A loss-of-coolant accident and a loss-of-pressure-control accident are closely related in some reactors. In a loss-of-coolant accident, either the physical loss of coolant (which is typically deionized water, an inert gas, NaK, or liquid sodium) or the loss of a method to ensure a sufficient flow rate of the coolant occurs.Many commercial reactors are contained within a 1.2-to-2.4-metre (3.9 to 7.9 ft) thick pre-stressed, steel-reinforced, air-tight concrete structure that can withstand hurricane-force winds and severe earthquakes. The containment building is the last of several safeguards that prevent the release of radioactivity to the environment. Contemporary safety principles ofĭefense in depth ensure that multiple layers of safety systems are always present to make such accidents unlikely.
Nuke reactor meltdown series#
Failures in control systems may cause a series of events resulting in loss of cooling. The reason may be one of several factors, including a loss-of-pressure-control accident, a loss-of-coolant accident (LOCA), an uncontrolled power excursion or, in reactors without a pressure vessel, a fire within the reactor core. A core damage incident can occur even after a reactor is shut down because the fuel continues to produce decay heat.Ī core damage accident is caused by the loss of sufficient cooling for the nuclear fuel within the reactor core. If the heat from that reaction is not removed adequately, the fuel assemblies in a reactor core can melt. Nuclear power plants generate electricity by heating fluid via a nuclear reaction to run a generator. A meltdown is considered very serious because of the potential for radioactive materials to breach all containment and escape (or be released) into the environment, resulting in radioactive contamination and fallout, and potentially leading to radiation poisoning of people and animals nearby. Superheated steam and hot metal inside the core can lead to fuel–coolant interactions, hydrogen explosions, or steam hammer, any of which could destroy parts of the containment. Subsequent failures can permit these radioisotopes to breach further layers of containment.
Once the fuel elements of a reactor begin to melt, the fuel cladding has been breached, and the nuclear fuel (such as uranium, plutonium, or thorium) and fission products (such as caesium-137, krypton-85, or iodine-131) within the fuel elements can leach out into the coolant. Alternatively, an external fire may endanger the core, leading to a meltdown. A meltdown may be caused by a loss of coolant, loss of coolant pressure, or low coolant flow rate or be the result of a criticality excursion in which the reactor is operated at a power level that exceeds its design limits. This differs from a fuel element failure, which is not caused by high temperatures. It has been defined to mean the accidental melting of the core of a nuclear reactor, however, and is in common usage a reference to the core's either complete or partial collapse.Ī core meltdown accident occurs when the heat generated by a nuclear reactor exceeds the heat removed by the cooling systems to the point where at least one nuclear fuel element exceeds its melting point. The term nuclear meltdown is not officially defined by the International Atomic Energy Agency or by the United States Nuclear Regulatory Commission. Unit 2, which suffered a partial core melt, is in the background.Ī nuclear meltdown ( core meltdown, core melt accident, meltdown or partial core melt ) is a severe nuclear reactor accident that results in core damage from overheating. Three Mile Island Nuclear Generating Station consisted of two pressurized water reactors manufactured by Babcock & Wilcox, each inside its own containment building and connected cooling towers.
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