RADIATION CHEMISTRY IN PRESSURIZED WATER NUCLEAR REACTORS: H2 GENERATION BY 10B(n,α)7Li, AND THE REACTION OF BORATE WITH •OH

Abstract

Nuclear power plants (NPPs) are complex engineering systems, with malfunctions having enormous potential to lead to widespread and extreme impacts on society and the environment as a whole. Their safe operation depends on a multitude of factors such as intelligent planning, proper design, quality components, high-level safety operations, and economic viability. Due to requiring high temperature and high pressure of an NPP’s cooling fluid, one of the main concerns for further developing safe operating conditions and evaluating component lifetimes is improving our understanding on the issue of corrosion in nuclear systems. In the U.S., all commercially operated Pressurized Water nuclear Reactors (PWRs) are light-water reactors wherein their coolant waters can reach temperatures up to 350 °C. According to a report in 2005 in association with the U.S. Federal Highway Administration, an annual cost of $4.2 billion was directly attributed to corrosion in NPPs in 1998, out of a total $6.9 billion in the electrical utilities industry (Koch, et al., 2005). Boron is added into commercial PWR primary water in the form of boric acid as a soluble chemical neutron “shim” in order to compensate for fuel burnup and allow smooth long-term reactivity control. After a boron nucleus captures a thermal neutron and becomes unstable, the energy of the recoil ions resulting from its fission accounts for up to 33 % of the total dose to the primary water. This event is an important source for H2 and corrosive H2O2, so its product yields must be accurately included in models of the cooling water radiation chemistry. H2 produced in water from the 10B(n,α)7Li fission reaction has been measured up to 300 °C to aid in quantification of the corrosive H2O2 from the same reaction. Thermal energy neutrons from the Rhode Island Nuclear Science Center 2 MW reactor interacted with boric acid contained in N2O-saturated water in temperature-controlled high-pressure cells made from tubing of either titanium or zirconium alloy. After exposure for a minimum of one hour, the solution samples were extracted and sparged with argon. The H2 entrained by the sparging gas was sampled with a small mass spectrometer. A small amount of sodium was included in the boric acid solution so that after sparging, samples could be collected for 24Na activation measurements in a gamma spectrometer to determine the neutron exposure and thus the total energy deposited in solution. The G-value (µmol/J) for H2 production was obtained for water at a pressure of 25 MPa, over a temperature range from 20 °C to 300 °C. These results have been complemented with Monte Carlo N-Particle® (MCNP®) simulations in collaboration with the National Institute of Standards and Technology, and have been compared with previous experimental results at room temperature and simulated results up to 350 °C. Additionally, boric acid has thus far been accepted as a chemically nondisruptive additive, as it was confirmed long ago to have extremely low reactivity with the two main reactive species produced in reactor primary water by radiolysis, the solvated aqueous electron and the hydroxyl radical (e(aq)- and •OH). However, at the Electric Power Research Institute standard desired pH of 7.3 and the operational temperature of 350 °C, approximately 22% of the boron added in PWR primary water exists in the chemical form of the conjugate base, borate, not boric acid. Although borate was previously confirmed to have no appreciable reactions with e(aq)-, it was not adequately studied for reactions with •OH prior to this work. We have observed a clearly apparent reaction between borate and •OH. Current chemistry models are completely ignorant on both the existence of the resultant species and its reactions. The chemical reaction of [B(OH)4]- (borate) with •OH along with cross-reactions of the product species have been studied up to 200 °C to determine those reactions’ rate constants and the products’ spectra. The University of Notre Dame Radiation Laboratory’s 8 MeV electron linear accelerator (LINAC) was configured to perform pulse radiolysis with pulse widths between 4ns to 20ns providing doses between 5.5 Gy and 62 Gy. High-energy electrons from the LINAC interact with the borated solution which has been N2O-saturated and is continuously flowed through a 316 stainless-steel optical cell. The cell temperature was adjusted by resistive-heating silicon cartridges, and pressure was controlled by two syringe pumps to prevent boiling. The cell had two fused silica windows for transmitting light from a xenon arc lamp through the solution and out to a multichromatic spectrophotometer system. Time-resolved spectral data was obtained over nano- and micro-second timeframes, for wavelengths ranging from the deep UV and into the infrared spectrum (250 nm to 820 nm). The reaction rates and products’ spectra were then obtained by analyzing the data using computational aids, namely IGOR Pro by Wavemetrics and KinTek Explorer by KinTek Corp. The product species of the reaction between borate and •OH is conjectured to be •[BO(OH)3]-, on the basis of ab initio calculations, which likely reacts with boric acid or borate to form a polymer radical.