NUC K117 Chapter 10: Neutron Poisons
DepartmentNuclear Engineering Technology
Course CodeNUC K117
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I. Neutron Poisons!
1. Neutron poisons deliberately added to the core !
•Burnable Poison !
•Non-Burnable Poison !
•Chemical Shim Poison!
2. Consider a core of ‘new’ fuel!
•Much positive reactivity to deal with!
•Control rods add some negative reactivity, but not enough!
•Chemical ‘burnable poisons’ added to further compensate with more
3. Burnable Poison Deﬁned!
•Burnable poisons have high neutron absorption cross sections. In addition,
their neutron absorption cross section decreases as they absorbed neutrons!
•As neutrons are absorbed, the negative reactivity contributed by the poison
decreases - it is ‘used up’ as core ages!
•As fuel burned, negative reactivity added; as poison burned positive
reactivity added. Tend to cancel each other out!
4. Fixed Burnable Poison!
•Poisons added to fuel in the form of Boron or Gadolinium pins. As pins are
‘depleted’ over core life, positive reactivity added!
5. Soluble Burnable Poisons (chemical shim)!
•Poison chemical dissolved in reactor coolant. Boron ( B10 ) used to add
•Naturally adds reactivity as poison depleted over core life!
•Concentration may be adjusted to tweak reactivity as needed (‘Boration’ or
•Adjusting concentration similar in eﬀect to moving control rods (hence ‘shim’)!
•Maximum concentration of B10 limited due to positive temperature coeﬃcient
6. Non-Burnable Poison!
•Some materials (such as Hafnium) have multiple isotopes that remain
neutron absorbers - a single atom can absorb up to ﬁve neutrons in this case!
7. Moveable Control Rods!
•Also a neutron poison - to be covered in a later lesson!
8. Neutron ‘poisons’ make nuclear reactors a feasible way to make energy over a
long period of time. The positive reactivity added by new fuel if oﬀset by the
negative reactivity provided by the poison. This arrangement allows nuclear
reactors to run for years - even decades, in the case of some military reactors.
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