25th anniversary of Chernobyl

The widows of Chernobyl victims hold portraits of their husbands who died following the clean-up operations for the 1986 Chernobyl nuclear explosion, at Chernobyl's victim monument in Ukraine's capital Kiev, Tuesday, April 26, 2011. On Tuesday, Ukraine marks the 25th anniversary of the Chernobyl nuclear disaster, which spread radiation over much of northern Europe. (AP Photo/Efrem Lukatsky)

Woman lights candles to honor the memory of the victims of the Chernobyl disaster in Kiev, Ukraine, early Tuesday, April 26, 2011. Ukraine marked the 25th anniversary since the Chernobyl power station exploded in the world's worst nuclear accident, endangering hundreds of thousands of lives and contaminating pristine forests and farmland. (AP Photo/Sergei Chuzavkov)

The widows of Chernobyl victims hold portraits of their husbands who died following the clean-up operations for the 1986 Chernobyl nuclear explosion, at Chernobyl's victim monument in Ukraine's capital Kiev, Tuesday, April 26, 2011. On Tuesday, Ukraine marks the 25th anniversary of the Chernobyl nuclear disaster, which spread radiation over much of northern Europe. (AP Photo/Efrem Lukatsky)

People lay flowers and light candles to honor the memory of the victims of the Chernobyl disaster in Kiev, Ukraine, Tuesday, April 26, 2011. Ukraine marked the 25th anniversary since the Chernobyl power station exploded in the world's worst nuclear accident, endangering hundreds of thousands of lives and contaminating pristine forests and farmland. (AP Photo/Sergei Chuzavkov)

People attend a memorial service for Chernobyl victims in Kiev, Ukraine, early Tuesday, April 26, 2011. Ukraine marked the 25th anniversary since the Chernobyl power station exploded in the world's worst nuclear accident, endangering hundreds of thousands of lives and contaminating pristine forests and farmland. (AP Photo/Sergei Chuzavkov)

A monument to victims of the Chernobyl explosion, surrounded by photos of the victims, in Ukraine's capital Kiev, Tuesday, April 26, 2011. On Tuesday, Ukraine marks the 25th anniversary of the Chernobyl nuclear disaster, which spread radiation over much of northern Europe. (AP Photo/Efrem Lukatsky)

Ukrainians hold candles near the monument to people who were evacuated from the town of Pripyat, which was badly affected by radiation in Chernobyl nuclear disaster, in Kiev, Ukraine, Tuesday, April 26, 2011. Ukraine marked the 25th anniversary since the Chernobyl power station exploded in the world's worst nuclear accident, endangering hundreds of thousands of lives and contaminating pristine forests and farmland. (AP Photo/Sergei Chuzavkov)

Guards of Honor march to lay a flower wreath at Chernobyl's victim monument in Ukraine's capital Kiev, Tuesday, April 26, 2011. On Tuesday, Ukraine marks the 25th anniversary of the Chernobyl nuclear disaster, which spread radiation over much of northern Europe. (AP Photo/Efrem Lukatsky)

Two girls light candles near the monument to Ukrainians who were evacuated from the town of Pripyat, which was affected by radiation in Chernobyl nuclear disaster in Kiev, Ukraine, Tuesday, April 26, 2011. Ukraine marked the 25th anniversary since the Chernobyl power station exploded in the world's worst nuclear accident, endangering hundreds of thousands of lives and contaminating pristine forests and farmland. (AP Photo/Sergei Chuzavkov)

From left center, Russian President Dmitry Medvedev, Ukrainian President Viktor Yanukovych and Russian Orthodox Patriarch Kirill, partly visible, lay flowers at the monument to Chernobyl victims during a memorial ceremony outside Chernobyl's ravaged 4th nuclear reactor that exploded on April 26, 1986, at Chernobyl, Ukraine, on Tuesday, April 26, 2011. Medvedev called on the international community to work out unified guidelines in order to continue developing nuclear energy but prevent accidents such as the Chernobyl disaster 25 years ago. Second left front is Ukrainian and Kiev Metropolitan Vladimir. (AP Photo/RIA Novosti, Vladimir Rodionov, Presidential Press service)

An unidentified relative, right, consoles Natalia Pravik, the mother of Vladimir Pravik, a firefighter who worked at reactor No. 4 of the Chernobyl nuclear plant when it exploded on April 26, 1986, mourns over his grave at Mitino cemetery in Moscow, Russia, Tuesday, April 26, 2011. Hundreds of people came to Mitino cemetery, where those who died in the Chernobyl catastrophe are buried, to pay tribute to loved ones who perished in the world's worst nuclear accident 25 years ago. (AP Photo/Mikhail Metzel)

A woman wearing a "No Nukes" mask and badges attends a candle light vigil marking the 25th anniversary of Chernobyl nuclear disaster in Ukraine during an anti-nuclear protest in Tokyo Tuesday, April 26, 2011 in front of the headquarters of Tokyo Electric Power Co., the operator of the tsunami-stricken Fukushima Dai-ichi nuclear power plant in Fukushima Prefecture, northeastern Japan. (AP Photo/Koji Sasahara)

Ukrainian President Viktor Yanukovych, right, and Russian President Dmitry Medvedev unveil a corner stone of a monument for Chernobyl disaster victims in Chernobyl on Tuesday, April 26, 2011. (AP Photo/RIA Novosti, Vladimir Rodionov, Presidential Press service)

From left, Russian President Dmitry Medvedev, Ukrainian President Viktor Yanukovych and Russian Orthodox Patriarch Kirill lay flowers at the monument to Chernobyl victims during a memorial ceremony outside Chernobyl's ravaged 4th nuclear reactor that exploded on April 26, 1986, at Chernobyl, Ukraine, on Tuesday, April 26, 2011. Medvedev called on the international community to work out unified guidelines in order to continue developing nuclear energy but prevent accidents such as the Chernobyl disaster 25 years ago. (AP Photo/RIA Novosti, Vladimir Rodionov, Presidential Press service)

From left, Russian President Dmitry Medvedev, Ukrainian President Viktor Yanukovych and Russian Orthodox Patriarch Kirill are seen during a memorial ceremony at the monument to Chernobyl victims outside Chernobyl's ravaged 4th nuclear reactor that exploded on April 26, 1986, at Chernobyl, Ukraine, on Tuesday, April 26, 2011. Medvedev called on the international community to work out unified guidelines in order to continue developing nuclear energy but prevent accidents such as the Chernobyl disaster 25 years ago. (AP Photo/RIA Novosti, Vladimir Rodionov, Presidential Press service)

Anti-nuclear protesters light candles during a candle light vigil in Tokyo Tuesday, April 26, 2011, marking the 25th anniversary of Chernobyl nuclear disaster in Ukraine in front of the headquarters of Tokyo Electric Power Co., the operator of the tsunami-stricken Fukushima Dai-ichi nuclear power plant. (AP Photo/Koji Sasahara)

Vera Toptunova, the mother of Leonid Toptunov, who was a senior reactor control engineer at the Reactor No. 4 of the Chernobyl nuclear plant when it exploded on April 26, 1986, mourns over his grave at Mitino cemetery in Moscow, Russia, Tuesday, April 26, 2011. Hundreds of people came to Mitino cemetery, where those who died in the Chernobyl catastrophe are buried, to pay tribute to loved ones who perished in the world's worst nuclear accident 25 years ago. (AP Photo/Mikhail Metzel)

An unidentified relative, left, consoles Vera Toptunova, the mother of Leonid Toptunov, who was a senior reactor control engineer at the Reactor No.4 of the Chernobyl nuclear plant when it exploded on April 26, 1986, as she mourns over his grave at Mitino cemetery in Moscow, Russia, Tuesday, April 26, 2011. Hundreds of people came to Mitino cemetery, where those who died in the Chernobyl catastrophe are buried, to pay tribute to loved ones who perished in the world's worst nuclear accident 25 years ago. ( AP Photo/Mikhail Metzel)

From left: Russian Orthodox Church Patriarch Kirill, Russian President Dmitry Medvedev, Ukrainian Orthodox Metropolitan Vladimir and Ukrainian President Viktor Yanukovych ring bels after a service for Chernobyl disaster victims at St. Elias Cathedral near Chernobyl nuclear power plant on Tuesday, April 26, 2011. (AP Photo/RIA Novosti, Vladimir Rodionov, Presidential Press service)

Tatyana Ignatenko, the mother of Vasily Ignatenko, a firefighter who worked at reactor No. 4 of the Chernobyl nuclear plant when it exploded on April 26, 1986, mourns over his grave at Mitino cemetery in Moscow, Russia, Tuesday, April 26, 2011. Hundreds of people came to Mitino cemetery, where those who died in the Chernobyl catastrophe are buried, to pay tribute to loved ones who perished in the world's worst nuclear accident 25 years ago. (AP Photo/Mikhail Metzel)

Russian Emergency Situations Ministry soldiers salute as Luybov Vashuk, sister of Nikolay Vashuk, a firefighter who worked at reactor No. 4 of the Chernobyl nuclear plant when it exploded on April 26, 1986, places flowers on his grave at Mitino cemetery in Moscow, Russia, Tuesday, April 26, 2011. Hundreds of people came to Mitino cemetery, where those who died in the Chernobyl catastrophe are buried, to pay tribute to loved ones who perished in the world's worst nuclear accident 25 years ago. (AP Photo/Mikhail Metzel)