This investigation aims to exploit the physics behind the detonation of Nuclear weapons, what they are theoretically capable of, and ways they can be used to save lives.
Discussion
Basics of Nuclear Bombs
Nuclear bombs are not all the same, in fact, there are two different types of nuclear bombs which have different ways of releasing mass amounts of energy. One type of nuclear bomb is a Fusion bomb, which is fuelled by a fusion reactor. Fusion reactors rely on the fusing of two light elements (low atomic number). 1, 2, 5, 6 For a bomb, the most common elements would be tritium (H^3) and deuterium (H^2), which are fused together to create the nucleus of a Helium atom (He^4) and a Neutron. 3 This fusion can produce around 17.6 MeV, and unlike nuclear fission, has no limit to the amount of fusion that may occur. 3
On the other hand there are nuclear bombs which rely on the fission of atoms, or the splitting of atoms. The most common element used as fuel for fission bombs is uranium-235 (one of uranium's Isotopes with 143 neutrons). 3 It is used because uranium 235 is very excepting of more neutrons, which is useful because it can then be split apart by firing another neutron at the nucleus, which it then will accept and become unstable and break apart (refer to figure 1) causing the initial release of energy and a number of neutrons, as seen in the below equations which show two possible products of the fission reaction with uranium. 3, 5
Figure 1 - Uranium 235 Accepting a free neutron and splitting apart. 3
Critical Mass
To prevent premature detonation of the nuclear bombs all fissionable material within the bomb is kept at a subcritical mass, or below critical mass. 5, 6 Critical mass is the minimum amount of fissionable material required to initiate and sustain a nuclear fission reaction. 1 It is common in fission bombs for there to be two separate subcritical masses of fissionable material (such as uranium 235), which upon detonation are brought together to create a supercritical mass, which provides more than enough neutrons to support a fission reaction at the time of detonation. 1 This is most easily demonstrated in the Gun-Triggered fission bombs, as seen in Appendix A.
This method of detonation was used in the atomic bomb named "Little Boy", which was dropped on Hiroshima. Essentially what happens is a small 'bullet' of uranium is displaced by an explosion, which propels it towards and into a larger sphere of uranium (both at subcritical mass) through a canon, and thus creating the supercritical mass of uranium. This supercritical mass is then bombarded with neutrons and is kept under pressure, with external explosives and the bomb casing, to allow fissionable material to fully fission. This is then followed by a massive release of energy and radiation (Refer to Appendix A).
Radiation
With nuclear radiation comes nuclear decay. They are essentially the same concept. In the regards to fission electromagnetic radiation, in the form of gamma rays, is the most common type of radiation. 4 Gamma rays are the emission of high energy photons. These high energy photons move at a very high frequency of approximately 1019 Hz, and retain very short wavelengths of less than 10 picometers. 4 Often gamma rays retain energy measuring around 100MeV, but Gamma radiation decay photons retains exceedingly more (usually around a few hundred MeV more). What makes gamma radiation so harmful to living tissue (i.e. Humans) is that gamma radiation is a type of ionizing radiation. 7 The photon particles in gamma rays are so energetic that they are able to strip other atoms of their electrons, and in turn giving that atom a more positive charge (A.K.A. Ionizing the atoms). 7 This can lead to many health issues for any living organism causing skin damage and burns, radiation sickness, and forms of cancer. 4
Another form of radiation and nuclear decay commonly found in the by-products of fission reactions beta decay and radiation. 4 Beta radiation is the emission of electrons due to the decay of one element into another (Refer to Figure 2). This beta decay occurs when an unstable neutron forms a proton and a electrons, which forces the extra electron (beta particle to be released. 4, 7 An electron being a charged particle, in fact, makes the beta particles more ionizing than that of the photons emitted in gamma radiation. Though this ionization make beta particle more harmful, it also allows them to be easily stopped. 4 These strongly ionized particle in beta radiation allows even a thin sheet of aluminium to stop them from penetrating our skin because as it passes through the matter the beta particles are decelerated by the electromagnetic interactions occurring between the atoms of the aluminium and their electrons. 4
Figure 24- Thorium undergoes beta decay gaining a proton and electron from a single neutron, becoming Protactinium and releasing an electron (beta particle)
Efficiency
The bomb "Little Boy" was a 14.5 Kiloton nuclear bomb with an above ground detonation, which had an efficiency of 1.5%.3 This means only 1.5% of the fissionable material underwent a fission reaction before the initial blast blew away the remaining fissionable material. This is a very poor efficiency, but since, has been improved with modern schematics and methods of nuclear fission, it was still a devastating blow. As seen in Table 1(also in Appendix B), with an above ground detonation of a 10-20 kiloton bomb (similar to 'Little Boy') lethal blast effects ranged from 1.5 to 1.9 km. The reason 'Little Boy' was not as efficient and effective as it could have been is due to either its poorly designed casing or its slow ignition mechanism, the gun trigger.
General Ranges of Nuclear Bomb Effects
Table 1- Range of a 50% chance of fatality from an Airburst 6
Little Boy was said to contain 64kg of uranium. Approximately 1.5% of the 64 kg of uranium actually underwent fission; therefore 0.96kg of the uranium was involved in the fission reaction. The remaining 63 kg of uranium were vaporized by the fireball of heat and energy. Of that 0.96kg only 0.6g were released as energy and the rest were fission products as shown in the equations in Figure 1. So if 0.6g of uranium was converted to energy, by using Einstein's formula E=MC2 , we can assume that it would create 5.4x1013 joules of energy. While this may be a large release of energy, the bomb Little boy still remains inefficient.
To create a more efficient bomb the use of a tamper is highly recommended. A tamper is a heavy non-fissionable casing around the uranium core which serves two purposes. 1, 3 One being that while the uranium core begins to expand, due to the heat created by the fission reaction, the tamper contains the core to allow more time for fission to occur. The second reason for using a tamper is to increase the fission rate. The tamper being a non-fissionable material will not accept free neutrons, but rather reflects the free neutrons back into the fissionable uranium core as to induce more fission. The use of this tamper can be demonstrated in appendix C.
Modern Methods
More modern fission bombs such as the Teller-Ulam Fission Bomb use an implosion triggered fission system as seen in appendix D. In a Teller-Ulam Fission Bomb there are essentially two bombs: A miniature fission bomb to initiate the larger fusion reaction, and the larger fusion bomb, which after reacting begins to cause a second fission reaction with its own uranium 238 tamper. As demonstrated in figure 2, is the initial fission implosion trigger itself. Here what happens is the explosives are detonated, thus creating a shock wave that launches the plutonium pieces into the Beryllium/Plutonium core. This uniting of the core and plutonium pieces initiates a smaller scale fission reaction. This reaction emits X-rays which, before the fission reactions initial release of energy, heat up the fusion bombs interior and tamper. The tamper begins to expand and burn away, which increases the pressure of the lithium deuterate by approximately 30 fold. This initiates the fusion reaction in the plutonium rod which soon after reacting with the lithium deuterate forces the tamper to fission. Theoretically all three reactions are to occur at the same instant, and in turn exploding and releasing large amounts of energy. 3
Figure 2- Implosion-triggered fusion bomb. 3
Possible Scenarios
If "Little Boy" were 100% Efficient
Scientists constantly strive to make a bomb which is 100% efficient. This is theoretically possible but not without further advancements in technology and science. In the event that scientist were able to create this 100% efficient Nuclear bomb it would release far more energy. For example if the bomb Little boy were to be 100% efficient, approximately 0.04kg of the uranium would actually release energy. This 0.04kg of uranium would release 3.6x1015 joules of energy. (Refer to Appendix for Calculations) In the future the use of a highly efficient nuclear fission bomb could be used to destroy or divert a small meteor heading towards the earth.
Meteor on a Collision course with Earth
In the scenario that there were a small meteor that weighed around 100 million kilograms, and travelled towards the earth at a slow speed of around 11 kps (kilometres per second). 8 Using the equation Force = Mass x Velocity the meteor would require an opposing for of 1.1x1012 newtons to completely stop or divert it. Assuming it takes 0.00002 seconds for a fission bomb to release its energy (refer to Appendix C), at a speed of 11kps the meteor would have travelled 0.22 m during the release of energy. Knowing that Joules=Newtons x Distance, and that the bomb would have to put out 1.1x1012 newtons over a distance of 0.22 meters, the bomb would have to produce approximately 2.42x1011 Joules of energy directed at the meteors sides or front, to stop or divert the meteor. This concept is theoretically possible, but when taking into consideration that the release of energy from a fission reaction is directed outwards from the point of detonation; it would take a considerably larger bomb that could produce a greater amount of energy, for this task to be achieved. For the larger amounts of energy would make up for the significant amounts of energy lost from the bombs lack of directed energy. (Note that energy is assumed to be released over 0.22m as to make calculations and conversions between joules and newtons possible and to give a theoretical outcome/solution)
Conclusion
This investigation has identified and explained the two ways of releasing mass amounts of energy. Nuclear fission, which relies on the splitting of atoms; and nuclear fusion, which relies of the fusing of two light elements. Different methods of initiating fusion and fission reactions have been exploited with their various uses of conventional explosives to propel objects, and the use of X-rays to create pressure to initiate fusion reactions. All of the methods researched in this paper are still in the early stages of development and fail to measure up to the theoretical fission efficiency of 100% fission. Nuclear sciences still have a long way to go, but hold the power to enormous amounts of energy. There are many possible uses for fission in humanities future. For future research it is recommended that stronger and more effective tampers be explored and experimented upon as to achieve the most complete fission reaction physically possible upon detonation. Further research into detonation systems would greatly benefit efficiency of the bombs as to reduce fizzle. Research into airbursts as appose to ground detonations would make great benefit to the effectiveness of the bombs physical affects, as seen when comparing the range of the bombs thermal flash effect during airbursts and ground detonations (GZ)(Refer to appendix B, Table 2). Most importantly, further research into alternative uses and ways of harnessing the immense power and energy that these bombs hold would benefit human lives and possibly save them.
Bibliography
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Appendix A --> Gun-Triggered Fission Bomb Flow Diagram 3
Numbered parts:
- bomb casing
- detonators
- conventional high explosion
- pusher (aluminum, others) and reflector (beryllium, tungsten)
- tamper (uranium-238)
- fissile core (plutonium or uranium-235)
Sequence of events in explosion: Numbered parts:
- Multiple detonators (2) simultaneously initiate detonation of high explosives (3).
- As detonation progresses through high explosives (3), shaping of these charges transforms the explosive shock front to one that is spherically symmetric, travelling inward.
- Explosive shock front compresses and transits the pusher (4) which facilitates transition of the shock wave from low-density high explosive to high-density core material.
- Shock front in turn compresses the reflector (4), tamper (5), and fissile core (6) inward.
- When compression of the fissile core (6) reaches optimum density, a neutron initiator (either in the centre of the fissile core or outside the high explosive assembly) releases a burst of neutrons into the core.
- The neutron burst initiates a fission chain reaction in the fissile core (6): a neutron splits a plutonium/uranium-235 atom, releasing perhaps two or three neutrons to do the same to other atoms, and so on; energy release increases geometrically.
- Many neutrons escaping from the fissile core (6) are reflected back to it by the tamper (5) and reflector (4), improving the chain reaction.
- The mass of the tamper (5) delays the fissile core (6) from expanding under the heat of the building energy release.
- Neutrons from the chain reaction in the fissile core (6) cause transmutation of atoms in the uranium-235 tamper (5).
- As the superheated core expands under the energy release, the chain reaction ends; the entire weapon is vaporized.
- Total elapsed time: about 0.00002 seconds.
Appendix D - Teller-Ulam Fusion Bomb3
Glossary
Airburst - An above ground detonation of a bomb
Ground Zero (GZ) - An on ground detonation(point of detonation referred to as ground zero)
Kiloton(kt) - Equivalent in TNT (eg. 1kt = 1000 tons of TNT)
Fizzle- Occurs when fissionable material in a bomb is blown away before it can completely fission
