Fukushima Corium Experiment Creates Unexpected Result

As part of an ongoing series of corium experiments our research team has been doing, the newest experiment created a quite unusual result. The newest experiment, the fifth in a series conducted by Edano, took the top range of assumed melt mass and ran the experiment with a sustained high heat. Upon completion it was noticed that a modest amount of the lead had left the pedestal but what was in the pedestal was much more interesting! A perfectly formed elephant’s foot was found to have taken place in the pedestal. An elephant’s foot is the name given to an unusual corium melt structure found in Chernobyl.

 Elephant’s foot at Chernobyl

The elephant’s foot was also part of a rather bizarre story where liquidators wanted to shoot the formation with a gun to see if they could break it apart to see the interior.

The corium experiments at Brookhaven National Lab found some results that appeared to line up with ours. Brookhaven used varying levels of water as they dropped lead into the drywell. Our experiments with varying lead volumes but without water follow a similar pattern to Brookhaven’s illustrations. Experiments 3 and 4 (ours) had similar melt spreads to the first stages of the Brookhaven melt spreads. It appears that fuel melt volume plays a role in the formation of an elephants foot.

 

Brookhaven experiments corium formations.

 Brookhaven experiment (top), our experiment (below)

If you are not familiar with the previous experiments you can read about those here: The series of experiments have looked at both glass and lead as a stand in material for the melted fuel mass (corium). One key difference between lead and actual corium is the ability of corium to generate heat on an ongoing basis allowing it to continue to be fluid for much longer than lead might. Lead was still considered a sufficient corium stand it that it was used in the early corium experiments at Brookhaven National Lab in the US. Lead volumes of the lead experiments: corium weights (net) : exp#3 = 103g (41.2%) exp#4 = 138g (55.2%) exp#5 = 273g (109.2%) * given that 250g is 100% experiment #2 was not weighed, experiment #1 did not process.  Experiment Details: Corium model material: Lead Corium model weight: 300g of lead was used as the starting material Corium finished weight :was 273g that calculates to 110% of estimated core volume. 250g would be 100% core volume. This is the high range of our experiments. Temperature: 400c held for one hour * some minor shaking was included to simulate the intense and repeated aftershocks reported during the melt down phase of the reactors at Fukushima Daiichi. At this point this factor doesn’t appear to have been a deciding factor in the unusual outcome and might have actually deterred such an outcome. The lead mass was packed into the modified tea strainer mesh as with previous experiments. The tea mesh is suspended inside the kiln with a fitted sheet of garden mesh (fence). It is worth noting that the suspension of the tea strainer removes any contact with the pedestal. This removes the potential for heat transfer from the tea strainer to the ceramic pedestal, something you would likely see in a real world meltdown.  An aluminum foil tube was added to the top of the tea strainer to help contain the lead and to create a condition closer to a reactor vessel. The aluminum foil walls direct the lead towards the bottom of the tea strainer as would be found in a BWR reactor where the bottom head is full of holes from the control rod systems.  After the melt process in the metals kiln, about 25g of lead remained in the tea strainer set up. Previous experiments saw some amount of residue remain in the tea strainer. It is assumed the remaining lead in this experiment may have remained in the strainer due to the lack of any mass behind it to continue to push it through. The images that follow include explanation and other information about the experiment as commentary along with the images. Large format versions of these images can be found here. The video of the details of the lead melt and elephant’s foot can be found here. The video gives a much better idea of the details of the melt than the images can do.  The tea mesh with the foil tube prepared to go into the melting kiln.  Close up of the 300g of lead prepared for melting.

 Weight of the lead with the mesh strainer and foil container.  Pedestal made of common terra cotta ceramic. This is the same pedestal arrangement with doorway as used in previous experiments. In this one the top was cut down to accommodate the larger load of lead. The steel food tin lid is again used as the stand in for the drywell floor. In previous experiments it was noted that the lead was adhering and fusing with the terra cotta ceramic floor so the steel one was used on later experiments.  The experiment prepared in the kiln. The tea mesh assembly is suspended over the ceramic pedestal with a piece of garden fencing to act as a support and to keep it out of contact of the pedestal (prevent possible heat transfer).

Handwritten note of the experiment number, date, lead weight and temperature.

 Time clock & date plus ambient temperature.  After the one hour run at 400c this was the result seen as the door was opened.  View of the melt result from above with the mesh strainer assembly removed.  View inside the pedestal shows the raised mass of the elephant’s foot.

 Another view of the pedestal shows a considerable amount of the melt did exit the pedestal door into the drywell.

 Another view of the elephant’s foot inside the pedestal and the lead that escaped the pedestal doorway.  Overhead view shows how the layers of lead melt creeped around the pedestal in a circular fashion before cooling and stopping.  In this view the different layers and the homogeneous nature of certain parts vs. other parts of the melt. The portions of the elephants foot have a much more aerated and less uniform appearance.  The remaining 25g of lead melt in the mesh strainer with the foil tube removed.  Side view of the tube and strainer after the melt run.  Inside of the foil tube shows no lead adhered to the inside.

Finished weight of the mass that left the reactor vessel model. 273 grams.  Side view of the elephant’s foot with the portion that leaked out the pedestal to the right. Distinct layers can be seen in the melt.  Angled view, wrinkles and fissures can be seen in the intermediate layer.  Top view of the lead melt. Areas of the melt flow can be seen as they curved around the inside of the pedestal.      Underside of the melt, the elephant’s foot even has a clear division among the melt on the underside.  The underside of the strainer mesh shows the remaining lead.  Another overhead view of the melt. The amount that escaped the pedestal can be clearly seen along with the upper layers that stopped short of leaving the pedestal.  Overhead view columns dripping off of the elephants foot can be seen in the upper right.  More detail of the elephant’s foot.  Side view shows the difference between the bottom layer and the top layers in consistency.  Detail of the elephant’s foot top.   We are still reviewing the results of this experiment. Additional conclusions and additional experimental lead melts may be done as we continue to gather information.

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