A non-invasive measurement approach of boric acid concentration for anticipating reactivity excursion in light water integrated small modular reactors

Abstract

This article presents a viability study about a non-invasive and a higher accurate experimental approach to measure boric acid concentration in water under a vast combination of pressure and temperature conditions. This novel approach has been developed to be used in a test facility simulating integral Pressurized Water Reactor - Small Modular Reactors (iPWR-SMRs), that is under development and will be operated by the Brazilian Nuclear Energy National Commission. Besides, the passive acting of the main systems is one of the most important requirements that characterize Generations III + and IV nuclear reactors. The integration of such systems in a unique hermetic enclosure has brought new technological challenges to the nuclear engineering. Among such challenges is the need for adequate and efficient instrumentation for assessing operating conditions and phenomena present in iPWR-SMRs. The boric acid solution has been used in conventional PWRs as an important option to core reactivity control, demanding the need to characterize boric acid concentrations in reactor vessels any time, justifying the interest of this research. However, in an iPWR-SMR, water flow (with boron) between the pressurizer and the nuclear core occurs under passive convection regime and is invariably very slow and small. This situation may require a non-invasive measurement approach. The objective of this work is to analyze the viability of using ultrasonic technique to characterize the concentration of boric acid in a liquid medium non-invasively under experimental conditions of a specific test facility. Ultrasonic tests were performed relating different boric acid concentrations with the path time of the ultrasonic wave. Factors able to mask the characterization of these concentrations by ultrasonic technique were also evaluated. The results showed that this technique allows the characterization of boric acid concentrations in liquid medium under very simple conditions, requiring further studies for complex conditions like those found in nuclear reactors.