In a nuclear fission reactor, each fission of a uranium nucleus is accompanied b
ID: 1452627 • Letter: I
Question
In a nuclear fission reactor, each fission of a uranium nucleus is accompanied by the emission of one or more high-speed neutrons, which travel through the surrounding material. If one of these neutrons is captured in another uranium nucleus, it can trigger fission, which produces more fast neutrons, which could make possible a chain reaction (see figure below). However, fast neutrons have low probability of capture and usually scatter off uranium nuclei without triggering fission. In order to sustain a chain reaction, the fast neutrons must be slowed down in some material, called a "moderator." For reasons having to do with the details of nuclear physics, slow neutrons have a high probability of being captured by uranium nuclei.
A slow neutron induces fission of U-235, with the emission of additional (fast) neutrons. The moderator is some material that slows down the fast neutrons, enabling a chain reaction.
In the following analyses, remember that neutrons have almost no interaction with electrons. Neutrons do, however, interact strongly with nuclei, either by scattering or by being captured and made part of the nucleus. Therefore you should think about neutrons interacting with nuclei (through the strong force), not with entire atoms.
(a) Based on what you now know about collisions, explain why fast neutrons moving through a block of uranium experience little change in speed. (Select all that apply.)
Collisions cannot involve the strong force.
The radius of a neutral uranium atom is quite large.
The mass of a neutron is much smaller than the mass of a uranium nucleus.
A uranium atom has 92 electrons
The speed of a small mass object will not change very much in a collision with a large mass object.
A neutron cannot interact with uranium because it is electrically neutral.
Fast neutrons are moving too fast to interact with uranium nuclei.
Explanation / Answer
Correct option is,
The probability that fission or any another neutron-induced reaction will occur is described by the neutron cross-section for that reaction. This may be imagined as an area surrounding the target nucleus and within which the incoming neutron must pass if the reaction is to take place. The fission and other cross-sections increase greatly as the neutron velocity reduces from around 20,000 km/s to 2 km/s, making the likelihood of some interaction greater.
But high-energy (> 0.1 MeV) neutrons are travelling too quickly to have much interaction with the nuclei of uranium. We therefore say that the fission cross-section of those nuclei is much reduced at high neutron energies relative to its value at thermal energies (for slow neutrons).
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