India is pursuing the design of Advanced Heavy Water Reactor (AHWR) to achieve the objective of thorium utilization with enhanced safety and economic competitiveness. AHWR is 920 MWth pressure tube type boiling water reactors with light water as coolant and heavy water as moderator. The reactor is designed to remove the core heat by two-phase natural circulation mode not only during startup, power rising or shut down cooling but also during normal full power operation. In addition, the design incorporates many passive safety features such as advanced accumulators, gravity driven cooling system for ECC and LOCA incidences respectively and passive containment cooling system for containment cooling. In many ways AHWR will be addressing stringent safety needs of the next generation nuclear reactors. Thermal hydraulics of two-phase natural circulation reactors pose stiff challenges while establishing stable operation at all power level, establishing thermal margin, thermal hydraulics related to refueling, channel burn out caused by small break LOCA, flow instability etc. In core detectors such as SPNDs are very sensitive to perturbation to neutron flux. By the definition of 1970s, neutron flux is made of the global component generated by the reactivity noise that fluctuates in phase with the whole reactor, and the local component that originates from the axially propagating perturbations. Perturbation changes the neutron field in its neighborhood, called the local component. The paper deals about effective use of SPND signal for detecting in core thermal hydraulics during steady state operation and during reactor shut down.