Chernobyl Nuclear Disaster - One Minute History

Soviet engineers were tasked with a safety test at Chernobyl's reactor 4, but untrained staff and protocol failures led to a catastrophic explosion, spreading radioactive contamination. Firefighters worked to extinguish the fire, exposing many to lethal radiation. By December 1986, reactor 4 was encased in a sarcophagus to limit contamination. In 2017, a new structure was added to aid cleanup, expected to finish by 2065.

What measures were taken by December 1986 to contain the radioactive contamination?

What is the purpose of the additional structure built over the sarcophagus in 2017?