Thorium nuclear power is gaining traction as a safe alternative to the dangers of conventional uranium reactors. Unlike uranium reactors, thorium reactors produce less high-level radioactive waste and are safe enough to stop the nuclear reaction in the event of an accident, making them a viable technology.
The nuclear power generation industry, which utilizes the fission reaction of uranium, has been growing even after the Three Mile Island accident and the Chernobyl disaster due to its “economic feasibility.” However, after the Fukushima disaster in Japan, safety concerns have become more widespread, and countries such as Germany and Taiwan have announced policies to phase out nuclear power. One technology is gaining traction in this situation. This is thorium nuclear power plants, which use the fission reaction of thorium instead of uranium to generate electricity. Thorium reactors were studied alongside uranium reactors until the 1970s, the early days of nuclear power plant technology, but were abandoned due to the technological and political economic conditions of the time. Now that uranium reactors are on the decline, thorium reactors are once again gaining traction as their disadvantages have turned into advantages. Let’s take a look at the principles, characteristics, and reasons for the recent resurgence of thorium reactors and how they can be realized.
Thorium reactors are different from uranium reactors because they use different materials, and the reactions that take place inside the reactor are different. All thorium in nature exists as 232Th with a mass number of 232. When a neutron is shot at a 232Th nucleus in a reactor, the nucleus absorbs it and becomes 233Th. This material is very unstable and soon decays to 233Pa. 233Pa decays back to 233U at a slower rate, with a half-life of about 27 days. The resulting 233U with a mass number of 233 is capable of fissioning even relatively low-energy neutrons, such as the 235U used in uranium reactors. Thorium reactors derive their electrical energy from the heat energy generated by the fission of this 233U.
Thorium reactors have several advantages over uranium reactors. First, the world’s thorium reserves are four times larger than uranium reserves. In addition, uranium can only be used in nuclear reactors to produce 235U, which exists in very small amounts in nature, whereas thorium can be used to produce 232Th, which is the only form of thorium that exists in nature. Uranium reactors produce high-level radioactive waste, such as plutonium, which remains toxic for tens of thousands of years, making disposal a major problem. Thorium reactors, however, do not produce high-level radioactive waste. Even the radioactive waste that is produced is reduced in toxicity after a few hundred years to the level of an ordinary coal mine.
The best feature of thorium reactors is that they can stop the nuclear reaction on their own in the event of an unexpected accident, such as the Fukushima disaster. In uranium reactors, the nuclear reactions are continuous, with the nuclei of uranium atoms absorbing neutrons, fissioning and releasing more neutrons. This is called a “chain reaction. However, in thorium reactors, the reaction process produces fewer neutrons than the number of neutrons that were initially introduced, meaning that the nuclear reaction stops unless more neutrons are supplied from the outside or more neutrons are released during the reaction.
Decades ago, when thorium reactors were first being studied, the fact that they produce no high-level radioactive waste like plutonium, and that the reaction stops without a neutron supply, were fatal drawbacks. During the Cold War, one of the purposes of building nuclear power plants was to obtain nuclear materials such as plutonium for use in nuclear weapons, and thorium reactors were far from ideal. Furthermore, thorium reactors were perceived as clearly “inferior technology” to uranium reactors because they were unable to sustain their own reactions and would shut down, in an era when efficiency was paramount. However, it has since been shown that the advantages of uranium reactors, with their self-sustaining chain reactions and constant burning, turn into disasters when humans lose control. The 1986 Chernobyl power plant disaster irradiated an estimated 5 million people in Russia and Ukraine, while the Fukushima disaster in Japan a few years ago killed nearly 800 people and still threatens the safety of our food supply. As the dangers of uranium reactors have been revealed over the decades, the disadvantage of thorium reactors has become the advantage of safety.
From a safety perspective, it’s a good thing that the reaction stops when neutrons are not supplied, but it shouldn’t happen during normal operation. Two main methods have been studied to solve this problem. The first is to use a mixture of uranium or plutonium, which are used in conventional nuclear power plants, along with thorium as nuclear fuel. Uranium and plutonium are good at producing chain reactions by releasing more neutrons than they put in, so they make up for the neutrons lost during the nuclear reaction of thorium. However, this method has inherent limitations. The resulting reactor is not a true thorium reactor, although it is technically less challenging, but rather a compromise between a conventional uranium-plutonium reactor and a half-baked system, which loses many of the unique advantages of thorium reactors. It doesn’t use or produce uranium and plutonium. Furthermore, while the degree to which the chain reaction occurs can be controlled by adjusting the mixing ratio, the nuclear reaction in a mixed reactor will continue in the event of an accident, fueled by the neutrons released by the chain reaction. In other words, this method does not fully realize the advantages of thorium reactors, but only utilizes thorium that has no other use.
The second method, called a “proton accelerator,” involves firing protons at high speeds and bombarding metals such as tungsten to produce large amounts of neutrons, which are then used in nuclear reactions. Thorium reactors are very safe because if there is an accident and the proton accelerator loses power, the nuclear reaction will slowly stop. Italian physicist Carlo Rubia first proposed the idea in 1995, but it went unnoticed for years. To produce enough neutrons to reliably trigger a chain reaction, an accelerator power of about 1 GeV is required, which requires a very large amount of power. Efficient accelerator design is difficult with current technology, resulting in a situation where the power generated by a nuclear power plant is comparable to the power required to run an accelerator. Therefore, developing a highly efficient accelerator is a major challenge for the proton accelerator method. In addition, because of the nature of this method, fission occurs with very fast neutrons, and the fission reaction with fast neutrons produces dozens of times more cadmium per unit mass than with slow neutrons. Cadmium is a class 1 carcinogen and a very harmful metal for humans.
With the nuclear power industry in crisis, we looked at an alternative technology: thorium reactors. Using thorium instead of uranium as nuclear fuel, thorium reactors undergo a completely different nuclear reaction process and have advantages over conventional nuclear power plants. However, a lot of research is still needed to commercialize thorium nuclear power plants. The United States and India, which have abundant thorium reserves, are leading the way in thorium nuclear power plant research, and India is actively promoting the export of ‘improved heavy water reactors’. At a time of transition not only for nuclear power plants but also for the energy industry as a whole, thorium nuclear power is worthy of serious consideration and research.
