Potential adversaries and others have built hardened underground facilities to protect keyassets. Conventional weapons are unable to destroy many such facilities, though opinions differ onwhether nonnuclear means can disable or isolate them. Surface-burst nuclear weapons would havelimited effectiveness against deeply buried facilities. In contrast, a nuclear weapon that burrows intothe ground a few meters before detonating would be more effective because it would transfer muchmore of its energy into the ground. The United States has one type of nuclear earth penetrator, the B61-11 bomb, which wasaccepted into the stockpile in September 2001. (1) That weapon, though, according to an article by several scientistsfrom Los Alamos National Laboratory, "cannot survive delivery into certain types of terrain in whichsuch [hardened underground] facilities may be located." (2) The Administration proposes RNEP as a study of modificationsto convert existing B83 nuclear bombs to an earth penetrator configuration. Another bomb, the B61,had also been under consideration earlier. The Air Force has been leading the study, which startedin May 2003, with NNSA in charge of studying modifications of specific warheads. Accordingly,the Department of Defense (DOD) and the National Nuclear Security Administration (NNSA), whichis the Department of Energy (DOE) agency in charge of the U.S. nuclear weapons program,requested funds to study the Robust Nuclear Earth Penetrator (RNEP). RNEP has been the most controversial nuclear weapon program in Congress for the past fewyears. Supporters argue that it is needed to attack hard and deeply buried targets (such as leadershipbunkers or WMD production facilities) in countries of concern, thereby deterring or defeatingchallenges from such nations; critics assert that RNEP would lower the threshold for use of nuclearweapons and prompt other nations to develop nuclear weapons to deter U.S. attack. Secretary ofDefense Donald Rumsfeld said in May 2003 that RNEP "is a study. It is nothing more and nothingless." (3) The plan was thatthe RNEP study would cost $15 million a year for FY2003-FY2005. While Congress appropriatedthe FY2003 request of $15.0 million, the FY2004 request met much criticism. The House rejectedan amendment by Representative Tauscher to transfer funds from RNEP to conventional means ofattacking buried targets. The Senate tabled an amendment by Senator Dorgan and another by SenatorFeinstein to bar funds for RNEP, and adopted an amendment by Senator Nelson (FL), and a similaramendment by Senator Reed, to require congressional authorization to start development engineering(discussed below) or later phases of RNEP. (The Nelson amendment became Section 3117 of P.L.108-136 , the FY2004 National Defense Authorization Act.) Congress reduced the FY2004appropriation to $7.5 million. In response to this reduction, NNSA planned to spend almost all of the $7.5 million to studythe B83 as an RNEP candidate, and little on the B61 study. (4) The B83 study was being conducted by Lawrence LivermoreNational Laboratory, in Livermore, CA, and Sandia National Laboratories' Livermore branch; theB61 study was the responsibility of Los Alamos National Laboratory, Los Alamos, NM, and SandiaNational Laboratories' headquarters site in Albuquerque, NM. For FY2005, Congress eliminatedRNEP funding. For FY2006, the Administration requested $4.0 million for NNSA to conduct a sledtest, in which a penetrator body would be shot down a test track and slammed into a material, suchas a concrete block, to help design and evaluate penetrators. For FY2006, the Administration alsorequested $4.5 million for DOD to study linking RNEP to the B-2 bomber. The appropriation was$4.0 million for DOD to conduct the sled test. To address some technical issues that have arisen in the RNEP debates, the Bob StumpNational Defense Authorization Act for FY2003, P.L. 107-314 , Section 1033, called for a NationalAcademy of Sciences report on effects of nuclear and conventional earth penetrator weapons(EPWs). The report, released in April 2005, had nine key conclusions: (1) many high-value buriedfacilities can be held at risk by nuclear but not conventional EPWs; (2) penetration to a depth of 3meters captures most effects of EPWs on buried targets; deeper penetration puts the weapon atgreater risk; (3) EPWs cannot penetrate deeply enough to contain nuclear weapon effects fully; (4)casualties from a nuclear weapon burst at shallow depth or on the surface are essentially the same;(5) detonating a nuclear weapon at shallow depth increases the energy transmitted to a buried target,permitting a reduction in yield by a factor of 15 to 25; (6) attacks using nuclear EPWs near urbanareas could produce thousands to over a million casualties, or hundreds to several hundred thousandfor attacks in rural areas; (7) a nuclear EPW could reduce civilian casualties in an urban area by afactor of 2 to 10 compared to a surface-burst weapon with 25 times the yield; (8) a nuclear weaponwould have to detonate within a chamber where chemical or biological agents were stored to destroythe agents; the same is true of nonnuclear "thermobaric" bombs, which generate high heat andpressure; and (9) in a nuclear attack on a chemical weapon facility, nuclear effects would probablykill many more civilians than would the released chemical agent, while a nuclear attack on abiological facility could kill similar numbers of civilians from nuclear effects and released biologicalagents, depending on weapon yield and amount of agent. (5) This report discusses the history of RNEP in Congress and the Administration for theFY2005 and FY2006 budget cycles. For a more extensive history and technical background, see CRS Report RL32130 , Nuclear Weapon Initiatives: Low-Yield R&D, Advanced Concepts, EarthPenetrators, Test Readiness , and CRS Report RL32599(pdf) , 'Bunker Busters': Sources of Confusion inthe Robust Nuclear Earth Penetrator Debate , both by Jonathan Medalia. Any consideration ofRNEP in the second session of the 109th Congress may hinge on whether the Administration requestsfunds for the program in the FY2007 budget. Congress required NNSA -- and DOE before NNSA was created -- to provide a five-yearbudget projection (current year plus four out-years) in the National Defense Authorization Acts forFY1997 ( P.L. 104-201 , Section 3155), FY2000 ( P.L. 106-65 , Section 3253), and FY2001 ( P.L.106-398 , Sections 3154 and 3155). The FY2005 budget cycle was the first in which NNSApresented detailed out-year projection with the current request. In the FY2005 request, the projectedfigures for RNEP were: FY2005 (request), $27.6 million; FY2006, $95.0 million; FY2007, $145.4million; FY2008, $128.4 million; and FY2009, $88.4 million, for a five-year total of $484.7million. (6) The FY2005request also presented a plan for RNEP. All figures for FY2005-FY2009 are subject tocongressional approval, rejection, or modification. The research program for FY2003-FY2009 was thus estimated to cost $498.3 million -- $6.1million spent for FY2003, $7.5 million appropriated for FY2004, and $484.7 million, as noted, forFY2005-FY2009. If RNEP were to proceed through development and production, additional costswould include completing production engineering, and manufacturing components to convert theselected weapon into an earth penetrator. These figures include NNSA costs, such as for thepenetrator case, but exclude Air Force costs for the guidance unit (tail assembly, guidance computer,etc.) NNSA cannot provide an estimate of total program cost because it has not completed the coststudy. By way of background, the Departments of Defense and Energy agreed years ago to a formalset of phases by which modified nuclear weapons move through research, development, production,deployment, and retirement, often called the Phase 6.X process. The key phases for RNEP are:Phase 6.2, feasibility study and option down select; Phase 6.2A, design definition and cost study;Phase 6.3, development engineering, in which the nuclear weapons laboratories produce a completedwarhead design; and Phase 6.4, production engineering, in which the design is adapted forproduction and a system to manufacture the weapon is created. In its FY2005 request, NNSA statedthe performance targets for RNEP as follows: FY2005: "Complete 56% of scheduled RNEP Phase 6.2/6.2A activity." Further, "In FY2005, subsystem tests and a full system test of the proposed design will becompleted." FY2006: "Complete 100% of scheduled RNEP Phase 6.2/6.2Aactivity." FY2007: "Report results of RNEP Phase 6.2/6.2A to Nuclear WeaponsCouncil [a joint Department of Defense (DOD)-DOE agency that coordinates nuclear weaponprograms] Obtain, if applicable, RNEP Phase 6.3 appropriate authorization. Complete initial 25%of scheduled RNEP Phase 6.3 activity (if authorized)." FY2008: "Complete 65% of RNEP Phase 6.3 activity (if appropriatelyauthorized)."