FILE – Tatyana Ignatenko, the mother of firefighter Vasily Ignatenko, grieves near his grave at Mitinskoye Cemetery in Moscow, Russia, on Wednesday, April 26, 2006, the 20th anniversary of the explosion and fire at the Chernobyl nuclear power plant in Ukraine. Vasily Ignatenko's crew was the first to respond to the disaster. (AP Photo/Alexander Zemlianichenko, File)

FILE - Lyudmilla Ignatenko, right, the widow of firefighter Vasily Ignatenko, mourns at his grave at Mitinskoye Cemetery in Moscow, Russia, on Thursday, April 26, 2001, the 15th anniversary of the explosion and fire at the Chernobyl nuclear power plant in Ukraine. With her at the cemetery is her son, who was not identified. (AP Photo/Alexander Zemlianichenko, File)

FILE - Relatives of firefighter Vladimir Pravik mourn at his grave at the Mitinskoye Cemetery in Moscow, Russia, on April 26, 2005. Pravik was among the first responders to the explosion and fire at the Chernobyl nuclear plant on April 26, 1986. (AP Photo/ Misha Japaridze, File)

FILE - Firefighters visit Mitinskoye Cemetery in Moscow, Russia, on Thursday, April 26, 2012, to honor those who died responding to the explosion and fire at the Chernobyl nuclear power plant on April 26, 1986. (AP Photo/Ivan Sekretarev, File)

FILE - The Chernobyl nuclear plant is seen in an aerial view, showing the damage from an explosion and fire on April 26, 1986, that sent a radioactive plume over Europe. (AP Photo/ Volodymyr Repik, File)