In the aftermath of the disaster at Chernobyl, a team of soviet scientists risked their lives by going back into the reactor to investigate the scale of the damage.

The Sarcophagus (darker area)

The investigation, dubbed Complex Expedition, was driven by the fear that there could be a second accident at Chernobyl if the missing nuclear fuel - plutonium, uranium and other extremely radioactive elements - was not located and contained.

Radiation emissions were still dangerously high and the scientists were exposed to levels of radiation that would be considered almost suicidal in the West. At the time, they had been worried that the uncontrollable reactor might explode again, and had to assume that a self-sustaining chain reaction was possible.

Clouds of radioactive dust escaped

To prevent this, the Red Army initially bombed the reactor with neutron absorbers and other chemicals. The intense radiation killed several pilots. It is now known that, despite those sacrifices, almost no neutron absorbers reached the core.

Inexplicably on May 6th, the emissions stopped. Something had happened in the core of the reactor.

Engineers began work on the construction of a concrete sarcophagus to surround the failed reactor to stop rain getting in and triggering a second explosion. Forced to build straight on to the damaged, red hot hull of Chernobyl Unit Four, the Russian energy authorities faced the biggest civil engineering task in history. A quarter of a million construction workers on the Chernobyl sarcophagus reached official lifetime limits of radiation.

Scientists recovered radioactive material by hand

When the problem seemed to be literally buried, scientists working inside the reactor continued their hunt for nuclear fuel. Collected manually, the cold rods of fuel that turned up could not account for the great heat still emanating from the core.

The scientists monitored rates of radiation in the building by drilling into the heart of the sarcophagus and inserting long metal detector tubes into the reactor to measure rates of radioactive decay. As well as radiation, they were exposed to high levels of radioactive dust.

Meanwhile, volunteers from the military were working on the exterior of the building, pushing nuclear fuel rods that had been spat out in the blast, back into the reactor ruins.

The hunt for nuclear fuel went on for six months before there was any result.

Lava, glass and crystal flow discovered in the base of the reactor

In December 1986, an intensely radioactive mass was discovered in the basement of Unit Four and scientists rigged up a crude wheeled camera to investigate. The mass was more than two metres across and weighed hundreds of tons. Because of its odd wrinkled shape, it was christened, "the elephant's foot". To approach it meant certain death.

Analysis of the material showed that it was composed of sand, glass and nuclear fuel, and the proportion of sand suggested to scientists that a large amount of fuel had escaped from the reactor in this form. Underneath the reactor, the investigation team found steaming hot concrete and, draining into the basement, lava and spectacular unknown crystalline forms - Chernobylite.

The findings meant that the risk of a second explosion had receded, but that serious problems remained.

The explosion dislodged the 2000 ton lid of the reactor

The walls of the sarcophagus are starting to fall down, having been built straight on top of the collapsed and unstable reactor walls. If the walls collapse, the resulting radioactive dust will escape. The explosion also threw the 2000 ton reactor lid into the air. It fell on its edge into the mouth of the core, and rests at a precarious angle half way down.

Scientists who worked on the Complex Exploration say that there are likely to be major collapses in the next ten years without further construction work.

Russian research into Chernobyl has now been cut back because of safety problems, and the high death rate - caused by radiation and the stress related conditions - of the scientists involved

An alliance of European companies has draw up plans to cover the reactor with a concrete structure designed to last longer than the pyramids, and big enough to allow the work of locating and packaging up radioactive material to continue.

So far, the money needed to recover the site has not been found but the contents of the Chernobyl tomb will remain radioactive for at least the next 100,000 years.
 

Back to top | BBC News Home | BBC Homepage	©