文档介绍：PROFESSIONAL(SPECIAL) ENGLISH COURSE NUCLEAR ENERGY CHAPTER 6 Fission (裂变) Out of many nuclear reactions known, that resulting in fission has at present (目前) the greatest practical significance (意义). In this chapter we shall describe the mechanism (机制) of the process, identify (识别, 鉴别) the byproducts (副产品), introduce the concept (概念) of the chain reaction (链式反应), and look at the energy yield (出产) from the consumption (消费) of nuclear fuels (核燃料). THE FISSION PROCESS (裂变过程) The absorption (吸收) of a neutron by most isotopes involves radiative capture (辐射俘获), with the excitation energy (激发能) appearing as a gamma ray. In certain heavy elements, notably (特别地,尤其) uranium ( 铀) and plutonium ( 钚), an alternate (交替的) consequence is observed---the splitting (分裂) of the nucleus into two massive fragments (重碎片), a process called fission. Figure 6. l shows the sequence (序列) of events, using the reaction with U-235 to illustrate (举例). In Stage A , the neutron approaches (接近) the U-235 nucleus. In Stage B , the U-236 nucleus has been formed, in an excited state (激发态). The excess (过度的, 额外的) energy in some cases may be released as a gamma ray, but more frequently (常常, 频繁地), the energy causes distortions (变形) of the nucleus into a dumbbell shape (哑铃形状), as in Stage C . The parts of the nucleus oscillate (振荡) in a manner analogous to (类似于) the motion of a drop of liquid. Because of the dominance (优势) of electrostatic repulsion over nuclear attraction , the two parts can separate , as in Stage D . They are then called fission fragments (裂变碎片), bearing (具有) most of the energy released. They fly apart at high speeds, carrying some 166MeV of ic energy out of the total of around 200MeV released in the whole process. As the fragments (碎片) separate, they lose atomic electrons, and the resulting high-speed ions lose energy by interaction with the atoms and molecules of the surrounding medium