1. Fangataufa atoll French Polynesia

3. Nuclear Fission
Breaking large / heavy unstable nuclei into smaller ones.
Lots of possible combinations of fragments from one initial nucleus
Eg:
When a large nucleus is split into smaller fragments or products, mass is lost in the form of an energy release.
WHY is this?

4. NCEA 2005 qu #3

5. It comes down to stability…
Mass number 50
100
150
200
B.E per nucleon
(MeV) 2 4 6 8
56Fe
238U
4He
7Li
Fusion
Fission
Squashing lighter atoms to form heavier ones (fusion) releases energy
Splitting very heavy nuclei (fission) releases energy
Heavy nuclei increase their stability by nuclear fission
Light nuclei increase their stability by nuclear fusion
It comes down to stability…

6. Nuclear Fission
Breaking large / heavy unstable nuclei into smaller ones.
Lots of possible combinations of fragments from one initial nucleus
Eg:
When a large nucleus is split into smaller fragments or products, mass is lost in the form of an energy release.
WHY is this?

7. Fission Reactions Why is mass lost & energy released?
Mass number 50
100
150
200
B.E per nucleon
(MeV) 2 4 6 8
56Fe
235U
4He
7Li
Fission Reactions
56Fe ~ 9 MeV BE / nucleon
235U ~ 7 MeV BE / nucleon 2H
Why is mass lost & energy released?
Large reactant nucleus (eg U-235) have a ‘lower’ binding energy per nucleon compared to Iron.
Why are the ‘daughter’ products more stable? – they have less protons and smaller in size / radius.
When a larger nucleus splits into smaller product nuclei they have a higher binding energy per nucleon and so less energy stored as mass.
This difference in mass between the reactant and products results in a release of energy (i.e. product’s may gain kinetic energy, and / or radiation is emitted).

9. The higher the binding energy per nucleon means lower mass per nucleon.
Lower mass per nucleon means lower overall energy (due to mass - energy equivalence).

10. Fission Summary… Mass-Energy is conserved!
A large nucleus has some of its mass-energy as binding energy. The rest (most) of its mass-energy is in the nucleons, as mass.
When the large nucleus splits into smaller product nuclei, they will have more binding energy per nucleon and less mass-energy then the original large nucleus.
The difference in mass-energy between the large nucleus and product nuclei is given off as energy, usually as Ek of the products and/or radiation.

11. NCEA 2005 qu #3

14. Adding one neutron to 235U results in the excited state 236U
Adding one neutron to 235U results in the excited state 236U* that quickly decays into unstable isotopes of xenon and strontium plus two extra neutrons. This process is known as:
Neutron decay.
Fusion.
Fission.
None of the above.

15. Adding one neutron to 235U results in the excited state 236U that quickly decays into unstable isotopes of xenon and strontium plus two extra neutrons. This process is modeled by a liquid drop nuclear model. Which of the two steps on the right is not included in this model?
1. The neutron digs a crater in the nucleus, splitting it in two.
2. The nuclei begins to oscillate violently after absorbing the neutron.
neutron 1. 2.
Both of them are included
Neither is included.

16. Adding one neutron to 236U92 results in the excited state 236U that quickly fissions into unstable isotopes of xenon and strontium plus two extra neutrons. Comparing the sum of the rest masses in initial and final states, you determine that: 1. 2. 3.

17. Adding one neutron to 235U results in the excited state 236U that quickly decays into unstable isotopes of xenon and strontium plus two extra neutrons. How much energy is released in the process? The binding energy/nucleon of the three participating nuclei are given below.
212 MeV
21 MeV
2120 MeV
None of the above