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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] |
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 | |
Figure 14-12d Enrichment Methods [view large image] |
of 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: |
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 is relatively cheap and easy to build. Figure 14-12g2 compares the cost of | |
Figure 14-12g2 Enrichment Techniques [view large image] |
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. |
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 | ||
Figure 14-12i First Reactor |
Figure 14-12j Modern Nuclear Reactor [view large image] |
elbow on the rail. A 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 |
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 at the rate of about 2000 tonnes/year (in the US alone), only if the politicians | |
Figure 14-12ka Spent Fuel [view large image] |
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 | |
Figure 14-12l Breeder Reactor [view large image] |
is an U-238 blanket to capture and reflect the neutrons back to the core, and liquid sodium is used as coolant in extreme temperatures surrounding the reactor. |
Reactor Type | Countries | # | Capacity (giga-w) | Fuel | Coolant | Moderator | Comment |
---|---|---|---|---|---|---|---|
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 |
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 |
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) : |
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 |
| |
Figure 14-12o European PWR |
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. In addition, it is a waste product of the growing rare-earth mining industry. | ||
Figure 14-12p Molten Salt Reactor [view large image] |
Figure 14-12q Thorium Deposits [view large image] |
China launched its molten-salt reactor programme in 2011, investing some 3 billion yuan (US$500 million). The experimental thorium reactor in Wuwei (武威), was due to |
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] |
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. |
(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 | |
Figure 14-12t Plutonium [view large image] |
of 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. |
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. |
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 |