Simulated experiments with TiO2 particles using a lab-designed single-stage impactor to evaluate impaction characteristics of particles leaked by steam generator tube rupture
- 19 Downloads
If a steam generator tube rupture (SGTR) occurs during a severe accident in a nuclear power plant, radionuclides can be released to the atmosphere as an aerosol. The release of radioactive compounds can be prevented if these compounds are deposited on the tube walls. To quantify the fraction of aerosol particles retained in the SG and to effectively trap the radioactive aerosols during a severe accident, characteristics of particle impaction on surrounding SG tube walls must be evaluated. In this study, TiO2 agglomerates were used for experiments. Particle breakup and bounce behavior due to impaction were evaluated by measuring aerosol number concentration as a function of particle size and by analyzing transmission electron microscopy images before and after impaction.
KeywordsParticle breakup Radioactive compounds Steam generator tube rupture (SGTR) Single-stage impactor TiO2 agglomerates
Steam generator (SG) tubing is subjected to various degradation processes, which can lead to cracking, thinning, and potential rupture. Despite improvements in SG design, manufacturing, and modes of operation, SG tube rupture (SGTR) events occasionally occur during pressurized water reactor (PWR) operation. If an SGTR occurs during a severe accident in a nuclear power plant, radionuclides may leak from the primary circuit to the secondary side. The radionuclides would then bypass containment and be directly released to the atmosphere as an aerosol [1, 2, 3]. The release of radioactive compounds into the atmosphere can be prevented if these compounds are deposited on the tube walls during the SGTR event . To effectively trap the radioactive aerosols during a severe accident, characteristics of particle impaction on surrounding SG tube walls must be evaluated.
The Paul Scherrer Institute (PSI) coordinated an international project called ARTIST (Aerosol Trapping In a Steam Generator) to quantify the fraction of aerosol particles that are retained in the SG under selected severe accident conditions . In studies for the ARTIST project [1, 2], aerosol retention in a scaled steam generator tube bundle was investigated using TiO2 agglomerates. The aerosol deposition and breakup characteristics were evaluated by measuring aerosol concentration and particle size distribution at the inlet and outlet of the ARTIST facility. Ihalainen et al.  devised a method to study both agglomerate breakup and bounce during impaction. Ihalainen et al.  evaluated the particle breakup and bounce behavior due to impaction by using a micro-orifice uniform deposit impactor (MOUDI) and a sampling chamber. Ihalainen et al. [5, 6] used TiO2 agglomerates that had the branched chain-like structure. However, Kissane  reported that fission products were observed to be agglomerates in closely packed shape. Kissane  sampled the fission products inside a nuclear reactor and investigated the morphology of the sampled fission products by using TEM images.
In this study, TiO2 agglomerate particles were generated through an atomizer and passed through a tube furnace at high temperature so that the particles might be sintered and formed into a dense cluster structure. A single-stage impactor designed by Hyun et al.  was used for the particle impaction test. The impaction characteristics of the TiO2 agglomerates were studied by transmission electron microscopy (TEM) and by measuring the size distribution of the TiO2 agglomerates before and after impaction.
2 Materials and methods
The size distribution of the aerosolized TiO2 particles was measured by a scanning mobility particle sizer (SMPS) (see path ⓐ of Fig. 2). The SMPS system was composed of a classifier controller (3080, TSI, USA), a differential mobility analyzer (DMA; 3081, TSI, USA), a condensation particle counter (CPC; 3776, TSI, USA), and an aerosol charge neutralizer (4530, HCT, Korea) with a sample flow rate of 0.3 L/min and a sheath flow rate of 3 L/min. The SMPS measures particles by their electrical mobility equivalent diameter and operates in a diameter range of approximately 10–700 nm. The SMPS results are shown in Fig. 1b. The GMD (geometric mean diameter) and GSD (geometric standard deviation) of the generated TiO2 particles were 210 nm and 1.93 nm, respectively (Fig. 1b). The TiO2 particles then passed through a tube furnace in which the gas temperature was controlled at 500 °C. After passing through the furnace, the TiO2 particles entered a DMA to classify the particles by their electrical mobility. With the DMA, monodisperse TiO2 particles with 476 nm GMD and 1.24 GSD were generated (Fig. 1c).
The DMA-classified particles were diluted in terms of concentration and entered a single-stage impactor that was designed to accelerate particles with variable jet velocities. The jet velocity of the impactor was varied from 48 to 260 m/s by changing the nozzle size. The particles were accelerated through the nozzle and impacted onto a plate of the impactor. The material of the impaction plate was stainless steel. A TEM grid located on the plate was used to analyze the deposited particles. At the outlet of the impactor, the pressure decreased to a value below 1 atm.
After the agglomerates were impacted on the impaction plate, they were delivered to a pressure recovery chamber located downstream of the impactor. The chamber introduced by Ihalainen et al.  was used such that the measurement of particle size distribution was taken with aerosol instruments at ambient pressure. Twenty minutes were required for the particle concentration to reach a steady state value in the chamber. After 20 min, the chamber was pressurized with filtered air for 30 s. The pressure in the chamber was then recovered to ambient pressure. Next, the particle size distribution in the chamber was measured using the SMPS. The sampling flow rate was 0.3 L/min.
3 Results and discussion
When the jet velocity was lower than 90 m/s, particles of 453 nm were mostly detected downstream the impactor. It was found that some of the particles entering the impactor nozzle exited the impactor. (Note that the mean size of particles entering the nozzle was 470 nm.) However, when the jet velocity was equal to or higher than 90 m/s, small sizes (for example, 52, 93, 166 nm) were detected. Therefore, the results of Fig. 5 show a quantitative evidence about potential existence of threshold kinetic energy (impaction) which causes the collision-induced breakup.
The increase in jet velocity did not affect the particle size distribution until the jet velocity reached 63 m/s. However, when the jet velocity increased to 90 m/s, small particles were measured. The degree of particle breakup increased when the jet velocity increased to 120 m/s but did not increase further when the jet velocity became 260 m/s.
As the TiO2 particles broke up after impaction at high speed (> 90 m/s), some of the particles were deposited while others exited the impactor. The GMD of the bounced particles was approximately 100 nm, which was less than that of the intact particles.
Therefore, in a SGTR situation in a nuclear power plant, some of the small radioactive compounds may leak into the atmospheric environment, and filtration systems may need to be installed to minimize the release of radioactivity from the SGTR.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science, ICT, and Future Planning) (No. NRF-2017M2A8A4015280).
- 4.US NRC, Severe accident risks: an assessment of five U.S. nuclear power plants, vol. 2. Sandia National Laboratory Report NUREG-1150 (1990)Google Scholar