Transactions of the Korean Nuclear Society Spring Meeting Jeju, Korea, May 29-30, 2014
An Investigation of Spray Performance to Remove Gaseous Iodine- Approach to mitigate the consequences of severe accident a
Irfan Younus a, Man Sung Yim a* Nuclear and Quantum Engineering Department, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Daejeon, South Korea * Corresponding author:
[email protected] 1. Introduction
The severe accident at nuclear power plant involving core degradation and containment over- pressurization can result in the failure of reactor containment building. Fukushima Daiichi Nuclear power plant accident in March 11, 2011 in Japan activated by an earth quake and tsunami is the recent example of such failure. This accident has created a massive impact on the nuclear industry today. As a result of reactor containment failure, significant amount of radioactive fission products was released into the open atmosphere. From such accident as Fukushima, we learn a lesson that possibility of occurrence of further severe accidents at nuclear power plants of any country in the world cannot be ignored. New technological approaches need to be in place to address such concern which has significantly deteriorated public confidence in nuclear power. Such technological approach must be capable of systematically mitigate the consequence of severe nuclear accidents involving radioactivity release. An example of such approach is spray technology. In case of an accident involving radioactivity release to the environment, it may possible to deploy spray system to quickly respond to the released radioactivity and to minimize the impact of accidental releases on humans and the environment. During early phase of Fukushima nuclear accident mitigation process, water spray operations were carried out through fire trucks and military helicopters, but the primary concern of such operations was to cool down the reactor and to extinguish the fire and not to minimize the spread of radioactive materials. The aim of this research is to investigate spray technology for effective and efficient capturing of fission products released from leaked/damaged nuclear reactor to the environment. For this purpose, a systematic approach with in depth information about release phenomena and spray features will be required. Based on the information regarding release phenomena including types of materials and their amount and size, release locations, release conditions such as rates, velocities, temperature, etc., requirements for spray application is being developed including spray material types (foam, mist etc.), spray solution additives, flow rates, pressure, drop size, spray coverage area and spray duration, etc. Subsequently the efficiency and effectiveness of spray system to reduce the dispersion of
radioactivity in the environment during the course of severe accident can be characterized. This paper is a summary of our initial investigation for the use of spray technology to reduce the consequence of severe nuclear accident. 2. Experimental Setup Under severe accident, the fission products released into the atmosphere are in the form of gases or aerosols [1]. From the radiological point of view, the 131I isotope is known to be one of the major radionuclides that would contribute to the dose rate during the short term after the accident. Although the released radioactive gases consist of particles, iodine and organic compounds [2], our initial investigation was for gaseous iodine. This was because it was expected that capture by spray is more difficult with gas than with particles. To evaluate the capture efficiency of gaseous iodine through chemical spray, a laboratory-scale experimental setup was developed as shown in Figure 1-2.
Fig. 1. Experimental setup to capture gaseous iodine
Fig.2. Experimental facility to capture gaseous iodine
The main objective of small scale laboratory work was to design larger scale testing facility set up and to gain spray operating experiences with iodine gas.
Transactions of the Korean Nuclear Society Spring Meeting Jeju, Korea, May 29-30, 2014
2.1 Preliminary experiments to evaluate system performance Preliminary experiments were performed to evaluate iodine gas generation, iodine deposition in transmission pipes, spray nozzle flow rate and coverage in the spray chamber, Pump flow rate and valves function was also checked. Through these preliminary experiments, complete mass balance of iodine in the system was characterized. Iodine gas flow rate and temperature was determined to minimize the loss of iodine through deposition in the transmission pipes (Figure 3).
spray chamber was collected in two sampling cylinders attached with the outlet port of the spray chamber containing NaOH solution. The concentration of iodine in the collected samples was measured through uv-vis spectrometer and Ion selective electrode. When iodine reacts with NaOH and Na2S2O3, the following reaction occurs [3]: 4I2+10NaOH+Na2S2O3 8NaI+2Na2SO4+5H2O
(1)
In the current work, the effect of chemical spray (0.5 % NaOH, 0.2 % Na2S2O3) flow rate on gaseous iodine removal efficiency was examined.
Reduction in Deposited iodine
iodine Deposited
3. Results and Discussions
a
Pre-Experiment-1
b
Pre-Experiment-2
Fig.3 Iodine deposition reduction in pipes by improving Iodine gas flow rate and temperature
2.2 CFD simulations to improve the spray chamber design and spray nozzle selection
Table 1 and Figure 5 show the iodine removal efficiency at three different spray flow rates, i.e., 1.42, 1.68 and 2 lit/min. The pH of spray solution was 13 and carrier gas release rate in the spray chamber was 25 liter/min. The temperature of spray chamber was 23 0C. The experimental results show that the iodine gas removal efficiency increases from 77 % to 98.7% when the spray flow rate increasing from 1.42 lit/min to 2 lit/min. Table 1: Iodine removal efficiency by spray
Computational fluid dynamics method was used to improve the spray chamber design and to select the spray nozzle for better spray coverage inside the chamber. For this purpose, multiphase flow modeling was performed with the use of ANSYS CFX software. The result of this modeling effort is shown in Figure 4. The liquid fraction in the spray chamber was analyzed based on based Eulerian-Lagrangian approach.
Sr. No
Spray flow rate (lit/min) 1.42 1.68 2
1 2 3
Efficiency (%) 77 95.5 98.7
100
b
Iodine removal efficiency (%)
a
The spray experiment to capture iodine was performed using 5 grams of solid iodine in hot platebased iodine gas generator. The gaseous iodine was transported and released into the spray chamber through Teflon pipe by passing compressed air at the flow rate of 25 lit/min as a carrier gas through the iodine gas generator. The full cone 1/8G3 spray nozzle was installed in the spray chamber. The spray solution of pH 13 was prepared by adding 0.5 % NaOH and 0.2 % Na2S2O3 in water and placed in storage tank. The sprayed solution after capturing iodine gas was collected in a container. The uncaptured iodine gas in
90
85
80
75
Fig. 4. (a) Contour of liquid volume fraction at 2 lit /min flow rate, (b) actual spray from nozzle
2.3 Main spray experiment to capture gaseous iodine
95
1.4
1.5
1.6
1.7
1.8
1.9
2.0
Spray flow rate (lit/min)
Fig.5. Effect of spray flow rate on Iodine removal efficiency So far, our work demonstrated successful implementation of spray experiment to examine capture efficiency of iodine gas. The study is being continued to investigate the following:
The effect of spray solution pH, iodine concentration and release rate on removal efficiency Use of foam material and its spray performance to encapsulate gaseous iodine and aerosols
Transactions of the Korean Nuclear Society Spring Meeting Jeju, Korea, May 29-30, 2014
Computational fluid dynamics (CFD) simulation of removal efficiency of spray system for the design of large scale spray system Validation of the CFD spray model by using experimental data Examination of radiation dose reduction effect on the site workers and the population near the plant, Development of an integrated accident mitigation system based on spray technology and related cost analysis 4. Conclusion
An experimental investigation of iodine removal efficiency in a spray chamber is demonstrated. The spray solution of pH 13 was prepared by adding 0.5 % NaOH and 0.2 % Na2S2O3. At constant carrier gas flow rate, the iodine removal efficiency was found to increase with increasing the spray flow rate. The study is being continued for the development of an integrated accident mitigation system based on spray technology. Acknowledgments This work was supported by the Nuclear Power Core Technology Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20131510400050).
REFERENCES [1] Baklanov, J.H. Sorensen, “Parameterization of radionuclide deposition in atmospheric long range transport modeling” Phys. Chem. Earth (B), 26, 787799 (2001). [2] L Soffer, S. B. Burson, C. M. Ferrell, R. Y. Lee, J. N. Ridgely, “Accident Source Terms for Light-Water Nuclear Power Plants” NUREG-1465, 1995A. [3] T. H. Row, L. F. Parsly, H. E. Zittel, Design consideration of Reactor Containment spray systemsPart 1, Oak Ridge National laboratory, 1969.
