What is Coruim. What happened to the Chernobyl Coruim?

[Fledermaus]

This a diagram of what happened to the Three Mile Island Coruim

Corium, also called fuel containing material (FCM) or lava-like fuel containing material (LFCM), is a lava-like molten mixture of portions of nuclear reactor core,



The heat for melting the reactor may originate from the nuclear chain reaction, but more commonly a decay heat of the fission products contained in the fuel rods is the primary heat source. The heat production from decay heat drops exponentially as the short half-life isotopes providing the most activity decay. Another heat source are oxidation chemical reactions between the hot metals and atmospheric oxygen or steam.

Chain reaction and corresponding increased heat production may progress in parts of the corium if a critical mass can be achieved locally. This condition can be detected by presence of short-life fission products long after the meltdown, in amount too high to be remaining from the controlled reaction inside the pre-meltdown reactor. As chain reaction generates high amount of heat and fresh highly radioactive fission products, such condition is highly undesirable.

The temperature of corium depends on its internal heat generation (the amount of decay heat producing isotopes, the dilution by other molten materials) and its heat losses (the physical configuration and the heat losses to the environment; a compact mass will lose less heat than a thinly spread layer. Corium of high enough temperature can melt concrete. A solidified mass of corium can remelt itself if its heat losses drop, e.g., if it becomes covered by heat-insulating debris or if the water cooling it evaporates.

Crusts can be formed on the corium mass, acting as thermal insulators and hindering thermal losses. Heat distribution through the corium mass is influenced by different thermal conductivities between the molten oxides and metals. Convection in the liquid phase significantly increases heat transfer.<1>

The molten reactor core releases volatile compounds. These can stay in gas phase (e.g. molecular iodine, noble gases) or condense into aerosol particles after they leave the high-temperature region. High proportion of aerosol particles originates from the reactor control rod materials. The gaseous compounds may become adsorbed on the surface of the aerosol particles.



Reactor vessel breachingIn absence of adequate cooling, the inside of the reactor overheats, deforms as the portions undergo thermal expansion, then structurally fails once the temperature reaches the melting point of the structural materials. The melt then accumulates on the bottom of the reactor vessel. In case of adequate cooling of the corium melt, it can solidify and the spread of damage is limited to the reactor. However, corium may melt through the reactor vessel and flow out or be ejected as a molten stream by the pressure inside the reactor. The reactor failure may be caused by overheating of its bottom by the corium melt, resulting first in creep failure and then in breach of the vessel. High level of cooling water above the corium layer may allow reaching a thermal equilibrium below the metal creep temperature, without reactor vessel failure.<5>

If the vessel is sufficiently cooled, a crust between the melt and the reactor wall can form. The layer of molten steel on top of the oxide creates a zone of increased heat transfer to the reactor wall;<1> this condition, known as "heat knife", exacerbates probability of formation of a localized weakening of the side of the reactor vessel and subsequent corium leak.

In case of high pressure inside the reactor vessel, breaching of its bottom may result in high-pressure blowout of the corium mass. In the first phase, only the melt itself is ejected; later a depression forms in the center of the hole and gas is discharged together with the melt, resulting in rapid decrease of pressure inside the reactor; the high temperature of the melt also causes rapid erosion and enlargement of the vessel breach. If a hole is in the center of the bottom, nearly all corium can be ejected. A hole in the side of the vessel may lead to only partial ejection of corium, retaining its portion inside the reactor.<6> Melt-through of the reactor vessel may take from few tens of minutes to several hours.

After breaching the reactor vessel, the conditions in the reactor cavity below the core govern the production of gases. If water is present, steam and hydrogen are generated; dry concrete results in production of carbon dioxide and smaller amount of steam.<7>

http://en.wikipedia.org/wiki/Corium_(nuclear_reactor)#Reactor_vessel_breaching

[PamW]

1. Corium is...

What is Coruim. What happened to the Chernobyl Coruim?
==================

Corium is a term for melted fuel. At Chernobyl, the melted fuel
dispersed into the corridors and passageways underneath the reactor
and cooled solid. The leading edge went over ledge and formed a
structure called the "elephant's foot".

http://www.youtube.com/watch?v=z82GkhcqDKw

PBS's Nova science series did a good episode detailing this called
"Return to Chernobyl"

PamW

[Fledermaus]

2. The Cernobyl desgin had sand packed around the core. The sand

melted and mixed with the nuclear lava witches brew of death. It moderated the nuclear reactions and helped cool and solidify the Corium.

We don't know what is happening at this point, but we know there has been core melting.

[FBaggins]

3. You sure about that?

Was it that sand was part of the design, or thousands of tons of sand that was dropped on the core (or what they thought was the core) after the explosion?

[PamW]

4. This is not so...

It moderated the nuclear reactions and helped cool and solidify the Corium.
================

This wrong. When the core melted and flowed outside the reactor proper, then
the fuel was no longer in a critical configuration, and nuclear fission reactions
stopped. From then on, the only heat producing reactions were radioactive decay,
and sand neither moderates or slows down radioactive decay.

The solidification was due to the fact that much of the heat production was due
to short-lived isotopes, and they decayed away, and thus the heat production rate
dropped and natural cooling was sufficient to solidify the molten mass.

PamW

[Fledermaus]

5. Wheres the Corium now?

The moderator is surrounded by a cylindrical water tank , a welded structure with 30 mm thick walls, inner diameter of 16.6 m and outer diameter of 19 m, internally divided to 16 vertical compartments. The water is supplied to the compartments from the bottom and removed from the top; the water can be used for emergency reactor cooling. The tank contains thermocouples for sensing the water temperature and ion chambers for monitoring the reactor power.<4> The tank, sand layer, and concrete of the reactor pit serve as additional biological shields.

http://en.wikipedia.org/wiki/RBMK#Containment

[FBaggins]

6. But why do you believe that's the sand with the corium?

They dropped far larger amounts of the stuff from the air.


And could you by any chance explain what asking "where's the corium???" accomplishes?

Do you think it's hidden somewhere? Does Waldo have it?

[Fledermaus]

7. PBS's Nova science, Where the Corium now?

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[AtheistCrusader]

8. Probably still there, which is why they 'sealed' it in place with the sarcophagus

instead of cleaning it out and removing it all like they did with TMI-2.

[FBaggins]

11. Do you have trouble forming complete thoughts in your sentences?

Or just don't like to answer questions?

Where the Corium now?

It's certainly not on it's way to China. Do you think it gets up and takes a weekend off at the movied every now and then?

IOW... what on earth is your point?

[NNadir]

10. Well, even a broken clock is right two times a day.

It's pretty clear that you don't know what's happening.

It's also pretty clear that you zero about nuclear engineering.

[eppur_se_muova]

9. It was sold to China and used to make cheap jewelry and children's toys.

Ha ha. I hope.

[Dogmudgeon]

12. If you're actually interested ...

It has long since cooled and solidified, and is encased in the structure's "sarcophagus".

It's "cool" enough for periodic inspections without putting the inspection teams at undue risk -- but make no mistake, it's still hot enough to require that those teams use protective garments, radiation count badges, and diligent safety measures.



See also http://en.wikipedia.org/wiki/Chernobyl_disaster#Steam_explosion_risk">Chernobyl Disaster, steam explosion risk

--d!

[joshcryer]

13. I saw a documentary on this, apparently before they had it mapped out...

...you'd have to go one step at a time, and if you went around the wrong corner for even a split second you'd die. One guy did that, simply walked around a corner and pow, got a lethal exposure.