Wednesday, January 27, 2016
CHEM 418 Nuclear Chemistry, Winter 2016: Lecture 7 Fission
A general overview of nuclear fission is presented. The probability of fission is described based on developed models including the liquid drop model and shell corrections. Discussion on spontaneous fission and fissioning isomers is given. The transition nucleus and fission product distributions are discussed. The total kinetic energy, mass distribution, and charge distribution during fission are presented. Changes in fission product distribution with parent properties are introduced. Delayed neutrons from fission and their role in reactors are given. Proton induced fission is introduced.
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I sent the lecture 7 pdf quiz as an attachment through email.
ReplyDeleteI have the question on the slide 23. You mentioned the calculation showing why 239Pu can be fissioned by thermal neutrons but not 240Pu is through a equation on lecture 4? What equation is that?
Thank you very much
this is the comparison of Q values for the reaction, which is in lecture 2
ReplyDeleten+ (A)Pu--> (A+1)Pu
Consider the data for the reaction above
Pu isotope (A) Q value (MeV)
239 6.534
240 5.241
The fission barrier is generally between 5 and 6 MeV. The Q value for the 240Pu reaction does not overcome the fission barrier. You can do this reaction with a number of adjacent actinide isotopes
quiz sent through email. Thank you!
ReplyDeleteJust a small question on the meaning of thermal cross sections and its relationship to fissioning a nucleus.
ReplyDeleteIf I understand correctly, in order to induce fission in a nucleus. We shoot neutrons of defined energy at the nucleus of interest. Our intention is to have the neutrons penetrate into the nucleus disrupting/deforming the boundary and attractive forces of atom with the goal of breaking the atom into smaller nuclides.
So the likelihood of creating/starting a fissile process does somewhat depend on the bonding energy of the given nucleus, correct? The energy/effort required to fission an Fe nucleus would much greater than that required to fission a given Uranium nucleus.
Another factor I am trying to wrap my head around is the thermal cross section of the nucleus. In the event of small/tiny cross section, such as 238-U at 5 micro-barns, the probability of a neutron that is a member of a flux, making impact with this nucleus depends on the magnitude of the thermal cross section?
So is it correct to assume that the size of a thermal cross sections is a statement on the likelihood/probability of a given Neutron successfully hitting the nucleus? Like an average… So to fission 238-U we must use a high neutron flux to successfully make contact with the uranium nucleus. But if we compare a nucleus of much larger thermal cross section, say 237-Pu 2400 barns, we may use a lesser neutron flux for similar fissioning purposes (ignoring differences in bonding energy).
If I’m off base on any of these idea, please let me know.
Thanks
-Faruq
Submitted Quiz 7 via email
Deletequiz 7 submitted via email- Raghav
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ReplyDeleteI have emailed you PDF quiz 7.
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ReplyDeleteMeant to submit this comment last night; submitted me quiz via email yesterday.
ReplyDeleteWanted to ask, in the last question on the quiz, it looks like the cross-section for U-235 decreases with increasing neutron energies. Ie, the probability of fission decreases at higher energies. Is that the right interpretation? Intuitively, I would imagine the opposite would be true.
-Taylor
This is true, and is used to control reactors. If a undesirable situation occurs in a reactor the also increases neutron energy, then the reaction cross section decreases. This will prevent further reactions. Water is a coolant and moderator. So if the coolant is removed the neutrons don't slow down. The means the reaction cross section decreases with the fission reaction decreasing as a result.
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