Overview of Reactors Operating on Uranium and Thorium Fuels
1. Uranium Reactors
The fuel rods are loaded into the reactor core and the system made operational; here the kinetic energy provided by the fission of the U235 is converted to thermal energy as the neutrons collide with other atoms.
Cooling medium is circulated around the core and pumped through a heat exchanger in a closed circuit. The heat from the cooling medium is used to generate steam which powers steam turbines. The steam turbines drive electric generators, producing power to the national grid.
2 Thorium Reactors
Over the last four or five decades there have been several experimental reactors using thorium-233U, which were mainly successful, demonstrating the possibility of thorium powered nuclear reactors.
However only recently the advantages of using irradiated thorium has been realized and received the intense research and development that it deserves.
There are several design systems for thorium reactors, some using a first stage reactor to irradiate the thorium, and further processing it to Th-233, before using it in a thorium reactor.
Another more recent innovation is to use a seed and blanket system which has a central core of rods surrounded by a blanket.
The central seed core rods contain 20% enriched U-235; these are surrounded by a blanket containing thorium and U-238.
India is now a world leader in the design of thorium for nuclear reactors, its Bhabha Atomic research Centre has designed and is running a 30kW test reactor using U233 and has committed to using thorium to fuel their 300MW Advanced Heavy Water Reactor.(AHWR)
In this particular design, the fuel cells in the core will be assembled using 30 rods of Th-233/ U-233 oxide in the centre surrounded by 24 rods of Th-233/Pu-239 oxide. This will enable the system to be self-sufficient in U-233, with the thorium providing 60% of the nuclear reactors power output, with the design life of the AHWR expected to be 100 years.
Thorium Advanced Heavy water reactors will use the thermal heat generated in the core to produce steam to run steam turbines as per a normal AHWR.
Lastly there has been reactors which use liquid thorium salts, for example the Liquid Fluoride Thorium Reactor which uses a fuel/salt mix of ThF4-U233F4. This is circulated through the core and on to a heat exchanger where a medium such as helium is heated to 1200K (930C) and fed to a gas turbine, returning to the exchanger in a closed loop. The turbine is used to drive an electric generator producing power to the National Grid.
There is a bleed-off in the liquid salt circuit which diverts some of the liquid salt to a processing plant where it is purified and reintroduced to the main salt loop.
There is also an emergency system where if there is an emergency, a freeze plug will melt allowing the liquid salt to enter an underground dump tank.