核能发电-放射性核过程与核能

1 GG425, wk15 L41, S2008 Lecture 41 Radioactivity, Nuclear Processes, and Nuclear Energy NO READING!! Today d. Fission and Isotope Enrichment and Fission Products GG425, wk15 L41, S2008 Fission • The “chain reaction” concept • Isotope enrichment – Prerequisite for nuclear weapons/power – Natural fission • Fission Products and other radioactive materials – Products of neutron addition (irradiation) – Products of fission – Incidental production of radioactive material GG425 -- ENVIRONMENTAL GEOCHEMISTRY-- Week 15, lecture 41 2 GG425, wk15 L41, S2008 235U is a natural, highly fissile isotope • 235U mostly decays via a series of α and β decays to 207Pb • A very small fraction of 235U undergoes “spontaneous” fission (i.e., it splits into small fragments and some neutrons without provocation). • If these neutrons hit other 235U atoms they can induce additional fission reaction with great efficiency. GG425, wk15 L41, S2008 235U, continued • A key aspect of 235U fission is that each fission reaction produces several neutrons: 235U + n º fission products + νn. • ν (the neutron yield) is about 2.5 for 235U • Critical mass is the mass of 235U required to initiate a “runaway” fission reaction. If each fission yields two neutrons, causing two additional fissions, the rate of fission and energy release will increase exponentially. • The 1st 235U critical mass was humanly achieved 70 yrs ago 3 GG425, wk15 L41, S2008 Critical Mass Chain Reaction Also note that each neutron produced goes on to split another 235U atom. This is also an over simplification. See fission factor (η) In this cartoon the neutron yield (n) is set to 3 (artificially high for simplicity). GG425, wk15 L41, S2008 Isotope Enrichment • 235U half life = 0.7 x 109 yrs 238U half life = 4.46 x 109 yrs Age of Earth = 4.55 x 109 yrs • Much of the original 235U on Earth has decayed away. Today, only about 0.72% of natural U is 235U. The rest is 238U (a much less fissile isotope). • To sustain a nuclear chain reaction 235U must be enriched above natural levels. • This enrichment increases the probability that neutrons released by one fission reaction will go on to cause additional fission reactions. 4 GG425, wk15 L41, S2008 • U-235 enrichment increases the probability that neutrons released by one fission reaction will go on to cause additional fission reactions. • Enrichment to about 3% is needed for Nuclear power plant fuel. • A nuclear explosive needs at least 20% U-235. Ideally greater than 90% U-235 is used. • It is difficult to separate 235U from 238U in significant quantities because of the small fractional mass difference (1.3%). • Apparatuses for isotope separation (gaseous diffusion or centrifugation) are usually used. Isotope Enrichment GG425, wk15 L41, S2008 A gas centrifuge is an evacuated casing containing a cylindrical rotor which rotates at high speed in an almost friction-free environment. U is fed into the rotor as gaseous uranium hexafluoride (UF6) which also rotates. The centrifugal forces push the heavier U-238 closer to the wall of the rotor than the lighter U-235, so that the gas closer to the wall becomes depleted in U-235 and the gas nearer the rotor axis is enriched in U-235. Text modified from the Institute for Science and International Security website: http://www.exportcontrols.org/centrifuges.html Enrichment from a single centrifuge is small, so they are linked together by pipes into cascades of successive centrifuges, where the U-235 is gradually enriched to the required level. 5 GG425, wk15 L41, S2008 Natural Fission • Earlier in Earth history the natural 235U/238U ratio was much higher. • 2 to 3 billion years ago ore-grade quantities of naturally occurring U-oxide were suitable for a spontaneous, continuous fission chain reaction. • Such a deposit existed in Gabon, Africa. The Oklo mine has been intensely studied to determine the fate of the radionuclides formed during this episode of natural fission. GG425, wk15 L41, S2008 Natural Fission • Fission products and yields suggest parts of the ore body were critical for a few 10s of thousands of years • Mobile fission product elements migrated into nearby clay and shale beds but stayed mostly close to the source 6 GG425, wk15 L41, S2008 σ is the reaction cross-section (measured in area) - it is proportional to the reaction probability for fission or capture: The bigger the cross section the more likely the reaction. Compare 235U and 238U. Also compare thermal vs. fast neutrons. Neutron Cross Sections GG425, wk15 L41, S2008 The fission probability depends on neutron energy Low energy distribution represents case where neutrons dissipate energy by interaction with moderator molecules (often H2O) and come to thermal equilibrium. These subtleties are important when building a nuclear bomb or nuclear power generator. 7 GG425, wk15 L41, S2008 Issues for building nuclear weapons and power generators For a bomb one has to get enough fissionable material to initiate a “runaway” chain reaction. For a power plant a steady-state must be maintained to prevent a runaway reaction (‘melt down”). How? For each fission reaction that occurs only one neutron normally goes on to produce an additional fission. Fission factor (η) is an important concept : η= ν*(probability that a released neutron will go on to produce additional fission reactions) 1946 “Baker” submarine test at Bikini atoll GG425, wk15 L41, S2008 fission reactors : 3 ways radioactive material is produced. 1. Neutron addition pathways to additional heavy nuclides (Np, Pu) via irradiation 2. Fission products from splitting heavy nuclides 3. Incidental radioactive material from neutron irradiation of non-fuel reactor components. 8 GG425, wk15 L41, S2008 1. Neutron addition pathways to additional heavy nuclides (Np, Pu) via irradiation • This table that we saw a few slides ago also illustrates that other nuclides are formed during a fission chain reaction. • Neutrons can be added to the nuclei of other atoms that do not split but instead undergo subsequent gamma emission. • Most important is 239Pu production by neutron addition to 238U followed by double beta decay. These reactions are also central to breeder reactors and fuel reprocessing GG425, wk15 L41, S2008 Non-fission nuclear reactions • Irradiation reactions result primarily from neutron addition. The main result is the creation of 239Pu in reactors. This “creates” new fissionable material = “new” nuclear fuel. • Some reactors are designed to maximize production of such new fissionable material. • If fuel produced exceeds fuel consumed it is called a “breeder” reactor. • To “harvest” this new fuel requires reprocessing of “spent” fuel rods - highly radioactive material. • Leaving fissionable material behind as high level waste is problematic because it is much more prone to becoming “critical”. 9 GG425, wk15 L41, S2008 Irradiation reactions of U U-235 U-238 GG425, wk15 L41, S2008 About 5 to 10 kg of separated Pu is required to make a weapon. This is roughly what is produced in a typical US nuclear power plant each year. However this material is not separated from “spent” fuel rods in the US and we currently have no plans for large scale reprocessing of spent fuel. + 10 GG425, wk15 L41, S2008 2. Fission products from splitting heavy nuclides This plot shows the fission products of 235U. Note the mass range on the x-axis. It is centered at about half the 235U mass, but the distribution of nuclei produced is bimodal, with a minimum close to the 50% of mass 235. GG425, wk15 L41, S2008 Fission products are bimodally produced as short and long lived isotopes. The best way to immobilize one element may not be chemically/ geochemically appropriate for others in the waste mix. Long lived nuclides present the more difficult waste storage issues. The large number of elements complicates long term storage of waste because many elements with different chemistries must be immobilized for long periods. 11 GG425, wk15 L41, S2008 Standard practice is to store high level waste for some years (5 - 10). Approximately 99% of the initial radioactivity burden decays away in this time. Note that this plot is only for fission products. Many heavy nuclei (e.g., 239Pu) formed by neutron addition reactions have very long half lives and do not under go significant decay on these time scales. GG425, wk15 L41, S2008 Comparison of long lived fission product activity to short lived products. This explains why the chemistry of long term stored radioactive waste is fundamentally different from weapons detonation and/or accidents at active reactors