A Soviet-era top secret object Duga, an over-the-horizon radar system, once used as part of the Soviet missile defense early-warning radar network, in Chernobyl, Ukraine, Thursday, Nov. 22, 2018. The Duga was closed in 1989 after the 1986 fatal explosion in the 4th unit of the Chernobyl nuclear power plant. (AP Photo/Efrem Lukatsky)

A Soviet-era top secret object Duga, an over-the-horizon radar system once used as part of the Soviet missile defense early-warning radar network, in Chernobyl, Ukraine, Thursday, Nov. 22, 2018. The Duga was closed in 1989 after the 1986 fatal explosion in the 4th unit of the Chernobyl nuclear power plant. (AP Photo/Efrem Lukatsky)

Visitors walk in front of the construction of the 'Duga' Soviet over-the-horizon (OTH) radar system near Chernobyl on November 22, 2018. - The radar, which was a part of the former Soviet Union missile defence radar system and is now located in the Chernobyl Exclusion Zone, was closed after the Chernobyl catastrophe 1986. (Photo by Sergei SUPINSKY / AFP)

Visitors walk in front of the construction of the 'Duga' Soviet over-the-horizon (OTH) radar system near Chernobyl on November 22, 2018. - The radar, which was a part of the former Soviet Union missile defence radar system and is now located in the Chernobyl Exclusion Zone, was closed after the Chernobyl catastrophe in 1986. (Photo by Sergei SUPINSKY / AFP)

A visitor poses next to a sign with the radiation warning symbol in front of the construction of the 'Duga' Soviet over-the-horizon (OTH) radar system near Chernobyl on November 22, 2018. - The radar, which was a part of the former Soviet Union missile defence radar system and is now located in the Chernobyl Exclusion Zone, was closed after the Chernobyl catastrophe in 1986. (Photo by Sergei SUPINSKY / AFP)

A sign with the radiation warning symbol is seen in front of the construction of the 'Duga' Soviet over-the-horizon (OTH) radar system near Chernobyl on November 22, 2018. - The radar, which was a part of the former Soviet Union missile defence radar system and is now located in the Chernobyl Exclusion Zone, was closed after the Chernobyl catastrophe in 1986. (Photo by Sergei SUPINSKY / AFP)

Visitors walk in front of the construction of the 'Duga' Soviet over-the-horizon (OTH) radar system near Chernobyl on November 22, 2018. - The radar, which was a part of the former Soviet Union missile defence radar system and is now located in the Chernobyl Exclusion Zone, was closed after the Chernobyl catastrophe in 1986. (Photo by Sergei SUPINSKY / AFP)

Visitors feed a stray dog in front of the construction of the 'Duga' Soviet over-the-horizon (OTH) radar system near Chernobyl on November 22, 2018. - The radar, which was a part of the former Soviet Union missile defence radar system and is now located in the Chernobyl Exclusion Zone, was closed after the Chernobyl catastrophe in 1986. (Photo by Sergei SUPINSKY / AFP)

A visitor walks under antennas of the 'Duga' Soviet over-the-horizon (OTH) radar system near Chernobyl on November 22, 2018. - The radar, which was a part of the former Soviet Union missile defence radar system and is now located in the Chernobyl Exclusion Zone, was closed after the Chernobyl catastrophe in 1986. (Photo by Sergei SUPINSKY / AFP)

Antennas of the 'Duga' Soviet over-the-horizon (OTH) radar system are pictured near Chernobyl on November 22, 2018. - The radar, which was a part of the former Soviet Union missile defence radar system and is now located in the Chernobyl Exclusion Zone, was closed after the Chernobyl catastrophe in 1986. (Photo by Sergei SUPINSKY / AFP)

Antennas of the 'Duga' Soviet over-the-horizon (OTH) radar system are pictured near Chernobyl on November 22, 2018. - The radar, which was a part of the former Soviet Union missile defence radar system and is now located in the Chernobyl Exclusion Zone, was closed after the Chernobyl catastrophe in 1986. (Photo by Sergei SUPINSKY / AFP)