Both the Chernobyl and F Island 7 accidents were the worst stages when the reactor core was completely melted down and leaked. The root cause is that the most important of these is the failure of the cooling system of the nuclear reactor. The Chernobyl accident was an experiment in which the cooling system was shut down prematurely, followed by a series of human errors. Or it will automatically stop functioning due to external factors. The F Island accident was caused by a tsunami that caused the entire plant to lose power, and the cooling system and the control rod that made the response stop were inserted into the system, and all of them could not move because there was no electricity. The general reactor insertion control rod is designed to be directly above the core container, so that in case of power failure, the control rod can also be completely reduced to the inside of the reactor core by relying on its own weight after the brake is loosened, so as to achieve emergency shutdown. And the control rod on the F island side was actually inserted from the bottom up; once the power was completely cut off, the immortals couldn't help it! Such a brain C design is also really home. Once the cooling system is shut down or the control rod system fails.
The large amount of heat generated by the core continuing the nuclear fission reaction will continue to accumulate inside the core, quickly drying the coolant. When the zircon-tin alloy coated by the outer layer of the fuel rod is too high in the reactor without cooling, exceeding its melting point of more than 2,000 degrees Celsius, the nuclear fuel in the reactor fuel rod, that is, uranium oxide, begins to melt through the shell of the zirconium-tin alloy and leaks out. This is the beginning of the overall meltdown process of the reactor core. Inside the core of a typical megawatt reactor, the amount of fuel is very large, up to 160 to 260 tons. So much of the nuclear fuel, originally wrapped in a metal shell of each other, and there is a liquid coolant in the middle for continuous cooling and slowing down the reaction. And once the meltdown begins, and one or two hundred tons of heat and fuel melt into a whole piece of magma-like extremely highly radioactive material, then this is the beginning of a catastrophic nuclear accident of level 7. Zircon-tin alloys, which have been melted down and appear liquid, are no longer protective shells, but become dangerous substances that expand accidents and contribute to abuse. Because of the high temperature and pressure water vapor in the reactor core, it will react violently with zirconium tin alloy.
Produces large amounts of dangerous hydrogen. In fact, this kind of reaction will occur slightly in the normal operation of nuclear power plants, even near the cores of nuclear submarines and nuclear-powered aircraft carriers. There will also be a certain amount of hydrogen gathered on the top of the containment outside the core, so whether it is inside the containment of the nuclear power plant or the inside of the nuclear submarine, in addition to strictly prohibiting open flames, there is a hydrogen removal device that runs at any time to eliminate the small amount of hydrogen produced at any time. After the accident, a large amount of hydrogen gas accumulated could not be disposed of, and it would soon trigger a large-scale hydrogen explosion and blow up the already fragile containment. After the metal container and the containment shell outside the core are burned through by the molten cores that are gathered together like magma at temperatures above 2000 degrees Celsius, they will continue to settle further down the formation. First, it melts through the protective concrete underneath, and finally enters the natural formation and continues to burn down. This creates an artificial cave that continues to expand deeper into the formation. Some have even exaggerated that it may eventually burn through the earth. Hanhai Wolf Mountain (Xiongnu Wolf Mountain) believes that it is not necessary to burn through the earth, but it is possible to burn through the earth's crust. At the beginning of the Chernobyl nuclear accident, seizing the opportunity to the reactor above and in the underground layer, with a huge sacrifice, manually invested a large amount of boric acid and lead to slow down the reaction sinking rate of the molten reactor core and achieve "cold stop reactor", that is, after the fission reaction of the molten reactor core slowed down, the mixture gradually cooled and solidified, and gradually stopped sinking.
However, several cores in the F Island accident have all been melted down, and there is no manual measure to prevent the sinking. At most, seawater is poured from above, but seawater can only partially cool down the upper part of the molten reactor core, and there is no way to continue to burn through the formation of the hot melt reactor core below. So to what extent did several fukushima meltdown cores settle? The outside world simply doesn't know anything! If the sinking core encounters large-scale oil and gas, or artificial upwelling channels that trigger magma, it is a huge new disaster. So why can most of the normal nuclear fuel power generation last for about 3 years, while the F Island accident has occurred for more than 10 years, why has the reactor core not cooled? This is that: 3.5% of the abundance of uranium raw materials for normal power generation, 3 years later can only consume 40% of it, after three years, the core of the thermal neutron density is not enough, the reaction efficiency is seriously insufficient, and the core fuel rod has begun to gradually deform. Therefore, normal power reactors must be refueled. The remaining 60% remains unusable. After the accident melted down the core, the remaining more than 60% of the uranium fuel began to be in disorderly dense contact and continuous reaction. This process does not follow the normal routine. Its natural extinction time may be thousands of years, or tens of thousands of years to hundreds of thousands of years. How long is it harmful? No one has a final timeline.