The benefits of enriched boric acid in PWRs

The economics of operating pressurized water reactors (PWRs) demand increased fuel economy which in turn places increasing demands on the primary coolant chemistry. Higher energy production can be achieved by using increased enrichment of the nuclear fuel, higher fuel burn up and longer fuel cycles, which in turn requires increased amounts of neutron poison to maintain reactivity control. In addition, greater amounts of chemical shim are required when utilizing alternative, high-reactivity fuels such as mixed uranium/plutonium oxide (MOX) fuels. The B-10 isotope of boron has a high cross section for the absorption of thermal neutrons. However, this isotope comprises only ∼19.6% to 20 % of natural boron. If the boric acid is enriched in the ¹⁰B isotope, then the maximum amount of chemical shim can be increased without exceeding the chemistry limits set by the acidity of boric acid or the concentration limit of lithium (PWR) or potassium (VVER). With sufficient isotopic enrichment, it therefore becomes possible to operate the entire PWR fuel cycle with constant pH_300C at the favored value of 7.2-7.4. The use of Enriched Boric Acid (with ∼40% 10B instead of ∼ 20% in natural boron) consequently provides many advantages in enhancing operational safety and life time of several components, as well as facilitating dose rate reduction programs in PWRs and VVER Nuclear Power Plants. When lowering the total boron concentration in the primary coolant, it is possible to minimize: - dose rates during maintenance activities - optimal pH_T from the beginning of the fuel cycle (BOC) can mitigate corrosion products that become activated; - the risk of corrosion of various materials of the Reactor Coolant System - allowing a lower lithium (or potassium) concentration mainly at BOC; the sensitive materials are fuel cladding (zirconium alloy), stainless steels (AISI 304 type), nickel-based Alloy 600; - the risk of Axial Offset Anomalies - resulting from precipitation of boron compounds on the fuel cladding; - the need for design modification to support either larger volume or higher boric acid concentration of safety tanks. Finally, EBA may also be added in units operating with natural boron. As B-10 becomes depleted, a small quantity of EBA (rather than larger quantities of natural boron) can be added to adjust the B-10 proportion within the specified tolerances. In this manner, EBA may be applied to decrease the amount of waste discharged into the environment. If the benefits of EBA deserve economic and technical evaluation in operating plants (which are more difficult and costly to transition), then it seems clear that any new plant should consider a design with the EBA option. This paper provides an overview of the benefits of Enriched Boric Acid (EBA) on PWR radiation field development, increased safety margins, structural component corrosion, reduced volume or concentration of borated water system tanks and waste reduction in support of advanced fuel management trends. (authors)