People waiting for a nuclear renaissance expect that the new reactors on the drawing board should assure a very high level of safety and security; they must have the ability to perform with a lower level of technological infrastructure prevailing in several developing countries; they must have high fuel use efficiency and superior waste disposal options.
“The development of the Advanced Heavy Water Reactor, AHWR300-LEU, is an effort to realize these futuristic objectives through innovative configuration of present day technologies,” Anil Kakodkar and Ratan Sinha, the designers of India's innovative nuclear reactor wrote in the May 2010 issue of Nuclear Engineering International .
They called the reactor India's passive breeder.
“ As a result of its fuel mix and fuel breeding properties, the 300 MWe plant requires 42 per cent less mined uranium per unit of energy produced than a modern high burn up PWR”, they added.
AHWR300-LEU with an estimated design life of 100 years is a vertical, pressure tube type, boiling light water-cooled, heavy water- moderated reactor with reduced environmental impact. It has many features which are likely to reduce both its capital and operating costs.
The designers have eliminated primary coolant pumps and drive motors and related control and power supply equipment, thereby saving the electric power to run them. This helps to reduce cost and to enhance reliability.
The use of heavy water at low pressure reduces the potential for leakages. The heat generated in the moderator will be recovered and used for heating the feed-water.
Quick replacement
The shop assembled-coolant channels have features which enable quick replacement of pressure tubes alone without affecting other components.
The design objective of the reactor is to require no exclusion zone beyond the plant boundary. The reactor will use natural circulation to remove heat from its core under operating and shut down conditions. In case the primary and the secondary shut down systems are not available due to the failure of all active systems or malicious employee action, passive injection of a “poison” — a high neutron absorbing liquid, in to the moderator will shut down the reactor.
When the reactor operates, its core will be very hot. Coolant removes the heat. If coolant is not available due to a Loss of Coolant Accident (LOCA), the emergency core cooling system (ECCS) will remove heat by passive means.
If the primary coolant tube ruptures, a large flow of water from accumulators will cool the reactor initially. Later, the core will be cooled by the injection of cold water from a 7000 cubic metre Gravity Driven Water Pool (GDWP) located at the top of the reactor building. After that, the passive containment cooling system (PCCS) provides long term containment cooling. GDWP serves as passive water sink giving a grace period of three days.
The reactor has a double containment with an elegant design which assists the formation of a passive water seal in the event of a loss of coolant accident. The seal isolates the reactor containment and the external environment, preventing the spread of radioactivity.
Fission of Uranium-233
The reactor fuel on an average contains 19.75 per cent of enriched uranium and the balance thorium oxide. A significant fraction of the reactor power, about 39 per cent, comes from the fission of Uranium-233 derived from in-situ conversion of thorium-232.
The reactor physics design has inherent safety characteristics during all conditions likely to be encountered during startup, shutdown and LOCA.
During an interview, Dr Sinha has stated that the scientists and engineers at BARC have designed a novel advanced heavy water reactor to burn thorium ( IEEE Spectrum, 2008 )
“They say that because no reactor in the world today uses thorium on a large scale, they will be breaking new ground”, he added
Currently BARC has the facility for large scale validation work.
Partly as a result of this, the reactor can achieve commercial operation by 2020.Indian scientists have been exploring various fuel cycle options for improved versions of AHWR.
AHWR300-LEU has all the safety features of AHWR. It also helps in thorium utilization.
It produces much less plutonium and minor actinides compared to Pressurized Water Reactors(PWR) which is the mainstay internationally. In view of that, this reactor is more proliferation resistant.
Since minor actinides (which have relatively long half life) are less than those in PWR, it is a better choice from considerations of waste management.
AHWR300-LEU has better reactor physics characteristics.
K.S.PARTHASARATHY
Raja Ramanna Fellow, Department of Atomic Energy
(ksparth@yahoo.co.uk)
