Energy & Environment I NAME

Quiz #6

25 points

Closed book and notes

Answers in red.

Four (4) types of neutron-nucleus collisions (or reactions) occur. Three (3) of these are very important in nuclear reactors. Name the three types of important collisions (or reactions), and for each, explain its importance in the nuclear reactor.

TYPE #1

Elastic collisions.

These are essential for thermalizing (slowing down) the fast neutrons produced by the fission reactions. The collisions of interest are those that occur between fast neutrons and light nuclei, especially H (as in H₂O), D (as in D₂O), and C (as in graphite). These collisions are very effective in the transfer of KE from the fast neutrons to the relatively slow moving nuclei. Without thermalization, and thus, without the re-supply of the slow neutrons to collide with U-235 nuclei, the fission process cannot be sustained. The chain reaction is broken.

TYPE #2

Capture reactions.

Capture reactions create new isotopes, eg, n+^AN ®^A+1N.

Capture reactions affect the nuclear reaction process as follows:

• They remove thermal neutrons necessary for maintaining criticality in the reactor. FP nuclei that participate are called “poisons”.
• They convert the fertile isotope (U-238) into the fissile isotope (Pu-239), thereby creating new fuel. This occurs by the capture reaction n+²³⁸U®²³⁹U, following by two b-decay reactions that end up at ²³⁹Pu. (Note both fast and thermal neutrons are captured by ²³⁸U.)

TYPE #3

Fission reactions.

The two fission reactions of most importance to our study of nuclear reactors are:

• Fission of U-235 [n+²³⁵U®FP1+FP2+(2-3)n]
• Fission of Pu-239 [n+²³⁹Pu®FP3+FP4+(~3)n]