Critical/Threshold energy - Is this statement incorrect?

[Luchekv]

"Critical energy is the excitation energy level of the nucleus above which fission can occur.

• In fissile isotopes, like U-235, the critical energy is at or below the excitation of the nucleus when it has absorbed a neutron. So any absorbed neutron can cause fission in these nuclei, though they still have a capture probability (U-235 can absorb a neutron and become U-236 instead of fissioning).
• In fissionable and non-fissile isotopes, like U-238, critical energy is greater than the excitation of the nucleus when it has absorbed a neutron, so the neutron must bring additional energy to cause fission."

Was doing some research as to why U-235 is more suitable than U-238 and found the above information. Although I'm slightly confused by the second dot point.

From what I understand we need to achieve an energy greater than "critical energy" to achieve fission... then shouldn't fission be occurring according to this statement:

• In fissionable and non-fissile isotopes, like U-238, critical energy is greater than the excitation of the nucleus when it has absorbed a neutron, so the neutron must bring additional energy to cause fission."

Critical energy is greater than the nucleus excitation level after an impact with a neutron? Doesn't that mean fission occurs? Shouldn't it say it is less than the excitation value after absorbing a neutron? Meaning only high energy neutrons can fission 238 where as low energy neutrons would be sufficient for 235?

 

[drvrm]

Meaning only high energy neutrons can fission 238 where as low energy neutrons would be sufficient for 235?

actually fission is a dynamic process- a nucleus absorbs a neutron leading the oscillation in its shape and a neck formation of two nuclei the daughter ones - its a layman's picture.
the probability of a high energy neutrons to get trapped in a nucleus becomes low as its wavelength is smaller and it crosses the nucleus easily
whereas the thermal neutrons have good cross section.
a good fissile material is also determined from its neutron output which can help in sustaining the 'fission chain reaction'.
regarding criticality and excitation - the criticality of a reactor depends on the sustainable controlled chain reaction and excitation of individual nucleus is a theoretical projection-to be tested in sustaining a chain of events.
this is just my first input and will like to investigate/share further on the issue.thank you.

 

[drvrm]

Critical energy is greater than the nucleus excitation level after an impact with a neutron? Doesn't that mean fission occurs?

. visit the following source;
http://www.nuclear-power.net/nuclear-power/fission/critical-energy-threshold-energy-for-fission/
some of the relevant pieces have been given below;
QUOTE:

<The minimum excitation energy required for fission to occur is known as the critical energy (Ecrit) or threshold energy.The critical energy depends on the nuclear structure and is quite large for light nuclei with Z < 90. For heavier nuclei with Z > 90, the critical energy is about 4 to 6 MeV for A-even nuclei, and generally is much lower for A-odd nuclei. It must be noted, some heavy nuclei (eg. 240Pu or 252Cf) exhibit fission even in the ground state (without externally added excitation energy). This phenomena is known as the spontaneous fission.

The amount of excitation energy required for fission to occur can be estimated from the magnitude ofthe electrostatic potential barrier and the dissociation energy of the fission.

It can be seen that for fission of 238U or 232Th the neutron must have some additional kinetic energy (negative BEn – Ecrit value), while absorption of a neutron without kinetic energy can already cause fission of 235U (or 233U, 239Pu). For example, according to the table , the binding energy of the last neutron in 236U is 6.8 MeV (target nucleus is 235U), while the critical energy is only 6.5 MeV. Thus, when a thermal neutron is absorbed by 235U, the compound nucleus 236U is produced at about 0.3 MeV above the critical energy and the nucleus splits immediately. Nuclei such as 235U that lead to fission following the absorption of thermal neutron are called fissile nuclei..

 

[Luchekv]

So just to confirm...in U235 the energy it takes to bind that neutron is also above the critical value resulting in fission. Where as in U238 binding energy is below the critical value therefore the neutron would need additional energy (kinetic) to hit that threshold of 7MeV?