Neutron Poison - Control Poisons - Soluble Poisons

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. 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 occurs because there are no regions of depressed flux like those that would be produced in the vicinity of inserted control rods. This system is not in widespread use because the chemicals make the moderator temperature reactivity coefficient less negative. All commercial PWR types operating in the US (Westinghouse, Combustion Engineering, and Babcock & Wilcox) employ soluble boron to control excess reactivity. US Navy reactors and Boiling Water Reactors do not.

Soluble poisons are also used in emergency shutdown systems. During SCRAM the operators can inject solutions containing neutron poisons directly into the reactor coolant. Various solutions, including sodium polyborate and gadolinium nitrate (Gd(NO₃)₃·xH₂O), are used.

On 16 March 2011, South Korea said they will send 1 kg sample of their Boric Acid stock to Japan. If the sample works on the reactors in Japan, South Korea will ship over 50 tons of Boric Acid to Japan. This was requested by the Japanese government in an attempt to further prevent meltdown at the Fukushima Nuclear Power Plant.