The B61-based "Robust Nuclear Earth Penetrator:" Clever retrofit or   headway towards fourth-generation nuclear weapons?
  It is scientifically and technically possible to build an earth penetrating device that could bury a B61-7 warhead 30 meters into concrete, or 150 meters into earth, before detonating it. The device (based on knowledge and technology that are available since 50 years) would however be large and cumbersome. Better penetrator materials, components able to withstand larger stresses, higher impact velocities, and/or high-explosive driven penetration aids, can only marginally improve the device. It is concluded that the robust nuclear earth penetrator (RNEP) program may be as much motivated by the development of new technology directly applicable to next generation nuclear weapons, and by the political necessity to periodically reassess the role and utility of nuclear weapons, than by the perceived military need for a weapon able to destroy deeply buried targets. 
Andre Gsponer