CHERNOBYL TODAY: Missing Fuel Mystery

Missing Fuel Mystery
Scientists have been able to estimate the amounts of radioactive fuel blown into the atmosphere and into the remains of the Unit No. 4, but they leave about 10 to 50 tons of reactor fuel unaccounted for. Three different scenarios have been raised to attempt to explain this mystery:

some or all of the missing fuel may have been released in the explosion

the missing fuel may have been spewed out in the fire that occurred after the explosion

or perhaps the missing fuel is buried deep under the rubble in the reactor, yet to be discovered

"Finding the missing fuel is an extremely important question for us - the nuclear safety of the [reactor] depends on it," says Edward Pazukhin, head of the ISTC's department of nuclear safety. Pazukhin says this because a growing pool of water in the bowels of the reactor may cause the remaining fuel to go critical.

Most of the information about the damaged core was gathered by "stalkers" who actually probe the insides of the sarcophagus for clues. Former ISTC director, Vladimir Tokarevsky said, "their work was very dangerous, and practically illegal because of the fantastically high doses they received." Soon after the accident, these stalkers had determined that the reactor fuel existed in three states:

ejected fragments from the core (pieces of fuel rods, control rods...)
uranium- and plutonium-laced dust ("hot particles")
solidified lava (a mixture of uranium oxide, graphite from the core, and melted concrete and zirconium from the reactor building)

"From analysis of the lava's composition, Pazukhin estimated that the maximum temperature of the explosion was about 2255 degrees Celsius and that the lavas must have been held at a temperature greater than 1660 degrees for at least 4 days. He believes that when the reactor exploded, chunks of concrete from the walls collapsed into the reactor chamber. The fuel rods heated to the point that their zirconium cladding melted, exposing the uranium oxide fuel. About 4 hours later, melting concrete, zirconium, and uranium oxide began to mix with about 145 tons of the mineral serpentine which was packed around the reactor as heat insulation. Within half an hour the serpentine heated to more than 500 degrees Celsius and began to decompose, releasing 19 tons of superheated water as vapor. The melted materials cooked for another 60 hours, forming a lava that flowed into several damaged rooms beneath the reactor chamber."

It is thought that much of the missing fuel will be found in one of those rooms under the reactor. Scientists have drilled holes into the west wall of room number 307/2 through which probes measured heavy gamma radiation and a high neutron flux. They believe some of the missing fuel may be in that room.