Applications of Nuclear Fission :
Nuclear Fuel Production, Nuclear Reactors, Nuclear Bombs
Nuclear Fuel Production - Uranium is as common as tin or zinc on Earth. About 63% of the world's production of
uranium comes from Kazakhstan, Canada and Australia (Figure 14-12a). Worldwide production in 2009 amounted to about 50000 tonnes. This is the only nuclear fuel that can be mined naturally. Other nuclear fuels such as plutonium are derived from uranium in breeder reactor. The various production processes are illustrated in Figure 14-12b and explained in more details below :
Figure 14-12a Worldwide Uranium Production
Figure 14-12b Uranium Processing [view large image]
Uranium ore - The primary uranium ore is uraninite (UO2), and pitchblende (UO3, U2O5 - collectively referred to as U3O8). A range of other uranium minerals can be found in various deposits. Some of the highest grade uranium ores in the world were found in the Democratic Republic of Congo.
Milling and refining - After mining the ores, they are crushed and then leached with sulfuric acid in the mill. The uranium oxides are separated to yield a substance in dry power form called "yellowcake" (see Figure 14-12c), which is sold on the market as U3O8.
Conversion - Natural uranium is 99.274% U238, with the fissile U235 only constituting about 0.7205% of its
weight. The rude material has to go through a process to increase the U235 percentage. But many of the enrichment processes require the substance in gaseous form, which only exists in uranium hexafluoride (UF6). Uranium hexafluoride can be solid, liquid, or gas depending on temperature and pressure. It is a solid or gas depending on whether temperature is below or above 57oC. It is in liquid form at 1.5 atmospheric pressure or higher and temperature greater than 64oC. Thus, the yellowcake has to be converted to uranium hexafluoride by treating it with strong acids and alkalis. Figure 14-12c shows a sample of uranium hexafluoride in crystal form and gas phase in a vial.
Figure 14-12c U- Compounds
Enrichment - According to cost and efficiency the enrichment methods are referred to by number of generation. The 0th generation is the calutron (California University Cyclotron) used in World War II, the 1st generation
employing gaseous diffusion is being phased out presently, the 2nd one is the method of choice currently using the centrifuge techniques. Figure 14-12d shows these three methods schematically. Future generation is still in development using infrared laser to separate the isotopes. Regardless of
Figure 14-12d Enrichment Methods [view large image]
enrichment methods, uranium fuel has to contain 3 to 4% U235 for nuclear reactors. Bomb grade uranium requires purification of U235 greater than 90%. Following is a brief description for the above-mentioned enrichment methods:
Calutron - It is essentially a mass spectrometer. The device is shown in Diagram a, Figure 14-12d with a magnetic field B pointing toward the page. The method relies on the fact that charged particles with different mass move in different circular paths perpendicular to a magnetic field according to the formula:
R = (v/qB)m
where v is the velocity, q the charge, m the mass of the particle, and R the radius of the circular path. Uranium ions are generated from solid uranium tetrafluoride, UF4, that is heated to produce a vapor and then bombarded with electrons to produce U+ ions.
Gaseous Diffusion - This method forces the uranium hexafluoride through semi-permeable membrane under pressure gradient as shown in Diagram b, Figure 14-11d. Since the rate of effusion is inversely
proportional to the square root of mass, this process allows the smaller constituents with U235 to pass through more to one side of the membrane. Denoting the rate of effusion by R, then
R(U235)/R(U238) = (352/349)1/2 = 1.004
which implies 2000 stages to achieve a purity of 99% U235. The
Figure 14-12e Diffusion Plant at Oak Ridge
huge facility in Oak Ridge, Tennessee was built to enrich uranium through 2 steps of diffusion and 2 steps of Calutron (Figure 14-12e). The plant is to be demolished completely by the end of 2011.
Gas Centrifuge - This process uses rotating cylinder to create a strong centrifugal force F (a fictitious force associated with rotating frame). For angular velocity , distance r from the rotational axis, and m the mass of the object:
F = mr 2
so that the heavier UF6 molecules containing U238 move toward the outside and the lighter ones rich in U235 is collected close to the center (Diagram c, Figure 14-11d). This technique produces about 54% of the world's enriched uranium now. All the above methods require cascade of identical stages to successively produce higher concentrations of U235 until it reaches the specification as shown in Figure 14-12f for a gas centrifuge plant.
Figure 14-12f Cascade of Gas Centrifuges
Laser Isotope Separation - The atomic energy levels through (electron)spin-orbit interaction split
into fine structures of the order 10-3 ev, which in turn split further into hyperfine structures of the order 10-6 ev by (nuclear)spin-orbit interaction (Diagram a, Figure 14-12g1). The amount of hyperfine splitting depends on the mass of the nuclei inversely. By tuning a laser on the hyperfine
Figure 14-12g1 Laser Isotope Separation
levels of the U235 atom or its molecular compound, it can be excited or ionized selectively and gathered into collector by appropriate action (direction of the gas flow is perpendicular to the page in Diagram b, Figure 14-12g1).
The Atomic Vapor Laser Isotope Separation (AVLIS) feeds uranium-iron metal alloy into the system. Uranium vapor is produced by heating. It is eventually ionized by the tunable laser and collected into the negatively charged plate.
In Molecular Laser Isotope Separation (MLIS) the uranium hexafluoride gas is selectively excited for only U235F6 molecules. A second laser knocks out a fluorine atom leaving an uranium pentafluoride (U235F5) to be precipitates out of the gas.
No details is available for the technique of "Separation of Isotopes by Laser Excitation" (SILEX) as it is a commercial secret. No laser separation uranium enrichment plants are currently (2011) in operation.
On 11 July 2012, the US Nuclear Regulatory Commission (NRC) held a final hearing on a proposal by GE of Fairfield, Connecticut, and Hitachi of Tokyo, Japan, to build the first commercial laser-enrichment plant. A decision on the plant, to be built in Wilmington, North Carolina, is anticipated in September 2012 and is widely expected to be favorable in spite of lingering doubt about accelerating the proliferation of nuclear weapons because the new technology
Figure 14-12g2 Enrichment Techniques [view large image]
is relatively cheap and easy to build. Figure 14-12g2 compares the cost of running the 3 different types of fuel enrichment. The Separative Work Unit or SWU is a measure of the work expended during an enrichment process.
The NRC promptly issued a licence to GE Hitachi (a multinational company) for the new technique of laser-enrichment on September 2012. The application is approved only on consideration of physical security such as the height of the fence around the plant. It cites that proliferation is not within its mandate. Now the American Physical Society (backed by environment groups, and scientists) has proposed that the NRC change its regulations to require future applicants to submit a "proliferation review" (similar to the environmental-impact statements). Anyway, it may be too late to prevent the propagation of this new technique to other countries as the principle of its design is now an open secret.
Fuel Fabrication - The UF6 gas is chemically treated to form uranium dioxide (UO2) powder. This
powder is then processed into ceramic pellets and loaded into metal tubes (fuel rod), which are subsequently bundled into fuel assemblies (Figure 14-12h) for use in nuclear reactor. Weapon grade uranium was enriched to 90% U235 by two steps of gaseous diffusion and two steps of calutron at Oak Ridge in 1945 to produce the bomb dropped on Hiroshima. Since then the majority of fission weapons have used plutonium. Uranium enrichment is currently used to produce fuel (3 to 4% U-235) for civilian nuclear reactors.
Figure 14-12h Fuel Fabrication
Nuclear Reactors - For efficient production of energy in nuclear reactors, the neutrons must be slowed down by moderation to increase their capture probability in fission reactors. Specifically, the initial fission of U-235 produces neutrons with mean energy of 2 Mev (see Figure 14-11f). Neutron with this energy has a fission probability about 1000 times less than a neutron with energy of 0.025 ev. So they need to be slowed down, on average, by a factor of 100 million. Some nuclear reactors use heavy water (D2O) to slow down (moderate) the neutrons, while ordinary water can be used to minimize neutron loss (via the H2O + 2n D2O reaction), which is harmful to health. The control rods used to regulate the rate of reaction are mainly composed of stainless steel tubes encapsulating silver-indium-cadmium (neutron) absorber material.
The world's first atomic pile, now called nuclear reactor, was constructed inside a squash court at the university of Chicago. It consisted of 57 layers of pure graphite blocks to slow down the neutrons. Slugs of natural uranium (with only 0.7% of the fissile U-235) are located in between forming a cube-like lattice within the pile. Cadmium rods (the absorbers of neutrons) were used for controlling the reaction rate. The design has a final cadmium rod to drop down automatically into the pile should the neutrons rise above a certain level. If that fails to stop the reaction, another rod could be released from the balcony by cutting a rope. As an extra insurance against an unwanted nuclear explosion, three young physicists stood on an elevator platform above the pile, ready to flood it with a cadmium-salt solution, just in case something went very wrong. These three were known as the "suicide squad". The pile was activated successfully without accident on
December 2, 1942 at 2:20 p.m. after a lunch recess. No photographers were present to witness the arrival of nuclear age. Figure 14-12i is the re-creation of the occasion by an artist in 1957. Enrico Fermi (captain of the team of 42 scientists) is the half-bald individual standing next to Walter Zinn who is leaning with his elbow on the rail. A
Figure 14-12i First Reactor
Figure 14-12j Modern Nuclear Reactor [view large image]
schematic diagram of a modern nuclear reactor (type PWR) running on the same principle is shown in Figure 14-12j.
While uranium-235 is the naturally occurring fissionable isotope, there are other isotopes which can be induced to fission by neutron bombardment. Plutonium-239 is also fissionable, and both types have been used to make nuclear fission bombs. Plutonium-239 can be produced by breeding from non-fissionable uranium-238 (by absorbing a neutron and then transmuted via the beta decay process). Spent fuel is taken out of the reactor after four years and can be recycled in a reprocessing plant. Some of the nuclear reactors at Hanford, Washington and the Savannah-River Plant, SC are designed for the production of bomb-grade plutonium-239. Figure 14-12k shows the different pathways for the two different types of reactors.
Figure 14-12k Two Fission Pathways
As shown in Figure 14-12ka 95% of the radioactive materials is uranium-238 in the spent fuel. Theoretically, it can be
extracted and reprocessed into more fuel, however the nuclear industry prefers the more cost-efficient way of mining. The remaining 1% of the long half-life actinides can also be transmuted into less harmful elements (plus output energy surplus) by fast neutrons, which are in turn produced by protons (from accelerators) hitting a lead target. Such method is currently on trial with funding from the European Union. Thus, there are solutions to lessen the impact of nuclear waste, which is being accumulated
Figure 14-12ka Spent Fuel [view large image]
at the rate of about 2000 tonnes/year (in the US alone), only if the politicians can stop wrangling about the method of disposal and how best to protect our future generation from its harmful effects.
In a breeder reactor, the fuel consists of 90% U-238 together with 10% Pu-239. There is no graphite to moderate the reaction by slowing down the neutrons hence it is sometimes referred to as fast breeder - meaning fast neutrons. The fast neutrons are absorbed by U-238, which is then transmuted into fissionable (fissile) plutonium. The great advantage of this type of reactor is that, rather than merely burning uranium to create energy, the naturally abundant U-238 is used in a cyclical process that simultaneously generates both fission energy and more nuclear fuel than there was in the first place. It is called breeder because it breeds fuel. Figure 14-12l shows a breeder reactor. It is rather similar to the conventional nuclear reactor except that there is an U-238
Figure 14-12l Breeder Reactor [view large image]
blanket to capture and reflect the neutrons back to the core, and liquid sodium is used as coolant in extreme temperatures surrounding the reactor.
Table 14-03 below lists the types of nuclear reactors in commercial operation (in order of decreasing popularity) according to "World Nuclear Association". The classification is mainly by the kind of fuel, heat transportation, and moderating material. There is a total of 435 of these kinds of reactors in the world with 386.4 giga-watts capacity.
Reactor Type
Capacity (giga-w)
Pressurized Water Reactor (PWR)* US, France, Japan, Russia, China 265 251.6 Enriched UO2 Water Water 2 cooling circuits, 1st use in submarine
Boiling Water Reactor (BWR) US, Japan, Sweden 94 86.4 Enriched UO2 Water Water 1 cooling circuit, heat transported by steam
Pressurized Heavy Water Reactor (PHWR) Canada (known as CANDU), India 44 24.3 Natural UO2 Heavy
Water (D2O) Heavy
Water (D2O) 2 cooling circuits, 0.7% U-235 fuel
Gas-cooled Reactor (AGR) UK 18 10.8 Enriched UO2 CO2 Graphite 2 cooling circuits - CO2 and water
Light Water Graphite-moderated Reactor (RBMK) Russia 12 12.3 Low-enriched UO2 Water Graphite Tendency to overheat as the one in Chernobyl
Fast Neutron Reactor (FBR) Japan, Russia 2 1.0 PuO2 and UO2 (MOX) Liquid Sodium None Power from PuO2, plutonium from UO2
Table 14-03 Types of Nuclear Reactor
* The Westinghouse AP1000 is the generation III+ pressurized water reactor built on passive safety system, which relies on natural forces to supply cooling water for a few days (in the absence of electricity or human intervention). A 2013 report reveals that the US is building four AP1000 type reactors to be completed in 2015 -2017. China is building 28 reactors, many of them are type AP1000, one of which (in Zhejiang ) will come online in late 2013. It takes the first step to blaze the trail into the wilderness that makes US utility companies nervous about undertaking such a new, long, and costly project.
The incidents at Three Mile Island in 1979 and at Chernobyl in 1986 had slowed down the building of nuclear reactors. Currently, they generate 17% of the world's electricity. Global warming and rising energy needs has prompted a consortium of ten nations to plan the future reactor (~ 2035). The next generation reactors should be a lot cheaper to run, produce much less radioactive waste at accident-proof facilities, and perhaps eliminate reprocessing waste (into plutonium for assembling bombs). Unlike today's water-cooled reactors, which tend to run at about 300 oC, the new design will operate at temperatures from 510 oC to 1000 oC. This allows for more efficient
conversion of heat to electricity. But these higher operating temperatures mean that the reactors will need new coolants. One of the most popular concepts is the supercritical-water-cooled reactor, which uses extreme pressures to prevent water from boiling at temperature up to 500 oC. The most advanced concept is helium gas cooling, which can achieve temperature in the range of 700 - 900 oC. Hydrogen can be split from water thermo-chemically at this temperature. The hydrogen gas can be used as fuel to convert into electricity for cars and homes. Operating at high temperatures also rules out conventional fuel systems in the form of metal rods as they melt at fairly low temperatures. Instead, the gas-cooled reactors will hold fuel pellets either in a honeycomb graphite structure, or fused into billiard-ball-sized graphite spheres, known as pebbles (Figure 14-12m1).
Figure 14-12m1 Next Generation Nuclear Fuel [view large image]
Figure 14-12m2 Pebble Nuclear Reactor [view large image]
In spite of criticisms and problems with the pebbles bed design, China has constructed a 10 MWt prototype called HTR-10, which started to operate in 2003 (Figure 14-12m2).
By 2013, as Germany is abandoning nuclear power; the US industry is struggling with the pros and cons, while Japan is in the midst of soul-searching about its post-Fukushima intentions, the Russian Federation is aggressively selling reactors all over the world, raising safety and proliferation concerns. Its state-owned nuclear company called Rosatom organizes annual exhibition by the name of AtomExpo. They have been doing brisk deals with all kinds of business options including nuclear plant rental. It expects as many as 80 orders from countries around the world by 2030. Incidentally,
Figure 14-12n Nuclear Reactors, Russian Style
the opening reception is completed with open bar, disco ball, and show girls - very different from the other nuclear conferences. Anyway, they are exporting essentially three types of nuclear plants as shown in Figure 14-12n (the Russian model names are in red) :
VVER (Russian PWR) - It is similar to the Westinghouse AP1000 - generation III of the most popular PWR type (see Table 14-03). The new design is more compact and built with gravity drain water tank which can passively remove heat up to 72 hours. The Russian version adds a core catcher to prevent melting core from sinking to the underground water table below.
Russian Breeder - As shown in Table 14-03, there are only two built in the entire world. The one in Russia is the world's only commercially operating breeder reactor. The liquid sodium coolant (used for its high heat capacity) can easily catch fire making it very dangerous to operate. Another concern is the proliferation of weapon-grade plutonium. But the Russians have another idea, they argues that such reactor can reprocessed fuel, i.e., the spent fuel would not be dumped into unpopular storage sites. In other word, they envisage a closed fuel cycle with no nuclear waste.
Small Floating Plant - This is essentially putting two small PWRs on a vehicle such as the submarine. The novel idea now is to put them on a barge, which can move the small nuclear plant to any place near coastal water. It would be an easy target for terrorists and the towing is accident prone. The scenario has created the worst security and environmental nightmare in the West as the first nuclear floating plant (time bomb ?) is scheduled to operate in 2016.
Eventually on August 23, 2019 the floating nuclear reactor "Akademik Lomonosov" began its maiden voyage of some 3000 miles from Mumansk to Pevek. It will take 4 to 6 weeks to reach the port before the Arctic Sea fozen solid. Three tug boats are used to tow it to the destination (Figure 14-12n1). It has two reactors producing 35 mega-watts each for the settlement of 2 million inhabitants.
Figure 14-12n1 [view large image]
Click to see the fanfare to send off the nuclear barge
Meanwhile, the UK government is expected to sign a deal (in late 2013) to start building 2 European PWR (Figure 14-12o) at Hinkley Point on the Somerset coast. It will be half-owned by China, and will become operational after 2018. The new design is more efficient and safer. There are several advantages over previous reactors including :
The plant has 6 backup diesel generators widely spaced around the complex (instead of the usual 4 in most nuclear plants).
It is more efficient using less fuel, and rarely needs to be shut down for maintenance.
It can run on MOX from discarded weapon-grade plutonium, which is difficult to dispose.
The 2 outer reinforced concrete walls are over 1 meter thick to withstand an airliner crashing into the building (remember 911?).
The concrete base spreads out molten mess in the event of core meltdown. The base is also cooled from below.
Figure 14-12o European PWR
The Molten Salt Reactor (MSR, Figure 14-12p) is in a class completely different from those mentioned earlier.
It was in experimental stage through out the 1960s and 1970s. The funding support was terminated by competition from the more mature types. Interest in MSR (using thorium as fuel) has been revived after the Fukushima Incident as it is touted to be safer, cleaner and cheaper. The world wide deposits of thorium is estimated to be about 4 times more than uranium (Figure 14-12q) - enough to supply global energy for hundreds of thousands of years.
Figure 14-12p Molten Salt Reactor [view large image]
Figure 14-12q Thorium Deposits [view large image]
Nuclear fuels such as uranium-238 and thorium-232 are labeled as fertile as they cannot go critical (capable of maintaining a chain reaction such as uranium-235 and plutonium-239, which are called fissile). However, the fertile fuels can be converted to the fissile variety by capturing slow neutron. The conversion process for Th-232 involves :
Th232 + n (thermal or slow neutron) Th233 (half-life = 22 minutes)
Th233 Pa233 + e- (half-life = 27 days, Pa = protactinium)
Pa233 U233 + e- (half-life = 0.16 million years, U233 is fissile)
In thorium MSR the fuel and coolant are mixed together in molten state. Such design allows dumping of the liquid into tanks in case of over heating (see Figure 14-12p) - thus it is safer. The end products and in-purities can be separated by chemical means (also see Figure 14-12p) - hence it is cleaner. This type of reactor is usually designed as a breeder. If it breeds just as much new fissile from fertile to keep operating indefinitely, it is called a break-even breeder. In that case only the fertile material needs to be replenished to keep the reactor going - one of the reasons to make it cheaper to operate (see "Liquid fluoride thorium reactor" in the Wiki page for more detail).
It seems that thorium should be the wonder fuel of the future except that the same technology exposes the world to further nuclear proliferation because the design can be scaled down to lab-size evading detection. Running in parallel, the protactinium can be extracted from the molten fuel to produce enough U233 for making a bomb in much shorter time (than making U235 or Pu239). The extraction technique is well known as outlined in : "Acid-media Techniques". Figure 14-12r1 shows the current design of reactors together with the improved versions, and the number of nuclear reactors in the world (current, under construction, planned, and proposed).
Figure 14-12r1 Reactors, Old and New
The Traveling Wave Reactor (TWR) is another type of sustainable reactor using solid fuel for both the fertile and fissile materials (Figure 14-12r2). Unlike the liquid version, such configuration cannot be replenished by adding new fuel. The whole thing is made of mostly fertile material with small amount of the fissile fuel at the center to start up the reaction initially. The fertile material turns into fissionable as the reaction progresses outward according to the formula : , where the neutron is the end product of fission and the final product Pu-239 is fissile. The reaction zone moves like wave propagating outward hence the name TWR (see Figure 14-12r3, stop the motion by Esc key).
Figure 14-12r2 Traveling Wave Reactor
Figure 14-12r3 Traveling Wave [view large image]
Figure 14-12r3 illustrates the process graphically in terms of 4 zones :
Spent Fuel Zone - It is the inner-most zone (in black color) containing the end products of fission. As shown in the insert in Figure 14-12r2, the atomic weights of the two pieces of fission products are mostly in the ranges of 90 - 100, and 130 - 140, e.g.
Fission Zone - Energy is produced in this zone (in green color) by the fission of Pu-239, which also emits three fast neutrons to generate more Pu-239 from U-238 (see end products above, and conversion below).
Conversion Zone - The conversion from fertile to fissile materials occurs in this zone, i.e., .
The blue color indicates the neutron density in the process.
Depleted Uranium Zone - It contains the U-238 fertile fuel not yet used (in red).
The TWR is promoted by its ability to use fuel efficiently without uranium enrichment or reprocessing. The fuel can be depleted (low grade) uranium, natural uranium, thorium, or spent fuel removed from Light Water Reactors (not the Heavy variety). TerraPower, the company chaired by Bill Gates, has signed an agreement with China to build a demonstration plant by 2018-2020.
Nuclear Bombs - There are three steps to achieve a nuclear explosion, namely:
Keeping the fissile material in sub-critical state before detonation.
Bring together the sub-critical pieces into a super-critical state before starting the chain reaction.
Initiating the chain reaction with a neutron source when the configuration is at maximum super-criticality.
In order to increase efficiency, a tamper is usually wrapped around the nuclear explosive to retard the expansion and to augment the number of neutrons by reflection. There are two main types of nuclear bombs depending on the fissile material (Figure 14-12s). The two crude drawings in the image come from the "Los Alamos Primer" - The first lectures on "How to Build an Atomic Bomb".
Figure 14-12s Two Types of Nuclear Bomb
The formula below is very important in the design of the bombs:
Mc = (4 /3)(m/A)3/ 2
where Mc is the critical mass, m the mass of uranium nucleus, "A" the fission cross section, and the density of the fissile material.
Gun-type Nuclear Bomb - An ingenious method is to produce a sub-critical state from an U235 sphere with 3 critical mass Mc1, i.e., M = 3Mc1, by taking away a piece of one critical mass. The withdrawal leaves an empty spot (see drawing in Diagram a, Figure 14-12s), which reduces the average density to 2/3 of the original. Since the critical mass Mc varies as -2, the critical mass Mc2 for the hollow sphere of U235 is equal to 2.25Mc1, which is greater than the total mass of 2Mc1in the hollow sphere - and it is thus in a sub-critical state.
Detonation is initiated by firing the detached piece to the core of the hollow sphere. This process must occur in a time interval much less than the initiation of chain reaction by neutrons from spontaneous decay in the sub-critical mass. Otherwise the bomb will end up as fizzle.
A neutron would be released easily when beryllium-9 is struck by an alpha particle as shown below:
Be9 + He4 Be8 + n + He4
Thus, to create a neutron source on demand, a strong alpha emitter such as polonium-210 is placed at one end of the hollow sphere while the beryllium is attached to the other end of the "bullet" so that neutrons are only produced when the pieces are close to the optimum position.
The yield of a nuclear bomb y can be computed by the following formula:
y = (# of mole) x (# of nucleus / mole) x (energy released / nucleus) x (efficiency) x (ev to Joules conversion)
where (# of nucleus / mole) = 6.023 x 1023 is just the Avogadro number, and 1 ev = 1.602 x 10-19 Joules. The atomic bomb dropped on Hiroshima contains 63.5 kg of U-235 giving (# of mole) = 63500/235, each U-235 nucleus releases 166 Mev, and the efficiency is only 1.38% - meaning only 1.38 out of 100 U-235 nuclei actually underwent fission (the rest of the fissile material so painstakingly prepared is blown off into the thin air). Thus the yield computed by the formula for this particular case is 60 TJ. The actual yield is 63 TJ (1 TJ = 1012 Joules, 1 ton of TNT = 4.184x109 J).
Implosion-type Nuclear Bomb - Plutonium (Figure 14-12t) is the key fissile material for modern nuclear bomb. It has some peculiar properties that required special handling. This element has at least 7 allotropic phases
(different forms), every one of which has its own density and other physical properties including critical mass. The alpha phase occurs at room temperature, it has a density of 19.86 gm/cm3, a critical mass of about 10 kg., and very brittle - not suitable to shape by machine. The bomb makers now use 0.8% gallium alloy to stabilize the malleable delta phase at higher temperature. Despite of such shortcoming it has become the fissile material of choice, since its enrichment is susceptible of chemical treatment, the cost of manufacturing is greatly reduced. The silvery button in Figure 14-12t was used in the core of
Figure 14-12t Plutonium [view large image]
the bomb dropped on Nagasaki (a.k.a. Fat Man). The plutonium (in the same image) in the form of a ring is important for criticality safety. There is enough material in there to make a modern strategic nuclear bomb.
Another problem with Pu-239 is the isotope Pu-240. About 1 in 3 of the irradiating neutrons (as depicted in Figure 14-12k) are absorbed to become Pu-240. The trouble with Pu-240 is the tremendous amount of spontaneous neutrons re-emitted at the rate of 1 million per sec per kg (as shown by Table 14-02). In the gun-type bomb, the assembly into critical mass takes about 10-3 sec, which is too long. The bomb has not passed into super-criticality sufficiently before it blows itself apart resulting in a predetonation - a frizzle. Bomb-grade plutonium contains fewer than 7% Pu-240, while those used in reactor can have 20% or more Pu-240. In spite of minimizing Pu-240 contamination, Nuclear bomb using plutonium has to be re-designed.
The sketch in Diagram b, Figure 14-12r1 is one of the many possibilities considered for arranging the plutonium in the method of implosion, which requires precise firing of explosive surrounding the outer surface of the sphere. In the final design, shock wave from simultaneous detonations compresses a solid plutonium sphere from sub-critical to super-critical. The critical mass for delta phase plutonium is 16 kg. The "Fat Man" carried only a 6.2 kg plutonium pit (core). Evidently the combined effects of increasing density and tamper modified the critical mass to less than 1/4 of its value at normal pressure. The initiator contains beryllium and polonium separated by a barrier and placed at the center of the bomb. The barrier in the container is ruptured by the shock wave of the explosive to make it into a neutron source. The fission process proceeds quickly in 10-6 sec, so that the problem with frizzle is avoided. Diagram b, Figure 14-12r1 shows the structure of implosion-type nuclear bomb.
The world's first nuclear explosion occurred on July 16, 1945 at 5:30 a.m. at the Trinity test site in New Mexico. It is the cumulative efforts of up to 130000 people over 27 months in the Manhattan Project headed by J. R. Oppenheimer at Los Alamos, where the bombs were designed and made. Many thousands more were involved indirectly, working for big corporations such Union Carbide and Du Pont. Other sites include the Met Lab at Chicago; there was Oak Ridge, where U-235 was separated from U-238; and Hanford, the site of the reactors that created plutonium, and the facilities to separate this from the uranium fuel. The device consisted of a five-foot sphere containing shaped charges of high explosives around a core of plutonium no bigger than a tennis ball. The pieces of plutonium will be imploded to reach critical volume by the conventional explosion. The bomb was winched up a hundred-foot wooden tower at Ground Zero. In Figure 14-12u, a physicist in charge of assembling the high-explosive charges sits next to the bomb. Another image in the same figure shows the display of
spectacular fire ball and mushroom cloud seconds after the detonation. Unknown to the scientists on the project, the real purpose of the bomb was to target Japan and to intimidate the Soviets according to General Groves, the equivalent of political commissar in the project. Some team members consider such intent to be the betrayal of the original aim of using it on Nazi Germany, which was
Figure 14-12u First Nuclear Explosion [view large image]
believed to be on the verge of making similar weapon. Anyway, the development came too late as Germany has already capitulated on May 8, 1945 before the Trinity test.
The North Korean nuclear test in 2006 seemed to fizzle out with very low yield. Probable causes of the misfiring:
Non-symmetrical implosion - The implosion must be extremely symmetrical to be fully successful. Typically a combination of fast and slow conventional explosives surrounds a sphere of plutonium (the "core" or "pit"). Engineers must carefully machine all the pieces that make up this explosive shell into shapes that, when detonated simultaneously, produce a precisely spherical shock wave that compresses the plutonium to two to five times its normal density (the more compression, the greater the explosive yield). At the higher density, what was a subcritical mass of plutonium becomes supercritical, i.e., one in which a sustained chain reaction takes place, producing the blast.
Wrong timing of the initiator - The initiator is a small device at the center of the core that emits a burst of neutrons to start the chain reaction reliably at a precise stage of the implosion. An initiator going off early or late or not at all would reduce the yield.
Impurity - The fuel rods of nuclear reactors produce the desired isotope, plutonium 239, but the longer it
remains in the reactor, the more of it becomes plutonium 240. Plutonium 240 constantly emits tens of thousands of times more neutrons per second than plutonium 239. Although neutrons are the key particles in producing a nuclear chain reaction, an excess of them early in the implosion is a recipe for predetonation. Figure 14-12v shows the fuel rods from the Yongbyon nuclear reactor. They probably provided the plutonium 239 needed for the test. North Korea has conducted an underground nuclear test successfully on May 25, 2009.
Figure 14-12v NK Fuel Rods
North Korea has successful tested a supposedly H-bomb in September, 2017 (see news from "The Sun").