The original shelter, formally referred to as the "Shelter Structure" and often called "the sarcophagus", was constructed between May and November 1986. It was an emergency measure to confine the radioactive materials within reactor 4 at the Chernobyl nuclear power plant (ChNPP). The shelter was constructed under extreme conditions, with very high levels of radiation, and under extreme time constraints. The Shelter Structure was moderately successful in confining radioactive contamination and providing for post-accident monitoring of the destroyed nuclear reactor unit; it has been estimated that up to 95% of the original radioactive inventory of reactor 4 remains inside the ruins of the reactor building.