Uranium: surface disturbance and the amount of

Uranium: uranium
is a slightly radioactive metal which is present in the Earth’s crust. It is in
most rocks and soils as well as rivers and sea water. It is about as common as
tin 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 the
concentration of uranium in the ground is high enough that it is economically
reasonable to be extracted for use as nuclear fuel. Concentrations of this size
are called uranium ore.

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Uranium Mining: The
two main ways to mine uranium ore are excavation and in situ techniques.

Excavation can be both
underground or open pit, depending on how deep the ore is. If the uranium is
typically deeper than 120m, underground excavation will be used.

Open pit mines need holes on
the surface larger than the deposit of ore being mined, this is because the
walls of the pit must be sloped to prevent collapse. Underground mines cause a
smaller surface disturbance and the amount of material needed to access the ore
is considerably less than that of an open pit. However, some special
requirements, mainly more ventilation, is needed in an underground mine to
lower the risk of airborne radiation exposure.

More and more uranium is now
being mined the in situ leaching (ISL). This is where oxygenated groundwater is
circulated through a very porous body of ore to dissolve the uranium oxide and
bring it to the surface. ISL may be done with acidic or alkaline solutions to
keep the uranium in solution. Then the uranium oxide is recovered the same as
conventional mills.

An industry example of a uranium
mining company could be Kazatomprom, which had the leading uranium output
production of 2016, producing 12,986 metric tons with a net income of
$326million. The company uses the in situ method. The company has 54 deposits
based in Kazakhstan; of which 16 are currently in use, with 38 on standby.

 

Uranium Milling: Is
a process usually carried out close to a uranium mine, which extracts the
uranium from the ore or ISL Leachate. Most mines have a mill, however if there
are several mines close together, one mill may do the processing for all of
them. The milling process produces a uranium oxide concentrate which is then
shipped from the mill. This concentrate is often called ‘Yellowcake’ and in
most cases, contains over 80% uranium; whereas the original ore may contain as
little as 0.1%.

The process carried out by a
mill is as follows: ore is ground/crushed into a powdery slurry which is
leached in sulfuric acid or sometimes a strong alkaline. This allows the uranium
to be separated from the waste rock. It is then recovered from solution and
precipitated as uranium oxide (U3O8). After being dried
and heated, the concentrate is packed into 200-litre drums. The remaining ore
and rock which contains most of the radioactivity, is then placed in facilities
near the mine which is often a mined-out pit. This remaining rock and ore is
called tailings, which must be isolated from the surrounding environment due to
their radioactivity. However, these remaining materials have less radiation
than the original ore and will be active for much less time.

An example of a mill could
be the Church Rock uranium mill in New Mexico. This mill is most famous for its
spill which occurred on 16th July 1979 when a disposal pond breached
its dam. Mill tailings of around 1,000 tons of waste and more than 93million
gallons of acidic and radioactive solution flowed into the Puerco River and
travelled approximately 80miles downstream.

 

Conversion and Enrichment: The uranium oxide from the mill is not yet ready to be
used as fuel and must undergo more processing. Only 0.7% of natural uranium is
fissile. The isotope of uranium which is fissile is uranium-235. This
concentration of this isotope needs to be increased for most reactors,
typically to 3.5%-5% U-235.

The separation of isotopes
is a physical process where an isotope is concentrated/enriched relative to
others. The process requires uranium to be gaseous, so the uranium oxide is
firstly made into uranium hexafluoride.

At conversion facilities,
the uranium oxide is first refined to uranium dioxide. The uranium dioxide can
be used as fuel for the reactors which font need enriched uranium. Most is then
made into uranium hexafluoride and drained into 14-tonne cylinders to be taken
to the enrichment facility.

Enrichment separated uranium
hexafluoride into two streams. One is enriched to a required level and called
low-enriched. The other is depleted in U-235 and can be called tails, or just
depleted uranium.

 

Fuel Fabrication: Fuel
for reactors is mainly in the form of ceramic pellets. These are made from
pressed 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 precisely
to ensure consistency of the fuel.

A 1000MWe reactor uses a yearly amount of around 27 tonnes of
enriched fuel.

An example of a fuel fabrication site in industry could be
the Sellafield MOX plant which produced fuel consisting of plutonium and
natural/depleted uranium which behaves similarly to low enriched uranium and
can be used as an alternative. MOX also provides the use of excess weapons
grade plutonium to generate electricity.

 

Power Generation: In the core of a reactor, the U-235
isotope fissions/splits and produces lots of heat from a continuous process
which is called a chain reaction. Some of the U-238 in the core of the reactor
is turned into plutonium, with half of it also undergoing fission, which
provides around one-third of the energy output from the reactor.

This heat is then used to drive a turbine and in turn an
electric generator. A 1000MWe unit can provide over 8TWh of electricity in one
year.

One tonne of natural uranium will typically produce 44million
kilowatt-hours of electricity. To reach this same amount using fossil fuels
would require burning over 20,000 tonnes of coal or 8.5million cubic metres of
gas.

the largest nuclear plant in the world is the Kashiwazaki-Kariwa
Nuclear Power Plant in Japan. It is owned by Tokyo Electric Power Company, and
is situated on a 4.2 square-kilometre site on the coast of The Sea of Japan;
from which it gets cooling water.  All
its seven reactors currently use low-enriched uranium as fuel but there have
been plans made for some reactors to use MOX fuel by the permission of the
Japanese Atomic Energy Commission.

Used Fuel: After 18-36 months used fuel is removed
from the reactor (though still having potential) as it is no longer practical
to use due to the increase of fission fragments and heavy elements formed. Used
fuel usually contains around 1.0% U-235 and 0.6% fissile plutonium. The fuel is
still emitting radiation and heat when it is removed from the reactor so it is
put into a storage pond which allows the heat and radiation emission to
decrease. The fuel can be kept in these pools for months or even years, however
after around 5 years, it can be taken to naturally-ventilated dry storage.
Finally, the fuel must either be reprocessed to recover/recycle what is still
usable, or long-term storage and disposal without being reprocessed. The longer
the fuel is stored for, the easier it is to handle because of the decrease in
radioactivity.

 

Reprocessing: The used fuel can still contain around
96% 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% is
plutonium.

During reprocessing, uranium and plutonium is separated from
the waste products (including fuel assembly cladding) by chopping up the fuel
rods and dissolving them in acid so the various materials can separate. This
enables the uranium and plutonium to be recycled into fresh fuel. It also
produces a reduced amount of waste in comparison to treating all the used fuel
as waste. The 3% of radioactive waste that is left can be stored in liquid form
and then solidified.

THORP is an example of a nuclear reprocessing site. After the
rods are chopped and dissolved in nitric acid, it is chemically conditioned and
then passed to the separation plant. The wastes are treated and stored on the
plant and the uranium can then be made available for customers to manufacture it
into new fuel.

 

Wastes: Nuclear waste is categorized as high,
medium, or low-level, these categories correspond to the amount of radiation being
emitted.

The liquid high-level waste can be heated strongly
(calcined), which produces a dry powder that is mixed into Pyrex glass which
immobilises it. This glass is poured into canisters made from stainless steel,
with each canister holding 400kg of glass. The canisters are then able to be
moved and stored with appropriate shielding.

 

Final Disposal: Currently, there are no dedicated
‘disposal’ facilities for nuclear waste, only storage. There are a few reasons
for this, one being that although the actual technicalities of disposal are straightforward,
there is no real demand for disposal facilities to be established. This is
because the waste that is presently stored is a rather small amount. Another
reason is that the longer the waste is stored the easier it is to be handled in
the future because of the decrease in radioactivity over time.

There is also the fact that fuel that is being stored could
still act as an energy resource if reprocessed at later dates, enabling the
recycling of uranium and plutonium.

Despite there being no pressing requirement for disposal
facilities, multiple countries are studying the what would be the best approach
to disposing of used fuel. Currently the consensus is for the waste to be
placed into deep repositories (around 500m underground), which will be recoverable
for a time, later to be sealed permanently.