  Bulk reduction of reactor power within a small finite time interval under
abnormal conditions is referred to as step-back. In this paper, a 500MWe
Canadian Deuterium Uranium (CANDU) type Pressurized Heavy Water Reactor (PHWR)
is modeled using few variants of Least Square Estimator (LSE) from practical
test data under a control rod drop scenario in order to design a control system
to achieve a dead-beat response during a stepped reduction of its global power.
A new fractional order (FO) model reduction technique is attempted which
increases the parametric robustness of the control loop due to lesser modeling
error and ensures iso-damped closed loop response with a PI{\lambda}D{\mu} or
FOPID controller. Such a controller can, therefore, be used to achieve active
step-back under varying load conditions for which the system dynamics change
significantly. For closed loop active control of the reduced FO reactor models,
the PI{\lambda}D{\mu} controller is shown to perform better than the classical
integer order PID controllers and present operating Reactor Regulating System
(RRS) due to its robustness against shift in system parameters.
