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The Secret of the Meltdown: What Really Happens at the Core of a Nuclear Reactor?
Nuclear energy is an important resource that supplies clean electricity to the world, but its potential risks are often worrying. When a loss of coolant accident (LOCA) occurs, the core of a nuclear reactor can be severely damaged, which can lead to a cascade of crisis events. To better understand this process, we will explore the mechanism of LOCA and what happens to the core of a nuclear reactor in this situation.
Background of Loss of Coolant Accident (LOCA)
In a nuclear reactor, the presence of coolant is crucial. It is not only used to remove the heat generated inside the reactor core, but also ensures the stable operation of the nuclear fission reaction. Once the coolant flow is reduced, the reactor's emergency shutdown system will immediately activate to stop the fission reaction. However, even though the reactor is no longer fissioning, the nuclear fuel still generates large amounts of heat due to radioactive decay.
"Whether it is at full power shutdown or shutdown, the decay heat of nuclear fuel will continue for a period of time."
Danger chains in reactors
If all independent cooling systems fail, heating could cause fuel temperatures to spike to the point of damaging the reactor. When coolant is present, it can boil and build up high pressure, which is why nuclear power plants are equipped with pressure relief valves and backup cooling water supplies.
In some reactor designs, if graphite and air are present at the same time, the graphite may catch fire, spreading radioactive contamination. This mainly occurs in reactors that use graphite as a neutron modifier, such as AGR, RBMK and Magnox reactors (see the example of the Chernobyl disaster).
Risk of Meltdown
If the fuel and reactor interior melt, the molten mass may still remain critical, resulting in continued heat generation and possibly even melting through the bottom of the reactor, creating a so-called "nuclear meltdown" situation. In this case, the process of molten material penetrating down through the soil to the water level (or even deeper) is known as the "China Syndrome." But current knowledge shows that molten material becomes unstable before it reaches deep enough to continue to generate heat.
"Although the reactor core in the Chernobyl incident melted, the chain reaction was no longer possible because the material was widely dispersed."
Safety design of modern reactors
Many modern nuclear reactor designs have passive safety features designed to prevent a meltdown under extreme conditions. For example, a Pebble Bed Reactor can effectively withstand extreme temperature changes in the fuel. On the other hand, the CANDU reactor has two large volumes of relatively low-pressure cooling water, which also act as a buffer for the heat source.
In the event of a loss of coolant, the power output of some reactors can be passively adjusted and heat production reduced. This is measured by the coolant vacancy coefficient, which most modern nuclear power plants show as negative values.
Warning about the Fukushima nuclear disaster
The 2011 Fukushima Daiichi nuclear power plant disaster was caused by a loss of coolant accident. At that time, the power system failed, causing the reactor core cooling failure, which eventually led to three nuclear meltdowns and hydrogen explosions. The incident is another reminder of the dangers that can result from losing coolant.
Challenges of Fuel Cladding
At present, the fuel rod cladding of most nuclear reactors is made of titanium alloy material. Although it has good corrosion resistance, it will react violently with water to generate hydrogen when overheated. This reaction played an important role in the hydrogen explosion in the Fukushima nuclear disaster.
The development and future of technology
The development of anti-oxidation coating technology for fuel coatings is also particularly critical. If the Zirlo substrate is coated with the Ti2AlC MAX phase, its anti-oxidation ability at high temperatures can be significantly improved, which is undoubtedly a major step for nuclear safety.
Conclusion: The future and risks of nuclear energy
In reactor design, the continued introduction of new technologies to prevent accidents and reduce the severity of injuries is an important step forward. However, we still need to think about, as technology advances, are we ready enough to deal with the risks hidden in nuclear energy in the future?
