– Example - 238U eventually becomes stable 206Pb (lead)
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.
– 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.
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.
• 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）.
—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.
– 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.
• Generally, completely out of control reactions would lead to an explosion (as in nuclear weapons)
– Reactor materials are not concentrated enough to explode.
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
– 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.
– 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
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...
• 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.
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.
• •
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
• 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.
• 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.
• For nuclear fission: