Sodium spray fires

The experimental and theoretical evidence on the behavior of sodium spray fires in closed vessels is reviewed for applicability to LMFBR containment. Only when the liquid sodium is explosively ejected at very high velocities into air do the experimental pressure and temperature increases approach those predicted from thermodynamic considerations alone. In all other cases, the peak pressures are substantially less than the thermodynamic limit, especially when sodium droplets of the sizes expected in LMFBR accidents are used.     

Potential results of spray and pool fires

The studies on sodium pool inflammation have been conducted in a 3161 vessel, and those on sodium pool fires in three facilities: 4, 22 and 400 m³. The influence of the following parameters has been studied: sodium temperature, combustion area, humidity in the atmosphere and oxygen concentration. The experimental results have been used to validate the FEUNA code. The experimental studies on sodium spray fires have been conducted in a 3.7 m³ vessel. The experimental results have been used to validate a sodium spray fire computer code: PULSAR.     

Experimental investigation in the TOSQAN facility of heat and mass transfers in a spray for containment application

TOSQAN is an experimental program undertaken by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) in order to perform thermal hydraulic containment studies. The TOSQAN facility is a large enclosure devoted to simulate typical accidental thermal hydraulic flow conditions in nuclear-pressurized water reactor (PWR) containment. The TOSQAN facility which is highly instrumented with non-intrusive optical diagnostics is particularly adapted to nuclear safety CFD code validation. The present work is devoted to studying the interaction of a water spray injection used as a mitigation means in order to reduce the gas pressure and temperature in the containment, to produce gases mixing and washout of fission products. In order to have a better understanding of heat and mass transfers between spray droplets and the gas mixture, and to analyze mixing effects due to spray activation, we perform detailed characterization of the two-phase flow.     

Heat transfer of a nuclear reactor containment spray drop

Heat transfer rates to spray droplets under the conditions of a LOCA in a LWR have been evaluated by systematic solution of the governing partial differential equations subject to appropriate initial and boundary conditions. The numerical calculations are based on new correlations. The computations have been facilitated through the use of an efficient hybrid-difference scheme.Results have been provided for the average heat transfer and for the effects of the drop-size, droplet spray angle, initial injection velocity, the containment temperature and pressure on the heat transfer to the drop. The drop fall-heights before attaining thermal equilibrium with the containment atmosphere have been predicted for various conditions. The importance of accurately calculating the drag associated with a moving drop experiencing condensation has been discussed in the context of developing the results.     

An experimental investigation of a passive cooling unit for nuclear plant containment

A set of condensation experiments in the presence of noncondensables (e.g. air, helium) was conducted to evaluate the heat removal capacity of a passive cooling unit in a post-accident containment. Condensation heat transfer coefficients on a vertically mounted smooth tube have been obtained for total pressure ranging from 2.48×10⁵ Pa(abs) to 4.55×10⁵ Pa(abs) and air mass fraction ranging from 0.30 to 0.65. An empirical correlation for heat transfer coefficient (h), has been developed in terms of a parameter group made up of steam mole fraction (Xs), total pressure (P_t), temperature difference between bulk gas and wall surface (dT). This correlation covers all data points within 20%. All data points are also in good agreement with the prediction of the diffusion layer model (DLM) with suction and are approximately 2.2 times the Uchida heat transfer correlation. Experiments with an axial shroud around the test tube to model the restriction on radial flow experienced within a tube bundle demonstrated a reduction of the heat transfer coefficient by a factor of about 0.6. The effect of helium (simulating hydrogen) on the heat transfer coefficient was investigated for helium mole fraction in noncondensable gases (X_He/X_nc) at 15, 30 and 60%. It was found that the condensation heat transfer coefficients are generally lower when introducing helium into noncondensable gas. The difference is within 20% of air-only cases when X_He/X_nc is less than 30% and total pressure is less than 4.55×10⁵ Pa(abs). A gas stratification phenomenon was clearly observed for helium mole fraction in excess of 60%.     

Experiments on sodium fires and their aerosols

A number of new sodium fire and aerosol experiments were undertaken to provide data for LMFBR safety analyses: (1) Experiments on the burning of single drops of liquid sodium falling in air have been performed to aid in model development for sodium spray fire codes. (2) The leakage of sodium oxide aerosols through a straight smooth capillary tube, representative of the maximum size of a hypothetical gas leak in the wall of the secondary containment of an LMFBR, has been studied. Even in those cases in which the capillary did not plug, <11% of the entering mass was of a respirable size as it emerged from the capillary. In addition, there were a number of conditions under which the capillary plugged. (3) Experiments on the behavior of high temperature, high concentration aerosols have shown a rapid depletion of the aerosol concentration in the first 6 sec following injection of 800 g/m³ aerosols at 1000°C into a closed vessel. This depletion has been correlated with the early formation of 100 to 200 μm agglomerates which fall out promptly.     

Simulation of containment cooling with outside spray after a core meltdown

In the most severe hypothetical loss of coolant accident, the reactor core melts and falls into the containment sump, there evaporating much of the sump water and raising the pressure within the containment building. One possible method to remove the decay heat is to cool the steel containment shell with an outside spray system. To perform the structural analysis needed to confirm the integrity of the containment, the thermal profile in the containment wall must first be found. The purpose of this work is to develop a computer code to calculate this transient profile. Other aspects such as hydrogen build-up are not considered in this code.The method uses relationships for the natural convection-partial condensation phenomena occurring at the containment internal surfaces, iteratively coupled to a one-dimensional heat balance at the wall to solve for the wall temperature as a function of angular position. A differential calculation as a function of time treats the thermodynamic changes within the containment as quasi-steady state. The result is a quick-running code capable of analyzing the temperature transient for several hours following the LOCA with a few minutes of computing time.     

Sprays in containment: Final results of the SARNET spray benchmark

The influence of containment sprays on atmosphere behaviour, a sub-task of the Work Package WP12-2 CAM (Containment Atmosphere Mixing), has been investigated through benchmark exercises based on TOSQAN (IRSN) and MISTRA (CEA) experiments. These tests are being simulated with lumped-parameter (LP) and Computational Fluid Dynamics (CFD) codes. Both atmosphere depressurization and mixing are being studied in two phases: a ‘thermalhydraulic part’, which deals with depressurization by sprays (TOSQAN 101 and MISTRA MASPn), and a ‘dynamic part’, dealing with light gas stratification break-up by spray (TOSQAN 113 and MISTRA MARC2b).In the thermalhydraulic part of the benchmark, participants have found the appropriate modelling to obtain good global results in terms of experimental pressure and mean gas temperature, for both TOSQAN and MISTRA tests. It can thus be considered that code users have a good knowledge of their spray modelling parameters. On a local level, for the TOSQAN test, single droplet behaviour is found to be well estimated by some calculations, but the global modelling of multiple droplets, i.e. of the spray, specifically for the spray dilution, is questionable in some CFD calculations. It can lead to some discrepancies localized in the spray region and can thus have a high impact on the global results, since most of the heat and mass transfers occur inside this region. In the MISTRA tests, wall condensation mass flow rates and local temperatures were used for code-experiment comparison and show that improvement of the local modelling, including initial conditions determination, is needed.In this dynamic part, a general result, in both tests, is that calculations do not recover the same kinetics of the mixing. Furthermore, concerning global mixing, LP contributions seem not suitable here. For the TOSQAN benchmark, the one-phase CFD calculations recover partially the phenomena involved during the mixing, whereas the two-phase flow CFD contributions generally recover the phenomena. Moreover, one important result is also that none of the contributions finds the exact amount of helium remaining in the dome above the spray nozzle in the TOSQAN 113. Discrepancies are rather high (above 5%vol of helium). Results are thus encouraging, but the level of validation should be improved. The same kind of conclusions can be drawn for the MISTRA MARC2B tests.As a conclusion of this SARNET spray benchmark, the level of validation obtained here is encouraging for the use of spray modelling for risk analysis. However, some more detailed investigations are needed to improve model parameters and decrease the uncertainty for containment applications as well as to increase the predictability of the phenomena within the containment analyses. Further activities are well encouraged on this topic, such as numerical benchmarks on analytical separate-effect experiments.     

An investigation of kinetic reaction and decomposition of sodium uranate

For the management of severe accidents of sodium-cooled fast breeder reactor, the coolability of the fuel debris bed on a core support plate is a key concern during the post-accident heat removal phase. In an air ingress scenario, the reactions between the fuel and highly oxidized sodium are likely to form sodium uranoplutonate. This would negatively influence the coolability of the fuel debris bed due to a lowering of the thermal conductivity and density. This study has focused on the formation kinetics of sodium uranate from UO₂ and liquid sodium including oxygen at a high concentration. In this paper, the experiments on reaction initiation temperatures, reaction rates, and the decomposition of sodium uranate are reported.     

A study on sodium spray combustion

Sodium spray combustion was studied through experiments and analysis, in order to clarify the burning rate, pressure and temperature transients in a sodium spray fire. In the experiments, about 400 g sodium was sprayed in a closed vessel of 2 m³, containing nitrogen and 0–21 vol% oxygen. Pressure, temperature and oxygen concentration were measured during and after sodium injection. The experimental results revealed that the temperature in the spray outer region was higher than that of inner region and observed oxygen consumption was not more than 80% of that expected for complete combustion of sodium. To analyze the experiments, a computer program SOFIA-II was developed based on an analytical single droplet combustion model and a two-dimensional temperature and oxygen concentration distribution model in the vessel. The calculated pressure agreed with the experimental pressure on the whole and the peak pressure difference was within 10% error.     

Response of secondary containment to presence of sodium and hydrogen

As part of an effort to demonstrate that the risk to the public from extremely low probability events in liquid metal fast breeder reactors is bound within an acceptable envelope, containment pressurization by sodium and hydrogen was evaluated. Temperature and pressure histories are presented for typical sodium spray and pool fires and sodium vapor reactions. A review of mechanisms for hydrogen generation and recombination as well as limit for flammability and autocatalytic recombination is provided, and general containment design options to reduce risk are discussed.     

An overview of reactor containment structures

The purpose of this paper is to present an overview of reactor containment structures and to summarize the present state-of-the-art of containment design. The areas covered are types of containments used for nuclear power plants in operation and under construction, and their development. Also presented are codes which currently govern the design, materials, and construction of containments, as well as some thoughts on safety and methods of analysis.     

Experimental and numerical approaches of aerosol removal in spray conditions for containment application

TOSQAN is an experimental program undertaken by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) in order to perform thermal hydraulic containment studies. The TOSQAN facility is a large enclosure devoted to simulate typical accidental thermal hydraulic flow conditions in nuclear pressurized water reactor (PWR) containment. The TOSQAN facility, which is highly instrumented with non-intrusive optical diagnostics, is particularly adapted to nuclear safety code validation. The present work is devoted to study a water spray injection used as a mitigation means in order to washout aerosol fission products.     

Ensuring the long-term functionality of passive auto-catalytic recombiners under operational containment atmosphere conditions—An interdisciplinary investigation

In order to sustain the structural integrity of the containment and other safety relevant components i.e. to avoid a detonation of the hydrogen-air mixture generated during a severe accident in light water reactors, passive auto-catalytic recombiners (PAR) are used for hydrogen removal in many European nuclear power plants (NPP). In 1999, the German NPP Emsland (KKE) was equipped with 58 PAR of AREVA design as an internal accident management measure for a beyond-design accident. Since that time the recombiners are in a stand-by state. As the catalyst elements are exposed to various airborne substances during normal plant operation their function is controlled periodically by testing selected catalyst sheets in a specially designed device. Under the conservative test conditions during this procedure some catalyst sheets showed a delayed responding behavior. First internal analysis gave indication of a beginning fouling on the catalytic surface.The aim of a precautionary investigation performed in cooperation between KKE, Forschungszentrum Juelich and RWTH Aachen University was to characterize the composition of the fouling and to correlate it with potential sources within the containment.In the framework of the investigation the reports of the periodic inspections were analyzed and appropriate sample sheets were selected from the installation. These samples were subjected to a comprehensive chemical surface analysis in order to identify effects like thermal sintering, poisoning or a blocking of the catalytic surface (Baerns, M., 2004. Basic Principles in Applied Catalysis, Springer Verlag). Along with the chemical analysis the catalytic activity of the samples was assessed in several test series in order to correlate the chemically quantified deposition on the catalyst samples with the characteristics of the start-up and the steady-state performance of the recombination reaction. In a final step, possible sources of the fouling were analyzed with regard to their possible contribution to the phenomena. According to the results achieved, measures have been implemented at KKE in order to optimize procedures and to enhance the performance of the PARs.     

An in-pile sodium loop for investigation of local cooling disturbances in LMFBR's

In the safety analysis of Liquid Metal Fast Breeder Reactors, investigations of the fuel element behavior under local off-normal cooling conditions and the possible failure propagation are of special interest. In a common program, called “Mol 7C” the Gesellschaft für Kernforschung, Karlsruhe, and the Centre d'Etude de l'Energie Nucléaire/Studiecentrum voor Kernenergie, Mol, are performing related in-pile experiments in a sodium loop in the BR 2-reactor. The test section contains a 37-rod bundle of fresh UO₂-fuel. A local blockage within the fuel bundle will initiate a certain local damage to a few rods. The experiments are expected to obtain important informations with respect to the problems of pin to pin propagation and the long term behaviour of a fuel bundle with defect pins. The in-pile part of the loop contains the fully integrated primary sodium circuit. Total heat removal capacity is about 700 kW. The equipment for the first experiment is nearly manufactured. The first experiment will start in the beginning of 1977. At first three experiments are planned.     

钠喷射火灾实验研究

介绍了喷射钠火试验的设计思路和如何对钠流进行散流喷射,同时描述控制系统和数据采集系统的设计和建造,通过对比实验中和程序FEUMIX计算中的温度和压力曲线,得到如下结论:试验中钠流主要以柱状钠火燃烧,在钠喷射过程中燃烧区空气温度上升非常迅速,并观察到最初的钠喷射高温峰值,采用FEUMIX以10%雾化份额计算的空气温度与在...     

Study of heat and mass transfers in a spray for containment application: Analysis of the influence of the spray mass flow rate

During the course of a hypothetical accident in a nuclear power plant, spray might be activated in order to reduce static pressure in the containment. The IRSN has developed the TOSQAN experiment to give a better understanding of the heat and mass transfers between a spray and the surrounding confined gas. This article analyses the influence of the injected spray mass flow rate on the thermal-hydraulics of spray tests. The three tests compared in this article, to carry out this analysis, have the same boundary and initial conditions, except the spray mass flow rate ranges from 10 to 53 g s⁻¹. First, the scenario of these three tests used for this analysis and their results are presented. Then, our analysis is focused on the inter-comparison of the thermal-hydraulic behavior induced by spray mass flow rate variations. This inter-comparison is divided into two parts: a global and a local one.     

An experimental investigation on the local behaviour of steel-concrete interfaces at large openings in the PSC inner containment dome

An experimental study has been conducted to know the structural behaviour of a representative test specimen for a typical Prestressed Concrete Inner Containment dome with due regards to the local behaviour at the steel plate-concrete interfaces at large steam generator openings, under initial condition at prestress transfer. The primary thrust of the work has been in the objective of predicting possibilities of separation at the steel-concrete interface zones adjacent to the embedded plates of the steam generator openings of the inner containment dome. Important observations in line with the desired objectives have been made based on the results of the experiment.     

钠工艺间钠雾火模型适用性分析

钠冷快堆钠泄漏事故中,泄漏的钠以液滴形式在抛射过程中与空气剧烈反应,发生钠雾火现象。通过桑迪亚国家实验室（Sandia National Laboratories,SNL）的T3钠雾火实验结果,以及原子国际（Atomics International,AI）的J1～J4钠雾火实验结果,对改进的Tsai钠雾火模型适用性进行了评估。针对Tsai模型高估实际工况钠喷雾燃烧速率的问题,将Tsai模型中钠液滴下落模型改为单一速度模型,同时将喷雾燃烧对空间的影响改为瞬发。在T3实验中分析了钠雾火阶段对改进模型适用性的影响,在J1～J4实验中分析了初始氧气浓度对改进模型适用性的影响。结果表明：改进的钠雾火模型能够很好地预测空气中钠喷雾燃烧的升温升压现象以及钠消耗量,可以应用于预测钠工艺间内钠泄漏事故后发生在空气中的钠雾火行为。