India’s passive breeder

The threat of climate change and the importance of sustainable development has brought nuclear power in sharper focus in recent times. Growth of nuclear power worldwide, however, requires satisfactory technological responses to the challenges of a very high level of safety and security assurance (as dictated by a very large increase in the number of reactors), ability to perform with a lower level of technological infrastructure as it prevails in several developing countries, and a high degree of fuel-use efficiency and superior waste disposal options. The development of the Advanced Heavy Water Reactor, AHWR300-LEU, is an effort to realise these futuristic objectives through innovative configuration of present day technologies.

General description

AHWR300-LEU is a 300 MWe, vertical, pressure-tube type, boiling light water-cooled, and heavy water-moderated reactor. The reactor incorporates a number of passive safety features and is associated with a fuel cycle having reduced environmental impact. AHWR300-LEU possesses several features that are likely to reduce its capital and operating costs.

• Using heavy water at low pressure reduces potential for leakages

• Recovery of heat generated in the moderator for feedwater heating

• Elimination of major components and equipment such as primary coolant pumps and drive motors, associated control and power supply equipment and corresponding savings of electrical power required to run these pumps

• Shop-assembled coolant channels, with features to enable quick replacement of pressure tube alone, without affecting other installed channel components

• 100-year reactor design life

A design objective of AHWR300-LEU is to require no exclusion zone beyond the plant boundary. The AHWR300-LEU uses natural circulation for removal of heat from the reactor core under operating and shutdown conditions. All event scenarios initiating from non-availability of main pumps are, therefore, excluded. Another unique feature of its design is passive poison injection in moderator in the event of non-availability of both the primary and the secondary shut down system due to failure of all active systems, or malicious employee action.

How it works

The main heat transport (MHT) System transports heat from fuel pins to steam drum using boiling light water as the coolant. The MHT system consists of a common circular inlet header from which feeders branch out to the coolant channels in the core. The outlets from the coolant channels are connected to tailpipes carrying steam-water mixture from the individual coolant channels to four steam drums. Steam is separated from the steam-water mixture in steam drums, and is supplied to the turbine. The condensate is heated in moderator heat exchangers and feed heaters and is returned to steam drums by feed pumps. Four downcomers connect each steam drum to the inlet header.

The emergency core cooling system (ECCS) is designed to remove the core heat by passive means in case of a postulated loss of coolant accident (LOCA). In the event of a rupture in the primary coolant pressure boundary, the cooling is initially achieved by a large flow of water from accumulators. Later, cooling of the core is achieved by the injection of cold water from a 7000 m3 gravity driven water pool (GDWP) located near the top of the reactor building. After that, the passive containment cooling system (PCCS) provides long-term containment cooling following a postulated LOCA. GDWP serves as a passive heat sink yielding a grace period of three days. The core is submerged in water from GDWP long before the end of this period.

The AHWR300-LEU has a double containment with passive containment isolation. The reactor building air supply and exhaust ducts are shaped in the form of U-bends of sufficient height. In the event of LOCA, the containment pressure acts on the water pool surface and drives water, by swift establishment of siphon, into the U-bends of the ventilation ducts. Water in the U-bends acts as a seal between the containment and the external environment, providing necessary isolation between the two.

Fuel

The AHWR300-LEU fuel cluster contains 54 fuel pins arranged in three concentric circles surrounding a central displacer assembly. The Zircaloy-2 clad fuel pins in the three circles, starting from the innermost, contain 18%, 22% and 22.5% of LEUO2 (with 19.75% enriched uranium) respectively, and the balance ThO2. The average fissile content is 4.21%. The fuel also incorporates a multipurpose displacer assembly for the spraying of ECCS water directly on fuel pins during a postulated LOCA. This helps achieving negative void coefficient. The fuel is currently designed for an average burnup of 64 GWd/te. In comparison to a reference high burnup PWR considered for scenario studies under the IAEA’s International Project on Innovative Nuclear Reactors and Fuel Cycles (INPRO), the AHWR300-LEU requires about 42% less mined natural uranium for the same quantity of energy produced, thus making it a favourable option for efficient utilisation of natural uranium resources. The reactor is configured to obtain a significant portion of power by fission of 233U derived from in-situ conversion from 232Th. On an average, about 39% of the power is obtained from thorium. With uranium constituting less than 22% of the total fuel inventory, the reactor produces only about 17% of the plutonium and 42% of the minor actinides as compared to the reference high burnup PWR.

The reactor physics design has inherent safety characteristics, such as negative reactivity coefficients of all conditions likely to be encountered during startup, shutdown and LOCA.

AHWR reactor systems are currently under study at the Bhabha Atomic Research Centre. Test environments include a 3MW boiling water test loop, a scaled test facility (the integral test loop) based on power-to-volume scaling for thermal hydraulic simulation of AHWRs, a natural circulation loop for stability and start-up studies, a transparent setup for natural circulation flow distribution studies, and a facility at the Apsara reactor that uses neutron radiography to perform flow pattern transition studies. Partly as a result of this large-scale validation work, the reactor can achieve commercial operation by 2020.

Ratan K. Sinha, director, reactor design & development group; Anil Kadodkar, DAE Homi Bhabha Chair Professor, Bhabha Atomic Research Centre, Mumbai 400085. This article is based on a presentation at the 2009 IAEA general conference.