Nuclear fusion and fission

1. Nuclear fusion and fission • Fusion is … • combination of nuclides into a new and heavier nucleus • Happens in stars, such as our sun • H + H  He + energy • Fission is … • breaking apart (through decay) of an unstable heavy nucleus into smaller nuclei (and particles)

2. Discovery • Italian disccovery: Enrico Fermi was bombarding Uranium nuclei with neutrons, believed he had synthesized a “trans-Uranic” element, but could not verify it

3. Next • Lise Meitner worked out that the “transuranics” were merely radioisotopes of known elements • This proved that nuclei could be shattered

4. The natural conclusion • Because of the mass excess of heavy isotopes… • Einstein’s conclusion that mass and energy are equivalent… • Fermi and Meitner’s demonstration of the fission of Uranium… • …came the realization that the energy holding a nucleus together could be released if the nucleus itself was shattered

5. The Manhattan Project

6. Neutron release drives nuclear fission • In a nuclear reaction, the heavy nuclei is struck by a “slow” neutron • The neutron is absorbed, which renders the nuclei unstable • The nuclei disintegrates into two new nuclei • Three neutrons are released (they are “fast” neutrons”) • The binding energy of the nucleus is released

8. Chain reactions • Because more neutrons are released than began the process, each neutron is capable of impacting another nucleus and inducing fission in that nucleus • The reaction then is self-sustaining, a “chain reaction” • This released a spectacular amount of energy (binding energy + conversion of a portion of mass into energy)

9. fission

10. Driving the reaction • Critical mass – the minimum amount of fissile material needed to provide the number of neutrons needed to sustain the reaction • If each nuclei releases >1 neutron the reaction will accelerate out of control • If each nuclei release 1 neutron the reaction is controlled • If each nuclei releases <1 neutron, the reaction will slow down and stop

11. Nuclear energy

12. Nuclear power • Just like an old-fashioned steam engine – except: • Controlled release of neutrons achieves a slow steady release of energy (heat) • Energy heats water • Flow of neutrons determined by “control rods” • Does produce unstable (radioactive) waste nuclei