L.E. Herranz

A diffusion layer model for steam condensation within the AP600 containment

Abstract Steam condensation plays a key role in removing heat from the atmosphere of the Westinghouse AP600 containment in case of a postulated accident. A model of steam condensation on containment surfaces under anticipated accident conditions is presented and validated against an extensive and sound database. Based on the diffusion layer theory and on the use of the heat/mass transfer analogy, one can deal with large temperature gradients across the gaseous boundary layer under high mass flux circumstances. The thermal resistance of the condensate film, as well as its wavy structure, have also been considered in this model. As compared to Anderson et al. (1998) (Experimental analysis of heat transfer within the AP600 containment under postulated accident conditions. Nucl. Eng. Des. (submitted)) experimental database, an average error lower than 15%, within the experimental confidence range, has demonstrated its remarkable accuracy. In particular, the model has shown a good response to the influence of primary variables in steam condensation (i.e. subcooling, noncondensable concentration and pressure), providing a mechanistic explanation for effects such as the presence of light noncondensable gas (i.e. helium as a simulant for hydrogen) in the gaseous mixture. In addition, the model has been contrasted against correlations used in safety analysis (i.e. Uchida, Tagami, Kataoka, etc.) and occasionally to Dehbi’s database. This cross-comparison has pointed out several shortcomings in the use of these correlations and has extended the model validation to other databases.

Experimental analysis of heat transfer within the AP600 containment under postulated accident conditions

Abstract The new AP600 reactor designed by Westinghouse uses a passive safety system relying on heat removal by condensation to keep the containment within the design limits of pressure and temperature. Even though some research has been done so far in this regard, there are some uncertainties concerning the behavior of the system under postulated accident conditions. In this paper, steam condensation onto the internal surfaces of the AP600 containment walls has been investigated in two scaled vessels with similar aspect ratios to the actual AP600. The heat transfer degradation in the presence of noncondensable gas has been analyzed for different noncondensable mixtures of air and helium (hydrogen simulant). Molar fractions of noncondensables/steam ranged from (0.4–4.0) and helium concentrations in the noncondensable mixture were 0–50% by volume. In addition, the effects of the bulk temperatures, the mass fraction of noncondensable/steam, the cold wall surface temperature, the pressure, noncondensable composition, and the inclination of the condensing surface were studied. It was found that the heat transfer coefficients ranged from 50 to 800 J s −1 K −1 m −2 with the highest for high wall temperatures at high pressure and low noncondensable molar fractions. The effect of a light gas (helium) in the noncondensable mixture were found to be negligible for concentrations less than approximately 35 molar percent but could result in stratification at higher concentrations. The complete study gives a large and relatively complete data base on condensation within a scaled AP600 containment structure, providing an invaluable set of data against which to validate models. In addition, specific areas requiring further investigation are summarized.

Heat transfer modeling in the vertical tubes of the passive containment cooling system of the simplified boiling water reactor

The long term containment cooling of GEs passive BWR design is based on a new safety system called PCCS (passive containment cooling system). Performance of this system relies on the pressure difference between the drywell and wetwell in case of an accident and on the condensation of steam moving downward inside vertical tubes fully submerged in a water pool initially at room temperature. In this paper a model based on the resolution of momentum equations of both phases, the application of the heat and mass transfer analogy, and the consideration of the presence of a noncondensable gas by diffusion theory in a boundary layer is presented. Assumptions and approximations taken resulted in new methods to estimate film thickness and heat transport from the gas to the interface. Influence of phenomena such as suction, flow development, film waviness, and droplet entrainment has been accounted for. Based on this formulation, a computer programme called HVTNC (heat transfer in vertical tubes with noncondensables) has been built up. HVTNC results have been compared to the experimental data available. Experimental trends have been reproduced. Heat transfer has been found to be severely degraded by the presence of noncondensables whereas high Reynolds numbers of gas flow have been seen to enhance shear stress and therefore, heat transmission. The average error of HVTNC is essentially located at regions where only a residual fraction of heat remains to be transferred, so that minor deviations can be anticipated in the overall heat transfer in the tube. Comparison of HVTNC to other models show a substantial gain of accuracy with respect to earlier models.

Validation of severe accident codes against Phebus FP for plant applications: Status of the PHEBEN2 project

Abstract The European Commission-funded shared-cost action project PHEBEN2 brings together 13 partner organisations to understand the source term aspects of the integral Phebus FP experiments, to validate integral LWR severe accident codes against the test data, to develop and apply criteria regarding the strengths and weaknesses of the codes for plant applications, and to propose guidelines for their optimum use for this purpose. At the half-way point of the project, contributions to the final interpretation report of the first Phebus test FPT0 have been completed and work on the interpretation of the following test is proceeding. A detailed investigation by CFD and particle tracking appears to have identified the cause of the systematic underprediction of deposition in the steam generator tube of the Phebus circuit. Containment calculations using lumped-parameter codes have been supplemented by extensive CFD analyses, revealing complex circulation patterns within the relatively simple containment geometry of Phebus. Iodine chemistry studies have been made of both FPT0 and FPT1. Concerning criteria and code assessment for plant applications, a short list of safety-important phenomena explored in Phebus has been prepared, and partners have drafted a report analysing for each phenomenon its safety importance, the experimental data available, the modelling approach adopted in PSA codes, and the expected uncertainties.

Adequacy of the heat-mass transfer analogy to simulate containment atmospheric cooling in the new generation of advanced nuclear reactors: Experimental confirmation

Abstract Driving forces of passive cooling systems of advanced reactor containments are substantially weaker than those brought in by active systems of operating power plants. This fact along with the new geometries being used suggest the need either to develop new reliable simulation techniques or to adapt and validate traditional approaches. Suitability of the heat-mass transfer analogy for this purpose is investigated based on previous authors’ experience. Major analogy drawbacks are identified and overcome by supplementing it with analytically derived factors. By comparing against experimental data available, the heat-mass transfer analogy is demonstrated to be a sound, configuration-independent, and accurate-enough theoretical approximation.

Modeling condensation heat transfer on a horizontal finned tube in the presence of noncondensable gases

Abstract European designs for the next generation of nuclear reactors incorporate innovative passive systems in their containments to enhance heat removal by condensation under postulated accident conditions. These systems consist of several units of cross-flow finned tube bundles internally cooled with water. So far most of the studies that have been addressed to the issue of heat transfer onto finned surfaces under condensing conditions have involved refrigerants and pure vapor conditions. This study presents a model (HTCFIN) capable of predicting condensation of a cross-flow air–steam mixture onto a single horizontal finned tube. The comparison of HTCFIN predictions to the available databases shows its acceptable accuracy in a wide range of conditions and allows an interpretation of the influence of major variables acting on the scenario. As a consequence, HTCFIN model represents a step forward in the present theoretical capability to estimate heat transfer within containments of next generation of European reactors in the case of a hypothetical accident.

Aerosol retention near the tube breach during steam generator tube rupture sequences

Abstract Steam generator tube rupture sequences are identified as major contributors to the risk assessments of pressurized water reactors. Despite very low probability, they involve a direct pathway for radioactivity release into the environment. Nonetheless, fission products could be partially retained in the secondary side of the steam generator, even in the absence of water. This paper summarizes the main results of a bench-scale experimental program focused on the aerosol retention near the tube breach at the secondary side of a dry steam generator. The major variables investigated were the breach configuration (i.e., type, orientation, and location) and the gas mass flow rate. The results showed that near the breach, aerosol retention is low (<20%), and it generally decreases when the gas mass flow rate increases. Discussion of the experimental results suggested that certain phenomena, such as fragmentation and/or resuspension, as well as particle nature could have a large effect on the scenario studied, and they should be considered as potential issues and/or variables to be explored in future work.

Hydrogen removal from LWR containments by catalytic-coated thermal insulation elements (THINCAT)

Abstract In the THINCAT project, an alternative concept for hydrogen mitigation in a light water reactor (LWR) containment is being developed. Based on catalytic coated thermal insulation elements of the main coolant loop components, it could be considered either as an alternative to backfitting passive autocatalytic recombiner devices, or as a reinforcement of their preventive effect. The present paper summarises the results achieved at about project mid-term. Potential advantages of catalytic thermal insulation studied in the project are: • reduced risk of unintended ignition, • no work space obstruction in the containment, • no need for seismic qualification of additional equipment, • improved start-up behaviour of recombination reaction. Efforts to develop a suitable catalytic layer resulted in the identification of a coating procedure that ensures high chemical reactivity and mechanical stability. Test samples for use in forthcoming experiments with this coating were produced. Models to predict the catalytic rates were developed, validated and applied in a safety analysis study. Results show that an overall hydrogen concentration reduction can be achieved which is comparable to the reduction obtained using conventional recombiners. Existing experimental information supports the argument of a reduced ignition risk.

Aerosol Retention in the Vicinity of a Breach in a Tube Bundle: An Experimental Investigation

This article summarizes the main results of a bench-scale program focused on experimentally assessing the aerosol retention near the tube breach in a tube bundle. The major variables investigated were particle nature (polydispersed TiO 2 agglomerates vs. solid, monodisperse SiO 2 spheres) and Re D (0.8−2.7· 10 5 ). In addition, comparisons to other data sets provided insights into the particle aerodynamic size effect on retention efficiency. Results showed that particle nature substantially affects aerosol retention in the tube bundle: mass retention efficiency was low for TiO 2 agglomerates (less than 30%) whereas it was much higher for SiO 2 particles (around 85%). Retention efficiency is also affected by Re D : its sensitivity was found to follow a log-normal behavior with a maximum retention attained at Re D near 1· 10 5 . This evolution with Re D was similar for both types of compounds. Particle size also influences retention efficiency: the bigger the TiO2 agglomerates the lower retention efficiency (no data were available for SiO 2 ). Among all these variables, particle nature was noted to have a prime importance for in-bundle retention, whereas Re D and particle aerodynamic size, although also affect retention efficiency, did not play such a key role. In light of the results, the presence of retention-inhibiting mechanisms such as fragmentation, resuspension or bouncing has been discussed. The data recorded will enhance the overall understanding of the governing mechanisms involved and will serve as a database against which compare model predictions. Nevertheless, further experimental data would be desirable to set up a sound database.

Major challenges to modeling aerosol retention near a tube breach during steam generator tube rupture sequences

Postulated accident sequences of a pressurized water reactor, consisting of steam generator tube ruptures (SGTRs) in combination with a melting core, have been demonstrated to represent a dominant contribution to the overall public risk. However, it should be expected that even in the absence of any water in the secondary side of the steam generator (“dry” SGTR scenario), some radioactivity retention takes place as a result of the interaction of the carrier gas with internal structures. The region near the tube breach becomes a key region because it behaves as a sink for the radioactive particles entering the secondary side, and consequently, it changes size distribution of aerosols flowing toward upper structures. This paper identifies major issues that should be addressed to accurately estimate aerosol retention in the field near a tube breach during dry SGTR scenarios. By developing a simple Lagrangian model based on the filter-concept approach (ARISG-I), the specific aspects of fluid dynamics and aerosol physics involved have been explored and the major knowledge gaps highlighted. Inertial impaction and turbulent deposition have been demonstrated to be major particle removal mechanisms. Their respective collection efficiencies have been derived by gathering and correlating separate effect data on particle deposition on cylinders in a crossflow configuration. Comparisons of model predictions to experimental data taken in a mock-up facility of the break stage under similar conditions to those anticipated in dry SGTR scenarios have been set. The substantial discrepancies found and their analysis have provided insights into the significance of drawbacks of model fundamentals, the inaccuracy of specific equations of deposition mechanisms, and most importantly, the lack of consideration of key phenomena that hinder aerosol retention. According to this analysis the main areas where research is needed are: gas jet behavior across the tube bank; particle resuspension, erosion, and/or bouncing; and particle inertial impaction and turbulent deposition under foreseen conditions.