NEUTRON POISONS
DOE-HDBK-1019/2-93
Reactor Theory (Nuclear Parameters)
NEUTRON POISONS
In some reactors, neutron-absorbing materials called poisons are intentionally
designed into the reactor for specific purposes. Some of these poisons deplete as
they absorb neutrons during reactor operation, and others remain relatively
constant.
EO 3.1
DEFINE the following terms:
a.
Burnable poison
b.
Non-burnable poison
c.
Chemical shim
EO 3.2
EXPLAIN the use of burnable neutron poisons in a reactor
core.
EO 3.3
LIST the advantages and disadvantages of chemical shim over
fixed burnable poisons.
EO 3.4
STATE two reasons why fixed non-burnable neutron poisons
are used in reactor cores.
EO 3.5
STATE an example of a material used as a fixed non-burnable
neutron poison.
Fixed Burnable Poisons
During operation of a reactor the amount of fuel contained in the core constantly decreases. If
the reactor is to operate for a long period of time, fuel in excess of that needed for exact
criticality must be added when the reactor is built. The positive reactivity due to the excess fuel
must be balanced with negative reactivity from neutron-absorbing material. Moveable control
rods containing neutron-absorbing material are one method used to offset the excess fuel. Control
rods will be discussed in detail in a later chapter. Using control rods alone to balance the excess
reactivity may be undesirable or impractical for several reasons. One reason for a particular core
design may be that there is physically insufficient room for the control rods and their large
mechanisms.
NP-03
Rev. 0
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