Boosted fission weapon A boosted fission weapon usually refers to a type of nuclear bomb that uses a small amount of fusion fuel to increase the rate , and thus yield , of a fission reaction . The neutrons released by the fusion reaction s add to the neutrons released in the fission , as well as inducing the fission reactions to release more neutrons of their own . The rate of fission is increased so much that much more of the fissile material is able to undergo fission before the core explosively disassembles . The fusion process itself adds only a small amount of energy to the process , perhaps 1 % . The alternative meaning is an obsolete type of single - stage nuclear bomb that uses thermonuclear fusion on a large scale to create fast neutrons that can cause fission in depleted uranium , but which is not a two - stage hydrogen bomb . This type of bomb was referred to by Edward Teller as " Alarm Clock " , and by Andrei Sakharov as " Sloika " or " Layer Cake " ( Teller and Sakharov developed the idea independently , as far as is known ) . The idea of boosting was originally developed between Fall 1947 and Fall 1949 at Los Alamos . Gas boosting in modern nuclear weapons . In a fission bomb , the fissile fuel is " assembled " quickly by a uniform spherical implosion created with conventional explosives , producing a supercritical mass . In this state , many of the neutrons released by the fissioning of a nucleus will induce fission of other nuclei in the fuel mass , also releasing additional neutrons , leading to a chain reaction . This reaction consumes at most 20 % of the fuel before the bomb blows itself apart , or possibly much less if conditions are not ideal : the Little Boy ( gun type mechanism ) and Fat Man ( implosion type mechanism ) bombs had efficiencies of 1 . 38 % and 13 % , respectively . Fusion boosting is achieved by introducing tritium and deuterium gas ( solid lithium deuteride - tritide has also been used in some cases , but gas allows more flexibility and can be stored externally ) into a hollow cavity at the center of the sphere of fission fuel , or into a gap between an outer layer and a " levitated " inner core , sometime before implosion . By the time about 1 % of the fission fuel has fissioned , the temperature rises high enough to cause thermonuclear fusion , which produces relatively large numbers of neutrons speeding up the late stages of the chain reaction and approximately doubling its efficiency . Deuterium - tritium fusion neutrons are extremely energetic , seven times more energetic than an average fission neutron , which makes them much more likely to be captured in the fissile material and lead to fission . This is due to several reasons : Taking these factors into account , the maximum alpha value for D - T fusion neutrons in plutonium ( density 19 . 8 g / cm³ ) is some 8 times higher than for an average fission neutron ( 2 . 5×10 vs 3×10 ) . A sense of the potential contribution of fusion boosting can be gained by observing that the complete fusion of one mole of tritium ( 3 grams ) and one mole of deuterium ( 2 grams ) would produce one mole of neutrons ( 1 gram ) , which , neglecting escape losses and scattering for the moment , could fission one mole ( 239 grams ) of plutonium directly , producing 4 . 6 moles of secondary neutrons , which can in turn fission another 4 . 6 moles of plutonium ( 1099 g ) . The fission of this 1338 g of plutonium in the first two generations would release 23 kilotons of TNT equivalent ( 97 TJ ) of energy , and would by itself result in a 29 . 7 % efficiency for a bomb containing 4 . 5 kg of plutonium ( a typical small fission trigger ) . The energy released by the fusion of the 5 g of fusion fuel itself is only 1 . 73 % of the energy released by the fission of 1 . 338 kg of plutonium . Larger total yields and higher efficiency are possible , since the chain reaction can continue beyond the second generation after fusion boosting . Fusion - boosted fission bombs can also be made immune to neutron radiation from nearby nuclear explosions , which can cause other designs to predetonate , blowing themselves apart without achieving a high yield . The combination of reduced weight in relation to yield and immunity to radiation has ensured that most modern nuclear weapons are fusion - boosted . The fusion reaction rate typically becomes significant at 20 to 30 megakelvin s . This temperature is reached at very low efficiencies , when less than 1 % of the fissile material has fissioned ( corresponding to a yield in the range of hundreds of tons of TNT ) . Since implosion weapons can be designed that will achieve yields in this range even if neutrons are present at the moment of criticality , fusion boosting allows the manufacture of efficient weapons that are immune to predetonation . Elimination of this hazard is a very important advantage in using boosting . It appears that every weapon now in the U . S . arsenal is a boosted design . According to one weapons designer , boosting is mainly responsible for the remarkable 100 - fold increase in the efficiency of fission weapons since 1945 . Some early non - staged thermonuclear weapon designs . Early thermonuclear weapon designs such as the Joe - 4 , the Soviet " Layer Cake " ( " Sloika " , ) , used large amounts of fusion to induce fission in the uranium - 238 atoms that make up depleted uranium . These weapons had a fissile core surrounded by a layer of lithium - 6 deuteride , in turn surrounded by a layer of depleted uranium . Some designs ( including the layer cake ) had several alternate layers of these materials . The Soviet " Layer Cake " was similar to the American " Alarm Clock " design , which was never built , and the British " Green Bamboo " design , which was built but never tested . When this type of bomb explodes , the fission of the highly enriched uranium or plutonium core creates neutrons , some of which escape and strike atoms of lithium - 6 , creating tritium . At the temperature created by fission in the core , tritium and deuterium can undergo thermonuclear fusion without a high level of compression . The fusion of tritium and deuterium produces a neutron with an energy of 14 MeV — a much higher energy than the 1 MeV of the neutron that began the reaction . This creation of high - energy neutrons , rather than energy yield , is the main purpose of fusion in this kind of weapon . This 14 MeV neutron then strikes an atom of uranium - 238 , causing fission : without this fusion stage , the original 1 MeV neutron hitting an atom of uranium - 238 would probably have just been absorbed . This fission then releases energy and also neutrons , which then create more tritium from the remaining lithium - 6 , and so on , in a continuous cycle . Energy from fission of uranium - 238 is useful in weapons : both because depleted uranium is very much cheaper than highly enriched uranium and because it cannot go critical and is therefore less likely to be involved in a catastrophic accident . This kind of thermonuclear weapon can produce up to 20 % of its yield from fusion , with the rest coming from fission and is limited in yield to less than one megaton of TNT ( 4 PJ ) equivalent . Joe - 4 yielded 400 kilotons of TNT ( 1 . 7 PJ ) . In comparison , a " true " hydrogen bomb can produce up to 97 % of its yield from fusion , and there is no upper limit to its explosive yield .