3.1.1.1. Aerosol formation and growth

Aerosols are solid particles suspended in the gas phase or in liquid droplets, ranging in size from 0.01 μm to 20 μm. Behavior of most aerosols that appear during severe accidents show very complicated aspects ranging from continuum mechanics to free molecular physics. The physical aspects affecting the characteristics of the aerosols can be enumerated according to the formation, growth, and shape of the aerosol particles. In addition, deposition of particles on the surfaces and resuspension of aerosol particles are also deemed important aspects.

Formation of aerosol: aerosol particles can be formed during various interactions between materials and by steam condensation. The major processes that form aerosol particles during an accident include the following: entrainment of solids or liquid droplets in a high velocity gas flow, expulsion of droplets by gases bubbling through liquids, shock waves produced in energetic interactions of molten materials with coolants, and high pressure melt ejection from the RCS [17]. The sizes of aerosols formed by mechanical processes and suspended in the gas phase are not relatively large and typically do not exceed 1–2 μm. Thus, these types of aerosol tend to be ignored during the accident analysis. However, the formation of aerosol particles from supersaturated steam is deemed the most important source and is treated very critically in the accident analysis.

Growth of aerosol: the major mechanisms that explain the growth of aerosols are growth by coagulation, by condensation, and by hygroscopicity. Aerosols can grow through continuous steam condensation or coagulation of particles. Of these processes, coagulation is considered the dominant mechanism responsible for the growth of aerosols following nucleation. Coagulation of aerosols affects the size and mobility of aerosols. In addition to the condensation and coagulation, hygroscopicity of aerosols is considered an important growth mechanism. For example, CsI and CsOH, which show strong solubility, can grow by hygroscopicity until equilibrium pressure in the solution is achieved.

3.1.1.2. Aerosol deposition and resuspension

Deposition of aerosol: The major processes that result in the deposition of aerosols include gravitational settling, diffusion to surfaces, turbulent deposition, inertial deposition, and phoretic processes, but this is not a complete list. Gravitational settling and diffusion significantly affect the behavior of aerosols. Gas viscosity and particle density are the major parameters that govern gravitational settling. Deposition velocity according to diffusion is greatly affected by the structural geometry and flow conditions. It decreases when particle diameter increases, while the deposition velocity according to gravitational settling shows the opposite trend. Inertial deposition occurs when particles are captured or accelerated in their motion toward obstacles. In such circumstances, if the aerosol particles are sufficiently large, they collide with the obstacle surfaces and remain deposited thereafter. Another important mechanism in aerosol deposition is phoretic deposition. Thermophoretic and diffusiophoretic depositions are detailed mechanisms; the former is caused when the temperature gradient produces a movement of the particles towards the lower temperature zone and the latter appears in the direction of flux of a condensed vapor such as steam. The thermophoretic process can be significant when high temperature aerosols and gases confront a large area of cold structures, for instance containment walls or floors.

Resuspension: Aerosols deposited in a pool or on the surface of a structure can be physically entrained or evaporated with changes of gas flow, temperature, or concentration. A resuspension may occur in the core, the RCS, or the containment. Aerosols deposited on the surface of the primary system can be resuspended by turbulent flow. A sudden increase in steam flow due to core cooling or relocation of core debris is the main source of potential resuspension of aerosols. Aerosols transported to the containment can be deposited on the surface of the containment, and on structures and water pools in the containment. The gas turbulent flow due to hydrogen deflagration or steam explosion can be the main source that induces aerosol resuspension. In addition, aerosols deposited in the containment can also be resuspended if there is a sudden change of containment pressure due to the failure of the containment pressure boundary or containment discharge. Modeling of aerosol resuspension is based on adhesion and removal forces. For the dry aerosols, the van der Waals force, electrostatic force, and chemical cohesive force are the important adhesion forces to be taken into account. In the meantime, the lift force perpendicular to the surface and drag force parallel to the surface should be considered as important removal forces. The deposited aerosols are removed layer by layer by agitation on the surface and moved with a rolling behavior [17], [18].

3.1.1.3. Pool scrubbing

When gases including radioactive materials are ejected into the pool, some radioactive aerosols and gases are captured in the pool by pool scrubbing. This phenomenon is taken into account by defining the decontamination factor (DF), which is the ratio of the initial mass of the specific radioactive material to the final mass after it passes through the water pool, in a wet-type filtered containment venting system (FCVS), as well as in other gas sparging systems. When the carrier gases including radioactive aerosols and gases enter the pool through a vent, the carrier gases leaving the vent form large globules that break up into a swarm of small bubbles. Several physical processes are involved in transporting aerosols to the liquid-gas interface (equal bubble surface) when steam/gas mixtures are bubbled through a water pool [19]. The DF of aerosols can be calculated in three regions, that is, gas injection, bubble rising, and pool surface regions [20]. The total DF is obtained by the product of the values calculated in those regions. In the injection zone, aerosol removal occurs through several mechanisms such as: Stefan flow from steam condensation during gas equilibration to pool conditions; inertial impaction of aerosols in a rapid gas velocity decrease; and centrifugal, diffusional, and gravitational aerosol deposition during gas injection through small orifice or multihole vents. In the rising zone, aerosol removal occurs from centrifugal, diffusional, and gravitational aerosol deposition within a bubble. The pool scrubbing phenomenon has been modeled and embedded in several computer codes, such as SPARC (Suppression Pool Aerosol Removal Code), BUSCA (BUbble SCrubbing Algorithm), and SUPRA (SUppression Pool Retention Analysis) [19]. These codes aim at simulating the pool scrubbing process and estimating the DFs of the radioactive aerosols and of iodine gas in the water pool.

3.1.1.4. Leaching

During the Fukushima accident, radionuclides from the damaged fuel were continuously dissolved in the coolant and then accumulated in the containment and turbine building in the form of contaminated water. Thus, there could be leakage paths for these radionuclides that leached out from the turbine building to the sea. In particular, compared to the case of Cs-137, Sr and Ba releases into water were much higher than those into the atmosphere. However, knowledge of the leaching mechanism and accurate models for the prediction of release rates are limited [16].

3.1.2.1. Chemistry

Cs, I, and Te species caused most of the airborne dose during past severe accidents at NPPs due to their high volatilities in the containment under severe accident conditions. It was observed from PHÉBUS-FP tests that a large amount of gaseous iodine was released from the RCS to the containment when nuclear fuel was degraded. The existence of Cs in the absence of other elements allows the quick production of CsI, because there is no obstacle in the reaction of CsOH + HI ↔ CsI + H2O. CsI can be produced even without the existence of gaseous I2 because a much larger amount of Cs exists compared to the I species. However, when Mo is added to the Cs-I system, the reaction changes completely due to the high affinity of Mo for Cs; as a result, partial cesium molybdatesmake a complex to CsxMoOy, which is the most stable gaseous molybdate. Nevertheless, CsxMoOy cannot be produced in a hydrogen atmosphere, and this result is very similar to the case of the nonexistence of Mo, in which CsI aerosol production is dominant. In experiments with a steam atmosphere, a maximum of 80% of the iodine transferred from the primary circuit, under a steep temperature gradient and with a small fraction, was in the form of aerosols. Gaseous iodine is mostly composed of molecular iodine; the remaining portion is HI or HOI. When boron (B) is added to the Cs-I system, gaseous iodine is activated due to the production of CsBO2, but the degree is smaller than it is in the case of Mo. With control-rod materials such as Ag and Cd, it is surmised that Ag and Cd react with iodine and prevent the production of gaseous iodine [9], [11], [14], [21].

Iodine aerosols released from the RCS to the containment may sediment and settle on the walls in the containment. If they are soluble, they can be dissolved in the sump or become plated out on wet surfaces as ions in the form of I–. Then, the behavior of I– changes according to time and the pH of the aqueous solution. Several FPs dissolved in the solution cause water radiolysis and decrease the pH in the pool. At this time, a large amount of the dissolved I–changes to elemental iodine and moves to the containment atmosphere. Organic iodide is also produced by the reactions of I2 and I– with paints on the containment surfaces, and with organic materials decomposed radiolytically in solution. Gas-phase radiolysis is an important decomposition route, as well as solution phase radiolysis. Thus, radiolytic products of air, such as NO2, O3, or HNO3, oxidize molecular and organic iodine and produce IxOy in the containment atmosphere. However, the IxOy aerosols are deposited on the containment surface, and are then partially decomposed to molecular iodine under irradiation. Decomposition of IxOy by CO produced from degradation of B4C (control rod) may also affect the amounts of gaseous iodine [9], [11], [14], [22], [23].

3.1.2.2. Adsorption and desorption

Volatile iodine can be adsorbed by surfaces in the containment over time to form organic iodides. Painted surfaces act as sinks for I2 and as a source for volatile organic iodine. The I2 adsorption on surfaces depends on the gas and water temperatures and the gas flow velocity along the walls. Under wet conditions, the wall condensation rate drives I2 diffusion towards the surfaces. Radiation induces fast radiochemical reactions between iodine and the paint [22]. Models for the adsorption of I2 on paint, steel, and aerosols have been developed, but uncertainties remain concerning the effect of paint aging on the iodine volatility, deposition of iodine oxides on the surface and the subsequent decomposition and volatilization of iodine species, iodine adsorption kineticson representative multicomponent aerosols, and temperature and loading effects from H2 phenomena on FPs deposited on the wall [14].

3.1.2.3. Revaporization

After condensed species are deposited in the RCS, they can be revaporized by the displacement of the thermodynamic equilibrium in case of changes in temperature or carrier gas composition, or decreases in partial pressures in the gas phase. The reasons for the equilibrium vapor pressures exceeding the ambient partial pressures of the vapor species may include decay heating of deposited materials and changes in the chemical environment within the RCS. For instance, in a high pressure scenario, natural circulation with high temperature can cause a creep rupture of the RCS and revaporization of volatile material such as species formed from cesium and iodine. Furthermore, rupture of the RCS can radically change the chemical environment of deposits and raise the oxygen potential of the atmosphere, which can produce various volatile species of FP elements. Therefore, revaporization may be a source of delayed release to the containment. The revaporized materials may be redeposited in cooler regions of the RCS or be swept out of the system [9], [24]. PHÉBUS FP tests demonstrated this type of revaporization of several elements, with other evidence such as reaction of the deposited species with the substrate, stratification of deposits in the upper plenum samples, different stages of release, and effects of changes in the atmosphere. Recently, revolatilization and distribution of ruthenium deposited on the surfaces under irradiation and ozone action have been studied in several international programs [9].