能源与环境7-2015.04.16

– Fuel rods remain in the reactor core, undergoing fission.
• At below 1% the fuel is no longer efficient for use in the reactor
pellet 小球 rod 棒 encase 装入 stainless steel 不锈钢 vessel 容器
• Efficient production of 239Pu requires a ―breeder‖ reactor, operating with fast neutrons. • Surrounding the reactor is a blanket of ordinary uranium, in which the 239Pu is bred by the fast neutrons. • The reactor fuel must be enriched to the extent of 15~20 percent with a fissionable isotope, either 235U or 239Pu.
• The main difference with respect to an ordinary fission reactor is that the water coolant is replaced by liquid sodium（钠）. • The sodium atoms slow the neutron to a much smaller extent, while liquid sodium efficiently carries away heat from the reactor.
– Use
• Control rods – Rods, composed of either cadmium（镉） or boron（硼）, are inserted into the core to capture neutrons – Control the chain reaction when the reaction accelerates too quickly.
• After enrichment, the uranium is formed into ceramic pellets which are then encased in metal fuel rods.
– Bundles of rods are put into a stainless steel reactor vessel.
– Example - 238U eventually becomes stable 206Pb (lead)
Baidu Nhomakorabea
isotopes 同位素 proton 质子 neutron 中子 radioactivity 放射性 decay 衰变 half-life 半衰期
• Nuclear fuel uses the isotope of uranium, radioactive 235U, the only isotope of uranium that readily splits. — Pure uranium ore is mostly composed of 238U (only 0.7% 235U), which does not split readily. —Thus, most nuclear fuel needs to be concentrated (enriched) before it can be used (3%~4.5%). • Nuclear weapons require much higher enrichment in 235U >>WEAPON GRADE nuclear material— enrichment 15%~20% 235U. A big step up from preparation of reactor fuel.
– An out of control chain reaction would lead to overheating of the core, causing the fuel rods and vessel to melt
• ―Meltdown‖ = supercritical mass - Chernobyl, Ukraine
– The released neutrons collide with other 235U nuclei, repeating the process in a chain reaction (self-supporting)
strike 撞击 collide 碰撞
• Usually, in nuclear reactors, operators control this chain reaction so it does not get out of control.
– the only method in use today is through nuclear fission.
• Nuclear power plant generates electricity in a manner similar to a coal-burning power plant.
1.
Introduction to Nuclear Fission and Nuclear Power
• Nuclear fission: nucleus splits into two daughter nuclei, with a very large release of energy. • Nuclear power: any nuclear technology designed to extract usable energy from atomic nuclei via controlled nuclear reactions.
reactor 反应堆 fuel rod 燃料棒 control rod 控制棒 core 堆芯 moderator 慢化剂 shielding 屏蔽 • • • •
Fuel rods contain pellets of fuel for a nuclear reactor . Control rods control the chain reaction. Liquid water acts both as a coolant and as a moderator. Moderator slows down the neutrons to increase the fission probability.
2. Nuclear Fuel
• Isotopes – elements with the same number of protons, different number of neutrons (and different atomic mass)
– Example - Uranium (U) = 92 protons in nucleus...
• For nuclear fission:
– A neutron (non-charged particle) strikes the nucleus of a 235U atom
• This causes it to split apart, releasing
– heat – 2-3 other neutrons, and various kinds of fission products
– heating a fluid (usually water) – generates steam (directly or indirectly) – drives turbine blades connected to a power generator
– Difference —— the method for heating the water: • nuclear reactions vs. coal combustion
Reprocessing
• The 235U in a fuel rod of a pressurized light water reactor can not be completely used up. • After about a year, the fuel rods must be replaced with new ones. • The spent fuel can be reprocessed by chemical extraction of the fission products, separation of the accumulated plutonium（Pu, 钚）, and reconcentration （再浓缩） of the uranium. — Uranium: refabricated （再加工）into new fuel rods — Plutonium: blended into MOX (mixed uraniumplutonium oxide, 混合氧化物) fuel rods.
• The technology of the breeder reactor is far more complex than that of the ordinary fission reactor, and the technology is still being developed. • The most formidable （可怕的） problem of the breeder reactor seems to be associated with the plumbing（管道） of the heat exchanger, since leaks of liquid sodium can be disastrous（惨重 的）.
• Generally, completely out of control reactions would lead to an explosion (as in nuclear weapons)
– Reactor materials are not concentrated enough to explode.
—the probability of fission is maximal when the neutron energy is close to the average energy of the surrounding molecules.
Breeder reactor（增殖反应堆）
• Breeder reactor extends the nuclear fuel supply by converting 238U to 239Pu. • The conversion is accomplished by irradiating uranium with neutrons.
• Moderator – Used to slow neutrons and encourage more interaction with uranium fuel
• ―Critical mass‖ – the amount of U required to maintain a chain reaction in a nuclear reactor
Chapter 3 Nuclear Energy and Its Environmental Impact (核能及其环境影响)
• 3.1 Nuclear Fission and Nuclear Power （核裂变和核电厂） • 3.2 Nuclear Fusion （核聚变）
3.1 Nuclear Fission and Nuclear Power
• •
238U 235U
(146 neutrons) (143 neutrons)
• Radioactivity and radioactive decay (half-life)
– Not all isotopes are stable
– Radioactive isotopes undergo radioactive decay to eventually become a stable nucleus