Crystallization Of Boric Acid Research Articles

Effect of Boric Acid Solubility in Steam on the Process of Mass Transfer during Emergency Cooling of VVER-1200 Nuclear Reactor

The mechanisms of boric acid mass transfer in a VVER-1200 reactor core are studied in this work in the event of a major circulatory pipeline rupture and loss of all AC power. The VVER-1200's passive core cooling technology is made up of two levels of hydro accumulators. They use boric acid solution with a concentration of 16 g H3BO3/kg H2O to control the reactivity. Because of the long duration of the accident process, the coolant with high boron content starts boiling and steam with low concentration of boric acid departs the core. So, conditions could arise in the reactor for possible accumulation and subsequent crystallization of boric acid, causing the core heat removal process to deteriorate. Calculations were carried out to estimate the likelihood of H3BO3 build-up and subsequent crystallization in the core of the VVER reactor. According to the calculations, during emergency the boric acid concentration in the reactor core is 0.153 kg/ kg and 0.158 kg/kg in both the events of solubility of steam and without solubility of steam respectively and it does not exceed the solubility limit which is about 0.415 kg/kg at water saturation temperature. No precipitation of boric acid occurs within this time during the whole emergency process. Therefore, findings of the study can be used to verify whether the process of decay heat removal is affected or not.

Determination of the physicochemical properties of boric acid for VVER emergency modes

The results of experimental research of physicochemical properties of water solutions of H3BO3 are presented in this article. The literature review about the influence of hydrogen ion exponent of H3BO3 solutions on its heat and mass transfer, and corrosiveness of an acid in the VVER reactor core in is given. It is found that the existing data does not cover the entire range of parameters (acid concentration, temperature and pressure) characteristic of a possible Loss-of-Coolant Accident (LOCA) at nuclear power plant with a VVER. The problem of the accumulation and crystallization of boric acid in case of LOCA and the operation of passive safety systems (such as hydro accumulators of the first, second and third stages, and a passive heat removal system) is formulated. Test equipment and techniques of research are described. The generalizing equations for the pH and acidity constant of hydrous solutions of H3BO3 in range of the concentration (5-100 gkg−1 H2O) and temperature (25-50 °C) has been obtained.

Impact of the Precipitation of Boric Acid and its Salts on the Operability of Passive Safety Systems During Serious Accidents in NPP

One of the problems of accidents in NPP with VVER is the evaporation of the primary coolant in the containment. Boric acid H3BO3 can ingress into the containment together with steam and crystallize, independently or in the form of salts, on heat-exchange and other surfaces of equipment placed in the containment, thereby lowering the efficiency of passive systems removing heat from the containment and the removal of hydrogen. The results of an experimental investigation of the deposition of insoluble aerosols and salts of boric acid on heat-exchange surfaces of the passive heat-removal system and the surfaces of autocatalytic hydrogen recombiners during serious accidents in NPP are reported. It is shown that the impact of crystallization of boric acid and its salts on the operability of these systems is very small.

Calculation analysis of the processes of boric acid mass transfer in the WWER core in boiling emergency mode

In this paper the processes of boric acid mass transfer in a WWER-TOI nuclear reactor core in case of the accidents with main circulation pipeline rupture and loss of all AC power supply are considered. The heat removal from the WWER-TOI reactor in case of an emergency process is determined by the use of the passive heat removal and passive core cooling systems. Passive core cooling system of WWER-TOI consists of three stages of hydro accumulators. These systems provide reactor cooling by feed of boric acid solution with concentration of 16 g H3BO3/kg H2O. In view of length of accident process, the boiling of the coolant with high content of boron and considering low concentration of boric acid in the steam leaving the core, conditions may arise for the possible accumulation and subsequent crystallization of boric acid in the reactor that can lead to a deterioration of the process of core heat removal. To assess the chance of accumulation and subsequent crystallization of H3BO3 in the core of the WWER reactor, a hand calculation analysis of the change of concentration of boric acid in the reactor in case of emergency mode was carried out. In accordance with the calculation results, if a boric acid concentration in hydro accumulators of passive core cooling systems is 16 g / kg H2O, than a large excess of the final boric acid concentration in the core is observed in accidents after approximately 43 hours of an emergency process. The options of reducing the concentration of H3BO3 to 8, 4, 2 and 1 g/kg H2O are considered in the calculation process. The obtained results allow us to conclude that a decrease in the concentration of boric acid in the system of hydro accumulators of the third stage would allow to avoid the crystallization of boric acid in the core in the event of an accident.

Effect of mass transfer processes on accumulation and crystallization of boric acid in WWER core in emergency cases

In this paper the processes of boric acid mass transfer in a WWER-TOI nuclear reactor in case of the accidents with main coolant circuit rupture and operation of passive safety systems (the hydro accumulators systems of the first, second and third stages, as well as the passive heat removal system) are considered. The results of the calculation of changes in the boric acid solution concentration in the core for the WWER emergency mode are presented. According to the results of the calculation a significant excess of the ultimate concentration of boric acid in accidents with main coolant circuit rupture after 43 hours of emergency mode is observed. The positive influence of the boric acid droplet entrainment on the processes of its crystallization and accumulation in the core is shown. The mass of boric acid deposits on the internals is determined. The received results allow concluding that the accumulation and crystallization of boric acid in the core may lead to blocking the flow cross section and to deterioration of heat removal from fuel rods. The necessity of an experimental studies of the processes of boric acid drop entrainment under conditions specific to the WWER emergency modes is shown.

Analysis of Accumulation and Crystallization of Boric Acid in the Reactor Core during WWER Emergency Operation

The results of hand calculation of boric acid accumulation in the core in a new generation WWER-TOI reactor in case of LOCA are presented. Variants of reducing the H3BO3 concentration in the HA-3 system down to 1, 2, 4 and 8 g/kg are considered. The mass of boric acid deposits on the core internals depending on the value of boric acid concentration in the HA-3 system is determined. The obtained results allow concluding that the accumulation and crystallization of boric acid in the core is possible.

Effect of boric acid mass transfer on the accumulation thereof in a fuel core under emergency modes at NPPs with WMR

Boric acid mass transfer processes in the reactor facilities with WMR are considered for the case of an emergency with breaking of the main circulation pipeline (MCP) and the operation of the passive safety systems, such as first-, second-, and third-stage accumulator tank systems, and a passive heat removal system (PHRS). Calculation results are presented for a change in the boric acid concentration in the fuel core (FC) of a water-moderated reactor (WMR) in the case of an emergency process. The calculations have been performed for different values of drop entrainment of boric acid from the reactor (0, 0.2, 2%). A substantial excess of the maximum concentration of boric acid has been found to occur 24 hours after an emergency event with a break of MCP. It is shown that increasing the droplet entrainment of boric acid causes the crystallization and accumulation thereof in the FC to become slower. The mass of boric acid deposits on the elements of internals is determined depending on the values of drop entrainment. These results allow one to draw a conclusion concerning the possibility of accumulation and crystallization of boric acid in the FC, because the latter event could lead to a blocking of the flow cross section and disturbance in the heat removal from fuel elements. A review of available literature data concerning the thermal properties of boric acid solution (density, viscosity, thermal conductivity) is presented. It is found that the available data are of quite a general character, but it does not cover the entire range of parameters (temperature, pressure, acid concentrations) inherent in a possible emergency situation at nuclear power plants with WMR. It is demonstrated that experimental study of boric acid drop entrainment at the parameters inherent in the emergency mode of WMR operation, as well as the studies of boric acid thermal properties in a wide range of concentrations, are required.

Thermodynamic modeling of the solubility of boric acid in the systems boric acid + lithium sulfate + water, boric acid + sodium sulfate + water and boric acid + potassium sulfate + water at 293.15–313.15 K

In this contribution the experimental solubility of boric acid in sodium sulfate aqueous solution was measured at different temperatures from (293.15 to 313.15)K, and Na2SO4 concentrations ranging from (0 to 3.3795)molkg−1 H2O.The results were represented using an equation based on Pitzer model for the interactions of nonelectrolytes with electrolytes in aqueous solutions, given by Chanson and Millero (2006) [18]. The model parameter was estimated and validated estimating the solubility of boric acid in lithium sulfate, sodium sulfate and potassium sulfate aqueous solutions at different temperatures and salt concentrations. The model represented satisfactorily the data for the systems (SD=0.033molkg−1 H2O for H3BO3+Na2SO4+H2O, SD=0.020molkg−1 H2O for H3BO3+Li2SO4+H2O and SD=0.030molkg−1 H2O for H3BO3+K2SO4+H2O). The model parameters are valid to maximum concentration of the salts, 3.380molkg−1 for Na2SO4, 3.149molkg−1 for Li2SO4 and 1.245molkg−1 for K2SO4 from 293.15K to 313.15K.Based on the results it was determined that lithium sulfate is a precipitant agent for boric acid and its behavior is attributed to the salting out effect of Li+ ion; sodium and potassium sulfates increase the boric acid solubility; this salting in effect is due to the presence of Na+ and K+ ions. The presence of these salts can be unfavorable for the crystallization of boric acid due to the increase of solubility which decreases the supersaturation, therefore the yield of the process.Comparing the parameters for the system H3BO3+Na2SO4+H2O, H3BO3+Li2SO4+H2O and H3BO3+K2SO4+H2O, it was found that effect of these ions on the decreasing of the solubilty of boric acid in aqueous sulfate solutions follows the order: Li+>Na+>K+, which can be attributed to the increase of their ionic radii, coordinated with 6 water molecules therefore the capacity to form hydration shells.

Improvement of crystallization and particle size distribution of boric acid in the processing of a boron carbide precursor

Nano boric acid crystal fabrication from the combined effect of hexane and dispersant.

Sodium sulfate recovery from boric acid liquors

Boric acid has multiple uses in several industries, the pharmaceutical, glass and ceramic industries being some of them. Its obtaining involves the addition of sulfuric acid to slurry of calcium–sodium borate at 70°C and, after the removal of insoluble undesired material, the crystallization of boric acid by cooling the clear solution to 15–25°C. The residual mother liquor contains mainly sodium sulfate, sodium chloride and boric acid in concentrations depending on the composition of the borate ore and the temperature at which the boric acid is recovered. The addition of methanol or ethanol to these liquors is proposed to recover selectively most of the sodium sulfate by salting-out. This separation of the main component of the solution would not only produce a more easily disposable liquid waste but it would also reduce its over-all volume as it could be recycled into the main process. 98% of the sodium sulfate can be recovered with a 7:3 volume to volume ratio of alcohol to mother liquor. The recovery is improved when the concentration of boric acid is reduced before the salting-out. Ethanol proved to be 35% more efficient for the selective precipitation process than methanol. Sodium chloride and boric acid do not co-precipitate with the sodium sulfate.

Crystallization of Boric Acid through Reactive Dissolution of Oxalic Acid Crystals in Aqueous Borax Solution

AbstractIn the reaction between solid oxalic acid and aqueous borax solution, product boric acid precipitates in the solution phase after complete dissolution of oxalic acid crystals. This makes it possible to measure the dissolution and precipitation kinetics independently. The oxalic acid dissolution process was found to be diffusion controlled. Nucleation rates of product boric acid were influenced by heterogeneous primary nucleation and secondary nucleation, while growth rates were affected by both the bulk diffusion and surface integration processes. Dissolution of oxalic acid ctystals was described by a surface reaction model, and overall nucleation and growth rates were correlated in terms of the influencing process variables. Simulation analysis was carried out by linking the two processes of dissolution and precipitation in series. The trends of the predicted supersaturation profiles, number concentration, magma density and mean size of the product crystals are in good agreement with measured values.

Kinetics of gypsum formation and growth during the dissolution of colemanite in sulfuric acid

One of the most important boron minerals, colemanite, is dissolved in aqueous sulfuric acid to produce boric acid whereby gypsum is formed as byproduct. Filtration of gypsum has an important role in boric acid production because gypsum affects the efficiency, purity and crystallization of boric acid. The formation and growth kinetics of gypsum during the dissolution of colemanite in aqueous sulfuric acid were studied in a batch reactor by varying the temperature (60–90°C), stirring rate (150–400 rpm), and initial concentrations of the reactants. The initial CaO/H 2SO 4 molar ratio was varied between 0.21–0.85 by keeping the initial concentration of sulfate ion at [SO 4 2−] o=0.623 mol/l, and 0.85–3.41 by keeping the initial concentration of colemanite at [B 2O 3] o=0.777 mol/l. The crystallization of gypsum from the solution was followed by monitoring the calcium ion concentration in the solution as it is decreased by the formation of calcium sulfate precipitate. The calcium ion concentration in the liquid phase first undergoes a rapid exponential decay and then slowly approaches an asymptotic value of the saturation concentration at the respective temperature. The saturation concentration decreases with the increasing temperature from 5.2 mmol/l at 60°C to 3.1 mmol/l at 80°C, however, further increase in the temperature up to 90°C causes an increase in the saturation concentration to 5.1 mmol/l. The stirring rate was found to have no significant effect on dissolution in the range of 150–400 rpm. The minimum saturation concentration of the calcium ion was obtained at 80°C when the initial CaO/H 2SO 4 molar ratio is 0.85. The boric acid concentration in the solution decreases with the decreasing initial concentration of sulfuric acid. After the fast dissolution reaction of colemanite in aqueous sulfuric acid, the nucleation of the gypsum crystals first occurs from the supersaturated solution and then the crystals grow on these nuclei. The needle like crystals become wider and taller on prolong crystallization. The rate of gypsum crystallization reaction was second order with respect to saturation level. The evaluation of the kinetic data in an Arhenius plot gives an activation energy of 34±2 kJ/mol for the crystal growth of gypsum from the supersaturated solution obtained by dissolution of colemanite in aqueous sulfuric acid.

On the confluence of dissolution, reaction and precipitation: The case of boric acid production

Reactions of borax with organic acids were studied in a batch mode. Product boric acid resulting from the reaction of solid borax with liquid propionic acid was found to deposit on the surface of the unreacted borax core to form an ash layer. Sharp interface unreacted core model was used for simulation of this process. The analytical solution to the model involving diffusion through a liquid boundary film, diffusion through the solid reaction products, and surface reaction was obtained. Reaction of borax solution with oxalic acid crystals exhibited a different route, i.e., reactive precipitation process. Crystallization of boric acid took place in the solution phase after complete dissolution of oxalic acid crystals. Nucleation and growth rates were evaluated from a series of experiments by the method of s plane analysis. Relative nucleation kinetics were correlated in terms of the significant process variables.

Experimental and analytical studies of boric acid concentrations in a VVER-440 reactor during the long-term cooling period of loss-of-coolant accidents

Concentrating and mixing of boric acid (H 3BO 3) during the long-term cooling period of loss-of-coolant accidents (LOCAs) in the Loviisa VVER-440 reactors has been studied with the REWET-II and VEERA facilities. To get more detailed information on boric acid mass transfer, a specific facility was built to simulate boron mixing in the lower plenum of the reactor. The experiments with the VEERA facility showed that in the VVER-440 reactor fuel bundles the mixing is complete due to boiling and U-tube oscillations and, hence, concentration distribution of boric acid in the bundles is uniform. The U-tube oscillations proved to be an important mechanism in transferring concentrated boric acid from the core to the lower plenum. The experiments demonstrated that crystallization of boric acid in the reactor core simulator is possible, if a stable long-term cooling situation with water boiling in the core continues long enough. In the experiments the crystallization of boric acid in the core simulator led to a flow blockage of the fuel rod bundle and overheating of the rod simulators when the flow through the core ceased. Experimental results were used to develop a computational model for calculations of boric acid concentrations in the reactor during LOCAs. The development work was supported with a series of RELAP5/MOD3 small-break LOCA analyses. The results of the RELAP5/MOD3 calculations were used to determine the boundary conditions under which concentrating of boric acid might occur. Reactor analysis showed that the crystallization of boric acid in the reactor is not possible during the long-term cooling period of LOCAs. This is mainly due to the fact that the ice-condenser in the Loviisa plant contains sodium tetraborate Na 2B 4O 710H 2O (borax), which enters the reactor when emergency core cooling water is taken from the sump. Borax increases greatly the solubility of boric acid in water and, hence, decreases the risk of crystallization.