Leading nuclear scientists with top security clearances will gather next summer at a screening room east of San Francisco and witness the results of the greatest effort ever in supercomputing. Using <A HREF="http://story.news.yahoo.com/news?tmpl=story&cid=569&ncid=569&e=3&u=/nm/20041226/tc_nm/tech_supercomputer_dc">a computer doing 360 trillion calculations a second,</A> scientists at Lawrence Livermore National Lab will simulate the explosion of an aging nuclear bomb in three dimensions.