Uranium and the Uranium Fuel Cycle

Uranium, the 92nd element on the periodic table, has been the fuel of choice for commercial nuclear power plants for the past 55 years. Enriched uranium produces significant amounts of energy: one kilogram of uranium is roughly equivalent to 1500 tons of coal. 90% of the world’s uranium comes from only 7 countries: Canada, Australia, Kazakhstan, Russia, Namibia Niger and Uzbekistan.

The two main isotopes of natural uranium are fissile uranium-235 (approximately 0.7% of all natural uranium) and fertile uranium-238 (approximately 99.3%) Fissile material is capable of producing a self-sustaining chain reaction without the introduction of external neutrons. Fertile material on the other hand cannot sustain a reaction by itself but can absorb neutrons to become fissile, thus contributing to a chain reaction. The naturally available uranium-235 is not sufficiently concentrated to operate in a standard nuclear reactor and therefore must be enriched prior to use.

After being mined and milled, uranium proceeds through a complex 16 step nuclear fuel cycle, one that is necessary for use in the light water reactor (LWR) systems that dominate the nuclear market. This endeavour encompasses numerous chemical processes and complex robotics, as well as many different facilities.

Conventional pressurized light water reactors (PLWR) rely upon large quantities of uranium for fuel throughout their life cycle. The quantity is large because approximately 33% of the original uranium load needs to be added every 18 months.

Thorium and the Thorium Fuel Cycle

Thorium, the 90th element in the periodic table, will be the primary fuel for the DBI Thorium reactor. It has been estimated that the nuclear energy available in thorium is greater than that available from all of the world’s oil, coal and uranium combined.

Thorium is approximately three times as abundant as uranium in the earth’s crust, reflecting the fact that thorium has a longer half-life. In addition, thorium generally is present in higher concentrations (2-10%) by weight than uranium (0.1-1%) in their respective ores, making thorium retrieval much less expensive and less environmentally damaging per unit of energy extracted. Countries with significant thorium mineral deposits include: Australia, India, Brazil, USA, Canada, China, Russia, Norway, Turkey, Venezuela, Sri Lanka, Nigeria, South Africa, and Malaysia.

Naturally occurring thorium has one isotope- thorium-232. In the DBI reactor, the initial start up fuel mix is a combination of thorium and uranium-235. The uranium acts as the “seed” source of neutrons needed to achieve criticality for the first reactor. This combination of fuels decreases the time and capital required to start the thorium fuel breeding cycle. As the DBI reactor design begins producing electricity, Uranium-233, bred from the Thorium-232, increased core reactivity and power output. Over time, the original uranium-235 is burned up and subsequently the reactor is fuelled only with Thorium-232. Over the life of the DBI reactor design (approx. 60 years), about 3% of the original load mass (thorium only) will be added every 18 months. Depending upon operational choices available with the DBI designs, no or very little additional uranium will be needed.

One reason why thorium reactors have not made more progress in the past is that nuclear fuel breeding traditionally has been a very slow and capital-intensive process. For this reason, the DBI reactor is designed to be started-up using conventional nuclear fuels, with low enough capital and operating costs that it can compete with other conventional nuclear power plants and pay for its costs in the first few years, even before the bred Uranium-233 is available.

Another and perhaps the major reason why thorium use for energy production has not made more progress over the past decades is that thorium is not nearly as easy to weaponize. A 1997 international scientific symposium on nuclear fuel cycles concluded that the principal reason thorium had not been used more widely to date is that the ore contains no fissile isotope.

Source: http://www.dauvergne.com/technology/thorium-vs-uranium/