The airborne resuspension of particles is a subject

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1 2 2 Studies on the airborne resuspension of radioactive particle contamination O. WITSCHGER (IRSN) L. ALLOUL (IRSN) H. LECOQ (IRSN) A. RENOUX (UNIVERSITY OF PARIS XII) J. MONNATTE (COGEMA) 2 The airborne resuspension of particles is a subject that arises in many fields related to industrial processes or those with environmental applications. In the nuclear field, knowledge of the resuspension coefficients of radioactive particle contamination is essential to safety analysis and radiation protection. This is because the accident scenarios considered for fuel cycle laboratories and plants (earthquake, fire, explosion, falling objects, etc.) can lead to situations of contamination resuspension. Thus, for a hypothetical accident scenario, based on a quantity of potentially dispersible radioactive material (e.g. a heap of powder), resuspension coefficients can be used to calculate the amount that would actually be dispersed into the atmosphere of the containments and/or premises concerned, and possibly transferred to points from which it could be discharged into the environment, or even reach human beings. These coefficients are essential to both assessing the radiological consequences of normal or accident situations and optimizing the means of protection to be used in designing facilities. An analysis of the data available to date clearly shows that knowledge of resuspension coefficients is always partial, or even practically nonexistent for certain scenarios (Witschger, 1999), chiefly owing to the multiplicity of parameters that may be involved. Also, when data is available, it is often hard to extrapolate the results obtained with a good level of confidence from an experimental configuration to situations representative of those encountered in actual facilities. The lack of data in the area of interest of laboratories and plants led IRSN and COGEMA to launch an Airborne Contamination Common Interest Program (PIC) in The objective of this program is to set up a data library on resuspension coefficients, which are then used for a realistic (and non enveloping) estimate of potential releases. This data library is being prepared firstly by survey and critical analysis of the currently available data in reported studies, and secondly, and this is a major point of the program, through experimental studies defined on the basis of key scenarios. Of these, the study of the airborne resuspension of radioactive particle contamination, especially powder, appears to be of the first priority. This is the subject of this article. ( The study of the airborne resuspension of radioactive particle contamination, especially powder, appears to be of the first priority. 83

2 The airborne resuspension of particles still remains a complex subject, whose results in the nuclear safety field have mainly concerned the safety of reactors, especially in assessing the retention effects of aerosols in the primary ducting. This study focuses on the following two lines of research: an experimental study using inactive materials; an active experimental study on actual UO 2, PuO 2 and MOX radioactive powders and MOX pellets. The inactive study has two objectives: firstly, better knowledge of the phenomenology of the resuspension of particles originating from a powder deposit, and secondly, the development of a resuspension coefficient value database for a range of parameters relating to flow, the powder deposit and the environment in general. Moreover, the inactive study has the advantage of being able to study numerous experimental configurations more easily, without radiological risk. However, it is necessary for the analyses to have data relating to radioactive materials in order to verify that the main physical phenomena have been properly taken into account in the inactive experiments. The active experimental study, by using actual radioactive powders, will therefore supply data which will be used to validate the results obtained in the inactive study and will enable the inactive to be extrapolated to the active with a known level of confidence. In addition, the active study will broaden the sphere of application of the results for experimental configurations leading to very low resuspension coefficients. This is because the radioactive product analysis techniques are clearly more sensitive than those used for inactive powders. Finally, some experiments will be conducted on MOX pellets, for which there is no data available. ( Coefficients and Resuspension COEFFICIENTS Resuspension (which we will subsequently also refer to by the term re-entrainment ) is characterised using various coefficients with different dimensions. In our situation, the coefficient mainly used is the fraction in resuspension KR, the resuspension rate TR or the resuspension flux FLUXR. When the phenomenon studied changes little over time or at least over the period of interest, which is our case as we shall see in the chapter describing the results, the coefficient used is exclusively the fraction KR, which is the ratio of the resuspension quantity over the initial quantity of particulate matter characterizing the deposit; it is a dimensionless number. The rate TR is the fraction of deposited material entrained per unit of time [T -1 ]. It is determined experimentally for a period of exposure which may vary in length according to the methodology used and, generally, according to the number or mass of the particles. The flux FLUXR is defined by the fraction of material entrained per unit of area and unit of time [L -2 T -1 ]. It is used more specifically to describe the case of a homogeneous deposit over a surface. One or other of these coefficients is found in the literature for the same situation studied. STATE OF THE ART Experimental and theoretical studies have been conducted on resuspension, especially over the last fifteen years; most of them apply to the special field of contamination in the electronics industry. Hence, the vast majority of the cases studied are mainly characterised by more or less scattered single-layer deposits. However, very few of these studies present results in terms of resuspension coefficients. Resuspension is characterised using various coefficients with different dimensions. 84 INSTITUT DE RADIOPROTECTION ET DE SÛRETÉ NUCLÉAIRE

3 2 The study carried out relates to a configuration of deposit not yet studied. While the basic physical concept is the same whether dealing with an isolated particle or a set of particles (resuspension takes place if a sufficient amount of energy transmitted by air flow to the particle enables it to overcome its adhesion to the surfaces with which it is in contact), many effects complicate the physics enormously, to the point where the theoretical description to date still presents areas of obscurity. It is generally accepted that resuspension can be described from a statistical point of view. This description is associated with the turbulent nature of flow, as well as with the adhesion of particles to the surface and to each other (Ziskind et al, 1995). Allowing for different statistical distributions of (normal) aerodynamic forces and (log-normal) adhesion forces thus enables the evolution of resuspension over time to be described. We thus have a short-term rate TR, i.e. over the very first few, very important, seconds, then a much lower long-term rate TR (by several orders of magnitude), which decreases in inverse proportion to time over periods of several hours. Since the levels of resuspension rates are a function of the average relative influence of adhesion with respect to aerodynamic forces, this means that the particles entrained in the short term are particles characterized by rather weak adhesion. Thus, the fraction in resuspension KR changes rapidly to become stabilized after a certain period of exposure. With regard to the detailed description of re-entrainment, the most recent analysis uses a so-called rock n roll model around a surface irregularity to describe the detachment of a particle with a rough surface. This model is based on the action of moments of aerodynamic and adhesion forces. Recently used for interpreting several series of experimental data from previously published studies, it shows encouraging results (Biasi et al, 2001). However, here again, the experiments chosen are more concerned with singlelayer deposits. CONFIGURATION STUDIED The study carried out within the framework of the program relates to a configuration of deposit not yet studied. This configuration, shown in figure 1, is described as a deposit in the form of a cone-shaped heap of powder consisting of different-sized particles, deposited on a rough surface and exposed to a turbulent, horizontal air flow. This configuration corresponds, for example, to the scenario of a heap of powder that would be found accidentally at the bottom of a glove box. The heap is exposed to an air flow that varies in strength according to the accident scenarios studied; the range of air velocities may vary from a few centimeters per second up to 1000 cm/s. Finally, the size of particles involved in this study range from a few micrometers up to approximately 100 mm. This range of size covers the range of diameters of the particles of the radioactive polydispersed powders in question, namely UO 2, PuO 2 and MOX. ( Figure 1 Illustration of the configuration studied. 85

4 Figure 2 Diagram of the BISE Inactive facility. BISE Inactive Experimental Facility The BISE Inactive experimental facility (figure 2) was set up for the purpose of studying the various key parameters governing the re-entrainment by air flow of particles originating from a heap of powder, and quantifying this re-entrainment in terms of resuspension coefficients (Alloul et al, 2000). In addition, it was designed with a view to creating a similar facility for conducting active experiments. Finally, the experimental studies carried out show that resuspension should be studied under very controlled experimental conditions if the most important parameters are to be assessed. The small-scale facility has succeeded in producing such results with sufficient accuracy. BISE Inactive is an open circuit with a total length of approximately 5 m, within which the air is conveyed by suction using a vacuum pump. The main part of BISE Inactive consists of a horizontal parallelepiped duct (length 40 cm, width 12 cm and height 7 cm). These dimensions were determined allowing for the intended range of air velocity (between approximately 0.1 m/s and 10 m/s), which requires a maximum flow rate of the order of 350 m 3 /h, under a considerable pressure loss associated with the presence downstream of a particle collecting filter. The circular, removable test surface, on which the heap of powder to be studied is deposited, is located immediately upstream of the duct outlet. The upstream part, consisting of a very high-efficiency filter compartment, enables working with clean air, without infiltrated pollution in the rest of the facility. The downstream part consists of The experimental studies carried out show that resuspension should be studied under very controlled experimental (conditions. an adjustment system for setting the velocity of the air in the vicinity of the test surface and keeping it constant, together with an electropneumatic valve with an adjustable opening velocity for controlling air ramp rates [acceleration and deceleration] in the circuit. The positioning of the vacuum pump at the circuit outlet enables operation under a slight negative pressure with respect to the outside, and does not cause additional turbulence in the flow. The three environmental parameters of pressure, temperature and relative humidity of the air conveyed are measured with the aid of special devices. The air flow conditions within the main stream are evaluated by high-frequency, hotwire, anemometry. In addition to characterizing the air stream in the immediate vicinity of the test surface, these measurements have been especially used to determine the relationship linking the air velocity in the flow V and the friction velocity of the air at the surface Vf, thus accurately describing the flow close to the surface. For an experimental configuration, the amount of particulate material re-entrained is obtained by measuring the mass of particles that has escaped from the heap. An accurate gravimetric measurement on a microbalance (sensitivity 10 µg) was adopted, owing to its precision and the speed of analysis that it allows, unlike more cumbersome physico-chemical methods. Particular attention was paid to determining the detection limit of this method, including the effects of environmental parameters and the various powder sample operations. We ended up with a limit LD, expressed as a resuspension fraction KR equal to ; this value may be regarded as satisfactory compared with some of the experimental studies available. 86 INSTITUT DE RADIOPROTECTION ET DE SÛRETÉ NUCLÉAIRE

5 2 Experimental results obtained on BISE Inactive The experimental parameters adopted relate to the deposit of powder, air flow, surface and exposure time. The approach that was proposed for bringing out the effects of the parameters was to conduct global experiments on polydispersed powders. The powders selected for the inactive experiments are pure aluminum oxides whose characteristics (grain-size distribution, morphology, bulk density, etc.) are similar to those of the radioactive powders covered by the study. Figure 3 Evolution of KR and comparison with the Fromentin model, (air velocity V = 10 m/s (Vf = 0.52 m/s), particle diameter DP = 27 µm and 5 µm). KINETICS OF RESUSPENSION This first series of experiments was conducted for the purpose of characterizing the progress of reentrainment (expressed by the fraction KR) as a function of the time of exposure to the flow. The results are also compared with calculations performed using the Fromentin model (1989), the only modeling, to date, approaching our situation. This semi-empirical modeling is based on experiments carried out on multi-layered powder deposits exposed to air velocities V in the 5to25m/s range with diameters DP between approximately 0.5 µm and 5 µm. Figure 3 compares the experimental results (with their confidence interval of 95%) obtained in BISE Inactive with the predictions of the Fromentin model. It is found that the data obtained in BISE Inactive for a grain size DP = 5 µm are the closest to the model, but remain significantly different. Moreover, a considerable discrepancy is observed (difference in a ratio of 50 to 250) between the experimental results and the model for a grain size DP = 27 µm. This discrepancy is mainly explained by the fact that the model does not include the effect of particle size. The results also show that the fraction in resuspension changes relatively little as soon as the exposure time exceeds 900 s, to tend towards the value of 3.5x10-1, characteristic of the experimental configuration. This tendency agrees with the comments of Biasi et al, 2001: beyond a certain period, the resuspension rate TR decreases sharply (inversely proportional to time), since, at a constant air velocity over time, the particles liable to be re-entrained (therefore corresponding to a low adhesion) are less and less numerous. Accordingly, the re-entrained fraction KR tends towards a limit which is a Figure 4 Contribution of effects on the re-entrained fraction KR. function of the relative significance of adhesion with respect to the forces responsible for reentrainment. MAIN EFFECTS OF THE PARAMETERS A second series of experiments was conducted with the object of determining the main effects of the selected parameters. The variation ranges of each of the parameters were determined taking into account the characteristics of our applications, together with the methodological constraints. In view of the large number (eight) of adopted parameters capable of influencing reentrainment, the experimental matrix was defined by applying the experimental design method. This methodology can be used to optimize the number of experiments to obtain reliable conclusions efficiently. The parameters that were 87

6 Figure 5 Evolution of the re-entrained fraction as a function of air velocity V and particle size DP. adopted are as follows: air velocity V, particle diameter DP (grain size), exposure time, the packed density of the powder, dimension of the heap of powder, the water content of the powder, air acceleration on starting and the roughness of the surface. Figure 4 (page 87) shows the results of the statistical analysis of the experiments carried out according to a two-way, fractional factorial screening design. This analysis reveals that: V and DP have a very significant contribution; their effects each contribute more than 30% of the overall effect; these two parameters therefore play a dominant part in re-entrainment; the other six parameters are not significant under the experimental design conditions; a strong positive interaction between the two parameters V and DP is revealed; it also contributes approximately 30% of the overall effect on the KR. FRACTION KR PLACED IN RESUSPENSION AS A FUNCTION OF V AND DP A second series of experiments was conducted for the purpose of specifically studying the influence of the air velocity and grain size parameters on the re-entrained fraction KR. These experiments were motivated by the fact of expecting resuspension to actually manifest itself above an air velocity threshold, this having to be dependent on the grain size of the powder. Furthermore, for the purpose of establishing a semi-empirical calculation model of the resuspension fraction, it was necessary to accumulate enough results to be able to make a more refined analysis. The values of the secondary parameters were set to average conditions representative of the variation ranges previously established. Figure 5 shows the experimental results of the re-entrained fraction KR as a function of air velocity V, between 0.5 and 10 m/s, for different grain sizes of alumina powders characterized by mass median volume diameters of 17, 27, 46 and 59 µm. Each of the results is shown with its confidence interval, the set of results in the figure representing a hundred experiments. The results reveal that at a constant grain size the air velocity has a very significant positive effect; over the velocity variation range studied, the variation in KR attains five orders of magnitude. For a specified air velocity, the fraction KR also significantly increases with the powder particle size. Moreover, these results can be used to 88 INSTITUT DE RADIOPROTECTION ET DE SÛRETÉ NUCLÉAIRE

7 2 define an air velocity pseudo-threshold below which KR remains lower than a certain limit; in our experiments, the smaller the grain size the higher this threshold velocity is. From the theoretical knowledge of resuspension, we have developed a semi-empirical model based on a simplified analysis of the balance of adhesion and aerodynamic forces type. This approach predicts that the relationship linking KR to the friction velocity of the air at the surface Vf and to the grain size DP takes the form: KR = α(dp + ) β DP + denotes a dimensionless diameter integrating the friction velocity of the air and grain size and equals: DP + = DP Vf ν where ν denotes the kinematic viscosity of the air (1.515x10-5 m 2 /s at 20 C and 1 atm.). α and β are factors determined by adjustment. It should also be noted that this dimensionless number expresses the interaction that we observed between the two parameters Vf and DP. Figure 6 compares the fractions KR calculated using our semi-empirical model with the fractions KR obtained experimentally. The limits shown, more or less of the same order of magnitude, were defined based on an analysis of the experimental standard deviations of each experimental fraction KR. Under these conditions, 90% of the data fall within this interval. Allowing for the variability of the phenomenon studied and the wide range of variation of KR (in our experiment from 10-6 to 10 0 ), the model, although simple in description, may be regarded as quite satisfactory and represents suitably the results. not quantitative, is however very informative. In combination with a more detailed analysis of the results described earlier, it leads to the hypothesis that re-entrainment occurs in two concomitant phases, one of which is governed by a so-called surface resuspension similar to a multilayered deposit, and the other only starting beyond a velocity threshold responsible for the deformation of the powder heap. This hypothesis is reinforced by additional experiments that were carried out on different forms of heap: below a velocity threshold, which is a function of the grain size of the powder, the reentrained fraction KR does not depend on the shape of the heap. Figure 6 Comparison of the experimental fractions KR with those calculated using the semi-empirical model. Figure 7 Photographic sequence showing the evolution of a powder deposit as a function of time (air velocity V = 10 m/s, particle diameter DP = 59 µm). A COMPLEX PHENOMENON The photographic sequence in figure 7 reveals the complexity of the phenomenon of re-entrainment. It shows the evolution of the deformation of a powder deposit as a function of time for an alumina powder particle size DP = 59 µm and an air velocity V = 10 m/s. The sequence was taken from above, the air flow coming from the left in the figure. Two resuspension phenomena are clearly seen: a phenomenon on the surface facing the incident air flow where the resuspension is associated with the velocity of the air directly incident to this surface, and a phenomenon located downstream of the heap due to the presence of a zone of air recirculation (phenomenon similar to a slipstream). This observation, while 89

8 BISE Active Experimental Facility The object of the active experiments is twofold: firstly, to obtain data for validating the results of the inactive study, and secondly to extend the configurations studied especially to those leading to low resuspension fractions. An important part of our study is extrapolation from the study of non-radioactive powders to radioactive powders. This would enable the data from inactive experiments to be used for analyzing safety and radiation protection with a known level of confidence. It will also be important to be able to compare the active results obtained with the semi-empirical model that we have developed. Broadening the configurations studied with respect to the inactive study configurations, especially in terms of air velocity, was also considered important. This involves covering a lower air velocity range than that studied inactively (less than a meter per second), which is representative of a normal environmental situation. For these low velocities, the expected KR fractions may be broadly below the detection limit of 2x10-5 obtained for the inactive experiments. The analysis technique used for the active experiments (α, γ spectrometry) which has highly sensitive measurement, should lower the detection limit by a minimum factor of Active experiments meeting special needs are also planned. MOX pellets enter into this framework since, as the expected coefficients are extremely small, the inactive simulation of such experiments is not feasible. The results of these experiments will also be used to characterize the detection limit of the BISE Inactive facility (including analysis). Figure 8 shows a general layout of the BISE Active experimental facility, which has been installed since November 2001 on the Valduc site. This facility consists of a closed circuit, whose air is conveyed by a vacuum pump and regulated by various assemblies including mass flow meters. The circuit is installed inside two separate glove boxes, one housing the systems associated with the facility s air, the second integrating the nuclearized version of the main BISE section, which is shown in figure 2 (page 86). Figure 9 shows a photograph of the main area of the BISE Active facility. An air/water heat exchanger has also been installed on the facility s circuit outside both glove boxes for keeping the air inside the circuit at a constant temperature throughout the experiments. Tests have been carried out characterizing the air stream in the main area. The results show that the two facilities, BISE Inactive and BISE Active, are similar from the air-flow point of view. An initial inactive re-entrainment experiment has been conducted in BISE Active with alumina powder. The KR fraction obtained is comparable with those obtained under the same conditions in BISE Inactive. Figure 8 General layout of the BISE Active facility. 90 INSTITUT DE RADIOPROTECTION ET DE SÛRETÉ NUCLÉAIRE

9 2 Figure 9 View of the main area of BISE Active. The matrix for the active tests that will take place in 2002 has been defined, taking into account various constraints considered important, like the sequencing of the tests (associated with the residual pollution in the containment) or the number of tests that can be carried out (each test employing several people). The results of these tests carried out with MOX pellets and MOX, PuO 2 and UO 2 powders should be analyzed and published during the first half of ( Conclusion and Prospects The action conducted since the beginning of 1999 on airborne contamination is aimed at improving our knowledge on the complex phenomenon of the resuspension of particles originating from a powder deposit exposed to air flow. The results of the experiments carried out in the BISE Inactive facility clearly show that there are two vital parameters in the phenomenon of resuspension: air velocity and the powder s grain size. The effects of other parameters like the time of exposure to the flow remain secondary. These experimental results provide concrete answers to the questions posed by the safety assessment of facilities. The unsuitability of the existing models for interpreting the results obtained has led us to develop a satisfactory semi-empirical model. In order to broaden the range of application of the results acquired to date, experiments on actual radioactive materials (MOX pellets, UO 2, PuO 2 and MOX powders) will be conducted throughout 2002 in another experimental facility designated BISE Active, on the Valduc site. All the knowledge and results acquired within the framework of this study of resuspension will eventually be integrated into the resuspension coefficient database (BADIMIS). This database, which is already used in safety assessment, will form the end product of the IRSN/COGEMA cooperative program on airborne contamination. All the knowledge and results acquired will be integrated into the resuspension coefficient database (BADIMIS). References - L. Alloul, O. Witschger, D. Le Dur, A. Renoux and J. Monnatte, An Experimental Facility for Powder Reentrainment Studies, J. Aerosol Sci., 31, S835-S836, L. Biasi, A. de los Reyes, M. W. Reeks and G. F. de Santi, Use of a Simple Model for the Interpretation of Experimental Data on Particle Resuspension in Turbulent Flows, J. Aerosol Sci., 32, , A. Fromentin, Particle Resuspension from a Multi-layer Depo Deposit by Turbulent Flow, Doctorate thesis. Paul Scherrer Institut. PSI- Bericht Nr.38, September G. Ziskind, M. Fichman et C. Gutfinger, Resuspension of Particulates from Surfaces to Turbulent Flows, Review and analysis. J. Aerosol Sci., 26, , O. Witschger, Mise en suspension de contamination particulaire radioactive. Synthèse bibliographique, IPSN/DPEA/SERAC Report No ,