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1 Nuclear Fission Nuclear Reactors, BAU, 1 st Semester, (Saed Dababneh). Thermal neutron fission of 235 U forms compound nucleus that splits up in more than 40 different ways, yielding over 80 primary fission fragments (products) U n Rb Cs n U n Br La n U n Zn Sm n The fission yield is defined as the proportion (percentage) of the total nuclear fissions that form products of a given mass number. Revisit thermal and look for fast. 1Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

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2 Nuclear FissionAsymmetry Remember neutron excess. (A,Z) (A,Z+1) or (A-1,Z). Only left side of the mass parabola.

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3 Nuclear Fission 235 U + n 93 Rb Cs + 2 n Q = ???? What if other fragments? Different number of neutrons. Take 200 MeV as a representative value. 66 MeV98 MeV miscalibrated Heavy fragments Light fragments Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh). 165 MeV average kinetic energy carried by fission fragments per fission.

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Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh). 4 Nuclear Fission neutrons emitted per fission. depends on fissioning nuclide and on neutron energy inducing fission.

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5 Nuclear Fission Mean neutron energy 2 MeV. 2.4 neutrons per fission (average) 5 MeV average kinetic energy carried by prompt neutrons per fission. Show that the average momentum carried by a neutron is only 1.5 % that carried by a fragment. Thus neglecting neutron momenta, show that the ratio between kinetic energies of the two fragments is the inverse of the ratio of their masses. Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

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6 Nuclear Fission Distribution of fission energy Krane sums them up as decays. Lost … ! Enge Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

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7 Segrè Lost … ! Nuclear Fission How much is recoverable? How much is recoverable? What about capture gammas? (produced by -1 neutrons) What about capture gammas? (produced by -1 neutrons) Why c < (a+b) ? Why c < (a+b) ? Distribution of fission energy abcabc Nuclear Reactors, BAU, 1 st Semester, (Saed Dababneh). Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

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8 and emissions from radioactive fission products carry part of the fission energy, even after shut down. On approaching end of the chain, the decay energy decreases and half-life increases. Long-lived isotopes constitute the main hazard. Example? (poisoning). Can interfere with fission process in the fuel. Example? (poisoning). Important for research. -decay favors high energy ~ 20 MeV compared to ~6 MeV for. Why? Only ~ 8 MeV from -decay appears as heat. Why? Nuclear Fission

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9 Nuclear Reactors, BAU, 1 st Semester, (Saed Dababneh). A, Z i A, Z-1 k A-1, Z j A, Z+1A+1, Z - - (n, ) dN i /dt = Formation Rate - Destruction rate - Decay Rate f N i saturates and is higher with higher neutron flux, larger fission yield and longer half-live. 9Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

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Investigate both and giving full description for the buildup and decay of fission fragment i. 10 Nuclear Fission Nuclear Reactors, BAU, 1 st Semester, (Saed Dababneh). Shutdown HW 8 HW 7 Investigate the activity, decay and gamma energies of fission products as a function of time. Comment on consequences ( e.g. rod cooling). 10Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

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11Nuclear Reactors, BAU, 1 st Semester, (Saed Dababneh). per watt of original operating power. T = time of operation. Nuclear Fission Fission product activity after reactor shutdown? 11Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

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Nuclear Fission The fission gamma radiation Prompt within 0.1 s and with average energy of 0.9 MeV. delayed gammas. Investigate how prompt Investigate how prompt gammas interact with water, uranium and lead. 12Nuclear Reactors, BAU, 1 st Semester, (Saed Dababneh). HW 9 12Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

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13 Nuclear Fission HW 10 The experimental spectrum of prompt neutrons is fitted by the above equation. Calculate the mean and the most probable neutron energies.

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14Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh). Recoverable energy release 200 MeV per 235 U fission. Fission rate = 2.7x10 21 P fissions per day. P in MW. 3.12x10 16 fissions per second per MW, or 1.2x10 -5 gram of 235 U per second per MW (thermal). Burnup rate Burnup rate : 1.05 P g/day. P in MW. The fissioning of 1.05 g of 235 U yields 1 MWd of energy. Specific Burnup (pure 235 U !!). Specific Burnup = 1 MWd / 1.05 g MWd/t (pure 235 U !!). Fractional Burnup Fractional Burnup = ??? Thermal reactor loaded with 98 metric tons of UO 2, 3% enriched, operates at 3300 MWt for 750 days t U. Specific burnup MWd/t. Not all fissions from 235 U. Fast fission of 238 U. 238 U converted to plutonium more fission. Nuclear Fission Work it out, NOW! Actually much less.

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Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh). 15 Nuclear Fission Two neutrinos are expected immediately from the decay of the two fission products, what is the minimum flux of neutrinos expected at 1 km from the reactor. Capture-to-fission ratio: Consumption rate Consumption rate : 1.05(1+ ) P g/day. Read all relevant material in Lamarsh Ch. 4. We will come back to this later. Read all relevant material in Lamarsh Ch. 4. We will come back to this later. 4.8x10 12 m -2 s -1

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16Nuclear Reactors, BAU, 1 st Semester, (Saed Dababneh). Nuclear Fission 3.1x10 10 fissions per second per W. In thermal reactor, majority of fissions occur in thermal energy region, and are maximum. Total fission rate in a thermal reactor of volume V Thermal reactor power (quick calculation) 16Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

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Nuclear Fission It is necessary to evaluate the potential hazards associated with an accidental release of fission products into the environment. It is required to determine a proper cooling time of the spent fuel (before it becomes ready for reprocessing) that depends on the decay times of fission products. It is necessary to estimate the rate at which the heat is released as a result of radioactive decay of the fission products after the shut down of a reactor. The poisoning is needed to be calculated (the parasitic capture of neutrons by fission products that accumulate during the reactor operation). 17Nuclear Reactors, BAU, 1 st Semester, (Saed Dababneh). 17Nuclear Reactors, BAU, 1st Semester, (Saed Dababneh).

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