The 1200 block of New York Ave. NW is closed at this hour, following what appears to have been a fatal collision between a cyclist and a security vehicle associated with the Nuclear Security Summit. The cyclist, who is believed to be female, was struck at the intersection of 12th Street and New York Ave. NW, D.C. Fire/EMS spokesperson Pete Piringer said. Piringer could not confirm that the woman had died, but he said it…