Controlling fuel assembly stability in a boiling water reactor

A method of controlling the stability of the fuel assemblies in a boiling water reactor core comprising the steps of:

• (a) measuring on an on-line basis reactor parameters including, flow, temperature, control rod position and pressure,
• (b) operating a digital computer to generate from said measured reactor parameters, nodal distributions of selected reactor parameters with at least one node per fuel assembly in the radial plane and a plurality of spaced nodes in the axial direction;
• to calculate from said selected nodal distributions, a stability index for selected fuel assemblies, taking into account core physical parameters which are a measure of power level, axial power distribution, flow, enthalpy, void drift, detailed fuel rod dynamics, nuclear reactivity feedback and, where appropriate, flow cross-coupling in the radial and axial directions;
• to determine the least stable selected fuel assembly from said calculated stability indexes;
• and to interatively recalculate the stability index for an unstable least stable selected fuel assembly using assumed incremental changes in a selected one of the rod position and flow parameters until said recalculated stability index for the least stable fuel assembly represents a stable condition;
• and
• (c) generating output representations of the calculated stability indexes and of the cumulative change in said selected parameter required to make the least stable fuel assembly stable;
• and
• (d) adjusting the reactor controls to change said selected one parameter by said cumulative change to bring said least stable fuel assembly to a stable condition.

A system for controlling instability in the fuel assembly of a boiling water reactor comprising:

• means for measuring on-line, selected reactor parameters including:
• flow, temperature, control rod position and pressure;
• a digital computer programmed:
• to identify in real time from the selected reactor parameters those fuel assemblies having inlet power peaking and above average power levels;
• to calculate only for the identified fuel assemblies a stability index taking into account hydrodynamic and nuclear feedback effects;
• and to generate output signals representative of said stability indexes;
• and
• means for generating human readable representations of said identified fuel assemblies and their stability indexes from said output signals.

A boiling water reactor comprising:

• a reactor core having a plurality of fuel assemblies;
• control rods for adjusting the reactivity of said fuel assemblies in the reactor core;
• flow control means for controlling the flow of reactor coolant through the fuel assemblies of the reactor core;
• means for measuring selected reactor parameters including:
• flow, control rod position, temperature and pressure;
• digital computer means programmed:
• to generate nodal distributions of selected reactor parameters, with at least one node per fuel assembly in the radial plane and a plurality of spaced nodes in the axial direction;
• to select, using said nodal distributions, fuel assemblies having their peak axial power at a location below a selected location and having a power level above a preselected level to calculate from said nodal distributions a stability index for said selected fuel assemblies taking into account physical parameters which are a measure of power level, axial power distribution, flow, enthalpy, void drift, detailed fuel rod dynamics, nuclear reactivity feedback, and where appropriate, flow cross-coupling in the radial and axial directions;
• to determine the least stable selected fuel assembly from the calculated stability indexes;
• to iteratively recalculate the stability index for an unstable least stable selected fuel assembly using assumed incremental changes in a selected one of the rod position and flow parameters until said recalculated stability index for the least stable fuel assembly represents a stable condition;
• and to generate a control signal representative of the cumulative assumed changes in the selected one parameter;
• and
• control means to which said control signal is selectively applied to change the selected one of rod position or flow by the assumed cumulative amount.

Apparatus for controlling the stability of the fuel bundles in a boiling water reactor comprising:

• means for measuring on an on-line basis selected reactor parameters including flow, temperature, control rod position and pressure;
• a digital computer programmed to:
• (a) determine the radial and axial power distribution on a per fuel bundle basis using said selected parameters, (b) determine the peaking location for the axial power distribution for each fuel bundle and the average power level for the fuel bundles, (c) determine the stability margins only for those fuel bundles for which axial power peaking occurs below the average peaking location and for which the power level is above said average power level, and (d) iteratively repeat step (c) using incremental adjustments in a designated one of the flow and rod position parameters to determine a change in the designated parameter required to bring the stability margins for the selected fuel bundles within designated limits;
• and
• means for implementing said change in the designated parameter.