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

 Page 5 / 26

## 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:

Le chatelier's principle
how many isotopes does hydrogen have
3 isotopes
FLIMZEE
3 isotopes
FLIMZEE
an lmportant medical use of nuclear radiation is
the minimum amount of energy required for effective collosion btw reacting particle is known as
Faith
the minimum amount of energy required for effective collosion btw reacting particle is known as
Faith
hi
Best
hmmmmm
Aniebiet
the minimum amount of energy required for effective collosion btw reacting particle is known as
Faith
Hey
Faith
hi
Nag
how do I get MCQs and essay to work?
I want spectroscopy
Nipun
Check on play store Maybe you'll get an app for that
Idrissa
what is electrolysis
state the periodic law
the modern periodic state dat element are arranged in row and column according to their atomic number
lola
what is chemistry
chemistry is the brach of science which deal with composition and dicomposion of matter
ezekier
What are the branchs of chemistry
Blessing
What is matter
matter is anything that has mass or weight and accopies space
ezekier
what is endothermic
something that absorbs some form energy
mohammed
What is gas law
law is the rule of government
Dawite
gas law I have no idea
Israel
There are different Gas Laws There's boyles law, Charles law... Etc
Kanji
can you be more specific
Allyson
wat is hydroxyl
James
why is borontrihydride considered a Lewis acid
electronic configuration
What is chemistry
it's is a branch of science that deals with the nature and composition of various matters and how the under go changes
Kelly
Pls what is the structural formula for propanonitrile
chemistry is the study of the interaction, structure and properties of matter
Olaiya
If a man has a mass of 115 pounds,what is his mass in gram lb=453.6g?
Henok
chemistry laboratory apparatus
Burette,pipette,Bunsen burner,test tube
Olaiya