Why are nuclear power plants melting

As Meltdown This describes a process in a nuclear reactor in which the fuel rods in the reactor core heat up and fuse in an uncontrolled manner. A core meltdown can occur if the reactor cooling and backup systems fail. A core meltdown is a serious accident in which radioactive material can escape from the reactor into the environment in an uncontrolled manner.

Process of a core meltdown

Pressurized water reactors and boiling water reactors are controlled by control rods that regulate the neutron flow between the fuel elements (fuel rods grouped together). In the event of failure of the cooling system, the fuel rod control system or other causes of an accident, the decay heat can heat the fuel elements so that they bend. If this condition lasts long enough, the nuclear fuel can melt and converge at the bottom of the reactor vessel.

In the final stage, the melted core could, in the worst case, eat its way through the reactor vessel and all building shells, reach the groundwater and thus release large quantities of radioactive substances. An above-ground release through an above-ground building leak is of course even more dangerous for the population.


The other consequences of a core meltdown, such as steam and hydrogen explosions, are typically associated with a core meltdown, but do not require them.

A particularly serious variant of the course of the accident is High pressure core meltdownif it is not possible to reduce the pressure in the reactor sharply in the first time. It would then be possible that the red-hot melt of the reactor core weakens the wall of the reactor vessel and escapes from the reactor vessel with a simultaneous, also explosive pressure increase (e.g. through an oxyhydrogen explosion or rapid evaporation). The high pressure generated in the containment could lead to leaks, which releases radioactive material.

Avoidance of core meltdowns

In the case of newer reactor designs, special devices (Core catcher) catch the reactor core in the event of a core meltdown and prevent the release of the fissile material inventory.

Due to the devastating potential consequences of a core meltdown, the operation of inherently safe reactors, especially decentralized PBMR with reduced power, is now being successfully tested, especially in Asia. For all commercial nuclear reactors currently operated in Europe, however, the risk of a core meltdown can be significantly reduced by additional safety measures (e.g. negative vapor bubble coefficient), but cannot be excluded in principle.

Known core meltdown accidents

On April 26, 1986, a catastrophic reactor accident occurred in Block 4 (graphite-moderated pressure tube reactor) of the Chernobyl power plant. After an oxyhydrogen (hydrogen / oxygen) explosion inside the reactor core and the subsequent fire of the graphite, a large amount of radioactive substances was released. This disaster is known as the second worst nuclear incident in history.

"Western" type reactors have also been affected by core meltdown accidents. On March 28, 1979, a pump failed in the non-nuclear part of the 880 MWe nuclear power plant on Three Mile Island near Harrisburg, Pennsylvania. Since the failure of the emergency cooling system was not noticed in time, the reactor could no longer be controlled a few hours later. An explosion could be prevented by releasing the released radioactive vapor into the environment. Investigations of the reactor core, which were only possible three years after the accident due to the accident, showed a core meltdown, which, however, had come to a standstill before the reactor pressure vessel melted through.

In 1969 a serious accident occurred in Switzerland in a small experimental reactor near Lucens (8 MWe). A failure of the cooling system due to corrosion led to a core meltdown and a fuel element fire with subsequent release from the reactor tank. The radioactivity was essentially restricted to the cavern and the surrounding tunnel system. The clean-up work in the sealed tunnel lasted until 1973 and 2003, respectively, when the waste containers were removed from the site.

In 1977 some fuel elements melted in the Slovakian nuclear power plant Bohunice A-1 (150 MWe) due to errors during loading. The reactor hall was contaminated. The reactor is no longer in operation today, it should not be confused with the VVER nuclear power plants that are now in operation there. Almost the same incident occurred in 1980 in the French kiln Saint Laurent A-1 (450 MWe). This closed plant should not be confused with the pressurized water reactors that are operated there today.

See also

Category: Nuclear Technology