Core Melt Research Articles

Ablation of a solid by an immersed jet

Issues related to the occurrence of a severe accident with melting of the core are considered as early as in the design phase of nuclear reactors. For sodium cooled fast reactors (SFR-Na), two mitigation devices are set up in the reactor vessel. The first one is ”transfer tubes” that are used to relocate the corium in the lower part of the vessel; these tubes are arranged in the fuel assembly. The second one is called ”core-catcher” and is supposed to receive the corium melt in the lower part of the reactor vessel. This device must absolutely be correctly designed to resist to the thermal ablation which undoubtedly will occur since the corium melt temperature is very high. In this context, experimental studies are carried out to fully understand the ablation process with simulating fluids, the final goal being to accurately simulate it in realistic conditions. Thus, an experimental study of a simplified situation is exposed using simulating materials: the core-catcher material is replaced by transparent ice and corium by a hot water jet. In this paper, the ablation of an ice block by an immersed water jet is investigated. Experiments were performed with an initial nozzle-to-solid distance H/Dj=10 and the Reynolds number ranged between ∼104 and ∼1.5×105. First, heat transfer is characterized at the stagnation point by analyzing pictures taken during cavity formation. In a second step, the cavity shape evolution is deeply analyzed to get more information about heat transfer in the whole cavity. These results help understanding the complex mechanisms at play during ablation and will be used to validate numerical simulations.

IMPLEMENTASI SISTEM LABVIEW DALAM PENGUMPULAN DATA TEKANAN DAN ALIRAN DI FASILITAS PASCONEL

The unsafe incidents in the Nuclear Reactors at the Chernobyl and Fukushima Dai-ichi Nuclear Power Plants were caused by disruptions to the internal reactor core elements, such as nuclear fuel and the electrical flow supplying the cooling systems. As a result, a complete cooling system failure (blackout) occurred, leading to a significant increase in the reactor core temperature and even the melting of the reactor core. A testing facility has been constructed to study the utilization of large-scale passive cooling systems. This facility is known as the Passive Condensation Experimental Loop (PASCONEL). PASCONEL facility is equipped with components like Pressure Transmitters on Heat Pipe 1, Pressure Transmitters on Heat Pipe 2, Pressure Transmitters on the Main Steam Generator, as well as Inlet and Outlet Flow Meters on the Main Steam Generator. In this research, the approach employed involves static testing of the facility for a duration of 24 hours. The objective of this testing is to gather data that can be used to analyze the performance of all installed components within the facility. The results of this data analysis will indicate whether all components function as intended.

Study of contact melting of plate bundles by molten material in severe reactor accidents

In a severe reactor accident, a crust will form on the surface of the molten material during the core melting process. The crust will have a contact melting with the internal components of the reactor. In this paper, the contact melting process of the molten material on the austenitic stainless steel plate bundles is studied. The contact melting model of parabolic molten material on the plate bundles is proposed, and the rule and main effect factors of the contact melting are analyzed. The results show that the melting velocity is proportional to the slope of the paraboloid, the heat flux and the distance between two plates D. The influence of melt gravity and the plate width on melting velocity is negligible. The thickness of the molten liquid film is proportional to the heat flux and plate width, and it is inversely proportional to the gravity. With the increase of D, the liquid film thickness decreases at first and then increases gradually. The liquid film thickness has a minimum against D. When the width of the plate is small, the width of the plate is the main factor affecting the thickness of the liquid film. The parameters are coupled with each other. In a severe reactor accident, the wider internal components of reactor, which can increase the thickness of the melting liquid film and reduce the net input heat flux from the molten material to the components, are the effective measures to delay the melting process.

Critical heat flux model for heated horizontally oriented cylindrical vessel and its application to PHWR calandria under severe accident scenario

In-vessel retention (IVR) of molten corium is a key severe accident management strategy adopted in pressurized heavy water reactors (PHWRs) and is recognised as the only option to manage a complete core melt scenario. Since the PHWR calandria vessel is completely surrounded by a large pool of water, the vessel itself is expected to act as a ‘core catcher’. The success of IVR strategy depends up on whether the actual heat flux imparted by molten corium is less than the critical heat flux (CHF). In the present work, a hydrodynamics model is developed and applied to obtain the variation of CHF along outer surface of 220 and 700 MWe Indian PHWR calandria vessel and tube. The thermal margin available for success of IVR strategy is evaluated. The governing equations for external buoyancy driven boundary layer flow are derived and solved in non-dimensional form. The details of the mathematical model, benchmarking exercises, validation aspects and application of the model to PHWR calandria vessel and tube are discussed. Parametric studies are presented to bring out the influence of diameter, system pressure, subcooling of water, depth of submergence and slip ratio on the variation of CHF along the cylindrical vessel surface.

Experimental Simulation of Decay Heat of Corium at the Lava-B Test-Bench

During the development of a severe accident at a nuclear power plant (NPP), corium is formed—a melt of core materials. A distinctive feature of corium, due to the content of fuel elements in its composition, is the presence of decay heat, which makes a significant contribution to the nature of the interaction of the corium melt with the structural materials of the reactor plant. In this regard, the decay heat should be taken into account when conducting computational studies and physical experiments. For this reason, certain requirements are imposed on the methods of simulating decay heat in the corium prototype, which relate to both the uniformity of the volume distribution and its intensity. This paper presents the results of calibration experiments to substantiate the operability of the induction heating system of the Lava-B test bench, which is used to simulate decay heat in the study of processes occurring during an accident with the NPP core meltdown. So, in order to obtain optimal characteristics of the heating system, a series of experiments was conducted on heating the graphite block in the experimental section of the Lava-B test bench. In the experiments, the capacitance of the used oscillating circuit capacitor banks and the electrical power on the inductor varied. As a result of the analysis of the data obtained, the most optimal parameters of the inductor-load simulator system were determined. In general, the performed experiments confirmed the operability of the induction heater and the possibility of its use in experimental studies of the interaction of corium with the various structural elements of the NPP reactor core at the Lava-B test bench.

An experimental investigation on debris bed formation from fuel coolant interactions of metallic and oxidic melts

During postulated severe accidents in a light water reactor (LWR), the core melt (corium) may relocate to the lower head and fail the reactor pressure vessel (RPV). The corium is expected to undergo fuel coolant interactions (FCI) if the reactor cavity is flooded with water. Both FCI energetics and resulting debris bed coolability are of paramount importance to reactor safety, since the ex-vessel corium poses a threat to the containment integrity if steam explosion occurs or the debris bed is uncoolable, leading to release of radioactive fission products to the environment. The present study is intended to quantify the characteristics of a debris bed resulting from FCI, which are crucial to debris bed coolability. Different from the previous studies with only oxidic materials, various materials, including metallic ones of Sn, Sn-Bi and Zn as well as oxidic one of Bi2O3-WO3, were employed as the simulants of corium (mixture of UO2/ZrO2/Zr/Fe) in the present study to investigate the effects of melt materials, melt superheat and coolant subcooling on debris bed formation in a water pool. High-speed photography was applied to visualize melt jet breakup, droplets fragmentation, as well as fragments sedimentation on the pool floor. Other obtained data are debris bed shape (profile) and porosity, as well as morphology and size distribution of debris particles. The comparative results of various tests provided insights toward filling the knowledge gap on debris bed characteristics under different melt materials and compositions.

MODELING OF THE CORIUM AND METALS – COOLERS INTERACTION IN A CORE CATCHER OF A LIGHT WATER REACTOR

The core catcher is one of the mandatory elements of the reactor safety system, which prevents the release of reactor core materials in a severe accident. The core catcher is steel vessel filled with sacrificial materials (SM) and forming a tank where a corium melt coming from the core is formed. The trap is a steel body filled with sacrificial materials (LM) and forming a vessel where a corium bath is formed coming from the core. The melt formed in the core catcher is cooled by heat removal to the cooling water through the shell of the steel vessel, as well as by water supplied directly to the surface of the melt after the dissolution process of the SM in corium (gravitational inversion). The delay in the water supply to the melt is associated with the features of the component structure of corium and its interaction with water (the formation of explosive hydrogen and the possibility of its detonation, as well as the threat of a steam explosion). However, a certain amount of time is spent on the implementation of gravitational inversion, and it is desirable to start the water supply to the melt immediately at the moment when the corium enters the core catcher due to the danger of the system going beyond the permissible limits (the beginning of boiling of uranium dioxide) due to decay heat in the corium. In this regard, the authors have an idea – to use a fusible metal for additional cooling of the surface of the corium in order to organize heat removal and reduce the temperature of the corium in the period before the end of the gravitational inversion process. The article presents the results of modeling the interaction of corium with candidate low-melting metals – coolers. The modeling was conducted using the ANSYS software package. As a result of the conducted work, the time for which each of the considered cooling metals will reach the points of phase transitions of melting and boiling is determined. The analysis of the results allowed us to draw appropriate conclusions about the possible practical implementation of the proposed method of cooling corium.

Effects of oxide layers on zirconium alloy-steam reaction

In a severe accident of a BWR plant, zirconium alloy of fuel cladding reacts with steam becoming a heat source for core melting and becoming a hydrogen source for hydrogen explosion. There are a lot of publications giving empirical formulas for energy and hydrogen production due to the zirconium alloy-steam reaction. Most of the formulas are based on a parabolic law, which means the reaction rate and hydrogen production rate decrease monotonously with the reaction time due to the protective oxide layers developed on the alloy surface. However, when the oxide layers are defective for some reason, their effectiveness against the reaction is weakened and the reaction rate will increase. In order to confirm the stability of the oxide layers, an integral-type experiment on the zirconium alloy-steam reaction was set up, hydrogen production rate due to the reaction was measured as a function of exposure time and surface temperature, and then, the oxide layers on the specimen were examined after the exposure. It was confirmed that (1) the analytical results based on Cathcart's formula well explained the experimental results, and (2) the hydrogen production rate did not decrease simply with the exposure time but the decrease in effective oxide layer density resulted in loss of protectiveness and the reaction rate increased.

3D Printing of Collagen Nerve Scaffold with Multichannel Characteristics

Given the difficulty in preparing multichannel nerve scaffolds for peripheral nerve repair, a method of manufacturing multichannel collagen nerve scaffolds by multi-nozzle 3D printing technology was proposed. Gelatin material is used as the core melting material because of its temperature-sensitive characteristics. A collagen nerve scaffold with a multichannel structure was manufactured by temperature-rising removal after printing, and the scaffold with different collagen concentrations was analyzed and evaluated in terms of microtubule diameter and mechanical properties. The research results show that the mechanical properties of the scaffold are gradually enhanced with an increase in collagen concentration. When the concentration of collagen is between 5% and 6%, the quality of the microtubule is significantly improved compared with the 4% collagen scaffold. Combined with the distribution of microtubule diameter, a lower concentration of collagen will lead to the collapse and deformation of the print structure, while a higher concentration of collagen will lead to the existence of voids in the print structure. Considering the manufacturing process, microtubule diameter, and mechanical properties of collagen nerve scaffold, 5% collagen nerve scaffold is the best and is expected to meet the requirements of peripheral nerve regeneration.

Structure and properties of steels for manufacture of core catcher vessel of nuclear reactor

The Russian new nuclear reactors are provided with a special core catcher vessel device (cc-vessel) designed to minimize the consequences of a severe beyond design basis accident at a nuclear power plant, when the reactor pressure vessel collapses and the core melts. For manufacture of the cc-vessel structural elements, low-carbon unalloyed or low-alloyed steels are used. When a severe beyond design basis accident develops, the cc-vessel’s body is subjected to extreme temperature and force loads, which can lead to degradation of the structure, loss of strength and failure of the entire cc-vessel. To calculate the strength characteristics of the cc-vessel, which ensure its safe and reliable operation, the detailed data are required on the structure and mechanical properties of low-carbon steels at high temperatures and after extreme thermal actions simulating the development of a severe beyond design basis accident. The paper analyzes data on the structure and mechanical properties (tensile strength, crack resistance, toughness and cyclic strength) of a number of low-carbon steels under extreme temperature and force actions, including conditions simulating the development of a severe beyond design basis accident at a nuclear power plant, in order to select the material for the design of cc-vessel of nuclear reactor. New data on the structure, mechanical properties, and thermal diffusivity in a wide temperature range of a Cr – Mo steel (Russian Standard – 15KhM) as a candidate structural material for the manufacture of the cc-vessel body are presented. The low content of manganese and alloying with molybdenum and vanadium in 15KhM steel provides a finer grained structure and eliminates the steel’s tendency to temper brittleness.

Variation of heat load due to in-vessel debris bed in IVR condition

We performed experiments investigating the heat load imposed on the inner reactor vessel wall in the in-vessel retention of core melt condition, varying the debris bed height and configuration. Mass transfer method was adopted to achieve high RaʹH of order ∼1015 with compact test rigs. We tested three types of the debris configurations: cone, flat and skew debris beds. The results showed that Nusselt numbers decreased in both the uppermost and the curved surfaces with the increasing bed height. Due to the friction increased by the debris bed, the natural convective flow in the oxide pool was weakened, which intensified the impairment of the heat transfer. As the rising plume at the center was disturbed by the porous structure of the debris bed, the local average heat transfer of the uppermost surface was weakened at the center. The heat transfer was dramatically decreased at the curved surface where the debris bed was covered.

Selective laser melting of permalloy magnetic core with complex geometry

The additive manufacturing (AM) technology is theoretically feasible in fabricating components with desired geometry; however, the practice in the AM of the magnetic functional component with complex geometry is still rare. Here, we report the manufacturing of magnetic cores with complex geometry from Fe-50 %Ni permalloy powders using selective laser melting (SLM), and the mechanical and magnetic properties as well as the microstructure with annealing are also investigated. The grain size of samples is from 50 to 100 μm, which exceeds the diameter of the powder. The SLM-manufactured components indicate moderate soft magnetic properties, their maximum magnetization, initial permeability, and coercivity are ∼ 151 emu/g, 310, and 6.9 Oe, respectively; and more impressively, an extremely high strength of ∼ 1060 MPa are also shown through sand-blasting and annealing at 1300 °C. The combined excellent mechanical and soft magnetic properties make it more practical and durable in the application of complex magnetic functional components manufactured by SLM.

Comparison of Melted Corium Relocation during Severe Accident of High Temperature Reactor using Moving Particle Semi-Implicit Method

System failure in nuclear reactors can cause degradation of a reactor core, allowing melting and relocation of the corium to the lower plenum in the nuclear reactor system. In this study, a severe accident simulation was carried out using the Moving Particle Semi-Implicit (MPS) method. In this method, we model the relocation of molten corium on the reactor core (support plate) to the lower plenum for several conditions with variations: corium material, lower plenum conditions, temperature, viscosity, and density. Those treatments were carried out in order to be able to compare and analyze the characteristics of the corium melt by reviewing the velocity profiles. The formation of a corium pool and debris bed can result in significant temperature differences and high heat flux against the walls of the reactor vessel, causing a decrease in the integrity of the reactor vessel and reactor failure.Keywords: Corium, Uranium Dioxide (UO2), Zirconium Dioxide (ZrO2), Fluid Relocation, Moving Particle Semi-Implicit (MPS).

Director anchoring on a simple edge dislocation at the surface of induced smectic-C_{A} films.

We present a detailed analysis of c-director anchoring measurements on simple edge dislocations at the surface of smectic-C_{A} films (steps). Indications show that the c-director anchoring on the dislocations originates from a local and partial melting of the dislocation core that depends on the anchoring angle. The SmC_{A} films are induced on isotropic puddles of 1-(methyl)-heptyl-terephthalylidene-bis-amino cinnamate molecules by the surface field, while the dislocations are located at the isotropic-smectic interface. The experimental setup is based on the connection of a three-dimensional smectic film sandwiched between a one-dimensional edge dislocation on its lower surface, and a two-dimensional surface polarization spread over the upper surface. Applying an electric field produces a torque that balances the anchoring torque of the dislocation. The film distortion that results is measured under a polarizing microscope. Exact calculations on these data, anchoring torque versus director angle, yield the anchoring properties of the dislocation. A specificity of our sandwich configuration is to improve the measurement quality by a factor of N^{3/2}∼600, where N=72 is the number of smectic layers in the film. We fit a second-order Fourier series on the torque-anchoring angle data, which has the advantage of converging uniformly over the entire anchoring angle range, i.e., over more than 70^{∘}. The two corresponding Fourier coefficients, k_{a1}^{F2} and k_{a2}^{F2}, are anchoring parameters that generalize the usual anchoring coefficient. When changing the electric field E, the anchoring state evolves along paths in a torque-anchoring angle diagram. Two cases occur depending on the angle α_{∞} of E relative to the unit vector S, perpendicular to the dislocation and parallel to the film. When α_{∞}<130^{∘}, the operating point Q that represents the anchoring state in the diagram follows reversible and "at-equilibrium" paths. Its free displacement velocity is infinitely slow, so that we have to push it with electric torque steps smaller than the experimental error bar δΓ∼10^{-14}N. On the other hand, for α_{∞}>130^{∘}, Q describes a hysteresis loop similar to the usually encountered ones in solids. This loop connects two states that exhibit broken and nonbroken anchorings, respectively. The paths that join them in an out-of-equilibrium process are irreversible and dissipative. When coming back to a nonbroken anchoring state, both the dislocation and smectic film spontaneously heal back in the very same state they were before the anchoring broke. The process does not produce any erosion thanks to their liquid nature, including at the microscopic scale. The energy that is dissipated on these paths is roughly estimated in terms of the c-director rotational viscosity. Similarly, we can evaluate the maximum time of flight along the dissipative paths to be of the order of a few seconds, which is consistent with qualitative observations. In contrast, the paths located inside each domain of these anchoring states are reversible and can be followed in an "at equilibrium" manner all along. This analysis should provide a basis for understanding the structure of multiple edge dislocations in terms of parallel simple edge dislocations interacting with each other through pseudo-Casimir forces arising from c-director thermodynamic fluctuations between them.

Investigation of the source term and radiological consequences of small break LOCA with failure of the emergency core cooling system in Bushehr nuclear power plant

In the present study, the in-containment source term derivation, environmental dispersion and deposition as well as radiological consequences of Bushehr Nuclear Power Plant (BNPP) due to Small Break Loss of Coolant Accident (SB-LOCA) have been studied. First, the calculation of in-containment release and distribution of the source terms due to the severe accident of SB-LOCA with failure of the active part of the Emergency Core Cooling System (ECCS) has been carried out. In this calculation, two scenarios, including with and without operator control actions, have been considered. Then, the environmental leakage of radionuclides as well as their dispersion and deposition considering two meteorological scenarios have been investigated. Consequently, public exposure has been calculated for each meteorological scenario and the obtained results have been reported and discussed. For two accidents with the same meteorological conditions, at a distance of 400 m from the release point, the received dose in the first 24 h is 18 and 342 mSv without and with operator action, respectively. The results show that due to the intervention of the operator and the use of emergency cooling supply systems, more water enters the reactor. Therefore, if the operator's actions do not prevent the core from melting, more source terms have entered the containment after the core melts and worse conditions than the baseline scenario without operator action occurred.

Experimental investigation on debris bed formation from metallic melt coolant interactions

Motivated by risk quantification of severe core meltdown accidents which may occur light water reactors, an extensive DEFOR (DEbris FORmation) test series with various molten oxidic materials was conducted previously at Royal Institute of Technology to investigate the characteristics of debris beds formed from oxidic melt-water interactions, which are important to the coolability of debris beds and thus the retention of core melt. Yet, little attention has been paid to metallic melt which exist in the core melt. The present study is intended to fill in the knowledge gap, i.e., to characterize metallic melt-water interactions when a melt jet falls into a deep water pool. Nine DEFOR-M tests are carried out to clarify the effects of melt jet diameter, free fall height and water pool depth on metallic debris formation characteristics. Molten tin of 20 kg is employed as the simulant of metallic melt. A melt sensor and a mass sensor are installed to detect the discharging period of coherent jet and the mass accumulation of debris bed, respectively. A high-speed camera is applied to record the process of jet breakup, debris fragmentation and solidification, and debris bed formation. The experimental results show that the water pool depth has a significant influence on steam explosion and debris bed formation characteristics. Moreover, the jet breakup length is sensitive to jet diameter and free fall height. Finally, the features of metallic debris beds are compared with those of oxidic ones in respects of debris bed's configuration and porosity, particles' morphology and size distribution.

Neutronic and Thermal hydraulic evaluation of accident tolerant cladding materials in a WWER1000

The concept of Accident Tolerant Fuel (ATF) claddings is developed to improve the safety of light water reactors (LWRs). The main objective of this work is neutronic and Thermal Hydraulic (TH) analysis of potential cladding materials in a WWER1000. In this analysis, the ZrC, TiC, SiC and FeCrAl are evaluated as enhanced alternatives for traditional Zircaloy-4. For neutronic analysis, Monte Carlo particle transport method is used. For thermal hydraulic analysis, the performance of the materials in unmitigated small break LOCA is evaluated by MELCOR. Assessments show that ATFs can delay the failure of reactor pressure vessel (RPV) and core melting by reducing the heat generated from oxidation reaction. The TiC and FeCrAl claddings are more reliable in the accident, but they are not desirable from the perspective of the Neutronics. The calculations demonstrate that ZrC and SiC can be excellent alternatives to the Zircaloy-4. Also, simulations show using TiC and FeCrAl as a 0.1 mm thick covering of the Zircaloy-4 improves the multiplication factor, but it is not sufficient.

SIMMER modelling of accident initiation phase in sodium fast reactors

The SIMMER-III code was developed mainly for simulation of hypothetical reactor accidents after core melting, but can be applied also for accident initiation phase simulations in sodium and other liquid–metal-cooled fast reactors. New thermal hydraulic and neutronic simulation approaches and models have been developed for treatment of the initiation phase. Two examples of simulation of unprotected loss of coolant flow (ULOF) transients, in ESFR-SMART and FFTF reactors, are presented. It can be concluded that the SIMMER code has a large potential for application to initiation phase analyses.

A simple correlation to estimate agglomerated debris formation based on experiments of melt jet-breakup using a metallic melt

The molten core (corium or melt) relocation in severe accidents in a light water reactor can cause agglomeration by the merging of melt particles with others, and form massive debris (agglomerated debris) when the particles that detach from a melt jet accumulate on the floor of the containment vessel. This paper presents a modification of a simple correlation to estimate the mass fraction of the agglomerated debris proposed in the previous study. The modification is based on experiments of melt jet-breakup using a metallic melt. The mass fraction of the agglomerated debris needs to be estimated because the agglomerated debris can severely affect the debris bed coolability. The present study focused on the effects of changes in the experimental conditions for melt jet injection (nozzle diameter and inlet velocity) on the mass fraction of the agglomerated debris to expand the experimental database of the previous study that focused only on the effects of changes in the melt and coolant temperatures, and coolant depth on the mass fraction of the agglomerated debris. The visualized observations using a high-speed camera and morphological investigation of recovered debris revealed the effects of changes in the nozzle diameter and inlet velocity on the mass fraction of the agglomerated debris. The extrapolation of the modified simple correlation revealed the mass fraction of the agglomerated debris under the anticipated reactor conditions.

Plausible Constraints on the Range of Bulk Terrestrial Exoplanet Compositions in the Solar Neighborhood

Rocky planet compositions regulate planetary evolution by affecting core sizes, mantle properties, and melting behaviors. Yet, quantitative treatments of this aspect of exoplanet studies remain generally underexplored. We attempt to constrain the range of potential bulk terrestrial exoplanet compositions in the solar neighborhood (<200 pc). We circumscribe probable rocky exoplanet compositions based on a population analysis of stellar chemical abundances from the Hypatia and GALAH catalogs. We apply a devolatilization model to simulate compositions of hypothetical, terrestrial-type exoplanets in the habitable zones around Sun-like stars, considering elements O, S, Na, Si, Mg, Fe, Ni, Ca, and Al. We further apply core–mantle differentiation by assuming constant oxygen fugacity, and model the consequent mantle mineralogy with a Gibbs energy minimization algorithm. We report statistics on several compositional parameters and propose a reference set of (21) representative planet compositions for use as end-member compositions in imminent modeling and experimental studies. We find a strong correlation between stellar Fe/Mg and metallic-core sizes, which can vary from 18 to 35 wt%. Furthermore, stellar Mg/Si gives a first-order indication of mantle mineralogy, with high-Mg/Si stars leading to weaker, ferropericlase-rich mantles, and low-Mg/Si stars leading to mechanically stronger mantles. The element Na, which modulates crustal buoyancy and mantle clinopyroxene fraction, is affected by devolatilization the most. While we find that planetary mantles mostly consist of Fe/Mg silicates, the core sizes and relative abundances of common minerals can nevertheless vary significantly among exoplanets. These differences likely lead to different evolutionary pathways among rocky exoplanets in the solar neighborhood.