| 1. |
- Grishchenko, Dmitry, et al.
(författare)
-
Design and Commissioning Tests of the TALL-3D Experimental Facility for Validation of Coupled STH and CFD Codes
- 2014
-
Konferensbidrag (refereegranskat)abstract
- Application of coupled CFD (Computational Fluid Dynamics) and STH (System Thermal Hydraulics) codes is a prerequisite for computationally affordable and sufficiently accurate prediction of thermal-hydraulics of complex systems. Coupled STH and CFD codes require validation for understanding and quantification of the sources of uncertainties in the code prediction. TALL-3D is a liquid Lead Bismuth Eutectic (LBE) loop developed according to the requirements for the experimental data for validation of coupled STH and CFD codes. The goals of the facility design are to provide (i) mutual feedback between natural circulation in the loop and complex 3D mixing and stratification phenomena in the pool-type test section, (ii) a possibility to validate standalone STH and CFD codes for each subsection of the facility, (iii) sufficient number of experimental data to separate the process of input model calibration and code validation. Description of the facility design and its main components, approach to estimation of experimental uncertainty and calibration of model input parameters that are not directly measured in the experiment are discussed in the paper. First experimental data from the forced to natural circulation transient is also provided in the paper.
|
|
| 2. |
- Bandaru, S V Ravikumar, et al.
(författare)
-
Multi-nozzle spray cooling of a reactor pressure vessel steel plate for the application of ex-vessel cooling
- 2021
-
Ingår i: Nuclear Engineering and Design. - : Elsevier BV. - 0029-5493 .- 1872-759X. ; 375
-
Tidskriftsartikel (refereegranskat)abstract
- Spray cooling is a versatile technology for various cooling applications involving high surface heat fluxes. Experimental facility was built to study heat transfer performance of an upward multi-nozzle array of water sprays impacting a surface of heated plate made of reactor vessel grade steel. The effect of inclination angles of the steel surface on the cooling performance was investigated to assess heat transfer in complex semispherical/ semielliptical geometry of large reactor lower head and to address possible application of spray cooling in severe accident management (SAM) of light water reactors (LWRs) based on In-vessel melt retention with external reactor vessel cooling (IVR-ERVC). Joule heating of SA302B steel foil of 0.15 mm thickness and surface area of 96 cm2 enabled prototypic heat fluxes to be evacuated during reactor accident. A 2×3 array of full jet narrow-coned pressure-swirl spray nozzles was used to reproduce multi-nozzle cooling. The tests were conducted as a series of consequent steady states realized at stepwise increasing power and surface heat fluxes up to the maximum values of 29 kW and 2.97 MW/m2 limited in the specific facility design. Seven surface inclinations, between 0o and 90o were tested and no significant variations in spray cooling performance with the inclination of the heated surface was found. The results indicated a promising prospect of using a multi-nozzle array system for cooling of large surface area of reactor lower head. Much higher heat fluxes can be safely extracted by spray cooling in comparison with the critical heat fluxes that appeared at RPV water pool cooling at natural convection. The maximum value of heat flux at direct spray impact zones and its drop-off slightly from the center to the periphery of the spray cone was detected in the tests. The water flow rate and liquid subcooling significantly influenced maximum steel surface temperature but had no noticeable effects on surface temperature uniformity. The spray-to-spray interaction had no observable effects on local surface temperatures, however, the colliding zones where four spray cones have visible effects on local surface temperatures due to poor liquid momentum. The results also showed that more uniform liquid film distribution could be obtained for some inclinations because of improved liquid drainage, which in turn leads to maintaining low surface temperatures.
|
|
| 3. |
- Bandaru, S V Ravikumar, et al.
(författare)
-
Upward-facing multi-nozzle spray cooling experiments for external cooling of reactor pressure vessels
- 2020
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 163
-
Tidskriftsartikel (refereegranskat)abstract
- Cooling by water spray is a well-known technology that can reach significantly higher Critical Heat Flux (CHF) compared to other cooling methods. For the light water reactor safety, the in-vessel retention (IVR) by external reactor vessel cooling (ERVC) is a comprehensive severe accident management strategy to arrest and confine the corium in the lower head of the reactor pressure vessel. Heat fluxes up to 1.5 MW/m2 have already been assumed attainable in low-power nuclear reactors while cooling required in high-power reactors is expected to reach 2.5 MW/m2. Instead of reactor lower head flooding and relying on cooling due to natural convection, a viable and more efficient alternative is to spray the external surface of the vessel. Given all the advantages of spray cooling reported in the literature, a lab-scale experimental facility was built to validate the efficiency of multi-nozzle spray cooling of a downward-facing heated surface inclined at different angles up to 90o. The facility employed a 2×3 matrix of spray nozzles to cool the FeCrAl alloy foil with an effectively heated surface area of 96 cm2 using water as the coolant. Heat loads and surface inclinations were varied parameters in the test matrix. The results show that no significant variations in spray cooling performance concerning the inclination of the heated surface. A surface heat flux of 2.5 MW/m2 was achieved at every inclination of the downward-facing surface. The results also indicate that more uniform liquid film distribution could be obtained for some inclinations, which in turn leads to maintaining low surface temperature. The obtained surface heat flux margin by spray cooling indicates that it is feasible to adopt IVR-ERVC strategy for a large power reactor.
|
|
| 4. |
- Bian, Boshen
(författare)
-
CFD Study of Molten Pool Convection in a Reactor Vessel during a Severe Accident
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- During severe accidents in nuclear reactors, the core and internal structures can melt down and relocate into the reactor pressure vessel (RPV) lower head (LH) forming there a stratified molten corium pool. The pool generally consists of superheated oxidic and metallic liquid layers imposing thermo-mechanical loads on the RPV wall. The in-vessel retention (IVR) strategy employs external cooling with water to maintain RPV integrity. Investigating the thermo-fluid behaviour of corium and predicting heat flux distribution on the vessel wall are crucial. The molten pool exhibits natural convection, which can typically consist of two stratified layers. There exists internally heated (IH) natural convection in the oxidic layer and Rayleigh-Bénard (RB) convection in the surface metallic layer.This study starts by illustrating the mathematical models that involve the numerical study of natural convection flow in molten corium. A verification work of the model has been done using a previous direct numerical simulation (DNS) study, and the results show good agreement. In addition, a scaling theory of the natural convection flow is demonstrated to facilitate the pre-estimation based on the Rayleigh number (Ra) and Prandtl number (Pr). After that, the numerical approaches involved in the numerical simulation of the corium are illustrated, especially focusing on the DNS method. A DNS mesh strategy is proposed in the form of a pipeline from the pre-estimation to the post-check. A scalability study of Nek5000 is performed on four different HPC clusters based on a DNS case of the IH molten convection in a hemispherical geometry with Ra=1.6×1011. The results show a super-liner speedup property of Nek5000 on each cluster within a certain range.Then, three numerical studies focusing on turbulent natural convection flow within both the oxidic and metallic layers of corium are demonstrated and discussed. Through these simulations, the thermos-fluid behaviour of the system is examined in detail, including flow configuration, temperature distribution, heat flux profiles on cooling boundaries, and turbulent quantities.1. A DNS investigation is performed on the IH molten pool convection within a hemispherical domain, employing a Rayleigh number of 1.6×1011 and a Prandtl number of 0.5. The results show a turbulent flow characterized by three distinct regions, consistent with the observation from the BALI experiments. Detailed information regarding turbulence, including turbulent kinetic energy (TKE), turbulent heat flux (THF), and temperature variance, is presented. Furthermore, the study offers comprehensive 3D heat flux distributions along the boundaries, showing heat flux fluctuations along the top boundary due to nearby turbulent eddies and a nonlinear increase in heat flux along the curved boundary from bottom to top.2. A numerical study investigates the effect of Prandtl number on the natural convection of an IH molten pool in a 3D semi-circular test section. Prandtl numbers of 3.11, 1.0, and 0.5 are considered, with a Ra= 6.54×1011. Smaller Prandtl numbers result in more vigorous turbulent motion and a thicker layer of intense turbulent mixing in the upper region. The descending flow extends further down the bottom, creating a stronger circulation at the bottom with smaller Pr. Additionally, smaller Pr leads to more thermal stripping structures and less stable stratification layers. Comparing heat fluxes on the top and curved walls reveals higher fluctuation frequency with smaller Pr for heat fluxes to the top boundary. However, the maximum heat fluxes to the side walls are lower with smaller Pr.3. A numerical study investigates the turbulent natural convection in a 3D fluid layer based on the BALI-Metal 8U experiment. Different methods, including DNS and three Reynolds-averaged Navier-Stokes (RANS) models, are employed. The results are compared with experimental data, and the performance of the RANS models is evaluated using DNS as a reference. DNS reproduces a two-distinct region flow structure observed in experiments, while the k-ω SST model exhibits similar flow patterns and TKE profiles. However, all simulations overpredict temperature compared to experimental data, with DNS providing the closest results. The DNS results also achieve better agreement with experimental data in terms of heat flux distribution and energy balance, specifically capturing the transient maximum heat flux on the lateral cooling wall. This transient behaviour plays a crucial role in accurately estimating the ‘focusing effect’.
|
|
| 5. |
- Bian, Boshen, et al.
(författare)
-
Direct numerical simulation of internally heated natural convection in a hemispherical geometry
-
Annan publikation (populärvet., debatt m.m.)abstract
- Internally heated (IH) natural convection can be found in nature, industrial processes, or during a severe accident in a light water reactor. In this accident scenario, the nuclear reactor core and some internal structures can melt down and relocate to the lower head of the reactor pressure vessel (RPV) and interact with the remaining coolant. Subsequent re-heating and re-melting under decay and oxidation heat creates a transition from a debris bed to a molten pool. The molten pool, which can involve more than hundred tons of dangerously superheated oxidic and metallic liquids, imposes thermo-mechanical loads on the vessel wall that can lead to a thermal and/or structural failure of the vessel and subsequent release of radioactive materials to the reactor pit, and can possibly make its way to the environment. This study uses Direct Numerical Simulation (DNS) to investigate homogeneous IH molten pool convection in a hemispherical domain using Nek5000, an open-source spectral element code. With a Rayleigh number of 1.6×1011, the highest reached through DNS in this confined hemispherical geometry, and a Prandtl number of 0.5, which corresponds to a prototypic corium, the study provides detailed information on the thermo-fluid behaviour. The results show a turbulent flow with three distinct regions, consistent with the general flow observations from the BALI experiments. The study also presents detailed information on turbulence, such as turbulent kinetic energy (TKE), turbulent heat flux (THF), and temperature variance. Additionally, the study provides 3D heat flux distributions along the boundaries. The heat fluxes along the top boundary fluctuate due to the turbulent eddies in the vicinity, while along the curved boundary the heat fluxes increase nonlinearly from the bottom to the top.
|
|
| 6. |
- Bian, Boshen, et al.
(författare)
-
Direct numerical simulation of internally heated natural convection in a hemispherical geometry
- 2024
-
Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 220
-
Tidskriftsartikel (refereegranskat)abstract
- Internally heated (IH) natural convection can be found in nature, industrial processes, or during a severe accident in a light water reactor. In this accident scenario, the nuclear reactor core and some internal structures can melt down and relocate to the lower head of the reactor pressure vessel (RPV) and interact with the remaining coolant. Subsequent re-heating and re-melting under decay and oxidation heat creates a transition from a debris bed to a molten pool. The molten pool, which can involve more than hundred tons of dangerously superheated oxidic and metallic liquids, imposes thermo-mechanical loads on the vessel wall that can lead to a thermal and/or structural failure of the vessel and subsequent release of radioactive materials to the reactor pit, and can possibly make its way to the environment. This study uses Direct Numerical Simulation (DNS) to investigate homogeneous IH molten pool convection in a hemispherical domain using Nek5000, an open-source spectral element code. With a Rayleigh number of 1.6 × 1011, the highest reached through DNS in this confined hemispherical geometry, and a Prandtl number of 0.5, which corresponds to a prototypic corium, the study provides detailed information on the thermo-fluid behavior. The results show a turbulent flow with three distinct regions, consistent with the general flow observations from the BALI experiments. The study also presents detailed information on turbulence, such as turbulent kinetic energy (TKE), turbulent heat flux (THF), and temperature variance. Additionally, the study provides 3D heat flux distributions along the boundaries. The heat fluxes along the top boundary fluctuate due to the turbulent eddies in the vicinity, while along the curved boundary the heat fluxes increase nonlinearly from the bottom to the top.
|
|
| 7. |
- Bian, Boshen, et al.
(författare)
-
Direct numerical simulation of molten pool convection in a 3D semicircular slice at different Prandtl numbers
- 2022
-
Ingår i: Nuclear Engineering and Design. - : Elsevier BV. - 0029-5493 .- 1872-759X. ; 393, s. 111772-
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper, a Direct Numerical Simulation (DNS) of an internally heated (IH) natural convection in a 3D semicircular slice molten pool is conducted using Nek5000, a CFD solver with spatial discretization based on the spectral element method. The mesh requirements in the bulk and boundary layers are both fulfilled using known correlations. A calculation of a simplified internally heated box is first established with an excellent agreement to existing data. Next, simulation of the 3D semi-circular is performed showing qualitative agreement with the general flow observations from the BALI experiments. The velocity field shows that the flow domain is divided into three regions, i.e., intensive turbulent eddies in the upper domain, weak flow motion in the lower domain, and the descending flow along the curved boundary. Correspondingly, the temperature field in the upper domain is relatively homogenous, while that in the lower domain is characterized by stratified layers. Further, the heat flux distribution along the boundaries shows that the heat fluxes fluctuate along the top wall due to turbulent eddies, and the heat fluxes at the curved wall increase nonlinearly from the bottom to the top. Finally, the influence of Prandtl number indicates that smaller Prandtl number will lead to more turbulence eddies, deeper descending flow, and more even redistribution of heat thereby lowering the maximum heat flux to the curved walls.
|
|
| 8. |
- Bian, Boshen, et al.
(författare)
-
Scalability of Nek5000 on High-Performance Computing Clusters Toward Direct Numerical Simulation of Molten Pool Convection
- 2022
-
Ingår i: Frontiers in Energy Research. - : Frontiers Media SA. - 2296-598X. ; 10
-
Tidskriftsartikel (refereegranskat)abstract
- In a postulated severe accident, a molten pool with decay heat can form in the lower head of a reactor pressure vessel, threatening the vessel’s structural integrity. Natural convection in molten pools with extremely high Rayleigh (Ra) number is not yet fully understood as accurate simulation of the intense turbulence remains an outstanding challenge. Various models have been implemented in many studies, such as RANS (Reynolds-averaged Navier–Stokes), LES (large-eddy simulation), and DNS (direct numerical simulation). DNS can provide the most accurate results but at the expense of large computational resources. As the significant development of the HPC (high-performance computing) technology emerges, DNS becomes a more feasible method in molten pool simulations. Nek5000 is an open-source code for the simulation of incompressible flows, which is based on a high-order SEM (spectral element method) discretization strategy. Nek5000 has been performed on many supercomputing clusters, and the parallel performance of benchmarks can be useful for the estimation of computation budgets. In this work, we conducted scalability tests of Nek5000 on four different HPC clusters, namely, JUWELS (Atos Bullsquana X1000), Hawk (HPE Apollo 9000), ARCHER2 (HPE Cray EX), and Beskow (Cray XC40). The reference case is a DNS of molten pool convection in a hemispherical configuration with Ra = 1011, where the computational domain consisted of 391 million grid points. The objectives are (i) to determine if there is strong scalability of Nek5000 for the specific problem on the currently available systems and (ii) to explore the feasibility of obtaining DNS data for much higher Ra. We found super-linear speed-up up to 65536 MPI-rank on Hawk and ARCHER2 systems and around 8000 MPI-rank on JUWELS and Beskow systems. We achieved the best performance with the Hawk system with reasonably good results up to 131072 MPI-rank, which is attributed to the hypercube technique on its interconnection. Given the current HPC technology, it is feasible to obtain DNS data for Ra = 1012, but for cases higher than this, significant improvement in hardware and software HPC technology is necessary.
|
|
| 9. |
- Bian, Boshen, et al.
(författare)
-
Validation of DNS and RANS Approaches on Turbulent Natural Convection against the BALI-Metal Experiment
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- During severe accident scenarios in nuclear reactors, the core and internal structures can melt down and relocate to the lower head of the reactor pressure vessel (RPV), where they interact with any remaining coolant. This process can lead to the formation of a stratified molten pool, which is also called corium. It consists of dangerously superheated oxidic and metallic liquids, which imposes thermo-mechanical loads on the vessel wall. Typically, the molten pool separates into distinct layers, with a lighter layer of metallic materials on top and a denser layer of oxides at the bottom. The metal layer acts as a heat sink, absorbing heat from the heat-generating oxide layer and conducting it towards the inner wall of the RPV. This concentrated heat load to the vessel is known as the focusing effect.This study conducts numerical simulations of the turbulent natural convection flow in a fluid layer undergoing both top and lateral cooling based on the BALI-Metal 8U experiment. Different methods were employed, including Direct Numerical Simulation (DNS) and three Reynolds-Averaged Navier-Stokes (RANS) models: k-ω SST, standard k-ε, and Reynolds stress equation model (RSM). The simulation results are compared with experimental data, and the RANS models are assessed using the DNS results. The results reveal that DNS is able to reproduce a two-distinct region flow structure similar to the experimental observations. The k-ω SST model shows similar flow patterns and turbulent kinetic energy (TKE) profile as the DNS results. Regarding the temperature field, all simulations overpredict temperature compared to the experimental data, with DNS providing the closest results. The turbulent heat flux (THF) result shows the RANS models are incapable of accurately modelling THF in turbulent natural convection flow. The heat flux analysis demonstrates that DNS achieved good agreement with experimental data in terms of heat flux distribution and energy balance, while the RANS models underestimate the focusing effect. Furthermore, DNS captures the transient maximum heat flux on the lateral cooling wall, which is higher than the time-averaged value, an important factor for estimating the focusing effect.
|
|
| 10. |
- Chen, Yangli, et al.
(författare)
-
A sensitivity study of MELCOR nodalization for simulation of in-vessel severe accident progression in a boiling water reactor
- 2019
-
Ingår i: Nuclear Engineering and Design. - : ELSEVIER SCIENCE SA. - 0029-5493 .- 1872-759X. ; 343, s. 22-37
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a sensitivity study of MELCOR nodalization for simulation of postulated severe accidents in a Nordic boiling water reactor, with the objective to address the nodal effect on in-vessel accident progression, including thermal-hydraulic response, core degradation and relocation, hydrogen generation, source term release, melt behavior and heat transfer in the lower head, etc. For this purpose, three meshing schemes (coarse, medium and fine) of the COR package of MELCOR are chosen to analyze two severe accident scenarios: station blackout (SBO) accident and large break loss-of-coolant accident (LOCA) combined with station blackout. The comparative results of the MELCOR simulations show that the meshing schemes mainly affect the core degradation and relocation to the lower head of the reactor pressure vessel: the fine mesh leads to a delayed leveling process of a heap-like debris bed in the lower head, and a later breach of the vessel. The simulations with fine mesh also provide more detailed distributions of corium mass and temperature, as well as heat flux which is an important parameter in qualification assessment of the In-Vessel Melt Retention (IVR) strategy.
|
|
