The Indian Advanced Heavy Water Reactor (AHWR) is designed and developed to achieve large-scale use of thorium for the generation of commercial nuclear power. In post Fukushima scenario, incorporation of core catcher system (CCS) in all advanced nuclear reactors has become mandatory. One of the advanced features of AHWR is incorporation of core catcher system to retain and cool the molten core in beyond design basis accident (BDBA) to avoid damage to containment structures and activity spread to the soil, water and environment. To make CCS cooling fully passive, an innovative Passive Water Injection (PAWAN) scheme has been recently designed, developed and tested. The PAWAN scheme senses the core melt accident and injects water from Gravity Driven Water Pool (GDWP) passively into the core catcher. A mesh of Passive Thermal Sensors (PTS) developed in-house, has been deployed for sensing the core melt condition. The passive thermal sensor works on the principle of thermal expansion of fluid. It consists of a temperature sensing bulb/pipe, pressurized filled fluid system and a connecting capillary tube to the passive valve actuator. With increase in surrounding temperature, the fluid pressure inside the thermal sensing bulb increases to open the passive valves. Two numbers of 15 NB size passive thermal sensors made of SS-304 material, filled with helium gas, were developed in-house and were successfully tested inside the furnace at 600
C as well as in the experimental facility. Efforts were made to design the PAWAN scheme to be robust and sensitive enough to sense even partial or single channel melt. The CFD tools were used for the layout and depth optimization of passive thermal sensors inside the sacrificial concrete block of core catcher system. To validate the CFD results and to demonstrate the PAWAN scheme, experiments have been conducted placing two passive thermal sensors in overlapping mesh layout in a sacrificial concrete block of 1000 mm diameter and 300 mm depth. The core melt condition was simulated by heating the sacrificial concrete block using 24 numbers of instant infra-red (IR) heaters of 1 kWeach. Two numbers of passive valves were actuated at 13.2 bar & 19.1 bar pressure respectively by the two independent passive thermal sensors by heating the sacrificial concrete block at 600
°C temperature. This paper deals with design of passive thermal sensors, thermal sensor mesh layout, tests results, analytical approach towards sensor layout and depth optimization in sacrificial concrete and its experimental validation.