LUXEMBOURG -- Ukraine's president signed a sweeping trade and cooperation treaty with the European Union on Tuesday, but EU nations warned it may not come into force unless the former Soviet republic agrees to give up its nuclear weapons. The EU also made clear to President Leonid Kravchuk that Ukraine should shut down for ever its nuclear center at Chernobyl. The giant power plant there was the scene in 1986 of the world's worst nuclear disaster. The "partnership" accord offers Kiev most favored nation status, giving trade treatment enjoyed by most other countries, and lifts various quotas on Ukrainian goods.The 12 EU nations insist ratification of the accord by their parliaments depends on Kiev signing the international treaty controlling the spread of nuclear weapons, which would require it to give up its warheads. "The question of Ukraine's participation in the Nonproliferation Treaty is a condition of the ratification," of the agreement with the EU, the Union's standing president, the Greek European affairs minister, Theodoros Pangalos, told reporters. Such agreements are the keystone of the EU's policy towards the former Eastern bloc. They aim to open up markets to each other's goods and services and set up regular high-level political consultations. However, unlike broadly similar accords the Union has concluded with Poland, Hungary, Slovakia, the Czech Republic, Romania and Bulgaria, agreements with Ukraine and Russia do not hold out the prospect of eventual EU membership. Ukraine depends heavily on nuclear energy to power its impoverished economy. Kiev says it will need up to $8 billion to shut down Chernoby.  (AP, Reuters)