Washington ? President Barack Obama optimistically opened a 47-nation nuclear summit Monday, boosted by Ukraine’s announcement that it will give up its weapons-grade uranium. More sobering: The White House counterterror chief warned that al-Qaida is vigorously pursuing ingredients and expertise for a bomb.

At the same time, Obama and Chinese President Hu Jintao sought agreement on potential sanctions to discourage Iran’s efforts to come up with its own nuclear weapon.

Ukraine’s decision dovetailed with Obama’s goal of securing all vulnerable nuclear materials worldwide within four years — an objective that the White House hopes will be endorsed by all summit countries at a closing session today, even if the means to accomplish it are unclear.