Water chemistry control strategy of ammonia-containing coolant in the primary circuit

BackgroundIn some specific reactors, ammonia is added to regulate the pH of the primary coolant. The oxidizing species is effectively suppressed by radiolytic products of ammonia, thereby the coolant is maintained in a reductive chemical environment.PurposeThis study aims to develop a predictive model capable of simulating water chemistry behavior under various control strategies to achieve coordinated regulation of both pH and dissolved hydrogen concentrations.MethodsFirstly, a transport model for radiolytic species in reactor coolant was developed on the basis of the RETA reactor system analysis code, with applicability to a wide range of reactor types. The model demonstrated high predictive accuracy, with root-mean-square (RMS) errors of only 1.79×10-8 for NH₃ and 5.69×10-8 for H2 concentrations compared with experimental data. Then, the KLT-40S reactor was taken as a research object, three ammonia injection strategies were established and comparatively evaluated: initial dispersion, constant-rate injection, and constant-rate injection optimized with hydrogen removal. The simulations were conducted by defining initial coolant parameters and radiation field conditions, followed by stepwise adjustment of ammonia injection rates and the timing of hydrogen removal. Each strategy was simulated until the system reached a quasi-steady state (1.6×104 s), allowing assessment of its effectiveness in controlling pH and dissolved hydrogen levels.ResultsThe results indicate that the initial dispersed injection strategy, while simple and effective in maintaining a reductive chemical environment, is only able to sustain pH regulation for less than 5 h. Conversely, the constant-rate ammonia injection strategy enables sustained pH control but results in excessive dissolved hydrogen concentrations, necessitating an appropriate hydrogen removal scheme. The hydrogen removal-optimized constant-rate ammonia injection strategy is capable of simultaneously regulating pH and maintaining dissolved hydrogen concentrations. The ammonia injection rate is 1.64 g·s⁻1, and the hydrogen removal system is initiated 1 200 s after the start of ammonia injection, with a controlled removal rate of 0.014 g·s⁻1. When the system is stable, the coolant pHT stabilizes at 6.9, with dissolved hydrogen concentrations ranging from 30 mL·kg⁻1 to 35 mL·kg⁻1 (Standard Temperature and Pressure，STP).ConclusionsThis study is anticipated to provide valuable insights for the development of new reactor types and the optimization of water chemistry control strategies.