Detailed Instructions: In this task you will be given an answer to a question. You need to generate a question. The answer given should be a correct answer for the generated question.
Q: The disaster began during a systems test on 26 April 1986 at reactor 4 of the Chernobyl plant near Pripyat and in proximity to the administrative border with Belarus and the Dnieper River. There was a sudden and unexpected power surge. When an emergency shutdown was attempted, a much larger spike in power output occurred. The second spike led to a reactor vessel rupture and a series of steam explosions. These events exposed the graphite moderator of the reactor to air, causing it to ignite. The resulting fire sent week long plumes of highly radioactive fallout into the atmosphere over an extensive geographical area, including Pripyat. The plumes drifted over large parts of the western Soviet Union and Europe. According to official post-Soviet data, about 60% of the fallout landed in Belarus.
A:
what caused the nuclear disaster in chernobyl ukraine