Advanced heavy-water reactor

The Bhabha Atomic Research Centre (BARC) has set up a large infrastructure to facilitate the design and development of PHWRs in areas including materials technologies, critical components, reactor physics, and safety analysis.^[4] Several facilities have been set up to experiment with these reactors.

Thorium is three times more abundant in the Earth's crust than uranium, though less abundant in terms of economically viable to extract proven reserves, with India holding the largest proven reserves of any country.^[5] Thorium is also contained in the tailings of mines that extract rare earth elements from monazite which usually contains both rare earth elements and thorium. As long as demand for thorium remains low, these tailings present a chemical (thorium is a toxic heavy metal) and - to a lesser extent - radiological issue which would be solved at least in part by use of thorium in nuclear power plants. Thorium lacks a fissile isotope; unlike uranium, which contains 0.72% of fissile ²³⁵
U, thorium is composed almost only out of fertile ²³²
Th which can be transmutated into fissile ²³³
U. Unlike ²³⁸
U, which is transmutable into ²³⁹
Pu, thorium is capable of producing large quantities of fissile material in a thermal reactor. This allows a much larger share of the original material to be used without the need for fast breeder reactors and while producing orders of magnitude less minor actinides. However, as thorium itself is not fissile, it has to be "bred" first to obtain a ²³³
U, which can then be used in the same reactor that "bred" the ²³³
U or chemically separated for use in a separate reactor. The Prototype Fast Breeder Reactor is intended to breed fissile plutonium for use with thorium.^[6]

The AHWR is a pressure-tube reactor moderated by heavy water and cooled by boiling light water. Its core consists of a calandria filled with heavy water, with pressure tubes containing fuel, however unlike most PHWRs the tubes are vertical rather than horizontal.^[7] The reactor core contains 452 coolant channels, of which 424 contain a fuel cluster. Each cluster contains 54 fuel pins containing a mixed oxide of ThO2 and either ²³³
U or ²³⁹
Pu.^[8] Each fuel element also contains an amorphous carbon moderator. The use of the heterogenous carbon and heavy-water moderator combined with the mixed oxide fuel enables the reactor to achieve a negative void coefficient.^[7] The remaining 37 channels are occupied by the shutdown system. This consists of 37 shut-off rods including 8 absorber rods, 8 shim rods, and 8 regulating rods. Each channel has a square pitch of 225 mm.^[9] The light-water primary coolant boils in the channels around the fuel.

The AHWR incorporates several features of the existing Indian PHWRs, including the pressure tube-type design, online refueling, and the availability of a large heat sink around the reactor core.^[7] It also incorporates passive safety through its boiling water coolant, which circulates via natural circulation and eliminates the need for primary coolant pumps. It also incorporates a large inventory of borated water in an overhead gravity-driven water pool to facilitate decay heat removal during a loss-of-coolant accident,^[7] as well as a passive containment cooling system.^[8]

The Indian Government announced in 2013 it would build an AHWR of 300 MWe with its location to be decided.^[10] As of 2017, the design was in the final stages of validation.^[11] However, as of 2025, no AHWR reactors are under construction.^[12]

Past nuclear meltdowns such as Chernobyl disaster and Fukushima nuclear accident have made the improvement of construction and maintenance of facilities to be crucial. These accidents were with the involvement of uranium-235 reactors and the poor structures of the facilities they were in. Since then, International Atomic nuclear Association has stepped up protocols in nuclear facilities in order to prevent these accidents from occurring again. One of the top security measures for a meltdown is containment of radioactivity from escaping the reactor. The Defence in Depth is a method used in nuclear facilities to acquire the most effective practice of radioactive containment. The AWHR has acquired the Defense in Depth process which is used in reactors adopting provisions and required equipment in order to retain the radioactivity within the core.

The Defense in Depth method establishes procedures that must be followed in order to reduce human error incidents and machine malfunctions.^[4] The procedures are the following:

• Level 1: Prevention of abnormal operation and failure
• Level 2: Control of abnormal operation and detection of failure
• Level 3: Control of accidents within the design basis
• Level 4: Control of severe plant conditions, including prevention of accident progression and mitigation of consequences of severe accidents
• Level 5: Mitigation of radiological consequences of significant release of radioactive materials.

The AWHR is an innovation in renewable energy safety as it will limit the use of fissile uranium-235 to breeding fissile uranium-233 from fertile thorium-232. The extraction of nuclear energy from the 90th element thorium is said to have more energy than the world's oil, coal, and uranium combined. The AHWR has safety features that distinguish it from conventional lightwater nuclear reactors. Some of these features consist of: strong safety systems, reduction of heat from core through a built in cooling system, multiple shutdown systems and a fail-safe procedure that consist of a poison that shuts down the system in the case of a technical failure (FBR).^[4] The potential threat scientists try to avoid in reactors is the buildup of heat because nuclear energy escalates when it reacts with high temperatures, high pressures and chemical reactions. The AHWR has features that helps reduce the probability of this occurrence through: negative reactivity coefficients, low power density, low excess reactivity in the core and proper selection of material attributes built in.^[13]