Rod Worth Minimizer

Rod Worth Minimizer (RWM) is a computer system which establishes an optimal control rod movements to minimize reactivity changes. It provides input to the rod block monitor system during low power operations.

The RWM enforces the banked position withdrawal sequence (BPWS) to ensure that the initial conditions of the CRDA analysis are not violated. The analytical methods and assumptions used in evaluating the CRDA. The BPWS requires that control rods be moved in groups, with all control rods assigned to a specific group required to be within specified banked positions.

In an operating reactor, there is a spectrum of possible control rod worths, depending on the reactor state and on the control rod patterns chosen for operation. Control rod withdrawal sequences and patterns are selected prior to operation to achieve optimum core performance and, simultaneously, low individual rod worths.

The RWM ensures the reactor operator adheres to a predetermined sequence of control rod withdrawals or insertions when the reactor is operating at low power levels. The predetermined sequence is designed to limit rod worths. Operation with low rod worths minimizes the effect of an accident such as a rod drop, thereby lessening the chances that such an accident would cause damage to the fuel or Primary Coolant System.

As core power increases, rod worth decreases; therefore, maintenance of low rod worths becomes critical only when operating at relatively low power. For this reason, the RWM enforces adherence to the rod sequence pattern only when core power is below the Low Power Setpoint (LPSP). When core power is below the LPSP, the RWM monitors each control rod selection and movement and compares them to the selection and movement called for by the sequencing enforced. If the operator deviates significantly from the sequence pattern, the RWM will alarm and may apply rod blocks to prevent further rod movement in one or both directions until the out-of-sequence condition is corrected. When core power is above the LPSP, the RWM does not block out-of-sequence rod movements, however blocks are still imposed if hardware fails, and certain alarm and error indications continue to be provided until core power rises above the Low Power Alarm Point (LPAP), which is set somewhat higher than the LPSP.