Reactor Theory (Nuclear Parameters)
The important information in this chapter is summarized below.
Neutron Poisons Summary
A burnable neutron poison is a material that has a high neutron absorption cross
section that is converted into a material of relatively low absorption cross section
as the result of neutron absorption.
A non-burnable neutron poison is a material that has relatively constant neutron
absorption characteristics over core life. The absorption of a neutron by one isotope
in the material produces another isotope that also has a high absorption cross
Chemical shim is a soluble neutron poison that is circulated in the coolant during
Burnable neutron poisons are used in reactor cores to compensate for the excess
positive reactivity of the fuel when the reactor is initially started up.
Chemical shim has several advantages over fixed burnable poisons.
Has a spatially uniform effect
Possible to increase or decrease amount of poison in the core during
Fixed burnable poisons have several advantages over chemical shim.
Can be used to shape flux profiles
Do not have an adverse effect on moderator temperature coefficient
Two reasons for using non-burnable neutron poisons in reactor cores are to shape power
and to prevent excessive flux and power peaking near moderator regions.
An example of a material that is used as a fixed non-burnable neutron poison is