Advanced Heavy Water Reactor

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The Advanced Heavy Water Reactor (AHWR) is a proposed heavy water moderated nuclear power reactor that will be the next generation design of the PHWR type. It is now being developed at Bhabha Atomic Research Centre (BARC) and aims to meet the objectives of using thorium fuel cycles for commercial power generation. The AHWR is a vertical pressure tube type reactor cooled by boiling light water under natural circulation. A unique feature of this design is a large tank of water on top of the primary containment vessel, called the Gravity Driven Water Pool (GDWP). This reservoir is designed to perform several passive safety functions.

The reactor design incorporates advanced technologies, together with several proven positive features of Indian Pressurised Heavy Water Reactors (PHWRs). These features include pressure tube type design, low pressure moderator, on-power refueling, diverse fast acting shut-down systems, and availability of a large low temperature heat sink around the reactor core. The AHWR incorporates several passive safety features. These include: Core heat removal through natural circulation; direct injection of Emergency Core Coolant System (ECCS) water in fuel; and the availability of a large inventory of borated water in overhead Gravity Driven Water Pool (GDWP) to facilitate sustenance of core decay heat removal. The Emergency Core Cooling System (ECCS) injection and containment cooling can act (SCRAM) without invoking any active systems or operator action.

The reactor physics design is tuned to maximise the use of thorium based fuel, by achieving a slightly negative void coefficient. Fulfilling these requirements has been possible through the use of PuO₂-ThO₂ MOX, and ThO₂-²³³UO₂ MOX in different pins of the same fuel cluster, and the use of a heterogeneous moderator consisting of amorphous carbon (in the fuel bundles) and heavy water in 80%-20% volume ratio. The core configuration lends itself to considerable flexibility and several feasible solutions, including those not requiring the use of amorphous carbon based reflectors, are possible without any changes in reactor structure.