More about the "controlled" meltdown...

Workers will place a fuel rod about a foot long inside of a stainless steel enclosure, which will prevent cooling water from coming in contact with the rod.
Additional fuel rods will then surround the capsule, releasing neutrons which will cause the test rod to melt once temperatures rise high enough.

Workers will record temperature, pressure, and water level information from the test. They will also place a camera in the capsule to record the melting process.

Once the fuel rod melts down the operators will allow the fission process to cease, and allow the molten materials to return to a solid state. Once solidified the fuel can be analyzed.

After the experiment the solidified fuel will be stored in a spent fuel pool.
[link to enformable.com]
.