Reactor Theory (Nuclear Parameters)
DOE-HDBK-1019/2-93
NEUTRON POISONS
To control large amounts of excess fuel without adding additional control rods, burnable poisons
are loaded into the core. Burnable poisons are materials that have a high neutron absorption
cross section that are converted into materials of relatively low absorption cross section as the
result of neutron absorption. Due to the burnup of the poison material, the negative reactivity
of the burnable poison decreases over core life. Ideally, these poisons should decrease their
negative reactivity at the same rate the fuel's excess positive reactivity is depleted. Fixed
burnable poisons are generally used in the form of compounds of boron or gadolinium that are
shaped into separate lattice pins or plates, or introduced as additives to the fuel. Since they can
usually be distributed more uniformly than control rods, these poisons are less disruptive to the
core power distribution.
Soluble Poisons
Soluble poisons, also called chemical shim, produce a spatially uniform neutron absorption when
dissolved in the water coolant. The most common soluble poison in commercial pressurized
water reactors (PWR) is boric acid, which is often referred to as "soluble boron," or simply
"solbor." The boric acid in the coolant decreases the thermal utilization factor, causing a
decrease in reactivity. By varying the concentration of boric acid in the coolant (a process
referred to as boration and dilution), the reactivity of the core can be easily varied. If the boron
concentration is increased, the coolant/moderator absorbs more neutrons, adding negative
reactivity. If the boron concentration is reduced (dilution), positive reactivity is added. The
changing of boron concentration in a PWR is a slow process and is used primarily to compensate
for fuel burnout or poison buildup. The variation in boron concentration allows control rod use
to be minimized, which results in a flatter flux profile over the core than can be produced by
rod insertion. The flatter flux profile is due to the fact that there are no regions of depressed
flux like those that would be produced in the vicinity of inserted control rods.
DOE reactors typically do not use soluble neutron poisons during normal operation. Some DOE
reactors do, however, include emergency shutdown systems that inject solutions containing
neutron poisons into the system that circulates reactor coolant. Various solutions, including
sodium polyborate and gadolinium nitrate, are used.
Fixed burnable poisons possess some advantages over chemical shim. Fixed burnable poisons
may be discretely loaded in specific locations in order to shape or control flux profiles in the
core. Also, fixed burnable poisons do not make the moderator temperature reactivity coefficient
less negative as chemical shim does. With chemical shim, as temperature rises and the
moderator expands, some moderator is pushed out of the active core area. Boron is also moved
out, and this has a positive effect on reactivity. This property of chemical shim limits the
allowable boron concentration because any greater concentration makes the moderator
temperature coefficient of reactivity positive.
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