Klivazh is the site of an underground peaceful nuclear explosion in the Soviet program of Nuclear Explosions for the National Economy. The event took place in the Donetsk oblast of the Ukrainian SSR on September 16, 1979. The site flooded with underground waters in 2018 after Russian military occupation during Russo-Ukrainian War, with a risk of radiological pollution of drinking water in the area.^[2]

The 0.2–0.3 kt (kiloton of TNT) explosion was performed in Ukrainian SSR in an eastern annex of «Yunkom» coal mine, near the town of Bunhe, city of Yenakiieve, at a depth of 903 m (2,963 ft) in between the coal layers «Deviatka» (l4) and «Tsehelny» (l21) on September 16, 1979, at 9:00 AM (GMT).^[1][3] The aim of the explosion was to lower interlayer tension and underground gas pressure, allowing more secure coal mining.^[4][5] Before 1979, on the «Yunkom» coal mine, there were frequent rapid coal and gas outbursts and sudden squeezing-out of coal by accompanying gas evolution, as the excavating layers were situated in the tectonic tension areas. The event details were classified in the time of the USSR.

Following the nuclear explosion a cavity formed with a radius of 5–6 m, around it there formed a crush zone with a 20–25 m radius. The radioactivity level in the mine excavation areas and sub-surface waters remained at the natural level when the measurements were regularly conducted during 1979–2000. After the PNE the coal and gas outbursts became less frequent.^[4] In 2002, the «Yunkom» mine was closed.^[6]

The danger of flooding the «Yunkom» mine was alarming for Ukrainian ecologists at the very announcement of the mine conservation as it may potentially cause radioactive pollution of the underground waters. The Ministry of Ecological Security held measures to ensure the hydrological and geological security of the site.^[7]

After the starting of Russo-Ukrainian War in 2014 the city of Yenakieve was occupied by pro-Russian forces of so-called Donetsk People's Republic.^[8]

In 2018 the DPR administration decided to flood the «Yunkom» mine. The drainage required maintenance of water pumps, which stopped along with the region's general infrastructure degradation after the Russian military occupation.^[4][5] The information was confirmed by OSCE Monitoring Mission.^[9] In April 2018 the DPR removed water pumps from the «Yunkom» mine, so it is gradually flooded by the natural waters. The radiological contamination was viewed as potentially spreading to the Mius River and then to the Azov Sea, threatening drinking and irrigation water supplies.^[10]

• Kramatorsk radiological accident
• Andreev Bay nuclear accident