function u = ctrl_shutdown(t, x, plant, ref)
% CTRL_SHUTDOWN  Hot-standby / cold-shutdown passive controller.
%
%  Holds the reactor deeply subcritical with rods fully in.  No feedback —
%  just a constant negative external reactivity that dominates any
%  temperature-feedback contribution the reactor could plausibly produce
%  in its accessible state set.
%
%  u = -5 * beta   (~5 $ subcritical)
%
%  Sign check: rho_total = u + alpha_f*(T_f - T_f0) + alpha_c*(T_c - T_c0).
%  At cold IC (T < T0) the feedback contribution is positive (roughly
%  +2.8 $ at T = T_cold0 everywhere), so -5 $ leaves the reactor at about
%  -2.2 $ — comfortably subcritical.
%
%  Inputs:
%    t, x, plant, ref - standard signature; all unused here.

    u = -5 * plant.beta;

end