![]() If it captures a neutron with a kinetic energy exceeding 1 MeV, then this energy plus the energy released by rearrangement can over come the binding energy and cause fission. no extra energy from its motion is needed to disrupt the nucleus), this is called "slow fission".īy contrast, when the abundant isotope uranium-238 captures a neutron it still has a binding energy deficit of 1 MeV after internal rearrangement. Since fission occurs regardless of the neutron's kinetic energy (i.e. The nucleus is then no longer stable and must either shed the excess energy, or split into two pieces. The binding energy of these three isotopes is so low that when a neutron is captured, the energy released by rearrangement exceeds it. The isotopes important for the large scale release of energy through fission are uranium-235 (U-235), plutonium-239 (Pu-239), and uranium-233 (U-233). If energy is absorbed, the binding energy increases. If energy is released by the rearrangement, the binding energy decreases. Any time a neutron or proton is captured by an atomic nucleus, the nucleus rearranges its structure. The stability of an atomic nucleus is determined by its binding energy - the amount of energy required to disrupt it. The overall capture cross-section can be subdivided into other cross-sections - the absorption cross-section and the fission cross-section. The probability that a particular nucleus will scatter or capture a neutron is measured by its scattering cross-section and capture cross-section respectively. Or it can capture the neutron, which in turn can affect the nucleus in several ways - absorption and fission being most important here. It can scatter the neutron - deflecting the neutron in a different direction while robbing it of some of its kinetic energy. The nucleus of an atom can interact with a neutron that travels nearby in two basic ways. A poorly designed or malfunctioning bomb may "fizzle" and release only a tiny fraction of its potential energy. The degree to which a bomb design succeeds in this race determines its efficiency. A fission bomb is in a race with itself: to successfully fission most of the material in the bomb before it blows itself apart. Two conditions must be met before fission can be used to create powerful explosions: 1) the number of neutrons lost to fission (from non-fission producing neutron captures, or escape from the fissionable mass) must be kept low, and 2) the speed with which the chain reaction proceeds must be very fast. If on average *more* than one neutron from each fission triggers another fission, then the number of neutrons and the rate of energy production will increase exponentially with time. If on average one neutron from each fission is captured and successfully produces fission then a self-sustaining chain reaction is produced. The nuclei of these isotopes are just barely stable and the addition of a small amount of energy to one by an outside neutron will cause it to promptly split into two roughly equal pieces, with the release of a great deal of energy (180 MeV of immediately available energy) and several new neutrons (an average of 2.52 for U-235, and 2.95 for Pu-239). Nuclear fission occurs when the nuclei of certain isotopes of very heavy elements, isotopes of uranium and plutonium for example, capture neutrons. 2.2.2 Basic Principles of Fusion Weapon Design.2.1.4 Basic Principles of Fission Weapon Design.2.1.3 Time Scale of the Fission Reaction.2.1.1 The Nature Of The Fission Process.Section 4 deals with the design and engineering of nuclear weapons in more detail, and the physics discussions there can be considered a continuation of Section 2. In this section I set forth the basic principles behind all nuclear weapons, although some familiarity with physics is assumed. The only authorized host site for the NWFAQ in English is the Nuclear Weapon Archive ()īack to Main Index 2.0 Introduction to Nuclear Weapon Physics and Designĭiscussions of physical principle, particularly nuclear physics, is unavoidable in most of the sections of this FAQ. Unauthorized host sites are expressly forbidden. ![]() Only authorized host sites may make this document publicly available on the Internet through the World Wide Web, anonymous FTP, or This material may be excerpted, quoted, or distributed freely provided that attribution to the author (Carey Sublette), theĭocument name (Nuclear Weapons Frequently Asked Questions) and this copyright notice is clearly preserved, and the URL of this website is included: Introduction to Nuclear Weapon Physics and Design Section 2.0 Introduction to Nuclear Weapon Physics and Design Nuclear Weapons Frequently Asked Questions
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