As a result of serious meltdowns in the Ukraine and Japan, safer alternatives to uranium-based nuclear energy are being sought. The chemical element thorium has come into the spotlight as a possible alternative.
What is thorium?
Thorium is a naturally radioactive chemical element with an atomic number of 90. Although it has just 2 protons less than uranium, it is 3 times as abundant, and 428.6 times as abundant as uranium-235, which comprises only 0.7% of all natural uranium.
Why is thorium better than uranium in fission reactors?
In theory, a molten salt reactor (MSR), using the thorium-232/uranium-233 conversion, could produce 250 times as much energy with thorium as a pressurized water reactor (PWR) could produce with the same amount of uranium. Those kinds of numbers make energy policy makers sit up and take notice.
Nearly all natural thorium is thorium-232, which has a half-life of roughly 14.05 billion years and degenerates naturally through alpha decay. Thorium-232 is not fissile itself, which is part of why it is considered a safer alternative to U-235. It is not capable of causing a fissile meltdown without a separate source of neutrons, which will convert it to U-233 by way of protactinium-233 after 2 beta decays. Uranium-233 is fissile, but there is never enough of it in the reactor to cause serious problems.
The energy is produced by separating out the new U-233 and running it in a separate reaction. Its purity means that the reaction will have little to no leftover depleted uranium to deal with.
The thorium nuclear process produces neutrons as well as absorbing them. These neutrons can be fed back into the nuclear cycle to convert more thorium to U-233, and so on. The cycle is not fissile and has nothing to do with critical mass, so there is no danger of it going out of control.
Is thorium nuclear waste better than uranium nuclear waste?
The thorium process needs to make uranium in order to work at all, so thorium nuclear waste is really uranium nuclear waste. However, it is a different kind of waste.
Materials used in the U-235 fissile process are considered waste when U-235 content is reduced to 0.3%. Other products of the U-235 process include americium, plutonium, technetium-99, and iodine-129, several of of which remain highly radioactive for thousands of years. Waste plutonium must also be guarded, because it can still be used for weapons.
Waste products from the thorium process includes protactinium and some waste U-232 which is produced as a side effect, as well as the previous technetium and iodine. Several of these products are also radioactive for thousands of years, but there are fewer of them. This process does not produce plutonium.
Does using thorium eliminate the risk of harvesting uranium for nuclear weapons?
Uranium-233 can be used to make nuclear weapons. However, the small amount of U-233 in the reactor core is constantly breaking down, which limits the amount of U-233 which can be produced directly by this process.
On the other hand, the thorium nuclear process also produces free neutrons, These could be absorbed outside the reactor’s core by a thorium or uranium blanket, to produce U-233 or plutonium-239 outside the core. Both of these elements can be used for nuclear weapons.
Is commercial thorium energy already here?
At the present time, thorium is not commercially viable without heavy government subsidy. Current nuclear reactor models were designed to meet the needs of the Cold War. Converting to commercial thorium would require a complete design overhaul with immense upfront capital costs, which few businesses are willing to risk on a next-generation technology which is still in the research phase. So far, only India has been willing to convert its Kakrapar-1 PWR reactor to thorium as part of its 3-stage nuclear power program, although several other countries have been running research programs.