The Advanced Heavy Water Reactor (AHWR) is designed and developed to demonstrate Advanced Safety features and utilization of thorium for the generation of commercial nuclear power. AHWR is a 300 MWe, vertical pressure tube type, boiling light water cooled and heavy water moderated reactor. It incorporates several passive safety features and inherent safety characteristics including many First Of A Kind (FOAK) systems. Additionally, AHWR will produce desalinated water utilising process steam and waste heat.

Thorium Utilisation

India has large thorium deposits, but limited uranium reserves which makes utilisation of thorium for nuclear power generation essential to achieve long term sustainability and energy security of India. As part of three stage nuclear power programme of India, AHWR will act as a gateway for demonstration of technologies for large scale utilisation of thorium for commercial nuclear power generation. It will help in initiating technologies on all aspects of thorium fuel cycle.

AHWR Fuel

AHWR utilises (Th, ²³³U)MOX and (Th, Pu)MOX as fuel and is self sustaining in ²³³U. The core consists of total 513 lattice locations arranged in square pitch of 225 mm. There are 452 coolant channel assemblies, 8 absorber rods, 8 regulating rods, 8 shim rods and 37 shut off rods in the core. The fuel cluster is having twenty-four (Th, ²³³U)MOX pins in the outer ring and thirty (Th, Pu)MOX pins in the inner and middle rings along with a displacer rod at the centre. Figure 2 gives the detail of fuel cluster of AHWR.

Brief description of AHWR reactor systems

AHWR is a 300 MWe, vertical pressure tube type, boiling light water cooled and heavy water moderated nuclear reactor. The main heat transport system removes heat from the core in natural circulation in passive mode during normal operation, shut down and accidental conditions. In case of loss of coolant accident (LOCA), the emergency core cooling system (ECCS) actuates in passive means and injects water directly inside coolant channels and on fuel pins. The Reactor Protection System comprises two independent fast acting shutdown systems. Shutdown System–1 (SDS–1) is based on mechanical shut-off rods and Shutdown System–2 (SDS–2) is based on a liquid poison injection into the moderator. The reactor has many passive safety systems which are discussed in next section.

Main design features of AHWR

Safety philosophy of AHWR

The broader safety objectives of AHWR are reducing Core Damage Frequency (CDF) and Large Early Release Frequency (LERF) to an insignificant level. The design also has enhanced robustness to malevolent acts. The reactor is designed to provide at least 7 days of grace period during postulated accident events. AHWR incorporates many passive safety systems and inherent safety characteristics to achieve enhanced safety as described below.

Inherent safety characteristics of AHWR:

• Negative fuel temperature coefficient of reactivity
• Negative power coefficient of reactivity
• Negative  coolant void reactivity coefficient of reactivity
• Adequate shutdown margin even without two rods
• Low specific power to facilitate energy removal by natural circulation
• Fuel design characteristics like radial grading in fuel cluster and axial grading (bottom peaking) fuel to have better thermal margins
• Conventional features like double containment, emergency planning zones etc.
• Thorium as robust nuclear fuel due to higher thermal conductivity, lower thermal expansion coefficient, higher specific heat, higher melting point, lower fission gas release characteristics and better dimensional stability at high burnups

Many test facilities have been established in various Divisions and Groups of BARC, wherein, extensive experiments have been conducted to conclusively validate the design and safety aspects of AHWR.