Nuclear energy is a viable long-term energy source given its abundance, nil contribution to global warming, and reliability. The main safety concern of operating a nuclear reactor for electricity generation is the possibility of design-based and other accidents. Given the psychological impact of any nuclear accident, advanced technologies have been adopted to make reactor operation failsafe and mitigate the consequences in the worst case of a failure. The melting of a reactor core under severe accident conditions has been a priority in reactor accident research studies. Understanding the phenomenology of an accident, engineering safety features, and taking corrective action after an unforeseen scenario have always been enhanced by information gained after accidents like Three Mile Island (1979), Chernobyl (1986), and most recently, Fukushima (2011), Sehgal (2011). In a core meltdown scenario, radioactive fission products and structural materials may be released into the atmosphere in aerosol form. These aerosols can then be transported by advective currents up to thousands of kilometers from the release point (Pöllänen et al., 1995). Concentrations of aerosol particles during a reactor accident can be as high as 100 g / m 3 (mass), corresponding to 1 0 13 / m 3 (Sher et al., 1994). An evaluation of radioactive surface contamination in the environment resulting from a theoretical nuclear research reactor incident indicates that radioactivity levels can reach 9.5 × 1 0 2 k B q / m 2 60 km from the site along the plume centerline (Xoubi, 2020). It is also known that radioactive aerosol particles from fallout can enter the food chain (Shahidi and Brown, 1998) and damage the DNA of cells, even leading to cancer at exceedingly high doses (Kamiya et al., 2015). The resuspension of radioactive particles in the post-accident phase and residual contamination on the accident site could lead to delayed hazard after a reactor accident (Kottapalli and Novosselov, 2019). Radioactive particles were found to be resuspended and transported over intercontinental distances due to forest fires in the Chernobyl region (lerko et al., 2021). Following the Fukushima Daiichi Nuclear Power Plant (F1NPP) disaster, investigations measured radiocesium levels in animals, plants, and fungi in Svalbard (Mezaki et al., 2019). The deposited insoluble radiocesium-bearing microparticles (CsMPs) released from the F1NPP accident in March 2011 become secondary contamination sources due to resuspension by atmospheric migration or other environmental transferring processes. These particles were detected 25 kms from the reactor even late in 2019, providing evidence of natural resuspension processes (Tang et al., 2022).

Understanding deposition and resuspension phenomena are important for source term evaluation under a severe reactor accident scenario. The deposition and resuspension of aerosol particles in reactor component sections in such cases determines the release of radioactivity into the environment. Radionuclides are less likely to be retained in the circuit when they are resuspended; when they are discharged directly into the environment in bypass sequences, the phenomena are of particular importance. The deposition of aerosol particles depends on factors like temperature gradient, gravitational settling, nature of flow, and aerosol geometries. The nature of the deposited layer (mono- or multi-layer), aerodynamic forces, deposited particle properties, and surface features are the parameters that affect resuspension processes. During a turbulent flow, the deposited layer of particles loses its ability to adhere to surfaces and becomes resuspended in flow. This happens when the aerodynamics forces (lift and drag) acting on the particle become larger than cohesive and adhesive forces for particle–surface interactions. Depending upon its size, gravitational force, and aerodynamic force strength, peeled-off aerosol particles follow a rolling motion or a combination of lifting and rolling in the resuspension phase. These mechanisms are modeled in two categories: force-and-moment balance models and energy balance models (Gradoń, 2009). Micro-videography-based investigation by Ibrahim et al. (2003) showed the rolling of particles and subsequent particle-to-particle collisions as two main mechanisms of detachment from the deposited surface, with rolling as the primary contributor. This phenomenon is particularly relevant in reactor components such as vacuum breaker lines, steam generator (SG) tubes, and decay heat removal piping, where deposited materials can re-entrain into the gas flow during system depressurization or emergency cooling operations. Aerosol retention during steam generator tube rupture (SGTR) severe accidents is significantly higher in a flooded steam generator than in a dry one (Lind et al., 2020). Mechanical resuspension within the reactor coolant system (RCS) is particularly important in bypass scenarios such as SGTR as it can directly contribute to the source term. It may occur due to events like core quenching from delayed emergency core cooling system (ECCS) activation, core collapse into residual water at the vessel bottom, or rapid RCS depressurization Parozzi (1992). Additionally, resuspension can happen under high-velocity turbulent gas flows, even under steady flow rates.

In the context of reactor accident aerosol research, specific studies have been conducted in different facilities around the world. Several large-scale experimental campaigns have played a foundational role in shaping our understanding of aerosol behavior in light-water reactor (LWR) severe accidents. The LWR Aerosol Containment Experiments (LACE), conducted at HEDL in Richland and coordinated by Oak Ridge National Laboratory, investigated aerosol retention under containment-scale thermal-hydraulic conditions and provided critical validation data for codes such as MELCOR and CONTAIN (Wright et al., 1986). The CODEX-Aerosol program, conducted at the KFKI Atomic Energy Research Institute, explored aerosol generation and resuspension during simulated reactor core degradation using small-scale fuel bundles. Aerosol emissions—monitored in oxidizing and steam environments—showed strong correlations with temperature rise and fuel cladding oxidation, highlighting the role of chemical speciation and morphology in transport behavior (Pintér Csordás et al., 2000; Hózer et al., 2006). The Aerosol Code Evaluation/Radioactive Tracer Facility (ACE/RTF) experiments further focused on iodine volatility and resuspension under high radiation fields and varying containment pH. These studies demonstrated that iodine speciation becomes independent of initial chemical form due to radiolytic interconversion and that stainless steel versus epoxy surfaces significantly affect sorption behavior. The series also highlighted modeling gaps in organic iodide prediction, surface reactivity, and radiation-induced transformations. Additionally, the Mod-Advanced Corium Experiments (MACE) program under ACE Phase D provided data on heat transfer during molten core–concrete interaction and the generation of aerosols and gases during quenching events—relevant for containment pressure and source term evaluation (Kupferschmidt et al., 1992; Sehgal and Spencer, 1992). Aerosol behavior under severe accident conditions has been studied by performing small and large-scale experimental programs such as PHEBUS FP (Gonfiotti and Paci, 2018; Kissane and Drosik, 2006), DEMONA (Liljenzin et al., 1990), WAVE (Hidaka et al., 2000), DRAGON (Suckow and Guentay, 2008), STORM (De los Reyes et al., 1999), THAI (Gupta et al., 2015), Falcon (Beard et al., 1992), and Wind (Maruyama et al., 1999; Freitag et al., 2018; Freitag et al., 2020). Such studies in context of the Indian Pressurized Water Reactor Program (IPHWR) have been conducted in the National Aerosol Test Facility (NATF) at the Bhabha Atomic Research Centre, India (Sapra et al., 2008; Modi et al., 2014; Dwivedi et al., 2019). To the best of our knowledge, there has been no research into the resuspension phenomenon in such medium- or large-scale facilities under dry conditions. The Indian Institute of Technology (IIT) Kanpur, has recently established the National Aerosol Facility (NAF), which aims to investigate the spatial and temporal evolution of metal oxide aerosols in simple and complex geometries incorporating the study of the deposition and resuspension behavior of aerosols in dry and wet conditions for the context of the IPHWR program. Studies such as the hygroscopic nature of nuclear-accident-relevant cesium-bound compounds (Mishra et al., 2019a; Mishra et al., 2019b), characterization of aerosol generator system (Dwivedi et al., 2020), and the methodology to perform measurements of droplet (Kumar et al., 2022) have been recently performed at NAF.

This study focuses on the deposition and resuspension characteristics of zinc metal oxide aerosol particles in a circular cross section straight piping system of stainless steel at NAF. The choice of a plasma torch aerosol generator (PTAG) as an aerosol generation system ensures that the characteristics of the aerosol particles in the test sections remain as needed for simulating the accident conditions. The choice of zinc oxide (ZnO) aerosols as the test material was based on a combination of operational and physical factors. Metal powders are mostly preferred in PTAGs (Venkatramani, 2002), resulting in controlled and stable aerosol generation, and zinc metal powder fulfils this requirement. In past, studies at NATF (Sapra et al., 2008) and NAF (Dwivedi et al., 2020) have been helpful in optimizing PTAG parameters for experiments such as the current study. In addition, resuspension and thermophoresis are mostly physical phenomena, so they accurately simulate the experiential behavior of any chemical/radioactive species as long as the size and concentration is normalized with respect to the demand conditions.

Despite extensive research on aerosol behavior in nuclear containments, a significant gap remains in understanding deposition and resuspension phenomena within reactor piping systems, especially under temperature gradients and dynamic flow conditions that arise during severe accidents. While existing models include resuspension modules, their validation is often limited to idealized geometries or small-scale test data. This study addresses that gap by conducting large-scale experimental simulations in a stainless-steel pipe representing a segment of reactor primary system piping using measured gas/wall temperatures and flow velocities as inputs. The central hypothesis is that thermal gradients and flow transitions directly govern deposition and resuspension dynamics and that models like SOPHAEROS can reproduce these effects when provided with realistic boundary conditions. The objective is twofold: first, to advance the physical understanding of aerosol behavior under accident-relevant thermal-hydraulic conditions, and second to evaluate the predictive capability of SOPHAEROS under such conditions. The study is designed to contribute to international efforts to refine source term predictions, improve safety code validation, and generate data relevant to severe accident management in light water reactors.

This work used numerical simulations performed with Aerosol Source Term Evaluation Code (ASTEC) to compare experimental and theoretical results. ASTEC, jointly developed by the French Institut de Radioprotection et de Sûreté Nucléaire (IRSN) and the German Gesellschaft für Anlagen und Reaktorsicherheit mbH (GRS), is used for simulating an entire severe accident sequence in a nuclear water-cooled reactor from an initiating event. It has been used for several studies/applications in varied contexts, such as source term estimation, probabilistic safety assessment, accident management, and phenomenological interpretations (Chatelard et al., 2014). The modular flexibility of ASTEC helps researchers design experiments for studying standalone as well as integral mechanisms that target specific reactor component systems (RCS) and/or a certain phase of the accident sequence. The SOPHAEROS module of ASTEC is intended to simulate major fission product vapor and aerosol phenomena in the RCS, composed of a 1D series of control volumes (Cousin et al., 2008).

The SOPHAEROS module solves aerosol and vapor transport equations using a volume-based iterative approach. Each control volume represents a segment of the physical pipe and is solved sequentially at every step using an implicit Newton–Raphson scheme. In each volume, SOPHAEROS first determines the vapor-phase chemical equilibrium based on the mass of elements and species transferred from the previous iteration. It then constructs a matrix system that includes fluxes and source terms related to key physical phenomena: homogeneous and heterogeneous nucleation, condensation/evaporation, coagulation, sorption, deposition, and mechanical resuspension. These fluxes are evaluated based on both volatile and non-volatile species. The model tracks aerosol and vapor mass in five physical states: (1) volatile species in the vapor phase; (2) suspended non-volatile aerosols; (3) volatile species condensed on walls; (4) deposited aerosols; (5) sorbed vapor species on surfaces. Aerosols are also divided into multiple particle size classes, allowing for dynamic updates due to coagulation and deposition processes. At each step, the solver balances source terms (from aerosol input, inter-volume transport, and surface interactions), linear transport and deposition rates, and nonlinear terms (such as coagulation and condensation). The resulting nonlinear system is solved using an iterative Newton update, with convergence achieved based on changes in mass across all tracked states and size bins. This approach allows SOPHAEROS to capture the evolving aerosol size distribution and deposition behavior along the pipe under varying thermal and flow conditions using experimentally measured temperatures and flow rates as fixed boundary conditions.

In this study, only the SOPHAEROS module of ASTEC was available and utilized in the standalone mode. As a result, no thermal-hydraulic calculations were performed using CESAR (the module responsible for two-phase thermal hydraulics in full ASTEC simulations). Instead, the wall and gas temperature profiles (Figure 4) obtained experimentally during the deposition phase were directly supplied as input boundary conditions in each control volume. These fixed temperatures influenced deposition and resuspension phenomena through thermophoresis and vapor equilibrium models but were not dynamically computed. The key aerosol parameters given as input to the code are MMAD = 0.5 μ , GSD = 2.01, and density = 5.6 g / c m 3 . The carrier gas flow rates used as initial conditions in the code are given in Table 1 along with the aerosol mass flow rate.

SOPHAEROS is a lumped-parameter code that employs a one-dimensional (1D) axial representation of the system in which the physical test pipe is modeled as a sequence of control volumes. While this approach allows for the simulation of axial aerosol transport and thermophoretically driven deposition, it inherently lacks the ability to capture three-dimensional (3D) flow characteristics, such as radial velocity gradients, parabolic profiles, and circumferential non-uniformities that naturally arise in pipe flow. Unlike full 3D computational fluid dynamics (CFD) codes such as RELAP5-3D or ANSYS Fluent, SOPHAEROS does not resolve detailed boundary layer development or wall shear stress distributions. As a result, while it cannot predict localized hot spots or asymmetrical deposition patterns, it can reasonably approximate overall deposition trends and axially averaged mass distributions. This dimensional simplification may introduce some uncertainties, particularly in entrance regions with steep gradients. However, when supplied with accurate thermal and flow boundary conditions, SOPHAEROS is capable of providing good agreement with experimental data and valuable insights into aerosol transport behavior.

Coagulation is an important phenomenon for the interaction and growth of aerosol particles. The coagulation of particles increases their size and hence enhances their deposition. The SOPHAEROS module integrates these phenomena in the aerosol dynamic equation. These depositional mechanisms are discussed in the subsequent sections. The mass balance equation for intra-volume mechanisms and inter-volume transport develops a nonlinear system which is solved by adopting an implicit numerical approach. All the phenomena are computed in each control volume. An implicit method of solution allows coupling between the condensation/evaporation of vapor on/from wall and aerosol dynamical processes (nucleation, agglomeration/coagulation, and drifts) which are solved in each control volume. The aerosol distribution is modeled using a sectional approach. In all aerosol size classes, the same species composition is taken into account. This section discusses the mathematical framework of the SOPHAEROS module adopted for performing theoretical simulation.

The experiments were conducted in two stages: depositional and resuspensional. Pilot experiments were used to establish the inlet concentration during the depositional phase and carrier gas flow rate during the resuspension phase. The mass concentration of inlet particles was kept at approximately 5 g / m 3 (measured using the gross filter sampler) so as to lead to sufficient particle deposition in the test volumes. It was also observed that no resuspension happened at low carrier gas flow rates (<500 L m i n − 1 ). Table 1 lists the operating parameters for the deposition and resuspension processes.

At the National Aerosol Facility of the Indian Institute of Technology, Kanpur, India, an experiment was conducted to investigate the deposition and resuspension of zinc oxide aerosol under dry environment conditions in a circular cross-section straight stainless steel pipe. The test aerosols were generated using a plasma torch aerosol generator by introducing zinc metal powder into the plasma flame, wherein aerosolization occurred via the evaporation–condensation technique. The XRD analysis confirmed the formation of zinc oxide particles with no traces of zinc metal powder. The deposition phase of the experiment took place at a high temperature gradient laminar flow, while resuspension took place at a zero thermal gradient turbulent flow. The experimental results are also compared with the computational results from the SOPHAEROS module of the ASTEC code.

During the deposition phase of the experiments, the temperature gradient between the bulk gas and wall temperature was measured in all eight test volumes of the test section. It was seen that the temperature gradient increased initially and then reduced after approximately 750 s, finally saturating to an equilibrium value. Higher temperature gradient results in higher thermophoresis, which was also captured by code simulation and thermophoresis, was found to be the major deposition process. Overall, the experimentally measured reduction of deposited mass in subsequent volumes was seen to match the ASTEC predicted results. In the resuspension phase, the striping of deposited aerosol and the transportation of resuspended aerosol caused mitigation in the kinetic energy of the carrier gas. The reduction in kinetic energy caused less stripping of deposited aerosol in the downstream test section. However, visual observation from the open end of the test section during the experiment revealed that the majority of the resuspension took place within a few seconds of the start of the resuspension process. Thus, it can be concluded that the default force balance model for resuspension in the SOPHAEROS module is highly dependent on the duration of the resuspension process. The trends obtained experimentally and computationally for the deposition and resuspension phases are in good agreement. However, quantitatively, the ASTEC result shows slight deviation from the experimental results. The main reason for these deviations could be due to the 1D-coded ASTEC limiting the use of the actual flow profile as the initial condition of the simulation. These results provide a good foundation for large-scale experiments and the development of severe accident simulation codes.

This study provides insights into aerosol behavior in nuclear-relevant pipe geometry under thermal gradient-driven deposition and high-flow resuspension. The experimental results serve not only to elucidate the dominant transport and interaction mechanisms but also to support the evaluation of SOPHAEROS predictions. While the model performs well in capturing overall trends when supplied with accurate thermal and velocity inputs, further development and validation, especially in resolving 3D effects, remain areas for future research.