Negative Flux Rate Trip

A Negative Flux Rate Trip (NFRT) aka High Negative Rate Trip is designed to detect a neutron changes from a fuel drop accident and trip the reactor to shutdown.

The original design basis for the NFRT function was to mitigate the consequences of one or more dropped rod cluster control assemblies (RCCAs). The intent was that in the event of one or more dropped RCCAs, the reactor trip system would detect the rapidly decreasing neutron flux (i.e. high negative flux rate) due to the dropped RCCA(s) and would trip the reactor, thus ending the transient and assuring that DNB limits were maintained.

In 1982, an evaluation prepared by Westinghouse Electric Corporation and documented in WCAP-10297-P-A, "Dropped Rod Methodology for Negative Flux Rate Trip Plants," determined that the NFRT function was only required when a dropped RCCA or RCCA bank exceeded a specific reactivity worth threshold value. Any dropped RCCA or RCCA bank which had a reactivity worth below the threshold value would not require a reactor trip to maintain DNB limits. An additional evaluation method, WCAP-11394-P-A, was developed by Westinghouse Electric Corporation in 1987, which determined that sufficient DNB margin existed for Westinghouse plant designs and fuel types without the NFRT function regardless of the reactivity worth of the dropped RCCA or RCCA bank, subject to a plant/cycle-specific analysis. The NRC subsequently reviewed and approved the Westinghouse analysis method and results and concluded that the analysis contains an acceptable procedure for analyzing the dropped RCCA event for which no credit is taken for any direct reactor trip due to the dropped RCCA(s) or for automatic power reduction due to the dropped RCCA(s). Therefore, the NFRT function is not required to maintain existing DNB limits and may be eliminated.

Removal of function

Many plants removed the original function because the trip may be false and cause a unwarranted plant scram.