blocks of graphite and other debris. The reactor itself was covered with bags of sand, lead and boric acid dropped from helicopters: some 5000 metric tons of material were dropped during the week that followed the accident. At the time there was still fear that the reactor could re-enter a self-sustaining nuclear chain-reaction and explode again, and a new containment structure was planned to prevent rain entering and triggering such an explosion, and to prevent further release of radioactive material. This was the largest civil engineering task in history, involving a quarter of a million construction workers who all reached their official lifetime limits of radiation. [ 52 ] By December 1986, a large concrete sarcophagus had been erected to seal off the reactor and its contents. [ 77 ] A unique "clean up" medal was given to the workers. [ 78 ] Many of the vehicles used by the "liquidators" remain parked in a field in the Chernobyl area. [ 79 ] During the construction of the sarcophagus, a scientific team re-entered the reactor as part of an investigation dubbed "Complex Expedition", to locate and contain nuclear fuel in a way that could not lead to another explosion. These scientists manually collected cold fuel rods, but great heat was still emanating from the core. Rates of radiation in different parts of the building were monitored by drilling holes into the reactor and inserting long metal detector tubes. The scientists were exposed to high levels of radiation and radioactive dust.