Answer the following questions. Uranium is the basic raw material of both civilian and military nuclear programs
UNIT 7
NUCLEAR FUEL CYCLE
Activity 1
Mining uranium
Uranium is the basic raw material of both civilian and military nuclear programs.
It is extracted from either open-cast pits or by underground mining. Although uranium occurs naturally all over the world, only a small fraction is found in concentrated ores.
When certain atoms of uranium are split in a chain reaction, energy is released. This process is called nuclear fission.
In a nuclear power station this fission occurs slowly, while in a nuclear weapon, very rapidly. In both instances, fission must be very carefully controlled.
Nuclear fission works best if isotopes - atoms with the same number of protons, but different numbers of neutrons - of uranium 235 (or plutonium 239) are used. These isotopes have almost identical chemical properties, but different nuclear properties. Uranium-235 is known as a "fissile isotope" because of its propensity to split in a chain reaction, releasing energy in the form of heat.
When a U-235 atom splits, it emits two or three neutrons. When other U-235 atoms are present, these neutrons collide with them causing the other atoms to split, producing more neutrons.
A nuclear reaction will only take place if there are enough u-235 atoms present to allow this process to continue as a self-sustaining chain reaction. This requirement is known as "critical mass".
However, every 1,000 atoms of naturally-occurring uranium contain only seven atoms of U-235, with the remaining 993 being denser U-238.
Activity 2
Explain the meaning of the following notions:
Open-cast pit, mining, fraction, ore, split, fission, isotope, collide, cause, critical mass
Conversion
Once extracted, uranium ore is taken to a mill to be crushed and ground into a fine powder. This is then purified in a chemical process and reconstituted in a solid form known as "yellow cake", due to its yellow coloring. Yellow cake consists of 60-70% uranium, and is radioactive.
The basic aim of nuclear scientists is to increase the amount of U-235 atoms, a process known as enrichment. To do this, the yellow cake is dissolved in nitric acid and chemically processed before being heated to become uranium hexafluoride gas.
Uranium hexafluoride is corrosive and reactive and must be handled very carefully. Pipes and pumps at conversion plants are specially constructed from aluminium and nickel alloys. The gas is also kept away from oil and grease lubricants to avoid any inadvertent chemical reactions.
Enrichment
The aim of enrichment is to increase the proportion of fissile uranium-235 atoms within uranium.
For uranium to work in a nuclear reactor it must be enriched to contain 2-3% uranium-235. Weapons-grade uranium must contain 90% or more u-235.
A common enrichment method is a gas centrifuge, where uranium hexafluoride gas is spun in a cylindrical chamber at high speeds. This causes the slightly denser isotope u-238 to separate from the lighter u-235.
The dense u-238 is drawn towards the bottom of the chamber and extracted; the lighter u-235 clusters near the centre and is collected.
The enriched u-235 is then fed into another centrifuge. The process is repeated many times through a chain of centrifuges known as a cascade.
The remaining uranium - essentially u-238 with all the u-235 removed - is known as depleted uranium. Depleted uranium, a heavy and slightly radioactive metal, is used as a component in armor-piercing shells and other munitions.
Another method of enrichment is known as diffusion.
This works on the principle that of the two isotopes present in uranium, hexafluoride gas, u-235 will diffuse more rapidly through a porous barrier than its heavier cousin, u-238.
As with the centrifuge method, this process must be repeated many times.
Answer the following questions.
What is “yellow cake”?
What is the main purpose of nuclear scientists?
What is enrichment?
What are the methods of enrichment?
Reactor
Nuclear reactors work on the principle that nuclear fission releases heat, which can be harnessed and used to heat water into steam to drive turbines.
A typical nuclear reactor uses enriched uranium in the form of fuel 'pellets', each roughly the size of a coin and about an inch long. The pellets are formed into long rods known as bundles, and housed inside a heavily insulated, pressurized chamber.
In many power stations, the bundles are submerged in water to keep them cool. Other types use carbon dioxide or liquid metal to cool the reactor core.
To function in a reactor – i.e. produce heat through a fissile reaction - the uranium core must be 'critical'. This means that the uranium must be in sufficiently enriched form to allow a self-sustaining chain reaction to occur.
To regulate this process, and allow the nuclear plant to function, control rods are inserted into the reactor chamber. The rods are made of a substance, typically cadmium, which absorbs neutrons inside the reactor.
Fewer neutrons mean fewer chain reactions are started, slowing down the fission process. There are more than 400 nuclear power stations across the globe, producing about 17% of the world's electricity. Nuclear reactors are also used to power submarines and naval vessels.
Activity 3