function u = ctrl_operation(t, x, plant, ref)
% CTRL_OPERATION  Stabilizing mode controller: hold T_avg at ref.T_avg.
%
%  Proportional feedback on external reactivity (rod worth) driven by
%  temperature error:
%       u = Kp * (ref.T_avg - T_avg)
%
%  Sign check: when T_avg > ref.T_avg, error < 0, u < 0 → rods insert,
%  power drops, T_avg falls.  Correct polarity for a PWR.
%
%  Gain Kp is a rough-out value.  Scale reasoning:
%   - alpha_c ~ -1e-4 /K means the moderator already supplies 1e-4/K of
%     restoring reactivity per Kelvin of temperature error.
%   - Kp = 1e-4 /K roughly doubles the effective moderator coefficient —
%     it damps transients without overwhelming the intrinsic feedback.
%   - Real PWR rod controllers use much smaller gains with a deadband.
%     We can retune or switch to PI once we care about steady-state offset.
%
%  Inputs:
%    t      - time [s]            (unused in this controller, kept for signature)
%    x      - state vector (10 x 1)
%    plant  - parameter struct     (unused here; reserved for future gain-scheduling)
%    ref    - struct with field .T_avg = target average coolant temperature [C]

    Kp = 1e-4;   % [dk/k per K]

    T_avg = x(9);
    e     = ref.T_avg - T_avg;
    u     = Kp * e;

end