Nuclear meltdown

As a meltdown refers to a major accident in a nuclear reactor, in which some ( "partial meltdown " ) or all the fuel rods in the reactor core heat up excessively and melt.

Principle of the danger of a meltdown are all nuclear reactor types used for commercial power generation affected.

A meltdown may occur if the reactor cooling and also any emergency cooling fails. The decay heat - it arises inevitably after interruption of nuclear fission - then causes the fuel to become very hot, melt and the melt ( corium ) converges at the bottom of the reactor.

In such an accident can highly radioactive material uncontrolled leakage from the reactor into the environment and endanger people and the environment - an accident, which is referred to as a worst-case scenario.

  • 4.1 Total meltdowns
  • 4.2 Partial meltdowns

Expiration of a meltdown


A core melt can occur, when the heat power generated by the fuel rods and can not be derived. Failure which can lead to a meltdown, are:

  • Loss of coolant
  • Cooling failure
  • Power excursion ( About criticality of the reactor )

The reaction of a reactor to incidents depends strongly on the design. From the design depends, for example, whether a fault, the chain reaction comes to a halt or not. Important here is the distribution of nuclear fuel, coolant and moderator. The moderator slows down the released during the fission neutrons. Without moderator the neutrons would be much too fast to trigger more fissions in low-enriched nuclear fuel.

  • In pressurized water reactors, boiling water reactors and the cooling water also serves as a moderator. If there is a loss of cooling water, missing the moderator, and the nuclear fission comes to a standstill (negative void coefficient ).
  • Pressure tube reactors often use graphite as a moderator. A graphite block encloses pressure tubes in which the fuel rods and the coolant water are. As coolant and moderator are separated here from each other, does not result in a loss of the coolant to an end of the chain reaction. In a loss of the refrigerant increases, even the number of neutrons, which may cause fission, so that the power generated by the nuclear fission can be extremely increased (positive void coefficient ).
  • Breeder reactors use enriched nuclear fuel and higher than coolant usually sodium. Here no moderator to maintain the chain reaction is required. Here, too, does not result in a loss of the coolant to an end of the chain reaction, and then this increases the power (positive void coefficient ).

Even if a reactor can be shut down and the chain reaction comes to a standstill, the risk of a meltdown has not been eliminated. During operation of the reactor, about 6.5 % of the power from the radioactive decay of the fission products formed during nuclear fission occur ( decay heat ). A reactor with 1300 MW of electrical power generated nearly 4,000 MW thermal power in the reactor core. Approx. 260 MW of this power comes from the decay heat. Decay heat decreases with time. One hour after shutdown of the reactor decay heat is still about 1.6% of the thermal power ( 65 MW), one day after being switched off 0.8 % ( 32 MW ), even several months after the shutdown is still about 0 produces 1 % of the power (4 MW) from the decay of fission products. This power must be dissipated. If this fails, the reactor core heats up more and more until it finally melts.

Example: meltdown by cooling loss in a light water reactor

If the cooling off ( eg failure of backup power during a power outage in the public network ), something like the following scenario can play

Excess pressure

At a cooling failure, the heat generated in the core can no longer be removed. Even if it is possible to shut down the reactor decay heat is sufficient to strongly heat the reactor core.

  • Increasing the temperature within the reactor core via the normal temperature, the pressure rises in the reactor pressure vessel. This increase in pressure can reach values ​​that threaten the stability of the reactor pressure vessel. To prevent rupture of the reactor pressure vessel, pressure from the reactor pressure vessel into the surrounding containment must be drained. Since the heat production stops from the decay of fission products, critical pressures are reached in the reactor pressure vessel, so that always pressure must be vented into the containment over again.
  • In this way the pressure in the containment. With repeated pressure discharge from the reactor pressure vessel, critical pressure values ​​arise that threaten the stability of the containment in the containment. Thus must be removed also from the containment pressure. Depending on the construction type of the reactor depressurization takes place either in a surrounding reactor building or directly into the atmosphere ( venting ).
  • By releasing the pressure from the reactor pressure vessel ( Venting ) cooling water is lost. If it is not possible to replenish cooling water, the level of the coolant drops in the reactor pressure vessel. This can eventually cause the fuel rods are no longer completely covered with water, so that the upper portion of the fuel rods protruding from the cooling water and surrounded only by water vapor. Water vapor carries heat away much worse than liquid water. Thus, the fuel rods heat up particularly strong in this area.

Formation of hydrogen

  • Be reached temperatures in excess of 900 ° C in the exposed areas of the fuel rod, the strength of the fuel rod decreases. The fuel rods begin to burst. Gaseous and volatile radioactive fission products from escaping from the fuel rods in the reactor pressure vessel. Must continue to pressure from the reactor pressure vessel and from the containment to be drained are reaching radioactive substances into the environment.
  • The cladding of the fuel rods consist of a zirconium alloy. At temperatures above 1000 ° C, the zirconium begins to react with the ambient water vapor. It forms zirconium oxide and hydrogen. This chemical reaction is exothermic, that is, it will lead to additional energy which heats the fuel rods. With increasing temperature, the reaction strength increases, the hydrogen production increases.
  • By additional heating of the water vapor and the formation of hydrogen, the pressure in the reactor pressure vessel significantly. In order not to damage the reactor pressure vessel, this pressure must be released into the containment. Because of the bursting of the fuel rods, the concentration of gaseous and volatile fission products has risen in the cooling water and thus increases the pressure draining the radioactivity in the containment.
  • By draining a hydrogen- steam into the containment, there is a danger that forms the hydrogen and the oxygen present in the containment air, an explosive oxyhydrogen gas mixture. If there is an explosion of oxyhydrogen mixture, not only the containment, but also the reactor pressure vessel may be damaged. For this reason, the containment is filled with an oxygen free inert gas, in some types of reactors. Even if an oxyhydrogen explosion can be avoided in the containment is increased by the discharge of the hydrogen-containing vapor, the pressure in the containment, so that critical pressure values ​​can be achieved.
  • If you let the excess pressure from the containment from, on the one hand increases the radioactive contamination of the environment, since reach the containment strengthened radioactive substances due to the bursting rods. On the other hand, is outside the containment of hydrogen with oxygen in the air in contact. It may come to the formation of an explosive oxyhydrogen gas mixture and subsequently to hydrogen explosions.

Destruction of the fuel elements

  • If the temperature of the exposed fuel rod ends continues to rise, burst from 1170 ° C, the fuel rods to a greater extent. The release of fission products into the reactor core increases. Likewise intensifies with increasing temperatures, the formation of hydrogen on the cladding tubes of the fuel rods, from temperatures of 1270 ° C, it is relatively strong, with the result that the hydrogen and fission product containing steam must be frequently discharged into the containment, since the reaction the fuel rod cladding with water vapor produces additional heat, the heating of the fuel rods accelerated.
  • At temperatures between 1210 ° C and 1450 ° C, the control rods start to melt. Neutrons can be trapped here no longer. A chain reaction is only because in these areas the water has evaporated and thus there is no longer a moderator. Would it be possible now to feed back more water into the reactor pressure vessel, should urgently be taken to ensure that this water is mixed with enough neutrons scavenging substances such as boron. Because stood by the water fed again a moderator is available, because of the melted control rods no longer a neutron absorber would be available. Without boron incorporation, the chain reaction would start uncontrollably with the risk that as a result, the reactor core is more damaged or even destroyed. Is the bottom of the reactor core still liquid water exists, here the control rod melt solidifies again.
  • At temperatures of about 1750 ° C, the cladding of the fuel rods begin to melt. The pellets of nuclear fuel, which are located inside the fuel rod tubes can then be released and move together with the molten fuel rod cladding down. If the molten fuel rod material in cooler areas, for example in the below water still present, it may solidify again.
  • At temperatures of about 2250 ° C, the structures of the fuel elements will be destroyed, fuel pellets melted cladding and all other fuel element materials are no longer stable and fall down. This fuel element debris collect on more stable fuel element parts that are still in the water, or even fall all the way down to the bottom of the reactor pressure vessel. Nuclear fuel accumulates thus the bottom of the reactor pressure vessel, where may still water.

Partial meltdown

  • The mountains of rubble from fuel pellets melted fuel rod cladding and other fuel element materials can put on yet undestroyed fuel element parts at the top, debris can fall between the fuel rods or collect at the bottom of the reactor pressure vessel. As this debris obstructing the passage of coolant, fuel pellets are inside the rubble cooled considerably worse than in intact fuel assemblies. The heat that is generated by the decay of fission products, can hardly be dissipated, the mountains of rubble heat up inside her further on.
  • If temperatures of over 2850 ° C is reached, start the fuel pellets to melt. It forms a meltdown. Located at the bottom of the reactor pressure vessel, water or succeeds again feed into the reactor pressure vessel of water, possibly the melting of the fuel can be initially restricted to the region of the reactor core, which protrudes from the water. The result is a partial meltdown. The molten material forms a melt lump is heated in its interior by the decay of fission products and which can be cooled only from the outside over its surface.
  • The heat power generated by such a melting lump depends on how big the lump is melting, so the quantity contained in decaying fission products in it. The heat generated power still depends on which time has passed between the shutdown of the reactor and the formation of the melt. With increasing time, the heat power generated decreases. The melting of a lump heat output depends on the size of the surface of the molten lump, it depends on the temperature and on the surface of the molten lump from the quality of the heat transfer at the surface of the molten lump. It forms an equilibrium state between the internally generated and delivered to the surface heat output. A poor heat transfer at the surface of the clot leads to a relatively high surface temperature is required to dissipate heat generated power of the surface. In good heat transfer, such as on the border with liquid water, ranging a relatively low surface temperature to dissipate heat generated power. , The surface temperature below the melting temperature of the surface of the clot and the clot is determined to remain stable. If the heat transfer poorly, such as on the border with air or water vapor, the surface temperature must be relatively high in order to transfer the heat capacity. If the melting temperature is exceeded at the surface, the lump is all in liquid and moves down.
  • If it is possible, after formation of a partial meltdown feed water and thereby cooling the melt to the extent that it is committed to the surface, the propagation of the core melt is initially stopped. Inside the melting lump remains liquid. This cooling must be maintained over months, at least until the heat power generated by the decay of the fission products is so far back that the melting lump remains fixed even without effective cooling. However, decreases the efficiency of cooling or the cooling is interrupted, the surface of the molten gob is again in liquid, and the lumps moves further downwards.
  • On the surface of a cooled partial meltdown the same processes take place as to overheat fuel rods. Are the surface temperatures of 900 ° C is exceeded, is formed from the zirconium present in the melt and water vapor of hydrogen which must be vented. Here again, there is the risk of explosive mixtures.
  • Failure to cool a partial core melt sufficiently, the melt moves down. Exceeds the melt water still present, it evaporates to a greater extent. The meltdown always detected large regions of the reactor core, the size of the molten lump grows. With increasing size the amount of generated heat increases fission products, the thermal power generated increases in proportion to the volume. The surface of the molten slug, however, does not grow to the same extent, that is, the generated power per unit surface area increases, the surface temperature of the melt slug increases. To stop the spread of the melt, that is, to lower the surface temperature below the melting point, greater cooling efforts are always required. For very large melting lumps it can happen in an extreme case that the heat generated power is so great that even under water, the surface temperature exceeds the melting point so that the melting lumps would be liquid even under water.

Full meltdown

  • If the entire fuel element material covered by the meltdown, one speaks of a complete meltdown. The molten material is then collected on the bottom of the reactor pressure vessel. A melting through the reactor pressure vessel it can only be prevented if it is cooled from the outside, for example by the surrounding containment is flooded.
  • Are cooling measures for the reactor pressure vessel is not successful, meltdown can melt the wall of the reactor pressure vessel and flow underneath the reactor pressure vessel to the inner layer of concrete of the containment. The behavior of concrete in this case strongly depends on whether the concrete is integrated into the melt or not. If the concrete melted and connects the molten concrete with the melt, thereby increasing the size of the molten lump and the size of its surface without the heat power generated increases. This reduces the surface temperature. The concrete layer is sufficiently thick, the size of the clump may grow to such an extent that on the surface of the melting temperature is exceeded. The melt would be stopped. But does not connect to the molten concrete with the fuel rod melt, eg by swimming as " slag " on the fuel rod melt, then the size of the chunk to be considered remains unchanged, the surface temperature of the lump does not change. The melt would continue to move through the concrete down. The melting lumps would pass through the concrete foundation, all radioactive substances contained therein would enter the ground.
  • Ways to bring such a melt to stop, would be: Surface magnifications (eg over shallow pans, in which such a lump pours ) ( core catcher ). By increasing the surface lowering the surface temperature can be achieved in the gut would be the case of the melting point on the surface below, and the melt would solidify at the surface.
  • Divide into as many small melting lumps. This one increasing the surface area is also connected. The surface temperature decreases, in the ideal case under the melting point.


A particularly severe variant of the accident sequence is the high pressure core melt. This occurs when we do not succeed in the first time, greatly reduce the pressure in the reactor. The red-hot molten reactor core can weaken the wall of the reactor vessel while simultaneously strong and even an explosive increase in pressure, for example by an oxyhydrogen explosion or rapid evaporation of the water ( physical explosion), escape from the reactor vessel. The high pressure generated in the containment eventually leading to leakage, which radioactive material can pass into the surroundings.

Such scenarios were published in 1989 in the " German Risk Study Nuclear Power Plants Phase B " and led to extensive discussions ( see Article nuclear power plant ). To reduce the risk of explosion, was prescribed the so-called Wallmann valve after the meltdown at Chernobyl in Germany, filtered with the gas can be discharged into the atmosphere. To avoid explosive gas explosions reactors must also be equipped with Recombiners (so-called " pottery Candles" ), which reduce the hydrogen concentration in the reactor.

The above-mentioned side effects of the meltdown, such as steam and hydrogen explosions, typically accompanied by a meltdown, but not necessarily presuppose it.

Even without an explosion, the regular cooling devices are unusable due to a melt in the rule. Since by further heating threatens melting through the outer protective container, the molten core must be cooled in order to prevent more serious damage to man and the environment of provisional measures under all circumstances. This cooling system is to maintain, where appropriate, over several months, until the decomposition products have decayed sufficiently that the residual decay heat causes no increase in temperature.

According to a study by the Max Planck Institute for Chemistry, the risk of catastrophic meltdowns like Chernobyl and Fukushima is much higher than previously estimated. This can once in 10 to 20 years, ie 200 times more common than previously thought, occur.

Avoid meltdowns

Because of the devastating potential consequences of a core meltdown is now, especially in the Asian region, the operation allegedly inherently safe reactors, especially from remote high-temperature reactors (HTR ) with reduced power tested. Critics of the HTR technology point out that there may be catastrophic radioactivity releases in HTR - specific accident types such as water or air intrusion and an inherent safety is therefore not met, despite avoiding meltdowns. For all currently operating commercial nuclear reactors in Europe that the risk of a meltdown, although significantly reduced by additional security measures, it can not be ruled out in principle.

Newer reactor designs are special devices, so -called core -catcher, catch the reactor core at a meltdown, prevent the release of fission product inventory in order to alleviate the consequences of a meltdown. In addition, the containment of pressurized water reactors of the third generation ( eg European pressurized water reactor ) are designed with a wall thickness of 2.6 m for hydrogen explosions. As a weak point remains at these concepts, the above High pressure core melt in a spontaneous failure of the pressure vessel could lead to the destruction of all barriers.

List of known core meltdown accidents

Accidents with core melt can be performed on the International Nuclear Event Scale Rating (INES ) at level 4.

Total core meltdown

With a total meltdown of the reactor core is completely destroyed and the reactor so far damaged that repair is impossible.

  • On March 28, 1979 fell in the reactor Unit 2 of the nuclear power plant at Three Mile Iceland (880 MWe ) in Harrisburg ( Pennsylvania) in the non-nuclear part of a pump. Since the failure of the emergency cooling system was not noticed in time, the reactor was several hours later uncontrollable. An explosion was prevented by draining the released radioactive steam into the environment. Investigations of the reactor core, which only three years after the accident were possible due to an accident, showed a meltdown, were melted at about 50 % of the reactor core and which had come to a halt prior to melting of the reactor pressure vessel. This accident was classified on the International Nuclear Event Scale evaluation of INES level 5.
  • (Since the dissolution of the Soviet Union in 1991 in Ukraine at that time in the Soviet Union ) a catastrophic reactor accident on 26 April 1986 occurred in the graphite-moderated pressure tube reactor of the reactor block 4 of the Chernobyl nuclear power plant. As a result of an uncontrolled power increase to more than hundred times the nominal power, there was a total meltdown and a hydrogen explosion within the reactor core. In the subsequent graphite fire large amounts of radioactive substances were released. This disaster is classified on the International Nuclear Event Scale evaluation of INES level 7 and is considered the worst nuclear accident in history. The effects were therefore so serious because the reactor was not equipped with a safety container ( containment ).

Partial meltdowns

For a partial meltdown of the reactor core remains partially intact. Individual fuel rods or entire fuel melt or be damaged by overheating. Most plants are shut down after such an accident ( especially older nuclear reactors ); some have been repaired in the past and continue to be operated.

  • On July 26, 1959, came at the Santa Susana Field Laboratory (USA) due to a blocked cooling passage to a 30 percent meltdown. The bulk of the cleavage products could be filtered off, but it was the release of large amounts of iodine 131
  • In February 1965, there were on the nuclear-powered icebreaker Lenin a loss of coolant accident. After the switch-off to fuel exchange the coolant of the second reactor was, probably by an oversight of the operator removed before the fuel elements were removed. Some fuel rods melted through the resulting decay heat in them; others were deformed.
  • On October 5, 1966, came in the prototype fast breeder ' Enrico Fermi 1' (65 MW) in Michigan (USA) in some parts of the reactor core to a core melt due to a fragment in the cooling circuit. The reactor was repaired, continue to be operated and decommissioned in 1972.
  • On January 21, 1969, arrived in the Swiss underground experimental nuclear power plant Lucens (8 MWe ) and a serious accident. A conditional corrosion failure of the cooling led to the meltdown and the fuel fire with subsequent release from the reactor tank. The radioactivity was mainly due to the cavern and limited the surrounding tunnel system. The reactor was shut down in 1969. The cleanup in the sealed tunnel lasted until 1973. 2003 the waste containers have been removed from the site.
  • On October 17, 1969 melted shortly after commissioning of the reactor 50 kg of fuel in gas-cooled graphite reactor of the French nuclear power station at Saint -Laurent A1 ( 450 MWe ). The reactor was then closed down in 1969. Today's reactors at the nuclear power plant are pressurized water reactors.
  • On February 22, 1977 rendered in the Slovak nuclear power plant Bohunice A1 ( 150 MWe ) due to faulty loading some fuel. The reactor hall was radioactively contaminated. The reactor was then closed down in 1977.
  • 1977 melted half the fuel in Unit 2 of the Russian nuclear power plant Beloyarskaya. The repairs lasted one year, the Unit 2 was shut down in 1990.
  • On March 13, 1980 melted in the second block of the nuclear power station at Saint -Laurent in France a focal element, within the plant radioactivity was released. The reactor block is repaired, continue to be operated and decommissioned in 1992.
  • On 28 March 2011, the Japanese government admitted that there has been a partial meltdown after a series of accidents at the nuclear power plant in Fukushima I, in block 1, 2 and 3.

List of lesser known meltdowns

  • On December 12, 1952 at the NRX reactor in Ontario, Canada
  • In 1955 in Idaho, USA, in the Experimental Breeder Reactor I ( EBR-I )
  • On October 10, 1957 in Windscale, England (see Windscale fire )
  • On January 3, 1961 at the military research reactor SL -1 ( Stationary Low -Power Reactor Number One ), Idaho Falls, United States

In addition, some Russian nuclear-powered submarines suffered meltdowns. This came from the submarines K -278 Komsomolets (1989 ), K -140 and K -431 (10 August 1985).

The term China Syndrome

In the U.S., a reactor accident with core melt which can be unrestrained melting into the concrete foundation and ground water, colloquially referred to as China Syndrome.

Often the origin of the expression is explained by the fact that the People's Republic of China considered from the U.S., according to popular opinion about on the opposite side of the earth ( antipodes ) is (which is actually not the case, since both states are north of the equator) and it is felt that the molten reactor core in the direction of China into melts deep into the earth. This term was popularized by the movie The China Syndrome.

Other assumptions are aimed at the formation of a porcelain -like ( called in English porcelain china) shell around the molten reactor core from.