Fuel
What is Thorium?
Thorium is a naturally-occurring, slightly radioactive metal discovered in 1828 by the Swedish chemist Jons Jakob Berzelius, who named it after Thor, the Norse god of thunder. It is found in widespread locations all over the world, and is about three to four times more abundant than uranium. Thorium exists in nature in a single isotopic form as 232Th which decays very slowly (its half-life is about three times the age of the Earth).
The most common source of thorium is the rare earth phosphate mineral monazite, which contains up to about 12% thorium phosphate, but 6-7% on average. World monazite resources are estimated to be about 12 million tonnes, two-thirds of which are in heavy mineral sands deposits on the south and east coasts of India. There are substantial deposits in several other countries with South Africa having an estimated 148 000 tons of Thorium.
Thorium as a Nuclear Fuel:
Thorium is not a fissile material but classified as a fertile material. To create a thorium fuel capable of producing energy, a driver component or material is required. During irradiation, thorium transmutes to uranium-233 which is an excellent fissile material that can then yield energy. Plutonium or uranium is used as fissile drivers which are readily found in all spent nuclear inventories. Thorium will absorb neutrons in a thermal reactor and produce 233U.
Thorium oxide (ThO2 ) has excellent material properties for serving as a nuclear fuel. ThO2 has a higher thermal conductivity, a higher melting point than uranium oxide (UO2 ) and it retains fission products better within its crystalline lattice. Thorium oxide fuels can therefore operate at lower temperatures and exhibit less fission gas release than uranium fuels (including MOX). It is therefore recognized that thorium oxide fuels can operate safely to high burn-ups.
The burning of thorium fuel generates smaller amounts of plutonium and minor actinides compared to uranium fuel. Thus thorium based fuels will achieve much greater net plutonium consumption than conventional uranium based fuels, which produces plutonium as it burns.
ThO2 has excellent properties from a waste point of view, even after irradiation. It is highly insoluble, it is non-oxidizable and it retains both fission products and actinides extremely well within its lattice. Thorium oxide would therefore serve as a good matrix for once-through fuel designed specifically to burn plutonium.
Thorium fuel cycles are more resistant to nuclear proliferative actions. Both open and closed thorium fuelling options exhibit higher proliferation resistance than a corresponding uranium based cycle.
During and after a worst case accident the evacuation of people from the surrounding area will not be necessary.
Spent fuel will be passively safe – this means that no active cooling should ever be required for fuel storage to prevent release of fission products from spent fuel.
HTMR Fuel:
The safety features of the HTMR100 start with the fuel. During nuclear fission certain radioactive fission products are formed which are mainly in a gaseous form. The longer fuel remains in a reactor the more fission products are produced. As these fission products are transformed from a solid to a gas, the container in which these fission products are encapsulated will start to experience a pressure build-up. The coated particles are designed to withstand these pressures and have been tested and demonstrated to not release fission products. This capsule for the fission products is the first protective boundary in ensuring that the harmful fission products are not released into the atmosphere.
Another important characteristic of the fuel sphere containing TRISO coated particles is the fact that every single material is a ceramic with a very high melting point; therefor it is suitable for high temperature environments, even during postulated reactivity incidents, making it intrinsically safe.
Pebbles/Fuel Spheres
Fuel Compacts
Retention of Fission Products in fuel sphere
In the fuel sphere it has been proven that 99.99% of the fission products are retained within the 1mm small fuel particles, which are in effect small pressure vessels, coated with multiple silicon carbide and pyrolitic carbon layers. These layers provide excellent pressure and fission product retention to temperatures in excess of 1600°C.