This blog post examines whether thorium nuclear power could become a safe alternative energy technology that reduces the risks of existing uranium reactors.
The nuclear power industry, which utilizes uranium fission reactions, has continued to grow by emphasizing ‘economic viability’ even after the Three Mile Island accident and the Chernobyl disaster. However, following the Fukushima incident in Japan, concerns about safety have spread further, leading to a slowdown in its momentum, with countries like Germany and Taiwan announcing nuclear phase-out policies. Amid this situation, one technology is gaining attention: the ‘thorium reactor,’ which generates electricity using the nuclear fission reaction of thorium instead of uranium. Thorium reactors were researched alongside uranium reactors until the early days of nuclear technology in the 1970s but were shelved due to the technological and political-economic conditions of the time. Now that uranium reactors are on the decline, the disadvantages of thorium reactors back then have turned into advantages, bringing them back into the spotlight. Let’s examine the principles, characteristics, reasons for their renewed attention, and methods for realizing thorium reactors.
Thorium reactors differ fundamentally from uranium reactors, starting with the fuel they use, and consequently, the reactions occurring inside the reactor core are also different. All naturally occurring thorium exists as thorium-232 (²³²Th) with a mass number of 232. When a neutron strikes a 232Th nucleus inside the reactor, the nucleus absorbs it and becomes 233Th. This material is highly unstable and quickly decays into 233Pa. 233Pa then decays slowly, with a half-life of about 27 days, into 233U. The resulting 233U, with mass number 233, undergoes fission even with relatively low-energy neutrons, similar to the 235U used in uranium reactors. Thorium reactors generate electrical energy from the thermal energy produced during this fission process of 233U.
Thorium reactors offer several advantages over uranium reactors. First, global thorium reserves are four times greater than uranium reserves. Furthermore, while uranium reactors can only use 235U, which exists in extremely small quantities in nature, thorium reactors can utilize the entire naturally occurring form, 232Th. Uranium reactors produce high-level radioactive waste, such as plutonium, whose toxicity persists for tens of thousands of years, making its disposal a major problem. However, thorium reactors do not produce high-level radioactive waste. The radioactive waste they do generate loses its toxicity to levels comparable to ordinary coal mines within a few hundred years.
The most significant feature of thorium reactors is their ability to automatically halt nuclear reactions during unforeseen accidents like the Fukushima disaster. In uranium reactors, the nuclear reaction continuously occurs as uranium nuclei that absorb neutrons undergo fission, releasing more neutrons in a repeating cycle. This is called a ‘chain reaction’. However, in the reaction process of a thorium reactor, fewer neutrons are produced than the number initially introduced. In other words, unless more neutrons are supplied from outside or more neutrons are released during the reaction, the nuclear reaction stops.
Decades ago, when thorium reactors were first researched, their characteristics—producing no high-level radioactive waste like plutonium and ceasing reactions without neutron supply—were seen as fatal flaws. During the Cold War, one purpose of building nuclear power plants was to obtain nuclear materials like plutonium for nuclear weapons; thorium reactors were far removed from that goal. Moreover, from the perspective of that era, where efficiency was the supreme value, thorium reactors—unable to sustain their own reaction and prone to shutting down—were clearly perceived as ‘inferior technology’ compared to uranium reactors. However, it later became apparent that the very advantage of uranium reactors—their ability to sustain a self-sustaining chain reaction—could turn into a catastrophe when human control was lost. The 1986 Chernobyl accident exposed approximately 5 million people in Russia and Ukraine to radiation, while the Fukushima disaster in Japan a few years ago caused nearly 800 deaths and continues to threaten the safety of our food supply. Due to the dangers of uranium reactors revealed over decades, the perceived disadvantage of thorium reactors became an advantage: ‘safety’.
From a safety-first perspective, the fact that the reaction stops if neutron supply is interrupted is indeed an advantage. However, under normal conditions, the reactor must never shut down. Two primary methods have been researched to address this issue. The first method involves using a mixed fuel containing both thorium and uranium or plutonium, materials traditionally used in existing reactors. Uranium and plutonium emit more neutrons than they absorb, readily sustaining a chain reaction. This compensates for neutrons lost during thorium’s nuclear reaction process. However, this approach has inherent limitations. While technically less challenging, such reactors are not true thorium reactors but rather a compromise system, a half-measure between existing uranium/plutonium reactors and thorium reactors. Consequently, many of the inherent advantages of thorium reactors are lost. The benefit of neither using nor producing uranium and plutonium is not realized. Furthermore, while the degree of chain reaction can be controlled by adjusting the mixture ratio, the nuclear reaction in a mixed reactor will continue due to neutrons released by the chain reaction even in the event of an accident. In other words, this method does not fully realize the advantages of a thorium reactor; it merely utilizes thorium that would otherwise have no use.
The second method involves a ‘proton accelerator’ approach, where protons are fired at high speeds to collide with metals like tungsten, producing large quantities of neutrons for use in nuclear reactions. A thorium reactor using this method is highly safe because if an accident occurs and power to the proton accelerator is cut off, the nuclear reaction gradually stops. In 1995, Italian physicist Carlo Rubbia first proposed this method, but it received little attention for years. Generating enough neutrons to sustain a stable chain reaction requires an accelerator output of approximately 1 GeV, which demands enormous power. Current technology struggles to design efficient accelerators, leading to a situation where the power consumed to operate the accelerator is nearly equal to the power produced by the reactor itself. It’s a case of the cure being worse than the disease. Therefore, developing a highly efficient accelerator is a major challenge for the proton accelerator approach. Furthermore, due to the nature of this method, nuclear fission occurs via extremely high-speed neutrons. In nuclear fission reactions triggered by high-speed neutrons, dozens of times more cadmium is produced per unit mass compared to reactions triggered by low-speed neutrons. Cadmium is a Class 1 carcinogen and a highly toxic metal to humans.
Today, as the nuclear power industry faces crisis, we examined ‘thorium reactors’ as a potential alternative technology. Thorium reactors, which use thorium instead of uranium as nuclear fuel and undergo a completely different nuclear reaction process, have advantages over conventional reactors. However, significant research is still required to commercialize thorium reactors. Countries with abundant thorium reserves, such as the United States and India, are leading research into thorium reactors. India, in particular, is actively pursuing exports under the name ‘Advanced Heavy Water Reactor’ (AHWR). At this juncture, where not only nuclear power but the entire energy industry is undergoing a transition, serious consideration and research into thorium reactors are well worth the effort.
