'Bunker Busters': Robust Nuclear Earth Penetrator Issues, FY2005 and FY2006 [Updated June 8, 2005] [open pdf - 95KB]
"Earth penetrator weapons, often called 'bunker busters,' burrow into the ground some tens of feet before detonating, greatly increasing their ability to destroy buried targets. The United States has several types of conventional earth penetrators. The current U.S. nuclear earth penetrator, the B61-11 bomb, cannot penetrate certain types of terrain in which hardened underground facilities may be located, so the Air Force and the National Nuclear Security Administration (NNSA) are studying a more effective penetrator, the Robust Nuclear Earth Penetrator (RNEP). While Secretary of Defense Rumsfeld said in 2003 that RNEP was a study, NNSA's FY2005 budget document showed a five-year total of $484.7 million if RNEP were to proceed beyond the study phase. NNSA stated that no decision had been made to proceed with RNEP and that out-year figures were shown to meet congressionally-mandated budgeting requirements and were not a request. RNEP requests are, of course, subject to congressional approval, rejection, or modification."
CRS Report for Congress, RL32347