As long as the United States continues to have nuclear weapons, we must ensure that they remain safe, secure, and effective without the use of underground testing
– Don Cook, NNSA Deputy Administrator for Defense Programs
In October 2015, USA completed testing of upgraded Nuclear Earth Penetrating bomb B61-12. The aim was to extend the life of B61 Mod 7 and Mod 11 strategic bombs by 20 years. The upgrades include scalable nuclear yield (The B61 family of weapons can be configured with a wide variety of yields, including 0.3, 1.5, 5, 10, 45, 60, 60, 80, 170, and 340 kilotons), precision guidance and advanced safety mechanisms.
It is understood that strategic assets like ballistic missile facilities, command, control & communication centers, shelters for political leadership etc are located in tunnels at depths varying between 200 meters to 700 meters. These have been termed as strategic “hard and deeply buried target (HDBT)” by NATO countries and it is against such targets that Nuclear Earth Penetrating bombs are intended to be used.
Conventional weapons have the capability to penetrate to depths as much as the nuclear earth penetrator weapons (NEPW) but they are not as effective against the HDBTs. The energy transfer of NEPWs into ground is far more effective than surface or aerial bursts of even nuclear weapons. It is said that a 300-kiloton NEPW is as effective as a 6-megaton surface burst against HDBTs. Further, the accuracy requirements (Circular Error Probable, CEP) for surface bursts are more stringent than NEPWs for HDBTs to achieve the same kill probabilities. This brings in to focus two facts viz. that NEPWs require much less radioactive material and that with increasing accuracy of hit the damage potential keeps on increasing.
Sandia National Laboratories have been carrying out research work on the Earth Penetrators since the 1960s. One of their newer programs is the feasibility study program “Robust Nuclear Earth Penetrator program (RNEP)”. The aim of this program is to study feasibility of designing RNEPs which, can tackle a larger number of targets than the B61-11. The general terms of reference indicate that RNEP should be capable of reaching a specified depth, should be able to survive and penetrate the target, and should perform better than B61-11in terms of functionability, safety, security, & reliability. Sandia National Laboratories have the credit of building the most complicated nuclear safety mechanism called the ‘Micro Guardian’ in 1990s. This ensures that the nuclear weapon does not detonate until a predefined sequence of events is completed. It is said that the size of this system is 10 mm x 6 mm x 5 mm, and it forms a part of the optical micro-firing system. These developments highlight the march of Micro electro-mechanical systems (MEMS) as well as the Nano electro-mechanical systems (NEMS) into the nuclear arsenal arena.
It need not be stressed that arming and detonation of a nuclear weapon should not take place accidentally, however it is also to be ensured that the bomb once armed must not only hit the designated target but also explode. These conditions present a formidable technological challenge in designing of the arming, fusing, and firing mechanisms of nuclear bombs. This requires requisite robustness and multiple redundancies as also assured reliability of functioning. The MEMS/ NEMS have gained credibility mainly due to their compactness and minimal moving components as compared to the early analogue as well as digital counterparts. Programs such as the RNEP of Sandia National Laboratories would not only benefit NEPs but also conventional weapons as well. The availability of such devices and the fact that they have improved the resistance to failure of key components in fusing, arming, detonators, and neutron generators by many magnitudes has spurred research into next generation of nuclear weapons.
Though the consensus over the term Fourth Generation of nuclear weapons is still debatable, it can be safely stated that it would invariably be those classes of nuclear weapons which are triggered using advance triggering mechanisms such as super lasers, magnetic compression or antimatter (this also under active research!!). This would than result in a thermonuclear explosion of a few liters of deuterium-tritium mixture (equivalent of hundreds of tons of TNT). The main source of yield would not be fission reaction of the first three generations but a distinct fusion reaction, which would classify the next generation.
The stage is set for, NEMS to usher in unprecedented robustness, reliability, and precision in CEP, nEMs to replace conventional explosives and provide much greater explosive power, and advanced triggering devices & fusion yields to herald fourth generation nuclear weapons. The possessor would not only be able to unleash a swarm of conventional weapons but also carryout devastating assault without breaching the kiloton/ megaton taboo of first strike!