Is it okay to dump spent fuel and nuclear waste from nuclear power plants into the crater of an active volcano? What kind of black technology is ADS?

Nuclear power plants will inevitably produce nuclear waste during operation, such as fuel assemblies and control rods that have gone through several cycles. There are many active volcanoes on the earth, and most of them are located in uninhabited areas.

Can these nuclear wastes be thrown into a volcano to be burned by the volcano? Or can they flow into the deep crust of the earth along with the magma to eliminate the radioactivity?

Nuclear power plants do produce some radioactive waste during operation, and high-level radioactive waste is mainly spent fuel that has been burned.

The main material used in nuclear fuel is uranium dioxide ceramic fuel core. The melting point of uranium dioxide is generally 2800 degrees Celsius. The fuel rod cladding is zirconium-niobium alloy. This is because zirconium-niobium alloy has good corrosion resistance and mechanical properties in high-temperature and high-pressure water at 300-400 degrees Celsius. It is often used to wrap fuel rods, but it will react with water if the temperature exceeds 400 degrees Celsius.

The temperature of volcanic magma is actually not as high as everyone imagines. The temperature of acidic magma is only 650-850℃, and the temperature of basic magma is only around 1100℃. That is to say, if the fuel rods are thrown into the crater, they will not melt at all, and will be ejected back to the ground as the volcano erupts.

Volcanic ash can provide fertile soil. Farmers in many countries grow crops near craters. The nuclear waste ejected will inevitably affect the health of people nearby. If it is a dead volcano with a deeper crater, it can be used as a nuclear waste burial point. After being buried deep underground, the nuclear waste will naturally decay over time and the radioactivity will weaken.

Figure: Nuclear fuel assembly

Another reason why nuclear waste is not suitable for storage in active volcanoes is that half-life is an inherent property of radioactive materials. Physical changes such as melting alone cannot change the half-life of radioactive materials or eliminate radioactivity.

Nuclear waste is radioactive because some of the radioactive substances in it are constantly decaying naturally. A physical quantity that measures the speed of decay is called half-life. The half-life of radioactive elements in nuclear waste ranges from decades to billions of years. Whether the radioactive substances exist in solid or liquid form, their half-life will not change, that is, the radioactivity intensity will not change.

Diagram: Radioactive decay

The reason why spent fuel is so harmful is that it contains highly radioactive elements that are extremely harmful to the human body. The nuclear waste that we usually talk about with fear mainly refers to this category. After the nuclear fuel undergoes a nuclear reaction in the pile after neutron bombardment, the fuel assemblies whose burnup depth reaches the unloading standard are unloaded from the pile, which is the spent fuel. Spent fuel contains a large amount of unused value-added materials 238U or 232Th, unburned and newly generated fissile materials Pu239, U235 or U233, and transuranic elements such as Np, Am, Cm produced during the irradiation of nuclear fuel, as well as fissile elements Sr90, Cs137, Tc99, etc.

Illustration: Composition comparison before and after combustion *Source: World Nuclear Society

So how do we properly dispose of this spent fuel?

The treatment can be roughly divided into two categories: one is an open cycle, such as the United States, Canada, Sweden and other countries. Generally speaking, it is to find a stable geological zone for deep burial storage; the second is a closed cycle, such as my country, Japan, France and other countries, which separates and extracts the spent fuel, and extracts the uranium and plutonium in it for use again. my country is still in the development stage.

Finally, let's talk about the famous advanced nuclear energy system ADS system, which is the accelerator driven sub-critical system. As can be seen from the figure above, the spent fuel after burning contains not only uranium and plutonium, but also other radioactive substances. After the uranium and plutonium are extracted from the spent fuel, there are still radioactive wastes such as actinides. The half-life of these radioactive substances ranges from decades to billions of years. The proton beam generated by the accelerator bombards the heavy metal target (such as liquid Pb or Pb-Bi alloy) in the subcritical reactor, causing spallation reaction, and then produces neutrons through intranuclear cascade and extranuclear cascade. A proton with an energy of 1 GeV produces about 30 neutrons on the thick target. The spallation neutron target provides exogenous neutrons for the subcritical reactor. These neutrons will trigger the transmutation of spent fuel in the transmutation system, reducing the half-life of radioactive substances from billions of years to hundreds of years. The ADS system is equivalent to a spent fuel incinerator, which is the most ideal spent fuel treatment technology. At present, my country has achieved good research results.

Last words

Spent fuel reprocessing technology is indeed a problem that needs to be solved in the development of nuclear power, and it is also the core target of many people's attacks on nuclear power. However, spent fuel will not be left with nowhere to go as anti-nuclear people say. There is no need to think that there are always unruly people who want to harm me. In the near future, the spent fuel problem will be solved. At present, there is huge pressure to achieve carbon neutrality and carbon peak. Under the trend of global warming, nuclear energy is not the best choice, but it is a good choice.