Most recently, this fact was confirmed by India: it exploded a twostage hydrogen
bomb in the series of five tests which was its second experimental campaign, 24 years after its first single experiment of 1974.13 According to an official Indian statement, the device worked as expected, providing a total fission fusion yield of about 43 kilotons, a value that was intentionally kept low “to meet stringent criteria like containment of the explosion and least possible damage to buildings and structures in the neighboring villages.” A sketch of a modern thermonuclear bomb is given in Fig.4. One recognizes the same elements as in Fig.3, which are found in all thermonuclear explosives, with a number of variations. For instance, while the secondaries of early H bombs were generally cylindrical, the most modern ones are probably of spherical shape. In that case, instead of being made of plutonium or U235, the spark plug could be a DT fuze which ignites the Li6D fuel when optimum compression is reached. Finally, by chosing for the pusher/tamper an appropriate heavy material it is possible to control the total yield of a thermonuclear weapon. This is because the high energy (i.e., 14MeV) neutrons produced by fusion have sufficient energy to fission any kind of transuranic material such as all isotopes of uranium and plutonium. Therefore, while the minimum yield of a given thermonuclear design will be provided by using a non fissionable heavy material such as lead or bismuth as the tamper, the use of depleted (i.e., mostly isotope 238), natural, or highly enriched (i.e., mostly isotope 235) uranium will provide a range of increasing yields because more and more fission reactions will be produced in the tamper. This is why the pusher/tamper is sometimes called the “third stage” of a thermonuclear weapon. For example, assuming as in Fig.4 that the yield is 150kt if the pusher/tamper ismade of U238, the yield will be 300kt if the pusher/tamper is made of U235. On the other hand, if the pusher/tamper is made of a lead or bismuth, the yield will be significantly lower, on the order of 50kt. This is most probably what the Indian scientists have done in 1998 in order to be able to detonate the device at a relatively low depth into the ground, and to minimize the background signals which may overload the measuring instrumentation. Finally, if plutonium is used for the pusher/tamper a yield somewhat higher than with enriched uranium is obtained. This is why using plutonium for the “third stage” is providing the highest possible yield for a given design. This option has been implemented in some of the French thermonuclear weapons, and was an important justification for the French fast breeder program,14 as it could be for the Indian plutonium reprocessing/recycling program [26]. Consequently, although plutonium has some environmental disadvantages because it is more radioactive than U235, it offers an attractive alternative to the large scale production of highly enriched uranium because plutonium of any grade can be used for the “third stage” of a thermonuclear weapon. Therefore, any country such as Japan which has a large stockpile of separated plutonium will not need access to large amounts of U235 should it decide to make high yield thermonuclear weapons
