05000458/LER-2001-002

LER-2001-002,

Docket Number
Event date:
Report date:
4582001002R00 - NRC Website

REPORTED CONDITION

On October 4, 2001, a review by River Bend engineering of a GE report determined that the station has in the past potentially operated slightly above the maximum core thermal power as authorized in the facility operating license. This event is being reported as required by River Bend's license condition 2.E under the 10CFR50.73 reporting format.

In September 2001, GE issued a report titled "Impact of Steam Carryover Fraction on Process Computer Heat Balance Calculations." The GE report details the condition in which a nonconservative error in the process computer software causes the calculation of core thermal power to be slightly low.

This error leads to a scenario in which actual core thermal power can, unknown to the operator, be slightly above the maximum value allowed by the operating license. It is estimated that the station has operated, at times, approximately three megawatts thermal over its licensed maximum limit.

INVESTIGATION

The performance specifications on most boiling water reactor (BWR) steam dryers require less than 0.1 percent steam carryover fraction (moisture content in the steam) leaving the reactor vessel. This specification value is commonly used in the process computer for core thermal power calculations.

Measurements of the carryover fraction produced by later generations of BWR's has shown that steam dryer performance has improved significantly as dryer designs have evolved.

At the time of this investigation, River Bend used a design value of 0.1 percent for the moisture carryover fraction in the core thermal power calculation performed by the plant process computer.

CORRECTIVE ACTIONS

General Electric has not provided a determination of whether the 0.1 percent value is a part of the licensed core monitoring methodology, and is currently working on a Service Information Letter to address this condition.

The moisture carryover constant in River Bend's process computer was previously defined to be 0.1 percent. As a conservative measure to assure compliance with the operating license, that value has been changed to 0.0. Any process computer heat balance subroutines will now utilize the new constant value of 0.0, pending the disposition of the forthcoming GE Service Information Letter.

SAFETY SIGNIFICANCE

As stated in the General Electric report, use of the carryover fraction of 0.1 percent in thermal power calculations, while nonconservative, does not represent a safety issue. This error in calculated core thermal power is an order of magnitude less than the precision in the minimum critical power ratio evaluation process. The 0.1 percent error in core thermal power is significantly less than the precision of the process computer core thermal power estimate. Additionally, River Bend's accident analysis assumes through various methodologies that core thermal power is 2 percent greater than maximum rated at the initiation of an event, in accordance with Regulatory Guide 1.49. The error induced by the nonconservative carryover fraction caused reactor power to be, at maximum, approximately 100.1 percent of rated. Therefore, this condition did not have the potential to cause fuel thermal limits to be exceeded. This condition was of minimal consequence with respect to the health and safety of the public.