Mel Gibson's ex-girlfriend, Russian-born singer, songwriter Oksana Grigorieva, gestures during a news conference in Moscow, Monday, April 19, 2010.  Grigorieva said Monday she will perform a private concert in Moscow in aid of the victims of the 1986 Chernobyl nuclear disaster.  Grigorieva and U.S. actor Mel Gibson are known to have separated as a couple and have a baby daughter together.(AP Photo/Mikhail Metzel)AP – The Russian-born singer who split from actor Mel Gibson earlier this month said Monday she will perform a charity concert in Moscow to aid victims of the 1986 Chernobyl nuclear disaster.