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Background <p>Keeping radioactive iodine within the containment in the event of accidents at NPPs with VVER reactors is one of the important tasks that needs to be solved. The key factor affecting the volatility of iodine is the pH of the aqueous medium inside the containment. According to EUR safety requirements, the pH should exceed 7.0 to suppress volatile forms of iodine. The lack of domestic calculation models for assessing pH dynamics during accidents significantly complicates the design process.</p> Aim <p>To develop and validate a&#xa0;model for calculating the pH values of the aqueous medium in the containment during design basis and beyond design basis accidents at NPPs with VVER-1200 reactors.</p> Materials and methods <p>The paper considers two scenarios with an initial event of a&#xa0;double-ended guillotine rupture of the main circulation circuit: a&#xa0;design basis accident without damage to the core and beyond design basis accident with significant damage to the core and failure of the active part of the emergency cooling system. The studied object is the pH of the aqueous medium in the containment. The research method is numerical simulation based on the developed mathematical model and software code for solving the equations of chemical equilibrium between components in aqueous solutions. Experimental methods for validation of the mathematical model involve potentiometric pH measurements in aqueous solutions with known concentrations of components at 25 °C.</p> Results <p>A&#xa0;mathematical model for calculating pH values of the aqueous medium in the containment is developed and validated. Calculations show that during a&#xa0;design basis accident, pH ranges from 4.2&#xa0;to 8.0; the maximum pH values are reached by the 75th&#xa0;minute. Ten hours after the onset of the beyond design basis accident, the pH drops to 3.2 due to the formation of nitric and hydrochloric acids. After the sprinkler system is turned on, the pH rises to 7.8 and remains at this level for up to 30&#xa0;days after the accident. Due to design solutions of introducing additional alkali starting from the 40th&#xa0;minute, the pH becomes higher than 7.0 and remains in the range from 8.0&#xa0;to 8.4 until the sprinkler system is turned on.</p> Conclusion <p>The adequacy of the proposed model is confirmed by experimental data. The performed verification of the model shows the engineering solutions adopted at NPPs with VVER-1200 ensuring compliance with the requirements for binding radioactive iodine in design basis and beyond design basis accidents.</p>

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Numerical simulation of changes in the pH of the aqueous medium in the containment sump during design basis and beyond design basis accidents at NPPs with VVER-1200 reactors

  • 期刊论文

  • 发表日期：

    2025年9月22日

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  • A. V. Gavrilov,
  • D. R. Nigmatullin,
  • N. A. Prokhorov,
  • V. G. Kritskii

摘要

Background

Keeping radioactive iodine within the containment in the event of accidents at NPPs with VVER reactors is one of the important tasks that needs to be solved. The key factor affecting the volatility of iodine is the pH of the aqueous medium inside the containment. According to EUR safety requirements, the pH should exceed 7.0 to suppress volatile forms of iodine. The lack of domestic calculation models for assessing pH dynamics during accidents significantly complicates the design process.

Aim

To develop and validate a model for calculating the pH values of the aqueous medium in the containment during design basis and beyond design basis accidents at NPPs with VVER-1200 reactors.

Materials and methods

The paper considers two scenarios with an initial event of a double-ended guillotine rupture of the main circulation circuit: a design basis accident without damage to the core and beyond design basis accident with significant damage to the core and failure of the active part of the emergency cooling system. The studied object is the pH of the aqueous medium in the containment. The research method is numerical simulation based on the developed mathematical model and software code for solving the equations of chemical equilibrium between components in aqueous solutions. Experimental methods for validation of the mathematical model involve potentiometric pH measurements in aqueous solutions with known concentrations of components at 25 °C.

Results

A mathematical model for calculating pH values of the aqueous medium in the containment is developed and validated. Calculations show that during a design basis accident, pH ranges from 4.2 to 8.0; the maximum pH values are reached by the 75th minute. Ten hours after the onset of the beyond design basis accident, the pH drops to 3.2 due to the formation of nitric and hydrochloric acids. After the sprinkler system is turned on, the pH rises to 7.8 and remains at this level for up to 30 days after the accident. Due to design solutions of introducing additional alkali starting from the 40th minute, the pH becomes higher than 7.0 and remains in the range from 8.0 to 8.4 until the sprinkler system is turned on.

Conclusion

The adequacy of the proposed model is confirmed by experimental data. The performed verification of the model shows the engineering solutions adopted at NPPs with VVER-1200 ensuring compliance with the requirements for binding radioactive iodine in design basis and beyond design basis accidents.