Blockage Accident Research Articles

Experimental Study on the Transport Behavior of Micron-Sized Sand Particles in a Wellbore

In the process of natural gas hydrate extraction, especially in offshore hydrate extraction, the multiphase flow inside the wellbore is complex and prone to flow difficulties caused by reservoir sand production, leading to pipeline blockage accidents, posing a threat to the safety of hydrate extraction. This paper presents experimental research on the migration characteristics of micrometer-sized sand particles entering the wellbore, detailing the influence of key parameters such as sand particle size, sand ratio, wellbore deviation angle, fluid velocity, and fluid viscosity on the sand bed height. It establishes a predictive model for the deposition height of micrometer-sized sand particles. The model’s predicted results align well with experimental findings, and under the experimental conditions of this study, the model’s average prediction error for the sand bed height is 12.47%, indicating that the proposed model demonstrates a high level of accuracy in predicting the bed height. The research results can serve as a practical basis and engineering guidance for reducing the risk of natural gas hydrate and sand blockages, determining reasonable extraction procedures, and ensuring the safety of wellbore flow.

Experimental Study on the Suspending Mechanism of Suspending Agent in Coal-Based Solid Waste Slurry for Long-Distance Pipeline Transportation

The transportation of coal-based solid waste filling slurry (CSWFS) through pipelines for underground goaf injection is essential for enhancing mine safety and promoting green, low-carbon coal mining. To address the issue of pipeline blockage caused by the suspension sensitivity of CSWFS during long-distance transportation, this study proposes the addition of the suspending agent hydroxypropyl methyl cellulose (HPMC) to transform the filling slurry into a stable suspending slurry. The mechanism by which the suspending agent modifies the rheological property of CSWFS was elucidated and verified. Firstly, an evaluation index system for the suspending state of CSWFS based on the “experimental test and theoretical calculation” was established. The values for layering degree, bleeding rate time-loss, and the corresponding average time-loss rate over 0 to 120 min of A1–A5 CSWFS were recorded as 24 mm–2 mm, 3.0–0.2%, 252.4–54.2%, and 149.6–14.6%, respectively. The concentration gradient evaluation result, C/CA = 0.91 (≥0.8), confirmed that the suspending agent maintained a stable suspending state over time for CSWFS. Secondly, it was demonstrated that the suspending agent HPMC modified the rheological property of A1–A5 CSWFS by increasing its plastic viscosity, which strengthened the viscous resistance to particle settling, thereby transforming a semi-stable slurry into a stable one. Additionally, the formation of a spatial suspending network by the suspending agent ensures that no pipeline blockage accidents occured in practical engineering applications. Furthermore, the XRD and SEM tests were utilized to verify the microstructure of the top (T) and bottom (B) samples in A4 block. It was concluded that the type of hydration products, occurrence forms, lapping compactness, and microstructural development were consistent, ultimately forming a high-strength, dense, hardened filling block. Finally, numerical simulation confirmed that the addition of suspending agent in A4 slurry formed a comprehensive spatial suspending network and a well-structured, unified system. This is one effective approach which could contribute to addressing the technical issue of pipeline blockage during long-distance pipeline transportation.

Numerical analysis of duct wall failure in a prototype FAIDUS design-based generation IV SFR fuel subassembly under flow blockage accident

An ingenious prototype Fuel Assembly Inner DUct Structure (FAIDUS) design is conceptualized under the Generation IV safety philosophy to prevent large-scale corium pool formation following total coolant flow blockage accident. The early failure of the duct wall provides a built-in guideway for molten material relocation out of the core region, ensuring inherent safety robustness. Therefore, understanding the duct wall failure mechanism is critical for evaluating accident mitigation potential of the FAIDUS design. In this regard, a transient enthalpy-based multiphase computational model is developed to investigate the heat transfer and fluid flow behavior involved during the duct wall failure in 3-D domain. The numerical model is employed initially to simulate wall failure in the EAGLE ID1 in-pile test. The model results are validated against the in-pile test data. Subsequently, FAIDUS duct wall failure in a prototype fast reactor fuel subassembly (SA) is investigated employing the present model. The study reveals that the large heat flux from the molten pool to the duct is the main reason for the duct wall failure. The duct failure starts with an initial rupture across the corners of the hexagonal-shaped duct and progresses to total failure of the duct wall by the expansion of the rupture over the whole duct wall area with time. The outer hexcan wall sustains only minor thermal damage during the duct wall failure process. The present study explores the associated thermal-hydraulics, molten pool dynamics, crust formation behavior, and event sequences involved in duct wall failure in great detail. The simulation findings suggest that the FAIDUS SA design facilitates premature failure of the slim duct wall prior to the hexcan wall failure. This could potentially enable early discharge of the molten corium away from the core region, limiting the accident progress.

Simulation Analysis Based on Sodium-Cooled Fast Reactor Fuel Assembly Under Blockage Accident

This paper employs computational fluid dynamics to investigate the fuel assemblies within a sodium-cooled fast neutron reactor. To begin with, we developed computational models for seven wire spacer fuel rods under both normal operating conditions and transient blockage conditions. We conducted separate analyses to assess the impacts of normal operation, blockage thickness, and blockage area. This work allowed us to acquire data on the heat transfer properties and the flow field variations for the coolant, blockages, and fuel rods across different conditions. Subsequently, we leveraged these flow field alterations to examine the resulting temperature distributions. Analyses were conducted to evaluate the effects of normal operation, blockage thickness, and blockage area. The research acquired the heat transfer characteristics and flow field distribution variations of the coolant, blockages, and fuel rods under different operational conditions and utilized these variations to analyze the temperature distribution. Through research analysis, the following conclusions have been reached. Under normal operating conditions, the temperature and flow fields of the fuel components exhibit cyclic variations along the axial length, corresponding to the pitch of the wire spacers. Heat exchange between the internal and external subchannels occurs independently, which further substantiates that the incorporation of wire spacers strengthens lateral flow disturbances. This effect is more pronounced within the internal subchannels, thereby leading to a marked difference in the flow fields on either side of the wire spacers. In the case of blocked conditions, an increase in blockage thickness and area both lead to higher temperatures for the coolant, the blockages themselves, and the fuel rods. The temperature in the recirculation zone behind the blockages also rises with increasing blockage thickness and area, although the magnitude of this increase is not significant. After the onset of blockage conditions, the instantaneous temperature tends to increase. As time progresses, the instantaneous temperature fields following the blockage do not display greater fluctuations in temperature change than those observed after the system has stabilized. For the temperature parameters of the convective field, differences arising from an increase in blockage area are more significant than those caused by an increase in blockage thickness. When blockage area and blockage thickness are increased by the same multiple, an increase in blockage area results in a higher temperature peak. Increasing the area of blockage necessitates a longer duration for both the temperature and the velocity fields to revert to equilibrium.

Study on the suspending mechanism of gangue particle in coal-based solid waste slurry

In China's coal mine backfill mining projects, certain mines demonstrate an extensive filling distance that exceeds 3000 m. However, there are few researches concerning the theoretical underpinnings for slurry long-distance pipeline transportation. Gangue particles suspending in filling slurry is the prerequisite for successful long-distance pipeline transportation. This paper conducts theoretical researches on the heightened sensitivity issue of particle settling within coal-based solid waste filling slurry (CSWFS) for long-distance pipeline transportation, which can easily lead to unstable slurry concentration then pipe blockage or bursting accidents. The mechanical model for particle settling and critical non-settling particle diameter for long-distance pipeline transportation are developed based on the "solid-liquid two-phase carrier suspension fluid", and reveal the intrinsical mechanism of particle settling within CSWFS for long-distance pipeline transportation. Through the calculation of the mechanical model for critical non-settling particle diameter and particle settling, coupled with the particle diameter screening experiment, the value of critical non-settling particle diameter is determined to be 0.0067 m, and the density of solid-liquid two-phase carrier suspension fluid is 1712.37 kg/m3. The CSWFS is classified as a semi-stable slurry, lacking time-dependent suspending state required for long-distance pipeline transportation, which could lead to pipe blockage accidents. Further research into effective methods is urgently required to ensure adequate safety and adaptability for long-distance pipeline transportation of CSWFS.

Mesh morphing-based pre-processing and numerical simulation of blockage accident in lead–bismuth fast reactor fuel assembly

This paper presents a CFD modeling approach for a 19-rod bundle fuel assembly with spiral semi-cylindrical ribs, which potentially can be used for liquid metal cooled fast reactor, as well as a numerical simulation of flow and thermal behavior under partially blocked conditions. The ribs on fuel rod surface are integrally formed with the cladding to enhance heat transfer while preventing detachment. However, the complex channel geometry presents challenges for grid generation, as using a large number of tetrahedral or polyhedral elements would consume significant computational resources. Therefore, this paper proposes a mesh morphing-based pre-processing method to establish a structured hexahedral mesh for such complex geometry. Using this approach, the 19-rods assembly is analyzed, and the result has been verified by conventional mesh scheme as well as experiment.

Fluid Dynamics Effects of a Porous Blockage on Laminar Flow in the Interior Subchannels of a 61-Pin Wire-Wrapped Rod Bundle

Flow blockages in Liquid Metal Fast Reactor (LMFR) fuel assemblies that can cause fuel pin cladding failures require further investigation for the development and optimization of these Generation IV reactor designs. The objective of this study was to experimentally evaluate the effects of a porous blockage accident scenario on laminar flow behavior in the interior subchannels of a prototypical 61-pin wire-wrapped rod bundle. The laminar flow condition is considered because of its importance to natural circulation and loss-of-coolant-accident operating scenarios as well as the limited availability of experimental data. The matched-index-of-refraction method was utilized to obtain two-dimension two-component time-resolved particle image velocimetry (TR-PIV) measurements at three planes centered on blocked and neighboring subchannel regions. Time-averaged first-order and second-order statistics, computed by Reynolds decomposition, were visualized including the mean and fluctuating streamwise and spanwise velocity components. Line profiles described the evolution of flow from upstream regions, to separated flow, and recombination downstream, while a modified Galilean decomposition distinguished differences between flow in the blocked measurement plane and flow in its neighboring counterpart region. Spatial-temporal cross-correlations were performed for the streamwise velocity fluctuations to characterize the convection velocity and decay of traveling vortices in the wake of the porous blockage. The isothermal TR-PIV measurements from this study provide high-fidelity experimental data sets for the validation of computational models and numerical studies to characterize complex fluid phenomena in wire-wrapped rod bundles during the potential accident scenario of a porous, interior flow blockage.

Multi-scale coupling analysis of the flow blockage accident in the typical LFR

Multi-scale coupling analysis of the flow blockage accident in the typical LFR

Safety analysis of pressurized water reactors with annular fuel during different transients

The dual-cooled annular fuel has two channels, with a shorter heat transfer path and a larger heat transfer area than solid fuel, which can effectively improve the safety of the reactor. It has excellent potential to improve the economy of the pressurized water reactor (PWR) and is considered to have a good development prospect. In this paper, the safety of the reactor with annular fuel assemblies under different transient conditions is analyzed using the system code NUSOL-SYS and the subchannel code NACAF, with the CPR1000 as the reference power plant. The analysis mainly includes the large break loss of coolant accident (LBLOCA), main steam line break accident (MSLB), loss of flow accident (LOFA), and rod ejection accident (REA) under nominal power and higher power density. The flow blockage accident and the combined accidents of LBLOCA and the flow blockage accident are also evaluated. The results show that during LBLOCA, the peak cladding temperature (PCT) of the core is only 833 K, much lower than that of the solid fuel. In addition, the departure from nucleate boiling ratio (DNBR) of annular fuel is larger during MSLB and LOFA, which means a more negligible risk of fuel damage. In REA, the average temperature of annular fuel is below 900 K, and the average enthalpy is below 200 kJ/kg, which is much lower than that of solid fuel. The results at 150 % power show that the performance of the core with annular fuel is still better than that of solid fuel at nominal power, except that the PCT during LBLOCA is slightly higher than solid fuel at nominal power. During the flow blockage accident, the results with different blockage areas show that a small proportion of blockage of the inner channel does not have a significant effect on the cladding temperature and that there is a risk of rupture when the inner channel is completely blocked. Finally, the results of the assumed combined accident show that the blockage of the inner channel before scram will significantly increase the PCT of the core and the time to complete the reflood during LBLOCA.

Identification and prediction of hydrate–slug flow to improve safety and efficiency of deepwater hydrocarbon transportation

The vast hydrocarbon resources (oil, gas, and gas hydrate) in deepwater areas are considered one of the most significant resources of the future. However, the dangerous flow patterns and blockage accidents caused by hydrates threaten the safety and efficiency of the transportation process. A mutual interaction exists between hydrate growth and multiphase flow. In this study, hydrate formation and gas–slurry two–phase flow tests were conducted in a pure water system with a constant pressure (4 MPa) and various liquid loadings (40–80%) in a visual flow loop. It was found that the hydrate growth process in the dynamic system included bubble promotion, subcooling promotion, and mass transfer inhibition. Meanwhile, two atypical slug patterns (rapid conversion slug and deposition slug) in the vertical upward flow were determined, which corresponded to the promotion and inhibition regions. In the rapid conversion slug flow, the churn segments replaced the original Taylor bubbles when the liquid loading was less than 60%. In the deposition slug flow, slurry shedding phenomena was observed. In addition, the hydrate particle deposition behavior was found to inhibit hydrate growth and cause flow pattern shifts, but did not trigger the differential pressure surge. The gas isolation caused by the liquid plug and energy dissipation due to the collision between hydrate particles and the tube wall may be responsible for the sudden increase in pressure drop. Furthermore, the feasibility of predicting hydrate blockage using the differential pressure method was explored. The experiment results showed that this warning method was effective when the liquid loading was below 76%. And an effective warning can improve the transport capacity by approximately 34%.

Development of a clogging probability model in simplified geometry for fast reactor flow blockage analysis

The occurrence of flow blockage accident in fuel assemblies is considered as one of the primary challenges to be addressed in liquid metal fast reactors. Flow blockage accidents are primarily caused by the accumulation of solid particles, such as corrosion products, wire spacer fragments, and chemical products between liquid metal and impurities like air or water. To better understand the mechanisms underlying the blockage formation, a project consisting of several series of simulated experiments is currently underway at Sun Yat-sen University and Harbin Engineering University. In the initial stage of the project, experimental study was conducted to investigate the influence of particle characteristics on the probability of clogging by discharging various solid particles into a reducer pipe. Preliminary experimental analyses revealed the formation of an arch structure at a specific location on the pipe. This arch structure causes the granular flow to cease, resulting in blockage. Based on the accumulated experimental database, in this study an empirical model is developed to predict the clogging probability, taking into account the above-mentioned influential parameters. The model demonstrates a reasonable agreement with experimental results within the current range of conditions, encompassing variations in the ratio between hopper outlet size and particle size, particle capacity, particle static friction coefficient, and particle release height. Although further extension of the empirical model is inevitably necessary under more realistic reactor conditions, such as incorporating realistic sub-channels, irregularly shaped particles, or a loop with a liquid phase, it is expected that the model will be beneficial for the improved design of sub-channels and safety analysis of fast reactors.

Numerical study of the impact of blockages on the Thermal-Hydraulic performance of 19 and 61-pin Wire-Wrapped bundles with conjugate heat transfer using RANS

Liquid metal fast reactors assemblies with wire spacers present a compact rod pitch to diameter ratio. Therefore, a typical off-normal condition that may occur is the blockage accident, caused by the accumulation of corrosion products or due to the detachment of broken wires. Blockage accidents may threaten the safety performance, leading to fuel rod damage. This work presents a thermal–hydraulic analysis using the RANS k-ε Realizable model to assess the impact of blockages. The model was validated on a blocked 19-pin wire-wrapped assembly comparing cladding and coolant temperatures and pressure drop with experimental data. Then, a numerical comparison between nominal and blocked conditions on a 61-pin wire-wrapped bundle was performed, analyzing solid and porous blockages. The greatest peak temperatures were obtained for the solid blockage case. Both types of blockages resulted in a similar velocity prediction upstream of the obstruction, while major differences were found in the recirculation region.

Modeling Flow Blockage Accident in Graphite-Moderated Molten Salt Reactors

Fuel channel flow blockage is a postulated accident scenario in graphite-moderated molten salt reactors (MSRs), caused by debris partially obstructing the flow in a hole inside a graphite moderator brick. This obstruction produces an increase in the fuel salt bulk temperatures in the blocked channel, resulting in a significant fraction of the deposited fission power being conducted through graphite from the blocked channel to the surrounding unblocked channels. In the blocked channel, a combination of forced convection and natural convection takes place, the latter being dominant at very strong flow rate reductions. At full blockage (with the fragment completely obstructing the flow), because of the poor thermal conductivity of fuel salt, correct estimation of the natural convection impact on heat transfer between the channel bulk and the walls is fundamental to determine whether boiling conditions are reached in the blocked channel. The proposed assessing approach, developed in collaboration with ThorCon in the frame of the ThorCon reactor core design and optimization, is presented in this work. Results from a case study based on the molten salt breeder reactor (MSBR) design and operating conditions are presented and discussed. These analyses show that with respect to full blockage accidents in other reactor types, the consequences of full channel blockage accidents in graphite-moderated MSRs are milder. For graphite-moderated MSRs, the main mechanical limits during the accident arise in thermal stresses in the graphite brick. Graphite thermal and mechanical properties, namely, bulk modulus, thermal expansion coefficient, ultimate strength, and thermal conductivity, are severely impacted by neutron irradiation through the life of the graphite. All these effects are taken into account in the current analysis and are discussed in the present work.

Diagnosing core local flow blockages in a VVER-1000/446 reactor using ex-core detectors and neural networks

According to the Defense in Depth (DiD) concept, it is crucially important to detect and mitigate the consequences of accidents in Nuclear Power Plants (NPPs) in the early stages. Due to the nature of local blockages, the timely and suitable detection of melting accidents is a challenging task. Therefore, developing early detection and accurate mechanisms to prevent nuclear accidents as well as mitigation of their consequences are an essential step to be taken into account. In the present study, an add-on system for detecting and monitoring local melting accidents which in the early stages do not lead to SCRAM, is developed using neural networks and neutron ex-core detectors. The results of the core MCNPX model in terms of local flow blockage location in different sub-channels are used as the network training inputs. The core is divided into four, eight, and twenty-four angular segmentations, along with seven radial and ten axial partitions. More than 50,000 local blockage accidents are simulated in the MCNPX code, and 500 networks are trained for achieving an optimal network topology with 90.1% accuracy. The results of trained network shows that the proposed monitoring system is capable of diagnosing local flow blockages in less than one-tenth of a fuel rod height. The sensitivity and noise analysis results can represent the reliability of the designed system considering blockage distances from the detectors. The obtained results demonstrate that by implementing the proposed add-on system, local blockage accidents can be diagnosed with high accuracy.

Three-dimensional thermal-hydraulic characteristics analysis of plate-type fuel reactor core based on OpenFOAM

In this paper, a full scale three-dimensional thermal-hydraulic analysis method for plate-type fuel reactor core is proposed through integrating traditional sub-channel analysis concepts into CFD platform. The coupled heat transfer model between fuel plates and narrow rectangular coolant channels is established. The three-dimensional thermal-hydraulic analysis code CorTAF-PT is developed based on open-source CFD platform OpenFOAM. The CorTAF-PT is validated against with the Qiu-TAKAHASHI experiment and IAEA 10 MW MTR. The code simulation results agree well with the experiment and MTR operation data with relative error within 5%. The full-scale MTR simulation under steady state and LOFA is carried out through channel-level resolution. In partial blockage accident, maximum temperature of coolant, fuel pellet and cladding increase by 9.75 K, 60.65 K and 60.52 K respectively. Furthermore, the double temperature peaks of fuel plates and local coolant reflux with −0.121 m s−1 are revealed under partial blockage accident. This work provides reference value for correlational studies on thermal-hydraulic characteristics of plate-type fuel reactor core.

Analysis of lead–bismuth eutectic-cooled solid reactor under flow blockage accident by 3D neutronics thermal-hydraulics coupling method

As the most common and serious accident in liquid metal-cooled reactors, the occurrence of a flow blockage accident may threaten the integrity of the cladding and the safety of the core. Therefore, the accurate prediction of fuel and cladding under blockage accidents is particularly important in reactor design and thermal–hydraulic analysis. This paper uses a three-dimensional neutronics thermal-hydraulics coupling method to analyze the blockage accident of a lead–bismuth-cooled solid reactor proposed by the Innovative Nuclear System Laboratory (INSL) of Shanghai Jiao Tong University. Based on the accurate and material models, the solid reactor was modeled, and 15 blockage accidents were carried out. The effects of the blockage's radial position, thickness and axial position on the fuel and cladding temperature were compared. Unlike the conventional open-channel blockage, the results of the closed-channel blockage accident analysis show that the thickness and axial position of the blockage has less influence on the maximum fuel and cladding temperatures but slightly affect the fuel and cladding axial temperature distribution. Meanwhile, the blockage accident in the closed channel is more serious, and the maximum temperature rise of fuel and cladding is higher, but both are lower than the limit value required by the design criteria.

Development and validation of a general two-fluid thermal-hydraulic analysis code for reactors with plate-type fuel assemblies

A general two-fluid thermal-hydraulic analysis code, TTHAC-PFA (Two-fluid Thermal-Hydraulic Analysis Code for reactors using Plate-type Fuel Assemblies), has been developed. The code was developed on basis of the fundamental governing equations. Reasonable mathematic and physical models for parallel rectangular channels and fuel plates have been built up to meet the command of the safety analysis for plate-type fuel reactors. The coolant distribution is conducted on basis of the pressure drop equation theory. Normal subchannel approach is adopted in the code. Fortran language is adopted. The RA-6 experiment is used to evaluate the heat transfer models. The max deviation of the simulations from the measured value is about 5.7 °C. Then, experiments on two-phase flow pressure drop are adopted to validate the code. Approximately 97% of the data points are within ±20% of the deviation range. In addition, CARR (China Advanced Research Reactor) is adopted as the benchmark for normal condition validation. Flow distribution validation and temperature profile validation are conducted. The flow validation presents negligible mean discrepancy and the temperature differences are within the scope of 6 °C. All validations are in good agreement, which indicates the developed code meet the requirements of analyzing thermal-hydraulic characteristics for parallel rectangular channels in reactors. Furthermore, TTHAC-PFA is applied to the investigate channel blockage accident occurred in the CARR. The mass flow is redistributed among channels due to the obstruction. In the obstructed channel and two nearby channels, the coolant temperatures increase a lot. The coolant temperature in the obstructed channel rises by 60 °C. In the rest of the channels, the coolant temperatures decrease with the increase of coolant flow. Besides, the two fuel plates, which form the blocked channel, are asymmetrically cooled due to flow maldistribution. The temperature difference between the cladding on both sides of the fuel plate can reach 40 °C. As a result of partial blockage, the cladding temperature on the side of the blocked channel rises by 63 °C. During the blockage accident, the plate temperature is under the melting temperature. The results indicate the significance of studying the effects of the adjacent channels near the obstructed channel for reactors with plate-type fuel elements.

CorTAF: A nuclear reactor core three-dimensional thermal-hydraulic characteristics analysis code based on OpenFOAM

In this paper, a thermal hydraulic analysis method based on open source CFD platform OpenFOAM was proposed. The models of coolant flow and heat transfer, fuel rod heat conduction and coupled heat transfer were established according to the bundle structure characteristics of PWR core, and then a nuclear reactor three-dimensional thermal–hydraulic characteristics analysis code CorTAF was developed based on finite volume method with the spatialresolution of subchannel level. The international benchmarks, including GE3 × 3, Weiss and PNL2 × 6 fuel assembly flow and heat transfer experiments, were selected to carry out the validation. The calculation results were basically consistent with the experimental data, illustrating that CorTAF is suitable for predicting the flow and heat transfer characteristics of coolant in the rod bundle fuel assembly. The thermal–hydraulic characteristics of fuel assembly and PWR core under full power operation and blockage accident conditions were simulated using CorTAF. The steady-state and transient spatial distribution of physical quantities in subchannel-scale including coolant and fuel rod temperature were obtained, and the influence of blockage condition on flow and heat transfer characteristics was analyzed. This work has reference significance for the development of core thermal hydraulic analysis tools for PWR.

The blockage risk in the elbow of the Bi-directional pig used for submarine pipeline based on the modified Burgers-Frenkel (MB-F) model

The blockage accident is a knotty problem for the pigs employed in pipeline inspection and dewatering operations. Compared with the onshore pipeline, the rescue operation of the blocked pig in the submarine pipeline is extremely difficult and needs enormous costs. It is important to evaluate the blockage risk of the pig when passing through the elbow, which is beneficial for ensuring the security of the pipeline and the pig. However, the blockage risk assessment in current engineering practice mainly relies on the structure size examination and experience judgment. In this paper, a novel dynamic analysis model which describes the traveling attitude and force state of the pig in the elbow is derived based on the modified Burgers-Frenkel (MB-F) model. Four typical blockage forms were introduced on the basis of a quantitative index system established according to the pig frictional contact characteristics after a series of calculations. In addition, a quantitative assessment method for predicting the blockage risk of the bi-directional pig was presented through the weighting approach of subjective and objective evaluation using the analytic hierarchy process (AHP) and entropy combinational method. Then, the sensitivity analysis was conducted to determine the importance and the influence rules of the different factors.

Study on anti-disturbance capacity of LBE cooled double loop natural circulation system under asymmetric conditions

Heat exchangers in natural circulation LBE-cooled fast reactors are used in harsh conditions such as extreme temperatures, significant pressure differences, high density, and corrosive environments. Therefore, heat transfer tube rupture and flow blockage accidents are common and result in an asymmetric heat load or asymmetric resistance thereby significantly impacting the safe and stable operation of the reactor.In this study, SNCLFR-10 has been used as the reference reactor and FLUENT simulation is used to establish the LBE-cooled dual-loop natural circulation system. Consequently, a theoretical solution of the natural circulation flow of a dual-loop system (TSNCD) is deduced using dimensionless analysis.The TSNCD theoretical solution is verified by using a dual-loop natural circulation system established via FLUENT simulation. Subsequently disturbance characteristics of the natural circulation system under different heat load differences or resistance differences are analyzed. The characteristic parameters representing the anti-disturbance capacity of natural circulation are determined using fitting approximation, and the best anti-disturbance interval is obtained.The results show that the TSNCD theoretical calculation formula of dual-loop natural circulation flow is extremely reliable, and the error between the calculations and the FLUENT simulation result is less than 5 %.When an asymmetric heat load disturbance and asymmetric resistance disturbance are incorporated into the system together or alone, the loop flow changes insignificantly in a certain interval; the anti-disturbance capacity of the system is strong in this interval. The resistance of the system to heat load disturbance is 0.2<k1<0.8, and the resistance disturbance is 0.6<B<4.8. When an asymmetric heat load and asymmetric resistance disturbance are simultaneously introduced into the system, an optimal resistance disturbance is achieved (k1∈[0.40,0.60] and B∈[0.7,2.9]).