Corium (nuclear reactor)

Heat distribution throughout the corium mass is influenced by different thermal conductivity between the molten oxides and metals.

These may be gas phase, such as molecular iodine or noble gases, or condensed aerosol particles after leaving the high temperature region.

High-pressure conditions push the cladding onto the fuel pellets, promoting formation of uranium dioxide–zirconium eutectic with a melting point of 1,200–1,400 °C (2,190–2,550 °F).

Hydrogen embrittlement may also occur in the reactor materials and volatile fission products can be released from damaged fuel rods.

The steam explosion resulting from such sudden corium-water contact can disperse the materials and form projectiles that may damage the containment vessel by impact.

[4] Brief re-criticality (resumption of neutron-induced fission) in parts of the corium is a theoretical but remote possibility with commercial reactor fuel, due to low enrichment and the loss of moderator.

Cooling water from above the corium layer, in sufficient quantity, may obtain a thermal equilibrium below the metal creep temperature, without reactor vessel failure.

The layer of molten steel at the top of the oxide may create a zone of increased heat transfer to the reactor wall; this condition, known as "heat knife", increases the probability of formation of a localized weakening of the side of the reactor vessel and subsequent corium leak.

[1] In the case of high pressure inside the reactor vessel, breaching of its bottom may result in high-pressure blowout of the corium mass.

Aerosols released during this phase are primarily based on concrete-originating silicon compounds; otherwise volatile elements, for example, caesium, can be bound in nonvolatile insoluble silicates.

The oxide phase, in which the nonvolatile fission products are concentrated, can stabilize at temperatures of 1,300–1,500 °C (2,370–2,730 °F) for a considerable period of time.

An eventually present layer of more dense molten metal, containing fewer radioisotopes (Ru, Tc, Pd, etc., initially composed of molten zircaloy, iron, chromium, nickel, manganese, silver, and other construction materials and metallic fission products and tellurium bound as zirconium telluride) than the oxide layer (which concentrates Sr, Ba, La, Sb, Sn, Nb, Mo, etc.

[8] The dynamics of the movement of corium in and outside the reactor vessel is highly complex, however, and the number of possible scenarios is wide; slow drip of melt into an underlying water pool can result in complete quenching, while the fast contact of a large mass of corium with water may result in a destructive steam explosion.

[9] The thermal load of corium on the floor below the reactor vessel can be assessed by a grid of fiber optic sensors embedded in the concrete.

[10] Some reactor building designs, for example, the EPR, incorporate dedicated corium spread areas (core catchers), where the melt can deposit without coming in contact with water and without excessive reaction with concrete.

Some samples contained a small amount of metallic melt (less than 0.5%), composed of silver and indium (from the control rods).

The inclusion of iron and chromium rich regions probably originate from a molten nozzle that did not have enough time to be distributed through the melt.

Striated interconnected porosity was found in some samples, suggesting the corium was liquid for a sufficient time for formation of bubbles of steam or vaporized structural materials and their transport through the melt.

The zirconium-rich phase was found around the pores and on the grain boundaries and contains some iron and chromium in the form of oxides.

[23] The molten corium settled in the bottom of the reactor shaft, forming a layer of graphite debris on its top.

Eight days after the meltdown the melt penetrated the lower biological shield and spread on the reactor room floor, releasing radionuclides.

The material is dangerously radioactive and hard and strong, and using remote controlled systems was not possible due to high radiation interfering with electronics.

The Elephant's Foot, hard and strong shortly after its formation, is now cracked enough that a cotton ball treated with glue can remove 1-2 centimeters of material.

Large amounts of residual stresses were introduced during solidification due to the uncontrolled cooling rate.

The alpha decay of isotopes inside the glassy structure causes Coulomb explosions, degrading the material and releasing submicron particles from its surface.

[39] The level of radioactivity is such that during 100 years, the lava's self irradiation (2×1016 α decays per gram and 2 to 5×105 Gy of β or γ) will fall short of the level required to greatly change the properties of glass (1018 α decays per gram and 108 to 109 Gy of β or γ).

From 1997 to 2002, a series of papers were published that suggested that the self irradiation of the lava would convert all 1,200 tons into a submicrometre and mobile powder within a few weeks.

Some of the surfaces of the lava flows have started to show new uranium minerals such as UO3·2H2O (eliantinite), (UO2)O2·4H2O (studtite), uranyl carbonate (rutherfordine), čejkaite (Na4(UO2)(CO3)3),[42] and the unnamed compound Na3U(CO3)2·2H2O.

At an estimated eighty minutes after the tsunami strike, the temperatures inside Unit 1 of the Fukushima Daiichi Nuclear Power Plant reached over 2,300 ˚C, causing the fuel assembly structures, control rods and nuclear fuel to melt and form corium.

[45][46][47] Unit 2 retained RCIC functions slightly longer and corium is not believed to have started to pool on the reactor floor until around 18:00 on March 14.