Question

Imagine that you are in the very first stages of
trying to figure out how to build a nuclear power reactor or a nuclear weapon
using fission. The first step you would consider is what kind of nuclei could
live for a reasonable length of time so that you could keep it around, but
would also be able to decay by some process that would give off lots of energy


1.Explore the conditions that produce a chain reaction (where a
substantial fraction of the nuclei fission) and those that do not. You want to
design a nuclear bomb. What are three design characteristics that are critical
to creating an effective bomb as oppose to a dud? Include the physics reasoning
behind why these are so important.

Answer 1

A chemical chain reaction proceeds by a sequence generally subdivided into three stages:

(1) Initiation, in which a reactive intermediate, which may be an atom, an ion, or a neutral molecular fragment, is formed, usually through the action of an agent such as light, heat, or a catalyst.

(2) Propagation, whereby the intermediate reacts with the original reactants, producing stable products and another intermediate, whether of the same or different kind; the new intermediate reacts as before, so a repetitive cycle begins.

(3) Termination, which may be natural, as when all the reactants have been consumed or the containing vessel causes the chain carriers to recombine as fast as they are formed, but more often is induced intentionally by introduction of substances called inhibitors or antioxidants.

A nuclear bomb is designed to release all its energy at once, while a reactor is designed to generate a steady supply of useful power.

Design characteristics

1) Nuclear fission fuel: Nuclear weapons employ high quality, highly enriched fuel exceeding the critical size and geometry (critical mass) necessary in order to obtain an explosive chain reaction. The fuel for energy purposes, such as in a nuclear fission reactor, is very different, usually consisting of a low-enriched oxide material. Detonation of a nuclear weapon involves bringing fissile material into its optimal supercritical state very rapidly. During part of this process, the assembly is supercritical, but not yet in an optimal state for a chain reaction

2) Fission reaction products: When a heavy atom undergoes nuclear fission it breaks into two or more fission fragments. Also, several free neutrons, gamma rays, and neutrinos are emitted, and a large amount of energy is released.

3) The effective neutron multiplication factor, k, is the average number of neutrons from one fission that cause another fission. The remaining neutrons either are absorbed in non-fission reactions or leave the system without being absorbed.