# 21.4 Transmutation and nuclear energy  (Page 5/26)

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## Nuclear fuels

Nuclear fuel consists of a fissionable isotope, such as uranium-235, which must be present in sufficient quantity to provide a self-sustaining chain reaction. In the United States, uranium ores contain from 0.05–0.3% of the uranium oxide U 3 O 8 ; the uranium in the ore is about 99.3% nonfissionable U-238 with only 0.7% fissionable U-235. Nuclear reactors require a fuel with a higher concentration of U-235 than is found in nature; it is normally enriched to have about 5% of uranium mass as U-235. At this concentration, it is not possible to achieve the supercritical mass necessary for a nuclear explosion. Uranium can be enriched by gaseous diffusion (the only method currently used in the US), using a gas centrifuge, or by laser separation.

In the gaseous diffusion enrichment plant where U-235 fuel is prepared, UF 6 (uranium hexafluoride) gas at low pressure moves through barriers that have holes just barely large enough for UF 6 to pass through. The slightly lighter 235 UF 6 molecules diffuse through the barrier slightly faster than the heavier 238 UF 6 molecules. This process is repeated through hundreds of barriers, gradually increasing the concentration of 235 UF 6 to the level needed by the nuclear reactor. The basis for this process, Graham’s law, is described in the chapter on gases. The enriched UF 6 gas is collected, cooled until it solidifies, and then taken to a fabrication facility where it is made into fuel assemblies. Each fuel assembly consists of fuel rods that contain many thimble-sized, ceramic-encased, enriched uranium (usually UO 2 ) fuel pellets. Modern nuclear reactors may contain as many as 10 million fuel pellets. The amount of energy in each of these pellets is equal to that in almost a ton of coal or 150 gallons of oil.

## Nuclear moderators

Neutrons produced by nuclear reactions move too fast to cause fission (refer back to [link] ). They must first be slowed to be absorbed by the fuel and produce additional nuclear reactions. A nuclear moderator    is a substance that slows the neutrons to a speed that is low enough to cause fission. Early reactors used high-purity graphite as a moderator. Modern reactors in the US exclusively use heavy water $\left({{}_{1}^{2}\text{H}}_{2}\text{O}\right)$ or light water (ordinary H 2 O), whereas some reactors in other countries use other materials, such as carbon dioxide, beryllium, or graphite.

## Reactor coolants

A nuclear reactor coolant    is used to carry the heat produced by the fission reaction to an external boiler and turbine, where it is transformed into electricity. Two overlapping coolant loops are often used; this counteracts the transfer of radioactivity from the reactor to the primary coolant loop. All nuclear power plants in the US use water as a coolant. Other coolants include molten sodium, lead, a lead-bismuth mixture, or molten salts.

## Control rods

Nuclear reactors use control rods ( [link] ) to control the fission rate of the nuclear fuel by adjusting the number of slow neutrons present to keep the rate of the chain reaction at a safe level. Control rods are made of boron, cadmium, hafnium, or other elements that are able to absorb neutrons. Boron-10, for example, absorbs neutrons by a reaction that produces lithium-7 and alpha particles:

#### Questions & Answers

What is rightful definition of element
an element is the group of (vertical columns) of the periodic table exhibit similar chemical behaviour.
Barnabas
okay
angela
is alkanes a saturated hydrocarbon?
yup
it's saturated cos it has single bonds
yh....because they don't undergo additional reactions which hydrogen and other atoms can add across the carbon-carbon or triple bond
patience
and me...I'm I wrong?
patience
how does metal looses electron
By oxidation and reduction
hamidat
by oxidation loss
Official
An acid is a proton donor.
what is an acid
an acid is a substance when dissolved in water produces hydrogen ion or hydroxonium ion
hamidat
good
Mudassir
thanks
hamidat
is a substance which dissolves in water to produce hydrogen ions as the only positively charged ions
Kwagala
what is ionic bonding
It involves the transferring of electron from a metal to a non mental
hamidat
that's right
Edward
bonding between a metal and a non metal
miriam
calculate the hydrogen ion concentration of the solution when pH=5
no ideas
hamidat
What is thermodynamics
what is the meaning this word twentieth
dhu
Is the branch of physics that deal with heat and temperature and their relation to work, energy and properties of matter
Edward
There are no topics on hydrocarbons
I don't understand
hamidat
they are there please check under organic chemistry in the contents.
Kwagala
its not making sense to me I still don't understand
How and why
Betrice
yes
megan
we need diagram for easy going and understand
serah
How can we easily differentiate between the 5 gas laws
first amd foremist me i only know 3 gas laws, so please list them here
Kwagala
the gas laws i know include pressure law boyles law charles law i differentiate these with this formular big take classy pork pigs. viral i read thus as 1 big classy pigs take pork viral big.....take means in boyles law,temp is constant clasy.....prk mns in chrls lw, press cons
Kwagala
classy.....pork means in charles law pressure is constant pigs.....viral means, in pressure law volume is kept constsng ,the rest is the same boyles states that vol of a gas is inversely prop to volume keeping temp constant charles law, state vol of a gas is directly prop to temp keepn pressure cns
Kwagala
how many carbon is present in alkene
it's the carbon to carbon being double bonded to each other that makes it an alkene, not the amount of carbon itself. ex: C=C, C=C=C. both are alkenes.
Phill
I need more light on alkene
chidera
an alkene is a hydrocarbon, you can find this under the sub topic of organic chemistry in this textbook, check it out please,but before you know about alkenes you should first know about alkanes, alkanes are saturated and dnt easily react while alkenes have double bonds and can react .
Kwagala
other usefullness of hydrogen apart from this, it is colourless, odourless and tasteless
it is neutral to litmus paper, it is insoluble in water
hamidat
the enthalpy of a system
changing in heat of a system which can be lost or gained
Yussuf
is the energy change that occurs when molar quantities of reactants as stated in a chemical equation completely react
Kwagala
enthalpy of reaction is the energy change that occurs when molar quantities of reactants as stated in a chemical equation completely react
Kwagala
what is the unit of pressure
what is pressure measured in?
Tim
Pascal
Stupid
millimeter mercury ,mmHg or ATM
hamidat
it is atm...in lower case not upper
Kwagala