(An) investigation of spray based system to mitigate severe accident consequences

Abstract

After Fukushima nuclear accident in 2011, the possibility of severe accidents occurrence at nuclear power plants of any country in the world cannot be ignored. Such disastrous situation involving the release of radioactive material such as radioactive iodine and particles to the atmosphere can create significant impact on human health through thyroid cancer. Therefore, it is highly desirable to have new technological approaches to be placed for containing and limiting the spread of radioactive fission products. The purpose of this study was to examine the use of spray technology to mitigate the consequence of severe accidents outside the containment. An emergency response spray system can be deployed outside the conventional physical barrier-the reactor containment to prevent the dispersion of hazardous radionuclide. In this study, the efficiency of spray method in removing the released fission products, in particular, gaseous iodine and particles were investigated under the hypothetical accident situation. Effects of different parameters such as spray flow rate, spray solution pH, spray materials types and gas release rate were investigated as part of the study. Laboratory-scale experimental systems combining spray chamber with a nozzle and gaseous iodine and aerosol particles release port was setup to examine the performance of the proposed system. The alkaline (aqueous NaOH.$Na_2S_2O_3$) solutions of varying pH combined with water and foam based spray material (sodium lauryl sulphate) were used in the spray chamber to examine capture efficiency of gaseous iodine and aerosol particles. The captured and un-captured fractions were determined by the UV-Visible spectroscopy and ion selective electrode methods. Experimental results showed that use of proper spray solution pH and flow rate was important in determining iodine removal efficiency. The removal efficiency was also affected by the release rate of gaseous iodine. The removal efficiency of foam based- sprays was observed significantly higher when compared with water based-sprays. Temperature variation was also found to have effect on the volatility of the captured iodine in the sprayed solution. A mathematical model based on two film theory was also used to calculate the removal efficiency and was validated by the experimental results. The computational fluid dynamics (CFD) simulations of aerosol particle removal efficiency by sprays in ANSYS CFX were also performed and results were compared with experimental results. The radioactive iodine source terms released from a breached nuclear power plant as a result of severe accident and projected radiation doses were also estimated using RASCAL 4.3 computer code and the impact of proposed spray system on radiation dose was analyzed. Furthermore, the concept of deploying spray system to mitigate the consequence of severe accidents outside the containment or other buildings of a nuclear power plant were explored. Moreover, an economic study was performed to estimate the cost of fire-truck based spray system during a real nuclear accident situation.