Uranium: surface disturbance and the amount of
Uranium: uraniumis a slightly radioactive metal which is present in the Earth’s crust. It is inmost rocks and soils as well as rivers and sea water. It is about as common astin and around 500 times more common than gold. It can be found at around 4ppm(parts per million) in granite; being 60% of the Earth’s crust.There are places where theconcentration of uranium in the ground is high enough that it is economicallyreasonable to be extracted for use as nuclear fuel. Concentrations of this sizeare called uranium ore. Uranium Mining: Thetwo main ways to mine uranium ore are excavation and in situ techniques.
Excavation can be bothunderground or open pit, depending on how deep the ore is. If the uranium istypically deeper than 120m, underground excavation will be used.Open pit mines need holes onthe surface larger than the deposit of ore being mined, this is because thewalls of the pit must be sloped to prevent collapse. Underground mines cause asmaller surface disturbance and the amount of material needed to access the oreis considerably less than that of an open pit.
However, some specialrequirements, mainly more ventilation, is needed in an underground mine tolower the risk of airborne radiation exposure.More and more uranium is nowbeing mined the in situ leaching (ISL). This is where oxygenated groundwater iscirculated through a very porous body of ore to dissolve the uranium oxide andbring it to the surface. ISL may be done with acidic or alkaline solutions tokeep the uranium in solution. Then the uranium oxide is recovered the same asconventional mills.An industry example of a uraniummining company could be Kazatomprom, which had the leading uranium outputproduction of 2016, producing 12,986 metric tons with a net income of$326million. The company uses the in situ method. The company has 54 depositsbased in Kazakhstan; of which 16 are currently in use, with 38 on standby.
Uranium Milling: Isa process usually carried out close to a uranium mine, which extracts theuranium from the ore or ISL Leachate. Most mines have a mill, however if thereare several mines close together, one mill may do the processing for all ofthem. The milling process produces a uranium oxide concentrate which is thenshipped from the mill. This concentrate is often called ‘Yellowcake’ and inmost cases, contains over 80% uranium; whereas the original ore may contain aslittle as 0.1%.
The process carried out by amill is as follows: ore is ground/crushed into a powdery slurry which isleached in sulfuric acid or sometimes a strong alkaline. This allows the uraniumto be separated from the waste rock. It is then recovered from solution andprecipitated as uranium oxide (U3O8). After being driedand heated, the concentrate is packed into 200-litre drums. The remaining oreand rock which contains most of the radioactivity, is then placed in facilitiesnear the mine which is often a mined-out pit. This remaining rock and ore iscalled tailings, which must be isolated from the surrounding environment due totheir radioactivity. However, these remaining materials have less radiationthan the original ore and will be active for much less time.An example of a mill couldbe the Church Rock uranium mill in New Mexico.
This mill is most famous for itsspill which occurred on 16th July 1979 when a disposal pond breachedits dam. Mill tailings of around 1,000 tons of waste and more than 93milliongallons of acidic and radioactive solution flowed into the Puerco River andtravelled approximately 80miles downstream. Conversion and Enrichment: The uranium oxide from the mill is not yet ready to beused as fuel and must undergo more processing.
Only 0.7% of natural uranium isfissile. The isotope of uranium which is fissile is uranium-235. Thisconcentration of this isotope needs to be increased for most reactors,typically to 3.5%-5% U-235.
The separation of isotopesis a physical process where an isotope is concentrated/enriched relative toothers. The process requires uranium to be gaseous, so the uranium oxide isfirstly made into uranium hexafluoride.At conversion facilities,the uranium oxide is first refined to uranium dioxide.
The uranium dioxide canbe used as fuel for the reactors which font need enriched uranium. Most is thenmade into uranium hexafluoride and drained into 14-tonne cylinders to be takento the enrichment facility.Enrichment separated uraniumhexafluoride into two streams. One is enriched to a required level and calledlow-enriched.
The other is depleted in U-235 and can be called tails, or justdepleted uranium. Fuel Fabrication: Fuelfor reactors is mainly in the form of ceramic pellets. These are made frompressed uranium oxide (UO2) being sintered at temperatures over 1400°C. These pellets are then encased in metal tubes which forms fuel rods.Fuel rods are then arranged into a fuel assembly ready to be used in a reactor.The shape, size and characteristics of fuel pellets are measured very preciselyto ensure consistency of the fuel.A 1000MWe reactor uses a yearly amount of around 27 tonnes ofenriched fuel.
An example of a fuel fabrication site in industry could bethe Sellafield MOX plant which produced fuel consisting of plutonium andnatural/depleted uranium which behaves similarly to low enriched uranium andcan be used as an alternative. MOX also provides the use of excess weaponsgrade plutonium to generate electricity. Power Generation: In the core of a reactor, the U-235isotope fissions/splits and produces lots of heat from a continuous processwhich is called a chain reaction. Some of the U-238 in the core of the reactoris turned into plutonium, with half of it also undergoing fission, whichprovides around one-third of the energy output from the reactor.This heat is then used to drive a turbine and in turn anelectric generator.
A 1000MWe unit can provide over 8TWh of electricity in oneyear.One tonne of natural uranium will typically produce 44millionkilowatt-hours of electricity. To reach this same amount using fossil fuelswould require burning over 20,000 tonnes of coal or 8.5million cubic metres ofgas.the largest nuclear plant in the world is the Kashiwazaki-KariwaNuclear Power Plant in Japan. It is owned by Tokyo Electric Power Company, andis situated on a 4.
2 square-kilometre site on the coast of The Sea of Japan;from which it gets cooling water. Allits seven reactors currently use low-enriched uranium as fuel but there havebeen plans made for some reactors to use MOX fuel by the permission of theJapanese Atomic Energy Commission. Used Fuel: After 18-36 months used fuel is removedfrom the reactor (though still having potential) as it is no longer practicalto use due to the increase of fission fragments and heavy elements formed. Usedfuel usually contains around 1.0% U-235 and 0.
6% fissile plutonium. The fuel isstill emitting radiation and heat when it is removed from the reactor so it isput into a storage pond which allows the heat and radiation emission todecrease. The fuel can be kept in these pools for months or even years, howeverafter around 5 years, it can be taken to naturally-ventilated dry storage.Finally, the fuel must either be reprocessed to recover/recycle what is stillusable, or long-term storage and disposal without being reprocessed. The longerthe fuel is stored for, the easier it is to handle because of the decrease inradioactivity. Reprocessing: The used fuel can still contain around96% of the original uranium, less than 1% of this is the fissionable U-235.
Close to 3% of the fuel is made up of waste product and the final 1% isplutonium.During reprocessing, uranium and plutonium is separated fromthe waste products (including fuel assembly cladding) by chopping up the fuelrods and dissolving them in acid so the various materials can separate. Thisenables the uranium and plutonium to be recycled into fresh fuel. It alsoproduces a reduced amount of waste in comparison to treating all the used fuelas waste.
The 3% of radioactive waste that is left can be stored in liquid formand then solidified.THORP is an example of a nuclear reprocessing site. After therods are chopped and dissolved in nitric acid, it is chemically conditioned andthen passed to the separation plant. The wastes are treated and stored on theplant and the uranium can then be made available for customers to manufacture itinto new fuel. Wastes: Nuclear waste is categorized as high,medium, or low-level, these categories correspond to the amount of radiation beingemitted.The liquid high-level waste can be heated strongly(calcined), which produces a dry powder that is mixed into Pyrex glass whichimmobilises it.
This glass is poured into canisters made from stainless steel,with each canister holding 400kg of glass. The canisters are then able to bemoved and stored with appropriate shielding. Final Disposal: Currently, there are no dedicated’disposal’ facilities for nuclear waste, only storage. There are a few reasonsfor this, one being that although the actual technicalities of disposal are straightforward,there is no real demand for disposal facilities to be established. This isbecause the waste that is presently stored is a rather small amount. Anotherreason is that the longer the waste is stored the easier it is to be handled inthe future because of the decrease in radioactivity over time.There is also the fact that fuel that is being stored couldstill act as an energy resource if reprocessed at later dates, enabling therecycling of uranium and plutonium.
Despite there being no pressing requirement for disposalfacilities, multiple countries are studying the what would be the best approachto disposing of used fuel. Currently the consensus is for the waste to beplaced into deep repositories (around 500m underground), which will be recoverablefor a time, later to be sealed permanently.
