1 / 16

Fission and Fusion

Fission and Fusion. 3224 Nuclear and Particle Physics Ruben Saakyan UCL. Induced fission. Recall that for a nucleus with A 240, the Coulomb barrier is 5-6 MeV

zaide
Télécharger la présentation

Fission and Fusion

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Fission and Fusion 3224 Nuclear and Particle Physics Ruben Saakyan UCL

  2. Induced fission • Recall that for a nucleus with A240, the Coulomb barrier is 5-6 MeV • If a neutron with Ek  0 MeV enters 235U, it will form 236U with excitation energy of 6.5 MeV which as above fission barrier • To induce fission in 238U one needs a fast neutron with Ek  1.2 MeV since the binding energy of last neutron in 239U is only 4.8 MeV • The differences in BE(last neutron) in even-A and odd-A are given by pairing term in SEMF.

  3. Fissile materials “Fissile” nuclei “Non-Fissile” nuclei (require an energetic neutron to induce fission)

  4. 238U and 235U Natural uranium: 99.3% 238U + 0.7% 235U 238U 235U 235U prompt neutrons: n  2.5. In addition decay products will decay by b-decay (t  13s) + delayed component.

  5. Fission chain reaction • In each fission reaction large amount of energy and secondary neutrons produced (n(235U)2.5) • Sustained chain reaction is possible • If k = 1, the process is critical (reactor) • If k < 1, the process is subcritical (reaction dies out) • If k > 1, the process is supercritical (nuclear bomb)

  6. Fission chain reactions • Neutron mean free path • which neutron travels in 1.5 ns • Consider 100% enriched 235U. For a 2 MeV neutron there is a 18% probability to induce fission. Otherwise it will scatter, lose energy and Pinteraction. On average it will make ~ 6 collisions before inducing fission and will move a net distance of 6 ×3cm 7cm in a time tp=10 ns • After that it will be replaced with ~2.5 neutrons

  7. Fission chain reactions • From above one can conclude that the critical mass of 235U corresponds to a sphere of radius ~ 7cm • However not all neutrons induce fission. Some escape and some undergo radiative capture • If the probability that a new neutron induces fission is q, than each neutron leads to (nq-1) additional neutrons in time tp

  8. Fission chain reactions • N(t)  if nq > 1; N(t)  if nq < 1 • For 235U, N(t)  if q > 1/n  0.4 In this case since tp = 10ns explosion will occur in a ~1 ms • For a simple sphere of 235U the critical radius (nq=1) is  8.7 cm, critical mass  52 kg

  9. Nuclear Reactors Core • To increase fission probability: • 235U enrichment (~3%) • Moderator (D2O, graphite) Delayed neutron may be a problem To control neutron density, k = 1 retractable rods are used (Cd) Single fission of 235U ~ 200 MeV ~ 3.210-11 j 1g of 235U could give 1 MW-day. In practice efficiency much lower due to conventional engineering

  10. Fast Breeder Reactor • 20% 239Pu(n3) + 80%238U used in the core • Fast neutrons are used to induce fission • Pu obtained by chemical separation from spent fuel rods • Produces more 239Pu than consumes. Much more efficient. • The main problem of nuclear power industry is radioactive waste. • It is possible to convert long-lived isotopes into short-lived or even stable using resonance capture of neutrons but at the moment it is too expensive

  11. Nuclear Fusion Two light nuclei can fuse to produce a heavier more tightly bound nucleus Although the energy release is smaller than in fission, there are far greater abundance of stable light nuclei The practical problem: E=kBT  T~3×1010 K Fortunately, in practice you do not need that much

  12. pep pp hep 8B 7Be The solar pp chain p+p  2H + e+ + ne p+p+e-  2H + ne + 0.42 MeV (0.23%) (99.77%) 2H+p  3He + g + 5.49 MeV (~10-5%) (84.92%) (15.08%) 3He+3He a+2p 3He+p a+ e+ + ne + 12.86 MeV 3He+a  7Be + g (15.07%) (0.01%) 7Be+e-  7Li + ne 7Be+p 8B + g 7Li +p  a+a 8B 2a+ e+ + ne Overall:

  13. Solar neutrino spectra

  14. Fusion Reactors Main reactions: Or even better: More heat Cross-section much larger Drawback: there is no much tritium around • A reasonable cross-section at ~20 keV  3×108 K • The main problem is how to contain plasma at such temperatures • Magnetic confinement • Inertial confinement (pulsed laser beams)

  15. Fusion reactors Tokamak Lawson criterion

  16. ITER Construction to start in 2008 First plasma in 2016 20 yr of exploitation after that

More Related