Why is U235 used for chain reaction but not U238

[T.Duncan]

i want to ask why only U235 is used for the chain reaction but not used U238

 

[Astronuc]

U²³⁵ is considered 'fissile', which means the nucleus is readily or more easily fissioned upon the absorption of a neutron, and it tends to more readily fission with neutrons of low energy (< 1 ev). U²³⁸ is considered 'fissionable', which means it may fission but that it requires an energetic neutron in order to fission - generally several MeV.

The other fissile nuclei also have odd masses, U²³³, Pu²³⁹ and Pu²⁴¹, as well as some heavier nuclei.

The stability, or rather instability, to fission has to do with the binding energy of the excited nuclear once the neutron is absorbed. For example, the binding energy (excess energy) of the last neutron in U²³⁶ is approximately 6.4 MeV, but the critical energy for fission is only 5.3 MeV, so when U²³⁵ absorbs a neutron, the resulting U²³⁶ nucleus has an excess of 1.1 MeV above the energy needed to cause fission.

U²³⁸ will absorb a neutron to form U²³⁹, which will eventually decay by beta emission to Np²³⁹, and this nuclide then subsequently decays by beta emission to Pu²³⁹, which is fissile. This is the process by which Pu²³⁹ is produced.

 

[T.Duncan]

U235 can decay after absorbing a very low energy but U238 needs a large energy to decay, so that a chain reaction used U235, right??

other questions, why U238 needs a large energy to decay but U235 only needs < 1eV. The stability of U235 is lower than U238 ?? or other reasons ~?
and a whole chain reaction only used U235? any other isotopes or elements and how can they decay?(absorbing the energy that produced by U235?)

 

[Astronuc]

U235 can decay after absorbing a very low energy but U238 needs a large energy to decay, so that a chain reaction used U235, right??

other questions, why U238 needs a large energy to decay but U235 only needs < 1eV. The stability of U235 is lower than U238 ?? or other reasons ~?
and a whole chain reaction only used U235? any other isotopes or elements and how can they decay?(absorbing the energy that produced by U235?)

Firstly, let's distinguish between 'fissioning' and 'decay'.

Fissioning implies the atom 'splits' or 'fissions' into two nuclei (fission products). Decay implies the emission of a particle, e.g. beta or alpha particles, or gamma-ray.

When a nucleus of U²³⁵ absorbs a neutron and becomes U²³⁶ in an 'excited' energy state, it will most likely fission, but there is a chance that it will decay by gamma emission to U²³⁶ (relatively stable). The stabilities of U²³⁵ (t_1/2 = 7.038E+8Y) and U²³⁶ (t_1/2 = 2.34E+7Y) are less than U²³⁸ (t_1/2 = 4.468E+9Y).

See - http://wwwndc.tokai.jaeri.go.jp/CN04/CN024.html for properties of U isotopes.

The binding energy of the last neutron in U²³⁹ is ~4.9 MeV vs a critical energy for fission of ~5.5 MeV. So, U²³⁹ is more likely to spontaneous emit a gamma ray to dump the extra energy. Then at some point, the nucleus emits a beta particle to become Np²³⁹.

A chain reaction is sustained as long as the rate of neutrons absorbed by fissile species, e.g. U²³⁵, and causing fission is constant. In a nuclear reactor, some of the U²³⁸ will be converted to Pu²³⁹ and that will also contribute to the fission process. This is also the purpose of a breeder reactor - which converts more fertile material, i.e. U²³⁸ to Pu²³⁹ than it consumes.

 

[T.Duncan]

1) is the fissioned U235 caused the U238 to fission in a chain reaction (since i know that the % of U238 is higher than that of U235)?

2) why can U235 fission by low energy, U238 need a large energy to fission. is it relative to the critical energy for fission?

3)In a chain reaction, why not only used U235?

4) what are nuclear reactor and breeder reactor?

thanks for ur answering !

 

[Nomy-the wanderer]

2) why can U235 fission by low energy, U238 need a large energy to fission. is it relative to the critical energy for fission?

Odd numbered heavy nuclei, are naturally fissionable u don't need a lot of energy to split them, such as U235 or Pu 239, this has something to do with the repulsive and binding forces inside the nucleus. Usually natural uranium contain 0.7% only of U235 which is fissile, and the rest of U238.And that's why some times u need the enrichment process...
U-238 can capture a neutron and decay to Pu239 which is also fissile, but u need some more energy to do that.

3)In a chain reaction, why not only used U235?

It's a very hard chemical procedure to separate isotopes...

4) what are nuclear reactor and breeder reactor?

There are fast breeder reactors and thermal reactors, fast reactors use fast neutrons (energetic), and thermal reactors operate with thermal (slow) neutrons...

Thermal reactors also use moderators, which are intended to slow the neutrons until they approach the average kinetic energy of the surrounding particles. Thermal neutrons have a far higher probability of fissioning U-235, and a lower probability of capture by U-238 than the faster neutrons that result from fission do.
Examples can be Candu heavy water reactor, PWR(pressurized), LWR(light), BWR(boiling)..etc. Naval reactors can be PWRs.

For fast breeders there's hardly any need for moderators, u only need highly enriched uranium (sometimes weapons-grade), or plutonium in order to reduce the amount of U-238 that would otherwise capture fast neutrons. Some are capable of producing more fuel than they consume, usually by converting U-238 to Pu-239.
An example is sodium cooled reactor, and generally LMFBRs (liquid metal fast breeder reactors...

I hope this has answered your question, i didn't try to get into much details...

 

[Nomy-the wanderer]

I think there's a question I've missed...

1) is the fissioned U235 caused the U238 to fission in a chain reaction (since i know that the % of U238 is higher than that of U235)?

I'm not sure if i quite understand your question, but U-238 is fertile, if it absorbs a neutron with the right energy it decays to U239>Np239>Pu239(beta decay), the last product is naturally fissile...

Again fertile means that it can produce fissile nuclides under neutron irradiation, but it's not fissionable itself...

 

[Nomy-the wanderer]

Oh and fissile means, that the nuclide experience fission process by capturing a neutron...

Fission means the product of 2 nuclei from one heavier nucleus + energy of around 200Mev and a number of neutrons...

 

[Astronuc]

Please read my previous posts which address some of the content of the questions herein.

1) is the fissioned U235 caused the U238 to fission in a chain reaction (since i know that the % of U238 is higher than that of U235)?

Some U-238 will fission in a neutron flux, but the probability is very low. As Nomy-the wanderer indicated, the odd nuclei (mentioned in one of my previous posts) are more readily fissionable (i.e. they are fissile) because the addition of a neutron adds energy in excess of what is required to fission the resulting nucleus.

Most (>99%) of fissioning occurs in U-235 (or Pu-239) in a uranium fueled reactor. The U-238 absorbs neutrons and eventually becomes isotopes of Pu (239, 240, 241, 242), Am (241, 242m, 243) and Cm (242, 243, 244, 245) depending on time and neutron spectrum.

2) why can U235 fission by low energy, U238 need a large energy to fission. is it relative to the critical energy for fission?

U-236, with an excess of energy is much less stable than U-239. U-238 can fission, but generally requires neutrons of ~ 5 MeV or higher. Neutrons released from fissioning will have energies on the order of several MeV.

I would like to address this question in the Nuclear Engineering forum since it seems to come up often.

3)In a chain reaction, why not only used U235?

Well in some nuclear warheads, nearly pure U-235 was used. Modern nuclear warheads use Pu-239. Highly enriched U-235 or Pu-239 would be difficult to control in a reactor configuration.

Uranium which contains about 0.71% U-235 and 99.2% U-238 (with traces of U-234) can be enriched to increase the concentration in U-235. Modern commercial plants used fuel with up to 5% U-235 and ~95% U-238 in the form of ceramic UO₂ which is encapsulated in a metal tube of a zirconium alloy sealed at both ends.

The cost of enriched U increases with enrichment, so utilities use as low an enrichment as possible. On the physics side, it is a matter of distributing the thermal energy in a reactor as well as maintaing safety margins which dictates the enrichments used.

During the course of operation, fission products accumulate in the fuel, which perodically must be removed from the reactor, and fresh (unirradiated) fuel added.


Fast reactors, which use fast neutrons (keV-MeV energy range), tend to use fuel with about 20% Pu-239 dispersed in U-238, also in the form of a 'mixed' oxide (PuO₂-UO₂). The cladding is usually stainless steel, e.g. SS316L (as well as other special stainless steels). Special breeder assemblies are used to produce extra Pu-239. Rather than water in the core, fast reactors typically use liquid metal, particularly sodium.

4) what are nuclear reactor and breeder reactor?

A nuclear reactor is a device which enables the controlled fissioning of a fissile material (e.g. U-233, U-235 or Pu-239) to produce thermal energy. The thermal energy is then converted to mechanical energy (usually via a steam turbine) which is then transformed (via a generator) into electrical energy.

The vast majority of commercial nuclear power plants are light water reactors (LWRs) of which there are Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs). Some reasonably good descriptions of nuclear reactors can be found here - http://en.wikipedia.org/wiki/Nuclear_reactor - but I must caution the reader that there are some inaccuracies in the Wikipedia article.

Specifically -

"They [fast reactors] require highly enriched fuel (sometimes weapons-grade), or plutonium in order to reduce the amount of U-238 that would otherwise capture fast neutrons. "

This claim is misleading since weapons-grade (WG) can mean highly enriched (> 70% U-235 or Pu-239) as well as the Pu isotopic vector. In fast reactors, as I pointed out above, the enrichment is typically about 20% fissile (usually Pu-239). If natural U is used as the diluant, then the fuel will also have 0.71% U-235 in the matrix with U-238 being the balance.

The CANDU is a special type of pressurized water reactor which uses natural or slightly enriched (typ. < 1.5% enriched) U and heavy water D₂O rather than light water H₂O in the core as moderator.

 

[Nomy-the wanderer]

But Astronuc, my professor once said that they used to use weapons-grade (70 ~ 80% enrichment) fuel, before they decided- for safety reasons of course- to limit the enrichment to 20% only...

So yes i think it's not a claim, it's a fact. Just doesn't work anymore.

 

[Astronuc]

Certain research reactors and small cores such at the TRIGA reactor have used fuel with 70% enrichment, but the Uranium dispersed in a U-Zr-H matrix, and they had relatively high leakage, and a different negative Doppler coefficient. To my knowledge, commercial cores never used 70% enrichment.

Even fast reactors do not use, nor do they need fuel with 70% enrichment. MOX fuel with about 20% is satisfactory and essentially standard.

Again, it depends on what is meant when the term WG is used. Fast reactors generally use fuel with Pu from reprocessed reactor fuel (or so-called reactor grade (RG) Pu).

I believe some fuel made for graphite moderated gas reactors was enriched up to 90-93%, but again it was dispersed in a matrix. Such high enrichments are used when the physical density of the fuel is relatively low compared to 100% TD, and when the fuel is 'dispersed' in an inert matrix.

So in a sense, some fuel may be derived from WG U or Pu, but it is inaccurate to say or imply that a reactor requires WG fuel.

 

[Nomy-the wanderer]

The fuel derived from weapon grade plutonium gets reprocessed and used as a mox fuel, That's a different matter. i'll ask my rpofessor again for more details about what he was talking about, he mentioned a type of reactor that used 70 ~ 80% (or weapon grade) uranium and not plutonium...But i forgot what it was.

 

[Astronuc]

Certainly 90% enriched U is considered WG, but IIRC the cut off is about 70%. HEU is considered 20% or greater.

There is a discussion here - http://www.uic.com.au/nip04.htm

But it seems different groups or organizations use the term somewhat loosely.

 

[Nomy-the wanderer]

That was a great link, it's a usefull site. thanks Astronuc.

 

[Astronuc]

Another useful site is the World Nuclear Association
http://www.world-nuclear.org/

Here is a paper on the US-Russian Agreement on HEU
http://world-nuclear.org/trade_issues/tbriefings/heu/index.htm

On Weapons Grade material - see

http://world-nuclear.org/education/ne/ne7.htm

http://world-nuclear.org/uiabs96/haas.htm

Various forms of U and Pu and fuel cycle information.
http://world-nuclear.org/info/inf14.htm

Military Warheads as a Source of Nuclear Fuel - October 2005
http://world-nuclear.org/info/inf13.htm

 

[T.Duncan]

1)Would you give me more examples on 'fissile' and 'fissionable' elements? i want to know more ~

2)and why we use U but not other 'fissile' elements in a chain reaction?

 

[Nomy-the wanderer]

1)Would you give me more examples on 'fissile' and 'fissionable' elements? i want to know more ~

For example thorium-232 is fertile if bombarded with neutrons, it'll decay several times to form Uranium that is fissionable material.

Uranium 235 is fissile, when capturing a thermal(i.e slow) will experience fission and will produce 2 fission fragments and neutrons...
The type of these fragments depends on the surrousing conditions.

2)and why we use U but not other 'fissile' elements in a chain reaction?

This is a matter of properties, for example plutonium can be used, also thorium, but it's all connected to irradiation behaviour, strength, required mass, economics, availability, related reactor materials such as coolants, moderators, cladding...Also their state(solid, liquid), form (powder, metal, ceramic?).

You know why they developped a plutonium bomb instea dof a uranium one, cause this would make its size smaller, cause u basically need only 1/2 of the mass(when using uranium) to reach criticality.

So it depends on what kind of properties you are looking for, the application, costs...etc

 

[Astronuc]

1)Would you give me more examples on 'fissile' and 'fissionable' elements? i want to know more ~

2)and why we use U but not other 'fissile' elements in a chain reaction?

Nomy-the wanderer gave some examples fissile and fissonable, and I believe I mentioned others in previous posts, as did others.

See this thread - https://www.physicsforums.com/showthread.php?t=107527 - in our Nuclear Engineering section.

The reason uranium is used is that 1) it occurs in Nature, and 2) uranium ore has a much higher fraction of a fissile isotope (U-235) than thorium ore (U-233).

The main disadvantage of the thorium cycle is the lack of U-233 in nature, which necessitates the use of a fissile material such as U-235 or Pu-239 to prime the thorium fuel.

from the publication cited below.

On thorium, see technical publications at - http://www.thoriumpower.com/english/technologies/tech_publications.htm

particularly - "On the Use of Thorium in Light Water Reactors" - M.S. Kazimi, K.R. Czerwinski, M.J. Driscoll, P.Hejzlar and J.E. Meyer -- Massachusetts Institute of Technology (April 1999)

One can download pdf files. Just right click and use "save target as".

 

[skywolf]

is 235 easyer to fission because there's less neutrons to keep the protons from expelling each other?
or is that only a small part of a larger picture I am missing?

 

[Grogs]

is 235 easyer to fission because there's less neutrons to keep the protons from expelling each other?
or is that only a small part of a larger picture I am missing?

I think it has more to do with the odd number of neutrons in U-235 than the total number. Atoms with an even number of protons and/or neutrons tend to be more stable than those with odd numbers. A U-239 atom, for example, has about a 40% chance of fissioning if it captures a thermal neutron, versus a small fraction of a percent for U-238. All of our common fissionable isotopes (U-233, 235; Pu-239, 241) are even-Z, odd-N configurations.