what are delayed neutrons and why are they so important in r

what are delayed neutrons and why are they so important in reactor control? For a typical U-235 fueled reactor, what percentage of neutrons are delayed?

Solution

(1) What are delayed neutrons and why are they so importanat in reactor control?

It is known the fission neutrons are of importance in any chain-reacting system. Neutrons trigger the nuclear fission of some nuclei (²³⁵U, ²³⁸U or even ²³²Th). What is crucial the fission of such nuclei produces 2, 3 or more free neutrons.

But not all neutrons are released at the same time following fission. Even the nature of creation of these neutrons is different. From this point of view we usually divide the fission neutrons into two following groups:

While the most of the neutrons produced in fission are prompt neutrons, the delayed neutrons are of importance in the reactor control. In fact the presence of delayed neutrons is perhaps most important aspect of the fission process from the viewpoint of reactor control.

The term “delayed” in this context means, that the neutron is emitted with half-lifes, ranging from few milliseconds up to 55 s for the longest-lived precursor ⁸⁷Br. These neutrons have to be distinguished from the prompt neutrons which are emitted immediately (on the order of 10^-14 s) after a fission event from a neutron-rich nucleus. Despite the fact the amount of delayed neutrons is only on the order of tenths of percent of the total amount, the timescale in seconds plays the extremely important role.

key characteristics of delayed neutrons:

(2) For a typical U-235 fueled reactor, what percentage of neutrons are delayed?

138I,89Br,

(93,94)Rb

139I,(93,94)Kr,

143Xe,(90,92)Br