Dynamics of heat transfer in the melt pool at nuclear severe accident conditions (original) (raw)
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Modeling of natural convection phenomena in nuclear reactor core melt
A failure of reactor core cooling and major safety systems may cause melting of nuclear fuel and reactor vessel equipment. In the reactor vessel, the melt flows down into the lower plenum, where it is accumulated. In the past, the common opinion was that the melt would break through the reactor vessel and start to desintegrate the reactor concrete base. However, recent investigations revealed that the core melt can be safely retained in the reactor vessel lower plenum if it is properly cooled. The processes in the reactor core melt in the lower plenum are specific and not yet fully understood. As revealed by a comprehensive overview of the lower plenum cooling problem, natural convection is the most important phenomenon that controls heat transfer from the melt. In the case of natural convection, fluid motion is caused by volumetric forces and density gradients. If these are strong enough, thermal instabilities may result in hydrodynamic instabilities. It was discovered that transition from laminar to turbulent flow occurs at the value of Rayleigh number Ra=5e5 in the case of Rayleigh-Bénard convection and at Ra=1e6 in the case of fluid with internal heat generation. The main problem of turbulent phenomena modeling is the size of turbulent fluid flow structures, which are in general too small to be described accurately using a discrete numerical mesh. The base of the Smagorinsky model is the assumption that the smallest flow structures, which are separated and modeled as a subgrid term, are isotropic and homogeneous. Therefore, viscous dissipation is equal to the production of turbulent kinetic energy. As the Smagorinsky model is too dissipative in the vicinity of the walls, turbulent viscosity wall functions have to be implemented. Natural convection in the melt of nuclear reactor core was modelled as natural convection in a fluid with internal heat generation in a rectangular cavity. The value of Rayleigh number was Ra=1e10 and the value of Prandtl number was Pr=1.2. Numerical simulations were restricted to two-dimensional space, due to computer hardware limitations. The finite volume method was used for spatial discretisation and a combination of Adam-Bashford method and projection scheme was used for time integration. As the calculation of heat transfer in the form of dimensionless Nusselt number revealed, the most severe thermal loads occur on the side walls in the vicinity of the cavity upper boundary. Calculated values of heat transfer can be safely extrapolated to higher values of Rayleigh number.
Dynamic behavior of the melt pool at severe accident conditions
Prediction of thermal loads on lower plenum walls after core melting and relocation during severe accident conditions requires knowledge about the core melt behavior, especially the circulation pattern. To analyze the heat transfer dynamics on the lower plenum walls, two-dimensional numerical simulations of a fluid flow with internal heat generation were performed for Rayleigh numbers 10^6, 10^7, 10^8, 10^9, 10^11 and 10^13 at Prandtl number 0.8. For subgrid motion modeling, the Large Eddy Simulation (LES) Smagorinsky model was implemented. Time and boundary-averaged Nusselt numbers were calculated. Results show that differences between minimum, average and maximum Nusselt number increase in exponential manner when the Rayleigh number is increased beyond 108. Probability densities of Nusselt number were also calculated to realistically assess unsteady thermal loads. The calculated probability density functions indicate that time-average Nusselt numbers usually do not coincide with most probable values. The study also discloses the appearance of multiple Nusselt number probability peaks.
A possible severe accident scenario is a general meltdown and relocation of the reactor core during which molten core material accumulates in the lower plenum of the reactor vessel. The decay heat generated in a radioactive material would have to be removed through the walls of the lower plenum in order to ensure the integrity of the reactor pressure vessel. Numerical simulations of turbulent natural convection in a geometry representing the lower plenum cavity of a reactor pressure vessel were conducted. A two-dimensional numerical code based on a finite-volume method was developed to simulate turbulent natural convection in a fluid with internal heat generation using large-eddy simulation. Simulations were performed at Rayleigh numbers 1e10 and 2e11 and Prandtl numbers 1.2, 7 and 8, which corresponds to conditions in the numerical investigations made by Nourgaliev et al. (1997) and in the experimental work done by Asfia and Dhir (1996). The results are shown to be in satisfactory agreement.
The turbulent natural convection of air flow in a confined cavity with two differentially heated side walls is investigated numerically up to Rayleigh number of 10 12 . The objective of the present work is to study the effect of the inclination angle and the amplitude of the undulation on turbulent heat transfer. The low-Reynolds-number k-e, k-x, k-x-SST RANS models and a coarse DNS are used and compared to the experimental benchmark data of Ampofo and Karayiannis [F. Ampofo, T.G. Karayiannis, Experimental benchmark data for turbulent natural convection in an air filled square cavity, Int. J. Heat Mass Transfer 46 ]. The k-x-SST model is then used for the following test-cases as it gives the closest results to experimental data and coarse DNS for this case. The mean flow quantities and temperature field show good agreement with coarse DNS and measurements, but there are some slight discrepancies in the prediction of the turbulent statistics. Also, the numerical results of the heat flux at the hot wall are over predicted. The strong influence of the undulation of the cavity and its orientation is well shown. The trend of the local heat transfer is wavy with different frequencies for each undulation. The turbulence causes an increase in the convective heat transfer on the wavy wall surface compared to the square cavity for high Rayleigh numbers. A correlation of the mean Nusselt number function of the Rayleigh number is also proposed for the range of Rayleigh numbers of 10 9 -10 12 .
Nuclear Technology, 1992
Analyses of unprotected loss-of-flow accidents for medium-size cores of current liquid-metal fast breeder reactors have shown that the accident proceeds into a transition phase where further meltdown is accompanied by recriticalities and secondary excursions. Assuming very pessimistic conditions concerning fuel discharge and blockage formation, a neutronically active whole-core pool of molten material can form. Neutronic or thermohydraulic disturbances may initiate a special motion pattern in these pools, called centralized sloshing, which can lead to energetic power excursions. If such a whole-core pool is formed, its energetic potential must be adequately assessed. This requires sufficiently correct theoretical tools (codes) and proper consideration of the fluid-dynamic and thermohydraulic conditions of these pools. A series of experiments has been performed that serves as a benchmark for the SIMMER-II and the AFDM codes in assessing their adequacy in modeling such sloshing motions. Additional phenomenologically oriented experiments provide deeper insight into general motion patterns of sloshing fluids while taking special notice of asymmetries and obstacles that exist in such pools.
Numerical prediction of cooling margins for a fluid with internal heat generation
Advanced Computational Methods in Heat Transfer V
Reactor pressure vessel lower plenum retention problem was studied to determine external cooling margins of the plenum walls. The accumulated melt was modelled as an incompressible fluid with internal volumetric heat generation in a rectangular cavity. A Smagorinsky type of Large-Eddy Simulation model for buoyancy flows was implemented. Because of uncertainty about the upper wall thermal boundary conditions, isothermal and adiabatic boundary conditions were used to assess heat transfer margins (Nusselt number) at each boundary of the simulation domain. It was found out in both calculated cases that the Nusselt number is the lowest at the bottom of the simulation domain and increases with height. In the future nuclear safety studies, the most severe wall thermal conditions from both simulated cases will have to be considered.
Preliminary Study of Turbulent Flow in the Lower Plenum of a Gas-Cooled Reactor
A preliminary study of the turbulent flow in a scaled model of a portion of the lower plenum of a gas-cooled advanced reactor concept has been conducted. The reactor is configured such that hot gases at various temperatures exit the coolant channels in the reactor core, where they empty into a lower plenum and mix together with a crossflow past vertical cylindrical support columns, then exit through an outlet duct. An accurate assessment of the flow behavior will be necessary prior to final design to ensure that material structural limits are not exceeded. In this work, an idealized model was created to mimic a region of the lower plenum for a simplified set of conditions that enabled the flow to be treated as an isothermal, incompressible fluid with constant properties. This is a first step towards assessing complex thermal fluid phenomena in advanced reactor designs. Once such flows can be computed with confidence, heated flows will be examined. Experimental data was obtained using three-dimensional Particle Image Velocimetry (PIV) to obtain non-intrusive flow measurements for an unheated geometry. Computational fluid dynamic (CFD) predictions of the flow were made using a commercial CFD code and compared to the experimental data. The work presented here is intended to be scoping in nature, since the purpose of this work is to identify improvements that can be made to subsequent computations and experiments. Rigorous validation of computational predictions will eventually be necessary for design and analysis of new reactor concepts, as well as for safety analysis and licensing calculations.
Modeling of Transient Turbulent Natural Convection in a Melt Layer With Solidification
Journal of Heat Transfer, 1997
The phenomenon of turbulent natural convection in a horizontal heat-generating melt layer with solidification taking place at the cooled upper and lower boundaries is investigated theoretically. The objective is to determine the transient behavior of the crust at the upper and lower surfaces and the effect of crust formation on the turbulent natural convection process in the melt layer. Various surface temperatures, latent heats, and the heat source strengths are considered along with the effects of the Stefan number and Rayleigh number. Special attention is given to the interaction between the melt pool heat transfer and the crust dynamics. Numerical results are presented for the transient crust thickness, transient temperature distribution, eddy heat transport, and the heat transfer characteristics at the solid-liquid interface during the freezing process. The present study provides basic information needed to predict the transient behavior of a melt pool in a reactor lower head f...
2020
For the past decades, many researchers have been focusing in the area of severe accident in nuclear power plants, which has notable impacts in environment. One of its causes is long absence of the core cooling, which results to overheating and the possibility of relocation of melt pool to the lower plenum of reactor vessel. Corium, the molten mixture, can be stratified with a metallic layer coming from debris particles of reflector, steel, iron and zircaloy, above an oxide layer which is made up of ZrO2 and UO2. Heat transfer phenomena and fluid behavior in these layers play a vital role for the vessel integrity. One of which is natural convection involving internal heat source. The complexity of phenomena occurring inside the corium requires high-fidelity numerical simulation, as various CFD researchers take account into the unsteadiness of the flow, near-wall modelling, constant transition of the boundary layer regions, and lastly the turbulent kinetic energy production due to the...