Why Does Fission of U235 Release More Energy Than Calculated?

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Discussion Overview

The discussion centers on the energy release during the fission of Uranium-235 into Barium-144 and Krypton-89, exploring the discrepancies between calculated and reported energy values. Participants examine the binding energies of the involved isotopes and the implications for energy release in nuclear reactions.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant calculates the mass difference in the fission reaction and finds a release of 173 MeV, which aligns with some sources but does not match the commonly cited value of over 200 MeV.
  • Another participant explains that binding energies are negative and that higher binding energies of the products indicate energy release, clarifying the concept of binding energy as a measure of energy required to separate nucleons.
  • A later reply notes that Barium-144 and Krypton-89 are not stable isotopes, suggesting that additional energy is released during their subsequent decays.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the discrepancy in energy values, with no consensus on the reasons for the difference between the calculated and reported energy release in fission reactions.

Contextual Notes

The discussion highlights potential limitations in the calculations, including assumptions about binding energies and the treatment of decay processes, which may not be fully accounted for in the initial analysis.

bphiz
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Hi all

I am struggling with the fission of U235 into Barium 144 and Krypton 89.

U= 235.044u +(1.0087 mass of neutron)=236.053

Ba=(143.923) + (Kr=88.917) +3(1.0087u)

This leaves 0.186u missing on the products side.

If this is converted to MeV I gain the value, 173MeV. This agrees with various sources on the difference in binding energy of the different isotopes.

HOWEVER most sources then go onto say that in a single fission reaction more than 200 MeV is released per fission reaction. I can not reach this number at all. Even accounting for neutrino's making some of that energy inaccessible.My second problem is I can not really wrap my head around the concept this releasing energy...

With fusion, the mass of the products and the binding energy required is less than that of the constituent particles, therefore the missing mass manifests itself as energy, easy.

However even though the binding energy per nucleon is less for Uranium:

Binding energy for an entire uranium atom 1783MeV,
Krypton 766.909MeV
Barium 1190MeV

(I looked up those binding energies and they came out slightly differently to how I calculated them using (no of protonsx1.0073)+(no of neutrons) - mass of nuclues)

The binding energy required to hold the products together is HIGHER, than the energy required to hold uranium together.

Somebody please help explain this to me:(
 
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Binding energies are negative, so the higher binding energies of the products mean that energy is released.

Binding energy is not something you have to put into 'glue' the constituents together, it is actually a measure of the energy you would have to put into drag them apart. The (strong) nucelar force is sufficient to overcome the electrostatic force with a little bit to spare; that difference is what constitutes the binding energy.
 
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Thankyou

So why the MeV different then? 173 produced to upwards of 200?
 

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