Nuclear Reactors 140 - Experiments on Nuclear Core Meltdowns
Corium is a name that was coined for the molten material that results when the fuel of a nuclear reactor melts down in a nuclear accident. It contains nuclear fuel, fission products, control rods, structural material, and chemical reaction products with air, water and steam. If the reactor core is breached, then there will also be molten concrete in the mix. There is currently a debate about the fate of the corium created during the nuclear disaster at Fukushima in March of 2011.
In order to better understand exactly what happens when a nuclear reactor core melts down, researchers have carried out experiments with substitutue materials like lead and glass. Tests at U.S. Brookhaven National Laboratories used varying levels of water as they dropped molten lead into a hole in the ground referred to as a "drywell." Unlike lead, corium continues to generate heat on its own and so it remains liquid for a longer time. However, the behavior of molten lead can be useful in the study of corium.
Approximately ten ounces of lead were used in the tests, heated to seven hundred and fifty degrees Fahrenheit. The lead was placed in a modified tea strainer with used which had a more open mesh. The strainer was put on a ceramic pedestal. An aluminum tube was placed on top of the tea strainer to confine the lead like a reactor vessel. The holes in the tea strainer mimics the control rod holes in the bottom of a boiling water reactor.
Experiments in Japan with molten lead had similar results although the researchers did not use any water. The molten lead flowed through the mesh in the tea strainer and pooled in the terra cotta flower pot being used for a pedestal. When some of the molten lead flowed out through a hole in the flower pot, it created a lump of cooled lead that had the shape of the foot of an elephant. This shape was also seen in corium that oozed out of the containment vessel at Chernobyl.
At Fukushima, three of the reactors experienced a meltdown on March 11, 2011. The corium from the three reactors melted through the bottom of the reactrors during the first day of the disaster and is still sinking into the gound beneath the reactors. They think that the corium may be as much as thirty feet below the bottom of the reactors now. It is thought that there are three one hundred tons blobs of corium beneath Fukushima at around three thousands degrees Fahrenheit. Given the level of radioactivity and the ability of current technology, it will take thousands of years before the corium is cool enough for researders to find its exact location.
A great deal of cooling water has been pumped into the wreckage of the reactors to prevent more explosions. This water becomes contaminated with radioactive materials and has been being captured and stored. Recently, the volume of water has exceeded the ablity of the operators of Fukushima to decontominate and they have started pumping the contaminated water directly into the Pacific Ocean.
Three Mile Island Reactor after Meltdown:
1.
1. Inlet 2B
2. Inlet 1A
3. Cavity
4. Loose core debris
5. Crust
6. Previously molten material
7. Lower plenum debris
8. Possible region depleted in uranium
9. Ablated incore instrument guide
10. Hole in baffle plate
11. Coating of previously-molten material on bypass region interior surfaces
12. Upper grid dam