. It was subsequently determined that there was no loss of safety function.

C. Cause of Event

The apparent cause for this event was that the procedure for determining the alarm setpoints did not have details of the methodology for the calculation.

The investigation identified an error in the methodology used in a December 1999 calculation of the setpoints for the control room outside air intake noble gas channels, making the setpoints non-conservative, (i.e., approximately 45% higher than the required high alarm setpoints). From a review, it was determined that the procedure did not contain sufficient instruction on methods to calculate the setpoints. Without the detailed instruction, the preparer is left to use his own judgment to determine the methodology and perform the calculation. Additionally, the reviewer had no procedure guidance to refer to for the methodology, and the procedure form documented the old setpoint, the new setpoint, and signatures of the preparer and reviewer, but not the actual calculation.

D. Safety Consequences:

There were no safety consequences impacting plant or public safety as a result of this event.

There are two trains of VC. Detectors OPR31B and OPR32B are interlocked with the A train, and detectors OPR33B and OPR34B are interlocked with the B train. On a high radiation signal for the detectors of the respective train, the train-specific ventilation systems realign to the Emergency mode to support control room habitability.

The applicable design basis accidents are main steam line break, reactor coolant pump shaft seizure, rod cluster control assembly ejection, steam generator tube rupture, loss of coolant accident, and postulated fuel handling accident. For control room habitability analysis, activity released during the initial 30 minutes of the accident is assumed not filtered to compensate for time to manually realign the VC system to the Emergency mode of operation.

The TS required monitor is the noble gas detector in the control room outside air intakes. The High alarm was set at 2.9 mrem/hr verses required 2.0 mrem/hr (noble gas exposure) for the automatic ventilation swap to Emergency mode. This yields a net 0.9 mrem/hr above the expected dose rate. Since the accident analysis calculations assume manual swap of ventilation within 30 minutes, the excess dose rate for the alarm would only affect the control room for a maximum of 30 minutes. This yields 0.9 mrem/hr x 0.5 hr = 0.45 mrem (noble gas exposure).

The radiation exposure consequence to control room personnel from the 45% higher trip alarm is approximately an additional 0.5 mrem for noble gas exposure.

Since the design basis accidents do not credit automatic actuation of the VC system into the Emergency mode from a high radiation signal, this event did not result in a safety system functional failure.

E. Corrective Actions:

The corrective actions include:

• An extent of condition review was performed and verified that required TS radiation monitor setpoints are set in compliance with TS requirements.

• The setpoint calculations were revised and the setpoints for OPR31B, OPR32B, OPR33B and OPR34B were reset to the correct values.

• Review the applicable radiation monitor procedures that perform setpoint calculations for the process radiation monitors, and revise them to address details on the method used for setpoint calculations, and include instructions for the basis of the review criteria.

F. Previous Occurrences:

There have been no previous, similar Licensee Event Reports identified at the Braidwood Station.

G. Component Failure Data:

Manufacturer� Nomenclature� Model Mfg. Part Number N/A� N/A� N/A N/A