Lect14 -- Nuclear Principles

07n energy net n yield 107 238u n 239pu and

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Unformatted text preview:   239Np beta-decays into 239Pu with half-life of 2.4 days –  now have another fission-able nuclide –  about 1/3 of energy in normal reactors ends up coming from 239Pu •  Reactors can be designed to “breed” 239Pu in a better-than-break-even way Neutron control in a breeder reactor Conventional reactor 235U + n -> A + B + 2.07n + Energy (net n yield = 1.07) 238U + n -> 239Pu + β- and then 239Pu + n -> A + B + 2.08n (net n yield = 2.08 - 2 = 0.08) Breeder reactor At high temp, 238U neutron yield is 2.74n (net n yield = 0.74), enough to sustain chain reaction BUT high temp cannot use water as coolant A, B = fission products, Pu=plutonium, n = neutron The pluses and minuses of breeders Advantages •  Could convert all available 238U into 239Pu, while getting electrical power out •  30 year resource is 140 times as much (not restricted to 0.7% of natural uranium), or 4200 yr Disadvantages •  Technological hurdle: need liquid sodium or other molten metal to be the coolant (but four are running in the world) •  Enough 239Pu falling into the wrong hands spells: BOOM!! (Pu is pre-enriched to 100%; need less for bomb) The “fissile” nuclides “Fissile”: able to undergo fission by slow neutron There are only three known nuclides (arrangements of protons and neutrons) that undergo fission when introduced to a neutron: 233U: hardly used (hard to get/make) 235U: primary fuel for reactors 239Pu: popular in bombs Others isotopes may split if smacked hard enough by a neutron (or other energetic particle), but not when hit by slow neutrons – so not practical for reactors The 232Th fuel option The principle 232Th + n ----> 233U 233U then used for nuclear fission Advantages •  There is lots and lots and lots of 232Th •  233U is hard to make into bomb •  Waste products from 233U fission have short half-lives Disadvantages •  Technology not well developed •  Requires running plants at higher T – need molten salt for cooling...
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