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The reason 235 U size 12{ {} rSup { size 8{"235"} } U} {} and 239 Pu size 12{ {} rSup { size 8{"239"} } ital "Pu"} {} are easier to fission than 238 U size 12{ {} rSup { size 8{"238"} } U} {} is that the nuclear force is more attractive for an even number of neutrons in a nucleus than for an odd number. Consider that 92 235 U 143 size 12{"" lSub { size 8{"92"} } lSup { size 8{"235"} } U rSub { size 8{"143"} } } {} has 143 neutrons, and 94 239 P 145 size 12{"" lSub { size 8{"94"} } lSup { size 8{"239"} } P rSub { size 8{"145"} } } {} has 145 neutrons, whereas 92 238 U 146 size 12{"" lSub { size 8{"92"} } lSup { size 8{"238"} } U rSub { size 8{"146"} } } {} has 146. When a neutron encounters a nucleus with an odd number of neutrons, the nuclear force is more attractive, because the additional neutron will make the number even. About 2-MeV more energy is deposited in the resulting nucleus than would be the case if the number of neutrons was already even. This extra energy produces greater deformation, making fission more likely. Thus, 235 U size 12{ {} rSup { size 8{"235"} } U} {} and 239 Pu size 12{ {} rSup { size 8{"239"} } ital "Pu"} {} are superior fission fuels. The isotope 235 U size 12{ {} rSup { size 8{"235"} } U} {} is only 0.72 % of natural uranium, while 238 U size 12{ {} rSup { size 8{"238"} } U} {} is 99.27%, and 239 Pu size 12{ {} rSup { size 8{"239"} } ital "Pu"} {} does not exist in nature. Australia has the largest deposits of uranium in the world, standing at 28% of the total. This is followed by Kazakhstan and Canada. The US has only 3% of global reserves.

Most fission reactors utilize 235 U size 12{ {} rSup { size 8{"235"} } U} {} , which is separated from 238 U size 12{ {} rSup { size 8{"238"} } U} {} at some expense. This is called enrichment. The most common separation method is gaseous diffusion of uranium hexafluoride ( UF 6 size 12{"UF" rSub { size 8{6} } } {} ) through membranes. Since 235 U size 12{ {} rSup { size 8{"235"} } U} {} has less mass than 238 U size 12{ {} rSup { size 8{"238"} } U} {} , its UF 6 size 12{"UF" rSub { size 8{6} } } {} molecules have higher average velocity at the same temperature and diffuse faster. Another interesting characteristic of 235 U size 12{ {} rSup { size 8{"235"} } U} {} is that it preferentially absorbs very slow moving neutrons (with energies a fraction of an eV), whereas fission reactions produce fast neutrons with energies in the order of an MeV. To make a self-sustained fission reactor with 235 U size 12{ {} rSup { size 8{"235"} } U} {} , it is thus necessary to slow down (“thermalize”) the neutrons. Water is very effective, since neutrons collide with protons in water molecules and lose energy. [link] shows a schematic of a reactor design, called the pressurized water reactor.

The figure shows a close-shielded vessel containing fuel rod and control rods along with a moderator in one chamber from which heat is taken out to the other chamber to change water to steam. Next, the steam is taken out from the vessel to run a turbine, and then it is condensed and sent back to the closed vessel.
A pressurized water reactor is cleverly designed to control the fission of large amounts of 235 U size 12{ {} rSup { size 8{"235"} } U} {} , while using the heat produced in the fission reaction to create steam for generating electrical energy. Control rods adjust neutron flux so that criticality is obtained, but not exceeded. In case the reactor overheats and boils the water away, the chain reaction terminates, because water is needed to thermalize the neutrons. This inherent safety feature can be overwhelmed in extreme circumstances.

Control rods containing nuclides that very strongly absorb neutrons are used to adjust neutron flux. To produce large power, reactors contain hundreds to thousands of critical masses, and the chain reaction easily becomes self-sustaining, a condition called criticality    . Neutron flux should be carefully regulated to avoid an exponential increase in fissions, a condition called supercriticality    . Control rods help prevent overheating, perhaps even a meltdown or explosive disassembly. The water that is used to thermalize neutrons, necessary to get them to induce fission in 235 U size 12{ {} rSup { size 8{"235"} } U} {} , and achieve criticality, provides a negative feedback for temperature increases. In case the reactor overheats and boils the water to steam or is breached, the absence of water kills the chain reaction. Considerable heat, however, can still be generated by the reactor’s radioactive fission products. Other safety features, thus, need to be incorporated in the event of a loss of coolant accident, including auxiliary cooling water and pumps.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Physics for the modern world. OpenStax CNX. Sep 16, 2015 Download for free at http://legacy.cnx.org/content/col11865/1.3
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