Boric acid is dissolved in the reactor coolant to con­trol neutron reactivity in the core, and is referred to as a soluble neutron absorber, soluble poison, or ‘chemical shim’ (Cohen 1964 [26]). Its concentration is adjusted throughout a fuel cycle over a range 0-2500 wppm boron. The high boron level corresponds to the start of a fuel cycle and compensates for excess reactivity in the core. The level is reduced through­out the fuel cycle in response to fuel burn-up and changing core reactivity, temperature and the build up of other poisons such as xenon and samarium. Typical boron concentrations at identified reactor con­ditions are given in Table 1.19.

It should be noted that other soluble neutron ab­sorbers have been considered such as gadolinium or cadmium nitrate. Such elements have large neutron capture cross-sections and could therefore be effective at low concentrations. However there are potential disadvantages in terms of solubility and hydrolysis reactions, and current practice is entirely with boric

d The use of boric acid has the advantages that sufficiently soluble in water to yield the required concentrations, it has sufficient chemicaland physical. ability over the required temperature range, and it has a low propensity for incorporation into oxide films which could result in local concentrations of

neutron poison and acidity.

The high purity boric acid used will consist of nat­ural boron which comprises the two isotopes boron 10 (19 6%), with a neutron capture cross-section of 4000 barns and boron И (80.4%) with a neutron capture cross-section of 0.05 barns. This implies that boric 10 acid levels could be reduced by a factor of up to 5 if enriched boron -10 boric acid was used.

The use of boric acid to control reactivity in this manner is therefore a requirement of reactor op­eration, and the main disadvantage is that it imposes additional difficulties in pH control and purification by ion exchange. Boric acid is a weak acid, and al­though at typical PWR concentrations the acidity decreases with increasing temperature, there is an interaction with the control of pH.

Boric acid H3BO3 is known to form polyborate ions in the presence of hydroxyl ions by the follow­ing equilibria:

H3BO3 + OH — <-► B(OH)4-

2H3BO3 + OH” <-► B2(OH)7-

ЗН3ВО3 + OH" «-> B3(OH)i0"

where only the monomer, dimer and trimer are sig­nificant.

In a study of the boric acid dissociation over tem­peratures relevant to a PWR, Mesmer et al (1971) [27] determined the equilibrium constants and showed that the first reaction of H3BO3 was the significant determinant of pH in primary coolant. This is due 10 the decreasing importance of the polyborates at high temperature, low boron concentration and low hydroxide concentration.

In the presence of LiOH hydrated species of lit­hium metaborate (LiB02) are formed which have a negative solubility coefficient (decreasing solubility) with increasing temperature.