Aerosol Behaviour in SGTR accidents S. Guentay 1, L.E. Herranz 2, V. Layly 3, T. Routamo 4, A. Auvinen 5

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1 1/10 Aerosol Behaviour in SGTR accidents S. Guentay 1, L.E. Herranz 2, V. Layly 3, T. Routamo 4, A. Auvinen 5 1 PSI, Switzerland 2 CIEMAT, Spain 3 IRSN, France 4 FORTUM, Finland 5 VTT, Finland Corresponding author: salih.guentay@psi.ch SUMMARY By-pass accident sequences are major contributors to the risk assessment of Light Water Reactors (LWRs) due to the large fraction of radioactivity that may escape from the plant. In the particular case of a Steam Generator Tube Rupture (SGTR), a sound understanding of potential mechanisms governing radioactivity retention is still missing. As a consequence, the source term is assumed to be released from the reactor coolant system to the environment with no or minimum credit to any potential retention within the secondary side of the steam generator. The SGTR project of the 5 th Framework Programme of Euratom is the first project started generating understanding in a systematic way for possible mechanisms for retaining aerosol particles in tubes and in the complex structures of the secondary side of a steam generator. In particular, PSAERO and HORIZON experiments were conducted for in-tube retention, whereas the secondary side retention was investigated in the ARTIST and PECA- SGTR experiments. In addition, modelling efforts were initiated. The EURSAFE project considered SGTR scenarios as an issue that needed investigation. The SARNET project is addressing the issue within the Source Term area (WP15) and providing further efforts in the area of modelling. This paper summarises the main observations from the set of available data and updates the advances achieved in their interpretation. In addition, some key aspects of aerosol behaviour closely related to their retention are discussed: particle clusters de-agglomeration under foreseen SGTR conditions, bubble hydrodynamics in wet scenarios, resuspension of particulate deposits, etc. Finally, the theoretical approaches presently under development are succinctly described and the main difficulties found highlighted. A. INTRODUCTION Concurrent with a tube rupture and stuck open steam relief valve during a severe accident steam generator (SG) is the last barrier to hinder the release of activity in the environment. Variety of degradation processes can lead to tube cracking, wall thinning and potential leakage or rupture [1]. Over the last decade, considerable effort has been spent to understand these degradation processes and to improve related preventive and corrective actions and operational aspects. However, steam generator tube leakage incidents have

2 2/10 proven that such occurrence cannot be completely ruled out. Due to the complex operator interventions as well as the duration of time frame available dictated by the spontaneously ruptured number of tubes such an accident can lead to core melt. Rupture of one or more tubes can also be induced if the tubes are subjected to long lasting large temperature gradients or large differential pressures during a severe accident initiated by another set of events. The release may be mitigated by deposition of fission products in the steam generator tubes and on structures or by scrubbing in the secondary coolant. The absence of empirical data, the complexity of the geometry and controlling processes, however, make the retention difficult to quantify and its full import is typically not taken into account in risk assessment studies. The EU 5 th framework project SGTR [2] is the first project which initiated a systematic study, although far from complete, of the aerosol deposition and resuspension process in tubes under high flow conditions, and aerosol removal in the tube bundle which may be dry or flooded. A further strategic goal of the project was to demonstrate the effectiveness of the accident management interventions in reducing the source term even for severe accidents that lead to a by-pass of the containment. Definition of the accident scenarios and establishing the boundary condition for vertical inverted u-tube steam generators and horizontal steam generators were made with MELCOR and SCDAP/RELAP5 analyses for reference nuclear power plants of western design, i.e., Beznau - Switzerland and Borssele - The Netherlands NPP, and VVERs, i.e., Loviisa - Finland and Dukovany - Czech Republic [2]. Key signatures of the plant analyses are the significance of the number of tubes being ruptured for the western PWRs and the collector head break on the onset of the core degradation. B. EXPERIMENTAL INVESTIGATIONS B1. Integral experiments B1.1 Integral Studies of Vertical Steam Generator in the ARTIST Facility The integral tests of vertical steam generators were conducted in a representative scaled-down model of the Beznau reference PWR steam generator (Switzerland) called ARTIST facility and operated by PSI. The facility consists of a bundle, shroud, flooding system, a full size steam separator and dryer and aerosol sampling stations. Only the bundle section of ARTIST was used in this project (Figure 1). Five tests comprised the experiments. The first three tests dealt with the aerosol retention in the break stage under dry and wet conditions. The other two tests addressed accident management (AM) issues whereby the SG bundle goes from a fully dry state to a fully flooded state. An axis-symmetric guillotine break was used and located 300 mm above the tube sheet in the middle of the bundle. The aerosol AMMD s at the inlet were in the µm range, while at the outlet, the AMMD s were in the µm range. When the bundle is dry, and the full break flow directed into the bundle, aerosol deposition takes place all over the bundle. There is strong evidence that the aerosols disintegrate into smaller particles because of the sonic conditions at the break. This obviously promotes particle escape from the secondary and lowers the overall DF, which is between 2.5

3 3/10 and 3. Further investigation needs to be performed to determine the influence of the type of aerosol. Bend section Far-field stages Support plates Gap filled with straight tubes Break stage Flooding water inlet Tube sheet Figure 1. Schematic and photo of the ARTIST test section For the far-field conditions, under a flooded bundle and in the presence of steam, the DF is between 482 and A large fraction of the aerosols is scrubbed at the break level because of strong impaction of the incoming jet on the water interface and fast steam condensation. The additional water head beyond the break stage has only a secondary influence on the magnitude of decontamination. When no steam is present, the DF increases exponentially from 124 to 5739 when the water height in the bundle increases from 1.30 m to 3.6 m. The aerosol removal rate is roughly constant with height, and hence the DF is solely a function of residence time in the water pool (water height). When steam is present in the carrier gas under flooded secondary, condensation inside the tube causes aerosol deposition and produces blockages near the break, with a subsequent primary pressure rise. This has implications for real plant conditions, as aerosol deposits inside the broken tube will cause more flow to be diverted to the intact tubes, with a corresponding reduction in the source term to the secondary. B1.2 Integral Experiments of Horizontal Steam Generator in the HORIZON Facility Integral experiments of horizontal steam generator were conducted in HORIZON facility (Figure 2), which is a scaled-down model of horizontal SGs used in VVER-440, with tube dimensions similar to real SGs. The objective of the studies was to gather data on aerosol behaviour in the primary side of the SG tubes at realistic pressure and temperature levels in different flow velocities representing either the defect or intact SG tubes. The importance of the intact tubes on the overall primary side deposition depends on the break location along the defect tubes. The data were used for the model development.

4 4/10 Figure 2. Picture of the HORIZON facility The results of aerosol deposition on the primary side of the horizontal SG were compared with the values obtained from the calculations with the existing deposition models. It appeared that the current models are adequate at low Reynolds numbers (Re) below 5000, but give too high deposition velocities at high Re above compared to the experimental results. The turbulent impaction is considered to be the main deposition mechanism at high Re. However, the effect of resuspension, which becomes significant at high Re, is not usually taken into account in the calculations, and it should be added to the models. When looking at the amount of deposited material as a fraction of the material injected into the tubes, it is seen that in all experiments the deposited fraction per unit length has a peak at the tube bend. The effect of flooding the secondary side with water was also shown to be significant. Still, majority of the aerosol injected into the tubes is transported as aerosol out of the tubes, and therefore, the effect on environmental releases is small. More important effect of the secondary side flooding comes from the secondary side mechanisms such as pool scrubbing. B2. Separate effect support program B2.1 Separate Effect Studies of Vertical Steam Generator in the PECA Facility An experimental program was carried out in the PECA rig of the Laboratory for Analysis of Safety Systems of CIEMAT [3]. The test section consisted mainly of a scaled mock-up of the tube bundle (Figure 3) of a steam generator that was introduced in the 8 m 3 vessel of the PECA facility. The test matrix was set up based on best estimate calculations for two real pressurised nuclear power plants [4]. The major variables to be analysed were type (guillotine and fish-mouth) and orientation of the breach and gas flow rate. TiO 2 particles were used to keep the tests as similar as feasible to ARTIST s and, as there, there were evidences of particle fragmentation during the tests. Particle retention within the breach stage of a steam generator was observed to be rather low (less than 20 %) under the studied conditions. This result looks consistent with those from ARTIST.

5 5/10 Figure 3. Picture of the bundle used in the PECA facility The results indicated that retention efficiency decayed with gas mass flow rate from 100 kg/h to 250 kg/h according to a very simple correlation: η 2 ζ 5 kg (%) = with ζ = ( 1.81 ± 0.7) 10 2 Φ The type of the breach and the breach orientation did not result in quantitative differences in the mass removed from the aerosol source coming into the secondary side, when gas flow rates were above 100 kg/h. Contrarily, when flow rates were lower the mass retained in all the break configurations did distinguish clearly from each other. The deposition pattern within the bundle was proved to be highly dependent upon breach type. Guillotine tests showed a squared-shaped deposition profile, while in fish mouth tests a triangularshaped one was observed. In addition, retention in guillotine type break was concentrated on the first nearest tubes, while in the fish-mouth configuration farther tubes as a whole gave a significant contribution to the total mass depleted. B2.2 Separate Effect Studies of Horizontal Steam Generator in the PSAERO Facility The separate effect experiments of horizontal steam generators were conducted in PSAERO facility (Figure 4). The separate effect experiments were designed to complement the integral experiments conducted with the HORIZON facility. In the experiments aerosol deposition and the movement of deposited particles were measured online using radiotracer technique. The objective of the experiments was to gain mechanistic understanding about aerosol behaviour in the steam generator tubes. Resuspension was observed to take place simultaneously to deposition in turbulent flow. Particles most probably resuspended as large agglomerates, since the deposition velocity of resuspended particles was observed to be much higher than that of the injected aerosol. It was also evident that the resuspended agglomerates mainly moved close to the surface. The relation with the deposition and time dependence of the process requires that resuspension should thus be modelled dynamically. h

6 6/10 Steam Mixing tube Deposition tube L = 3 m, D = 13 mm Filter N2 Mixer Flow Furnace Dry powder generator Gammadetectors Figure 4. A schematic picture of the PSAERO facility. The flow rate during the deposition phase had a very significant impact on the strength particles adhered to the surface. It was evident that particles deposited in a higher flow rate were much harder to resuspend than was the case with a lower flow rate. The probable reason for this was that impaction of large particles packed the deposit near the tube inlet. Therefore, resuspension was first observed close to the outlet of the tube, where the deposit was mainly formed by settling. Particle diameter was also observed to be an important parameter in determining the adhesion of deposit. When results from these experiments were compared to previous studies, it was noted that polydisperse aerosol adheres to the surface much better than monodisperse aerosol [6]. The reason for this is that particles in the deposit layer have more contacts to other particles than is the case with monodisperse aerosol. A major problem in resuspension modelling is that the effect of particle size distribution is generally not taken into account. Parameters derived from experiments, conducted with monodisperse particles, should be used with caution in models describing the behaviour of polydisperse aerosol. As a further complication, surface roughness of the particles significantly influenced the adhesion, even if the size distribution of aerosol did not change. The results from the experiments were adequately reproduced with a correlation model that used friction velocity as a variable. This can be understood, if it is assumed that turbulent bursts are the main mechanism for particle resuspension. The frequency of turbulent bursts in laminar sub-layer depends primarily on the friction velocity. The impaction of large particles must also have caused erosion. A significant fraction of the already deposited particles were knocked off from the surface and subsequently deposited further downstream. C. ANALYTICAL MODELLING OF AEROSOL RETENTION IN COMPLEX STRUCTURES C.1. Aerosol Deposition Model for the Primary Side of the Steam generator AERORESUSLOG code was developed to calculate the aerosol deposition on the primary side of the SG. It is a one-dimensional steady-state model, which is capable of estimating aerosol deposition and possible resuspension in a horizontal tube flow. Based on

7 7/10 the most important accident scenarios, the turbulent impaction, thermophoresis and gravitational settling were considered to be most important deposition mechanisms. These mechanisms are fairly well understood and validated with experimental data. At high Re, in turbulent flow another phenomenon, resuspension, becomes important. In contrary to deposition, the mechanisms responsible for resuspension are less well understood, and more difficult to predict. The quasi-static rock n roll model for resuspension developed by Reeks and Hall [7] was used in the AERORESUSLOG code. Integral experiments of horizontal steam generators were calculated with AERORESUSLOG. The calculated results were in good agreement with measured values at low (Re=940) or intermediate Reynolds numbers (Re=3800) where no resuspension is expected. However, the values calculated at high Re ( ) including resuspension were inconsistent with the experimental results of horizontal SG. The reason for different values is most likely that the resuspension model does not take into account several important parameters such as the dependence of adhesion on deposit physico-chemical properties, polydisperse particle size distribution and system geometry. C.2 Aerosol Deposition Model for the Secondary Side of the Steam Generator A model capable of predicting aerosol deposition in the near-field of tube breach as the secondary side is dry, was built-up and encapsulated into a FORTRAN code, hereafter called ARISG-I (Aerosol Retention in a Steam Generator). This should be considered as a first step towards modelling total aerosol retention in the secondary side of a steam generator. ARISG-I is based on the filter concept : an aerosol flowing through a bundle of obstacles is submitted to forces that tend to clean up the gas by removing particles onto obstacle surfaces. The retention efficiency may be written as [5]: 1 d η TB = η Ntubes Nbins t 1 exp 1 1 y(j) ST(i,j) 1 α i= 1 s+ dt j Namely, retention efficiency shows an asymptotic behaviour dependent on the tube bundle topology (α, the packing density; s, tube spacing; d t, tube diameter) and on the individual efficiency of single tubes (η ST ) conveniently weighed with the mass fraction of particles of diameter j (y(j)). Under the prevailing boundary conditions in dry SGTR sequences [4] no thermal gradients or steam condensation should be expected. The removal mechanisms potentially relevant were: inertial impaction, turbulent deposition, settling and interception. A sensitivity study demonstrated that only inertial impaction and turbulent deposition could play a significant role in the scenario. This approach is known to have important drawbacks, some of which are: empirical correlations used to simulate the impact of the mentioned deposition mechanisms are derived from conditions other than those foreseen in the SGTR scenarios; other aerosol processes like resuspension and/or bouncing that counteract the effect of removal mechanisms, should be necessarily considered for an acceptable estimate of retention; and, gas velocity in the bundle is roughly approximated by solving the mass and momentum conservation equations in a single direction.

8 8/10 D. CURRENT EXPERIMENTAL AND MODELLING STUDIES International consortium project ARTIST [8], owned and run by PSI, is progressing and providing a comprehensive separate effect data for in tube, bundle, and separator and dryer units under a wide range of representative boundary conditions. Separate effect tests have been planed using a dedicated bundle at CIEMAT in support of the ARTIST project. As a result of the conditions set forth by the ARTIST project the produced data can not be displayed in this paper. However, the project has further demonstrated and highlighted significance of various mechanisms which might have a potential to alter the aerosol behaviour: a) De-agglomeration of aerosol agglomerates subjected to high shear. b) Effect of high turbulence on the agglomeration process and hence the sedimentation velocity of the agglomerates. Some tests have demonstrated that the particles subjected a large shear force display a shift in the size distribution. Although the primary particles of the agglomerates are in the range of nm the AMMD of particles drops from 3-4 µm to about 2 µm. Large shear force is produced by very high (up to sonic) velocity or in the sonic front when the aerosol laden gas discharges from a break and is subjected to pressure drop ratio larger than 2. It is an important issue to be physically understood and modelled since deagglomeration can change the aerosol input size distribution significantly as the aerosols enter into the secondary side. The issue has currently been addressed in a framework of PhD within cooperation between PSI and University of Newcastle (UK). The second issue has been thought to be the responsible mechanism for the significant aerosol deposits found on the floor of the dryer unit although on the dryer panels no aerosol were deposited. Dryer panels apparently caused a large turbulence although the mean velocity was small to avoid any removal by impaction. This issue is currently formulated as a PhD subject to be conducted in a framework program to be conducted by PSI, the University of Udine (Italy) and the University of Lausanne (EPFL, Switzerland). The particle behaviour will be studied with a set of detailed flow field information to be generated by using a direct simulation technique developed at EPFL under well defined boundary conditions. Gas flow is recognized to have a great significance in aerosol transport and deposition through the tube bundle. A significant effort is addressing gas flow characterization across the tube bundle at the different SG stages by both separate effect tests and 3D aerodynamic simulations. Once validated against experiments, 3D simulations will provide very useful information to deeply understand gas behaviour and to quantify axial and radial gas velocity depletion within the tube bundle. Modelling of the decontamination in the flooded secondary side of a steam generator In the framework of ARTIST project IRSN performed calculations using the SPARC B/98 pool scrubbing code coupled with SOPHAEROS module of ASTEC code and also reviewed injection zone models. The predictions using the regular pool scrubbing models indicated a large underestimation of the decontamination factor. Several explanations have been pointed out. First, the available models for the break zone are not suited for the guillotine type break used in the ARTIST facility. Moreover, it has been found that, according to the present correlation, the droplets entrained in the turbulent jet are too small to induce

9 9/10 noticeable particles capture. This was expected to be a strong mechanism (Epstein model). It is possible that this correlation be not valid for the high injection velocities encountered in ARTIST (representative of SGTR scenario). On the other hand, analysis of the numerical results indicated difficulties in computing the aerosol removal from the primary bubbles at the jet end. One is linked to the estimate of the internal circulation velocity (promoting particle inertial deposition) during the primary bubble growth, the other is linked to addition of the retention efficiencies due to internal circulation and particle impaction at the jet end, since the former is partially included in the latter. Also, a major shortcoming of the break zone modelling is that the jet impingement on the neighbouring tubes is currently not understood well and hence not modelled in the code. Concerning the decontamination within the bubble rising zone, one can distinguish two terms. The first of them is linked to the bubbles rising velocity and size distribution versus water submergence, and is roughly exponential in terms of the latter. The second is due to the decontamination at the support plate level, not presently included in the modelling. It is difficult, from experimental data, to separate the exponential and singular parts of the decontamination factor. However, we have some indications that the latter should be important enough. The SPARC-B/98 modelling includes correlations for the decay of the primary bubble into small bubbles distribution, this modelling also taking into account swarming and finite pool lateral size effects. Nevertheless, these correlations have been established for a pool configuration and are not suitable in the case of a steam generator tubes bundle. Preliminary analysis of the ARTIST experimental data compared to the modelling, with the above mentioned limitation, seems to indicate that the bubbles size and rising velocity should be noticeably lower than predicted by the code. E. DISSEMINATION AND EXPLOITATION OF THE RESULTS The results of the SGTR project, although far from complete, are applicable for various PWRs, including both vertical and horizontal steam generators. The data is already used for developing sound models for predicting the aerosol retention in the bundle region of a vertical steam generator. The obtained results from the SGTR project have been presented in various conferences. In addition, they can be used by European nuclear safety regulators and utilities. The mobility program of SARNET has enabled a Spanish scientist to work with the PSI ARTIST team to model the CIEMAT bundle using the FLUENT code mainly to understand the flow fields. F. CONCLUSIONS The SGTR project was the first attempt to systematically understand the possible removal aerosols in the tubes and in the complex structures of the steam generator secondary side. It made an important step forward in better understanding various physical models, especially regarding aerosol mechanical resuspension. The project highlighted the significance of the flooding of the secondary side of a vertical steam generator and pointed out not more than a few meters water level above the break would very significantly retain the aerosol particles in the secondary side. The project has also indicated areas where future work should be concentrated. These include more focused, separate effect studies of aerosol retention in the break stage and far-field stages, including the effects of thermophoresis and aerosol type. Extension of the investigations to upper structures (separator and dryer) is also

10 10/10 advisable and will allow a thorough understanding of aerosol phenomena in the whole steam generator. These areas are the subjects of the international consortium project ARTIST. REFERENCES [1] P. E. MacDonald, V.N. Shah, L.W. Ward and P. G. Ellison, Steam Generator Tube Failures, NUREG/CR-6365, April [2] A. Auvinen, J.K. Jokiniemi, A. Lähde, T. Routamo, P. Lundström, H. Tuomisto, J. Dienstbier, S. Güntay, D. Suckow, A. Dehbi, M. Slootman, L. Herranz, V. Peyres and J. Polo, Steam Generator Tube Rupture (SGTR) Scenarios, Nuclear Engineering and Design 235 (2005) [3] L.E. Herranz, F.J.S. Velasco, C. López del Prá, "Aerosol Retention near the Tube Breach during Steam Generator Tube Rupture Sequences", Nuclear Technology, Accepted for publication, [4] P. Bakker, M. Slootman, J. Dienstbier, S. Güntay, L. Herranz, J. Jokiniemi, T. Routamo, Accident Management Aspects of EU-SGTR Project, Workshop on Implementation of Severe Accident Management Measures, NEA/CSNI/R(2001)20, [5] L.E. Herranz, C. López del Prá, F.J.S. Velasco, An Approach to Aerosol Retention in SGTR Sequences: from Modelling Fundamentals to Research Needs, 2005, submitted to publication to Nuclear Technology. [6] 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", Journal of Aerosol Science 2001, 32, pp [7] M. W. Reeks and D. Hal, Kinetic Models for Particle Resuspension in Turbulent Flows: Theory and Measurement, Journal of Aerosol Science 2001, 32, pp [8] S. Güntay, D. Suckow, A. Dehbi, R. Kapulla, ARTIST: Introduction and First Results, Nuclear Engineering and Design, 231(2004)