In this blog post, we will look at whether thorium nuclear power can be a safe alternative energy technology that can reduce the risks of existing uranium nuclear power.
The nuclear power industry, which uses the nuclear fission reaction of uranium, has grown by promoting “economic feasibility” even after the Three Mile Island accident and the Chernobyl disaster. However, concerns about safety have spread further since the Fukushima disaster in Japan, and the momentum has slowed down as countries such as Germany and Taiwan have announced policies to phase out nuclear power. In this situation, there is a technology that is attracting attention. It is a thorium nuclear power plant that uses the fission reaction of thorium instead of uranium to generate electricity. Thorium nuclear power plants were studied along with uranium nuclear power plants until the 1970s, when nuclear power technology was in its infancy, but they were abandoned due to the technical and political and economic conditions of the time. Now that the uranium nuclear power plant is on the decline, the disadvantages of the time are being turned into advantages, and the thorium nuclear power plant is once again in the spotlight. Now, let’s take a look at the principles, characteristics, and reasons for the recent resurgence of the thorium nuclear power plant and how to realize it.
The thorium nuclear power plant uses different raw materials than the uranium nuclear power plant, so the reactions that occur inside the reactor are also different. All thorium in the natural world exists as 232Th, with a mass number of 232. When a neutron is shot at the nucleus of a 232Th atom in a nuclear reactor, the nucleus absorbs it and becomes 233Th. This material is very unstable and soon decays into 233Pa. 233Pa decays back to 233U at a slow rate of about 27 days. The 233U with a mass number of 233 produced in this way also undergoes nuclear fission, even with relatively small energy neutrons, like the 235U used in uranium nuclear power plants. Thorium nuclear power plants obtain electrical energy from the thermal energy generated during the fission process of this 233U.
Thorium nuclear power plants have several advantages over uranium nuclear power plants. First, the world’s thorium reserves are four times larger than uranium reserves. Moreover, while only 235U, which is present in very small amounts in nature, can be used in nuclear power plants, the only form of thorium that exists in nature is 232Th, which can be used in nuclear power plants in its entirety. The disposal of high-level radioactive waste is a major issue for uranium-based nuclear power plants, as they produce high-level radioactive waste such as plutonium, whose toxicity does not disappear even after tens of thousands of years. However, no high-level radioactive waste is produced in thorium-based nuclear power plants. Even the radioactive waste produced is reduced to a level of toxicity equivalent to that of ordinary coal mines after only a few hundred years.
The biggest feature of thorium reactors is that they stop nuclear reactions on their own when an unexpected accident like the Fukushima disaster occurs. In uranium reactors, nuclear reactions occur continuously as the uranium nuclei that have absorbed neutrons release more neutrons in a process of fission. This is called a “chain reaction.” However, in the reaction process of a thorium nuclear power plant, fewer neutrons are produced than the number of neutrons initially injected. In other words, the nuclear reaction stops unless more neutrons are supplied from outside or more neutrons are released during the reaction.
In the decades when thorium reactors were first being studied, the fact that they did not produce high-level radioactive waste like plutonium and that they stopped reacting without a supply of neutrons was a fatal drawback. During the Cold War, one of the purposes of building nuclear power plants was to obtain nuclear materials like plutonium for use in nuclear weapons, and thorium reactors were far from that. Not only that, thorium reactors, which could not maintain their own reactions and shut down, were perceived as “inferior technology” compared to uranium reactors from the perspective of the time when efficiency was the top priority. However, the advantages of uranium reactors, which continuously burn with a self-sustaining chain reaction, have since been revealed to be a disaster when humans lose control. The Chernobyl nuclear power plant accident in 1986 exposed about 5 million people in Russia and Ukraine to radiation, and the Fukushima disaster in Japan a few years ago killed nearly 800 people and continues to threaten the safety of our food supply. Because of the dangers of uranium nuclear power plants that have been revealed over the decades, the disadvantages of thorium nuclear power plants have become advantages in terms of “safety.” From the perspective of emphasizing safety, the advantage is that the reaction stops when the neutrons are not supplied, but the disadvantage is that the nuclear power plant should not be shut down during normal times. Two methods have been studied to solve this problem. The first method is to use uranium or plutonium, which were used in existing nuclear power plants, in combination with thorium as nuclear fuel. Uranium and plutonium release more neutrons than the neutrons that are injected, which makes them good for chain reactions, so they compensate for the neutrons that are lost in the nuclear reaction process of thorium. However, this method has inherent limitations. The nuclear power plants created in this way are not truly thorium nuclear power plants, although they are less technically difficult to build. They are a half-baked system that is a compromise between existing uranium and plutonium nuclear power plants. Therefore, the unique advantages of thorium nuclear power plants are largely lost. They do not have the advantage of not using or producing uranium and plutonium. In addition, although the degree of chain reaction can be controlled by adjusting the mixing ratio, the nuclear reaction in a mixed nuclear power plant will continue even in the event of an accident due to neutrons released by the chain reaction. In other words, this method does not fully implement the advantages of thorium nuclear power plants, but rather only utilizes thorium that has no use.
The second method is called a “proton accelerator,” in which protons are fired at high speed to collide with metals such as tungsten to produce a large amount of neutrons, which are then used in nuclear reactions. This type of thorium-based nuclear power plant is very safe because if an accident occurs and the power supply to the proton accelerator is cut off, the nuclear reaction will gradually stop. In 1995, Italian physicist Carlo Rubia first proposed this method, but it went unnoticed for several years. In order to produce enough neutrons to cause a stable chain reaction, an accelerator output of about 1 GeV is required, which requires a very large amount of power. Since it is difficult to design an efficient accelerator with current technology, the situation arises where the power generated by the nuclear power plant becomes similar to the power required to run the accelerator. This is a situation where the belly is bigger than the belly. Therefore, the development of a highly efficient accelerator is a major challenge in the proton accelerator method. In addition, due to the nature of this method, nuclear fission occurs due to neutrons at very high speeds, and the nuclear fission reaction caused by high-speed neutrons produces tens of times more cadmium per unit mass than that caused by low-speed neutrons. Cadmium is a class 1 carcinogen and a metal that is very harmful to the human body.
Today, the nuclear power industry is facing a crisis, so we looked into thorium nuclear power, a possible alternative technology. Thorium nuclear power plants, which use thorium instead of uranium as nuclear fuel and undergo a completely different nuclear reaction process, have advantages over existing nuclear power plants. However, a lot of research is still needed to commercialize thorium nuclear power plants. Research on thorium nuclear power plants is being led by the United States and India, which have abundant thorium reserves, and India in particular is actively promoting the export of its “modified heavy water reactor” technology. As we are currently in the midst of a transition period for the energy industry as a whole, not to mention the nuclear power plant issue, serious consideration and research into thorium nuclear power plants is well worth it.
