Thermal-hydraulic Model Research Articles

Full 3D thermal-hydraulic and electric modelling of quench propagation in HTS conductors

Abstract A fully three-dimensional multi-physics model to simulate quench propagation in HTS conductors for fusion applications is presented. It accounts for thermal, electric and fluid dynamics throughout the entire transient. The need for high-fidelity models for quench simulations comes from the bulky layouts of many HTS conductors that are being proposed. The model is then validated against experimental data, showing a good agreement on all the relevant quentities (local voltage and temperatures). It is shown that the detailed model improves the quality of the agreement with the measured data with respect to more simplified models. It also allows an insight on the temperature distributions in the conductor cross-section, which can be relevant for the interpretation of experimental data as well as to support the design of quench detection strategies which rely on local temperature variations.

Thermal-hydraulic simulation of loss of flow accident for WWR-S research reactor

Abstract A thermal-hydraulic model has been developed by RELAP5 code to simulate the thermal-hydraulic behavior of a WWR-S research reactor under loss of flow accident (LOFA). The reactor power is 2 MW with downward flow direction and different types of fuel bundles of different power densities and different coolant flow-rates. The simulation is performed for two scenarios; protected and unprotected LOFA. In the protected LOFA scenario; Scram is triggered as soon as the coolant flow rate reaches 85 % of its nominal value while in the unprotected LOFA scenario; the reactor continues operation during pump cost-down and natural circulation flow. Once the flow is low enough, the buoyancy force increases in the core leading to flow inversion phenomenon and establishment of a natural circulation mechanism within the core coolant channels in both scenarios. In the protected LOFA scenario, the coolant remains subcooled by a vast margin during transient while bulk boiling is predicted at the upper part of the core for the unprotected LOFA scenario. The heat fluxes leading to the onset of nucleate boiling and the critical heat flux are predicted and the safety margins are determined. The results for both scenarios are analyzed and discussed.

RGB Mapping: A Dynamic Approach for Flow Pattern Identification and Classification

Thermal-hydraulic modeling and simulation are heavily dependent on the knowledge of flow regimes, which sort multiphase flow patterns into classifications with static boundaries that display characteristic features of flow patterns and phase distributions. Flow regime maps heretofore have been primarily developed mainly for vertical or horizontal pipe orientations by capturing images of flows at varying conditions and assigning them to different flow regimes based on visualization studies. As such, this method is inherently subjective. Furthermore, classifying two-phase flows into regimes with static boundaries is not physical, as two-phase flows undergo gradual changes as they transition from one regime to another. Therefore, such a static approach is not only unphysical but also inaccurate, resulting in the erroneous analysis of two-phase flows. Additionally, subjective observations at such flow conditions are prone to disagreement and inconsistencies in labeling. These difficulties are exacerbated in inclined two-phase flows. In this study, a novel two-phase flow pattern map employing the dynamic red, green, and blue (RGB) color model is presented. The model contains the embedded information of three normalized time-averaged statistics from signals obtained by a dual-ring impedance meter. A database of impedance meter signals from a 25.4-mm-inner-diameter adiabatic air-water inclinable test facility is used to compute statistics for each experimental flow condition. Multiple statistical measures are evaluated, and a principal component analysis is performed for feature selection. By mapping each parameter to intensities of red, green, and blue, a dynamic RGB flow pattern map can be produced in which groups of similar colors are representative of various flow regimes and the dynamic transition characteristics between the flow regimes are characterized physically. Therefore, this new method makes viable the observation of how flow patterns gradually change with respect to angle, as well as the fuzzification or quantification of the uncertainty surrounding experimental transition boundaries.

Aircraft Electrothermal Pulse Deicing

Abstract Ice formation and accumulation on aircraft is a major problem in aviation. Icing is directly responsible for aircraft incidents, limiting the safety of air travel and requiring expensive, and sometimes ineffective deicing strategies. Furthermore, electrification of aircraft platforms leads to difficulties with integration of legacy deicing methods such as pneumatic boots. In this work, we study electrothermal pulse deicing capable of efficient and rapid removal of ice from aircraft wings. The pulse approach enables the efficient melting of a thin (<100 μm) ice layer on the wing surface to limit parasitic heat losses. Only the interface is melted, with the rest of the ice sliding on the melt lubrication layer due to aerodynamic forces. To study pulse deicing, we developed a transient thermal-hydrodynamic numerical model that accounts for multiple phases and materials, specific and latent heating effects, melt layer hydrodynamics, as well as boundary layer effects. To identify optimal deicing strategies, we use our model to study the effects of heater thickness (50 μm < th < 1 mm), substrate electrical insulation thickness (10 μm < ti < 1 mm), pulse duration (0.4 s < Δtpulse < 4.5 s), and pulse energy. Optimum operating points are identified for large (Boeing-747), midsize (Embraer-E175), and small (Cessna-172) aircraft. The scale-dependent thermal-hydraulic model results are used to estimate input conditions required for deicing and integrated into an electrical model considering energy storage, power electronics, integration, and layout, to achieve overall volumetric and gravimetric power density optimization.

Analysis of thermal production effect of multi-horizontal U-shaped well

The multi-horizontal section hot dry rock (HDR) heat extraction system is proposed for efficient deep geothermal resources mining in this study. The thermal-hydraulic numerical model of the multi-horizontal section geothermal exploitation system is set up, the space-time evolution of temperature field is analyzed. The sensitivity factors including completion schemes, horizontal well diameter, quantity of horizontal sections and spacing of horizontal sections are investigated. The simulation results show that the production temperature and heating power of the multi-horizontal HDR production system decrease with the development of mining, and the decreasing speed gradually slows down. The open hole completion can be preferred when the formation conditions permit. The variation of horizontal well diameter has little effect on heat production when other conditions remain unchanged. The quantity of horizontal sections to be 3 or 4 for better heat extraction performance under the spacing of horizontal section remains 300m and the spacing of the horizontal section to be 150m is recommended within the scoped of this study. The results of this study can provide guidance for HDR mining research.

DEVELOPMENT OF LEAK DETECTION METHODS FOR USE IN FIELD PIPELINE TRANSPORT

Today, most existing leak detection methods are effective only for stationary operating conditions of pipelines. During transient (dynamic) pumping modes, a significant decrease in the sensitivity of the methods occurs, or inoperability of leak detection systems (LDS) based on them is observed.Accordingly, for the operation of the LDS in the presence of transient processes in pipelines, it is necessary to use special approaches that take into account the features and interrelation of the monitored indicators of the dynamically changing pumping regime with each other. One solution to this problem is the use of a dynamic thermal-hydraulic model of the pipeline, which makes it possible to calculate pumping indicators at intermediate and boundary points of the pipeline at each moment in time.The article presents the developed digital model of a pipeline transporting single-phase flows, as a system of equations obtained from the laws of conservation of mass, momentum and energy. The model describes all significant processes and effects during the movement of the pumped flow through a pipeline and allows you to calculate the main characteristics of the flow, including finding the pressure and temperature distributions along the pipeline route.The method for detecting leaks from field pipelines operating under transient (dynamic) conditions is to compare the actual measured flow rate at the end point of the pipeline with the flow rate calculated using a dynamic model of this pipeline, into which the measured pumping indicators at the inlet of a real object are previously transferred. If there are significant deviations between the calculated pumping flow rate at the outlet of the pipeline in the model and the flow rate measured at the pipeline, a leak is concluded.In order to check the performance of the proposed solutions, modeling of leaking pipelines was carried out in specialized software at various flow rates, media viscosities and leak volumes. Based on the data obtained, a conclusion was made about the efficiency of the developed method.Thus, thanks to the proposed approach, it will be possible to promptly detect and eliminate leaks and withdrawals through unauthorized taps on field pipelines operating in unsteady mode, which in turn will significantly reduce operating costs associated with eliminating the consequences of failures.

Non-equilibrium overlapping grid method with two-phase porous media model for printed circuit heat exchanger based steam generator

The printed circuit heat exchanger (PCHE) is a promising alternative for the once-through steam generator (OTSG) of small modular reactors due to its compact structure and high efficiency. To study the effect of flow maldistribution on the performance of heat transfer and thermal stress, it is necessary to conduct three-dimensional full-scale simulation of two-side fluid and solid domains in the printed circuit heat exchanger based steam genarator (PCHESG). However, there is a lack of mature thermal-hydraulic models for the PCHESG. This work proposes a non-equilibrium overlapping grid method with two-phase porous media model for the PCHESG. The temperature and flow fields of the PCHESG under high-pressure conditions are evaluated. Compared to traditional methods, the new method greatly reduces the order of magnitude of the grid number from 107∼109 to 106 and the computational time from days to hours. It is found that the flow maldistribution with a threshold cold-side mass flow rate of 0.08∼0.0944 kg/s induced by the header parts of the PCHESG deteriorates the total heat transfer rate by 58.8 % and increases the spatial non-uniformity of the temperature and phase distributions. The solid domain is solved simultaneously and its temperature non-uniformity becomes significant with a threshold cold-side mass flow rate of 0.04∼0.06 kg/s. The results demonstrate that this method has great potential in achieving rapid design and optimization for the PCHESG.

Advances from R2CA project on reactor simulations for burst rod number evaluation during LOCA

In the frame of the “Reduction of Radiological Consequences of design basis and extension accidents” European Union’s Horizon 2020 project, a specific effort was focused on the update and the development of methodologies to assess the radiological consequences of loss-of-coolant accidents. Evaluation of the radiological consequences associated to this accident is strongly linked to the prediction of the rod burst ratio and new approaches were investigated through research and development associated to accident simulation software. Complementarily to approaches chaining system thermalhydraulic simulation to fuel performance code, approaches with integral codes ASTEC, DRACCAR and ATHLET-CD were developed respectively by ENEA, IRSN and HZDR. These applications were demonstrated on LOCA simulation for PWR and highlighted the capabilities of the tools. The ASTEC PWR model was extended by ENEA, which proposes a 2D core model with an increased number of representative fuel rods. HZDR and IRSN proposes approaches based on 3D core model which are able to capture distinctive fuel assembly responses and predict the rod burst ratio according to the distribution of rod behaviors within the core. This work highlighted several possible core models to predict the rod burst ratio and which can be used in integral tools abled to couple thermal hydraulics and thermomechanics. Advanced core models simulating the thermomechanical response of several representative rods per fuel assemblies and using 3D thermal hydraulics model are recommended due to the heterogeneities on power distribution, which impacts flow distribution. The need to model RPV in 3D was underlined by ATHLET-CD large break loss-of-coolant demonstrative case, which exhibits non-symmetric core heat-up.In addition, demonstrative cases proposed by ENEA and IRSN compared predictions obtained with the specific burst criteria developed in the frame of R2CA project and more classical one devoted to the core coolability assessment. The strong sensitivity of the rod burst ratio prediction to the burst criteria was highlighted. The need to handle uncertainties within a global methodology for rod burst ratio evaluation was underlined.Finally, throughout the development of loss-of-coolant accident applications with ASTEC, DRACCAR and ATHLET-CD, some prospects of development were identified for the codes and should bring advances for LOCA simulation and RBR predictions.

Concept design, application and optimization of operational parameters for new forced safety injection tank system

Background The engineered safety systems are designed to execute fundamental safety functions encompassing reactivity confinement, reactivity control and decay heat removal. Failure of any one of these functions can result in severe accident conditions. Passive systems have been implemented as a better option for plant safety. This research proposes a modification of existing safety injection tanks to the new forced safety injection tanks (FSITs) that utilize the backpressure from the pressurizer or steam generator to drive coolant into the reactor core under high pressure conditions. Methods FSITs aim to extend the coping-time during accidents like Station Blackout, providing an extended timing window for deployment of FLEX systems. The mathematical design is proposed and implemented into the thermal-hydraulic input of a nuclear power plant to demonstrate the system’s applicability that was demonstrated by leveraging the conventional PRA approach and risk quantification. Results The suggested system useful when used in the accident scenario of Station black out in-coincident with turbine-driven pumps fail to run. As the proposed design is a conceptual design, the optimization of associated operational parameters and set points is necessary. This optimization is performed using the risk analysis and virtual control environment-based Dynamic Probabilistic Risk Assessment framework. The method and findings of this study affirm that the coping-time for Station Blackout can be significantly extended, ensuring a substantial margin for the effective deployment of the FLEX. Conclusions A concept design of a passive forced safety injection system was suggested and demonstrated by integrating the mathematical model into a thermal-hydraulic model of a nuclear power plant. The parameters were optimized and the results demonstrated that the new system was effective in recovering the nuclear power plant from the accidents such as SBO with turbine-driven pumps fail to operate.

Multipoint kinetics model with power reactivity defect for the axial offset control in the VVER-1200 nuclear reactor during the load following mode of operation

This article proposes the multipoint kinetics model consisting with different number of point kinetics model (two points, four points, six points, eight points, ten points) in the axial direction for the VVER-1200 nuclear reactor. Each node is coupled with others through the coupling coefficients determined from the diffusion approach. For more precise description of the dynamical modes of the reactor operation, the proposed model integrates the power reactivity feedback derived from temperature reactivity coefficients and Mann’s thermal hydraulic model which assumes one fuel node adjacent to two coolant nodes. On the model with four axial points, additionally was tested the influence of the different number of delayed neutrons groups on the accuracy results and model running time during the load-following mode of operation. Moreover, the novel model of the control rods is introduced, utilizing a combination of sign functions to sequentially influence all nodes during insertion or withdrawal. The computational results show that the accuracy of the proposed model is satisfactory, and general assumptions about transients align with their physical definitions. This research contributes to the advancement of the point-like nuclear reactor modeling for improvement of the automatic power controller synthesis.

Detailed effects of reservoir permeability distribution differences on enhanced geothermal systems performance

In enhanced geothermal systems, fractured reservoir permeability significantly affects geothermal exploitation efficiency. However, the detailed effects are not fully understood while most previous literature ignored the spatial differences of reservoir permeability because of the complexity and heterogeneity of fracture distribution. This study aims to reveal the quantitative relationship between the geothermal system’s heat extraction performance and the distributed permeability, through investigating heat extraction ratio and flow impedance by building a 3D thermal-hydraulic coupled model with discrete fracture network. The current study also compared the effects of twenty-nine kinds of fracture network distributions at various depths. It is found that higher fracture permeability around production well (800×10−12 m2) more significantly improves heat extraction rate by 30% rather than higher permeability in other areas in the first year. It is also found that fracture permeability changes on both sides of predominant flow regions cause a lower heat extraction rate. Lower fracture permeability at bottom and top layers (25×10−12 m2) increases the heat extraction rate by 1.17 MW in the 30th year. The proppant distribution affects the pressure distribution in fractured reservoirs. Permeability distribution variation has small effects on the heat extraction ratio. These results provided theoretical basis for fractured reservoir construction, optimal proppant pumping scheme and reservoir thermal output prediction.

An OpenFOAM solver for multiphysics modeling of fusion reactor design: The nemoFoam code

This paper introduces a multiphysics code, named nemoFoam, designed within the OpenFOAM environment to address the coupling of neutron and photon transport with thermal-hydraulics in nuclear reactor simulations. The code, conceived both for fusion and fission applications, employs a modular approach where the neutronic module is currently based on multi-group neutron diffusion equations, including a mono-kinetic treatment for photons. The thermal-hydraulic module is built on the standard OpenFOAM solver. The paper focuses on presenting the key features of nemoFoam and discussing the structure of the code, the nuclear and thermal-hydraulic models, and the coupling strategy between them.In order to assess the performance of the neutronic module, the code is applied to a fusion case study, indeed a benchmark against the Serpent Monte Carlo code for neutron and photon transport is performed, applying nemoFoam to the Affordable, Robust and Compact (ARC) fusion reactor design. Simulation results demonstrate good agreement with Monte Carlo benchmarks, emphasizing the potential of the code for multiphysics simulations. The modular design of nemoFoam allows users to extend the implemented model equations to study additional phenomena, focus only on selected aspects independently and, potentially, to add new solvers in each module.

High-resolution and reduced-order multi-physics simulation on thermal–hydraulic and thermoelectric characteristics of the thermionic space reactor core

The simulation of a thermionic space reactor core necessitates a comprehensive consideration of the intricate interplay between fluid flow, heat transfer, and thermionic energy conversion, constituting a multi-physics coupling challenge. Given the absence of a thermionic conversion model in contemporary CFD (Computational Fluid Dynamics) codes, accurately simulating the thermal–hydraulic and thermo-electric behavior of thermionic reactors in existing CFD software is particularly challenging. To analyze the multi-physics coupling phenomena in thermionic reactors, this paper revises the conjugate heat transfer solver in the proprietary 3D (three-dimensional) finite-volume CFD code, mFVM (modular Finite-Volume Method Code), to enable bidirectional coupling with the thermionic conversion model. This enhancement aims to predict the high-resolution three-dimensional temperature field and thermoelectric characteristics of the thermionic reactor core. Furthermore, a reduced-order multi-physics model of the thermionic reactor core is established using RESYS (Reactor System Simulation Code). The numerical simulation outcomes for both the high-resolution and reduced-order multi-physics models, pertaining to electrically heated and fission-heated TFEs (Thermionic Fuel Elements), are validated against experimental data. The results indicate that the emitter electron cooling effects caused by thermionic emission reduced the maximum temperature of emitter by about 200 K for both electrically heated and fission-heated TFE. Therefore, a bidirectional coupling between the thermal–hydraulic model and the thermionic conversion model is essential for precisely calculating the temperature field within the TFE and the thermionic reactor core. Thereafter, both the high-resolution model and the reduced-order model are utilized to investigate the thermal–hydraulic and thermoelectric characteristics of the TOPAZ-II reactor core. The calculated electrical power of the TOPAZ-II reactor is 5.39 kWe using the mFVM model and 5.47 kWe using the RESYS model. Finally, an analysis of the thermoelectric conversion characteristics based on the simulation results of the reduced-order model reveals that the TOPAZ-II reactor system is capable of load-following only when the load resistance exceeds 0.06 Ω under nominal full power operation conditions.

Influence of heat flux profile on two-phase flow instability in parallel channels of helical coil once-through steam generator

To enhance the heat transfer capability, the helical coil once-through steam generator (HCOTSG) has been widely utilized in the small modular reactor (SMR). There are hundreds of parallel helical channels in the HCOTSG, and the coolant undergoes the phase change from subcooled liquid to superheated vapor in the helical tube. The flow oscillation between parallel channels may be triggered during the phase change. In present study, the two-phase flow instability is investigated with different heat flux profiles under many operating conditions. The thermal–hydraulic model for parallel channels of HCOTSG is established based on our previous work, while the local heat transfer phenomenon such as subcooled boiling, critical heat flux, and wall conduction are considered in detail. The heat flux profile along the tube is ascertained by the steady-state analysis of HCOTSG under different operating conditions and applied to the outside surface of the helical channel. The heat flux value is increased step by step to trigger the out-of-phase oscillation between parallel helical channels. The thermal–hydraulic model and analysis method are validated against experimental results. The characteristics of flow instability in the helical channel are also analyzed. It is observed that the delay effect causing the flow instability is more obvious if the void fraction exceeds 0.7 in the starting stage. Then the boundary of flow instability is compared under different heat flux profiles for various operating conditions of HCOTSG. The results suggest that the distribution of heat flux in the two-phase region plays a more significant role in the occurrence of flow instability in the parallel helical channel system. The thermal–hydraulic characteristics during flow instability are governed by the combined effects of void propagation and delayed feedback in the system. In conclusion, the thermal–hydraulic condition of HCOTSG undergoes the unstable region to achieve the normal operating point, which can be avoided by adjusting the operating parameters.

Development of thermal-hydraulic coupling model for deep-geological disposal of high-level radioactive wastes

The buffer plays an important role in a engineered barrier system for the deep geological repository of high-level radioactive waste (HLW). High temperature may result in degradation of buffer performance. It is crucial to investigate the buffer temperature behavior so that its peak temperature is not higher than the criterion of 100 °C. In the present work, the thermal-hydraulic (T-H) coupling model is developed to simulate the histories of temperature and saturation in the disposal repository. This model is validated with the previous simulation works in the DECOVALEX 2015 Task B2 in order to simulate the geological repository of HLW with confidence. The whole repository of HLW in the present works can be reasonably simplified as a single hole with the symmetric boundary conditions on its peripheral sides. Effects of tunnel spacing and hole spacing of repository on the histories of buffer peak temperature are considered since the layout with various spacing is strongly related to the peak temperature of buffer. Based on the comparison of simulation results from the T-H coupling model and T-only model for the different layout of whole HLW repository, the maximum value of peak temperature in the buffer predicted by the T-H coupling model is lower than that from the T-only model. In other words, less area is needed for the selection of repository, rendering that the present T-H coupling model can assist in the design of HLW repository.

A Combined Computational Fluid Dynamics and Thermal–Hydraulic Modeling Approach for Improving the Thermal Performance of Corrugated Tank Transformers: A Comparative Study

In this study, we present a complete analysis of the thermal behavior of an oil-filled distribution transformer using two different approaches: numerically, by applying computational fluid dynamics (CFD) simulations; and analytically, by using thermal–hydraulic modelling (THM). The THM method becomes challenging when the corrugated transformer tank wall structures are complex, and therefore CFD simulations are required to provide in-depth details of the winding oil thermal–hydraulic behavior and to generate parameters for improving the THM calculation. The numerical and analytical thermal performance results were then compared with heat-run measurements of a case study transformer and the accuracy of both approaches on different thermal performance parameters was validated. This study has certain reference value for improving the thermal performance and operation efficiency of distribution transformers, and eventually aims to provide engineers with an effective tool to design the most efficient and reliable distribution transformers with corrugated tank walls for various applications.

Ensemble learning for predicting average thermal extraction load of a hydrothermal geothermal field: A case study in Guanzhong Basin, China

Accurate prediction of the average thermal extraction load (ATEL) in hydrothermal heating systems optimizes energy recovery, though numerical models are constrained by modeling accuracy and computational time costs. Data-driven modeling can efficiently screen alternative production plans and enhance daily operational decisions. However, ensemble learning applications for forecasting ATEL lack comprehensive comparisons and appropriate algorithm selection. This study evaluates six ensemble learning algorithms: random forest (RF), adaptive boosting (AdaBoost), gradient boosting decision trees (GBDT), extreme gradient boosting (XGBoost), light gradient boosting machine (LightBGM) and categorical boosting (CatBoost) on 1159 data points from a homogeneous thermal-hydraulic coupling numerical model. The superior performance of the CatBoost is characterized by the highest accuracy (with the lowest root mean square error (RMSE) of 150.6 kW–170.7 kW, minimal mean absolute percentage error (MAPE) of 0.66%–0.73%, and exceptional R2 values of 0.999), commendable stability, reasonable computational cost and low uncertainty. LightGBM, XGBoost, and GBDT are identified as suitable substitutes, whereas RF is a possible substitute. Moreover, the production rate is identified as the most significant factor contributing to the ATEL, followed by the well spacing and injection temperature. While perforation depth has the least influence on heat recovery.

Adoption of a fast dynamic model to simulate transient operations of MNSR

After developing a thermal–hydraulic model, it should be coupled to a point kinetic model to completely simulate transient behavior of a miniature neutron research reactor (MNSR). In this study introduction and validation of such a dynamic model which could be used as computing source of a micro-simulator, is presented. A time dependent point kinetic model in association to a simple control algorithm and reactivity feedbacks calculation (poisons and temperature) is adopted. In order to validate the model, it is compared against MNSR operational data in different scenarios such as normal fixed power operations, and reactivity insertion operations, and post shutdown scenarios. Result indicates that the coupled model could estimate the critical rod position for different power levels with an error less than 5%. Although the control algorithm is simple, it could predict the actual trend of control rod position accordance to the actual data. The coupled model simulates different reactivity insertion accident similar to the reactor operations in such a way that the error between estimated power peak, peak time, and long-term power is less than 5%.

Analysis of the integral zeolite molecular sieve process for helium CPS application

Zeolite Molecular Sieves (ZMSs) are commonly adopted in tritium handling facilities for impurity removal from gaseous streams, particularly for tritiated water trapping. Several activities have been conducted to characterize the adsorption and desorption behaviour of the sieving materials. Instead, less attention has been put on the analysis of the integral process comprising the adsorption of the tritiated humidity, the regeneration of the molecular sieve and the recovery of the desorbed water. Focusing on the application for the EU-DEMO helium Coolant Purification System (CPS), this work presents a process simulator relying on a MATLAB dynamic model of the ZMS bed and on a RELAP5/MOD3.3 model for the regeneration loop. The dynamic model can simulate both the adsorption and regeneration modes of the ZMS by providing the temperature and the adsorbate concentration profile along the column, while the thermal-hydraulic model is used to assess the conditions of the regeneration loop. By coupling the outcomes of the two models it is possible to establish the effective regeneration efficiency of the process and the amount of the tritiated water recovered. For the case of helium CPS, the analysis demonstrates the feasibility of the proposed operative scheme and the capabilities of the regeneration procedure to ensure a 74.4 % regeneration efficiency of the ZMS bed.

Multiphysics Analysis of PLOFC and DLOFC Accidents in the HTGR-TR

High-temperature, gas-cooled reactors (HTGRs) are promising advanced reactor designs. Leveraging passive safety features, higher core outlet temperatures, previous commercial operational experience, and potential coupling with nonelectrical systems, HTGRs present many advantages over traditional light water reactors and other advanced reactor designs. The High-Temperature, Gas-Cooled Test Reactor (HTGR-TR) is a point design developed by Idaho National Laboratory as a response to the need for the expansion of U.S. test reactor capabilities. The HTGR-TR was designed to serve as a test reactor for Generation-IV small modular prismatic block HTGRs and to provide irradiation data, two crucial contributions to the development of advanced reactors. As in any reactor design, it is necessary to understand the behavior of the reactor during accident scenarios to determine the overall safety of the design. Pressurized loss of forced cooling (PLOFC) and depressurized loss of forced cooling (DLOFC) are two accidents that can occur in HTGRs and that present challenges to decay heat removal. In order to understand the behavior of the fuel during these accidents, the resulting mechanical behavior and fission product migration in the fuel are evaluated. Using the HTGR-TR design, which utilizes tristructural isotropic (TRISO) fuel, a multiphysics analysis during the PLOFC and DLOFC transients was performed. This study utilizes neutronics, thermal hydraulics, and fuel performance modeling to analyze the behavior of TRISO particles during steady-state operation and the described transients. This analysis aims to characterize the hoop stress and strain during the transients, as well as predict fission product migration and radiological release. Due to the startup conditions of the HTGR-TR, the silicon carbide (SiC) layer for both of the transients and steady-state conditions are initially in compression for both the hoop stress and strain. The DLOFC transient at the beginning of cycle was the most challenging overall to the fuel, having the closest to tensile behavior and the closest to tensile strain. Because the integrity of the SiC layer was not challenged during any of the transients, the release of 90Sr and 137Cs was found to be negligible. The PLOFC transient at the end of cycle resulted in the largest 110mAg release and subsequent radioactivity increase. However, the increase in radioactivity was minor when compared to typical operating conditions due to the relatively large diffusivity of 110mAg though SiC at normal operating temperatures. Highlights include Neutronics and thermal-hydraulic modeling generating steady-state and transient conditions for use in fuel performance analysis.2. Detailed BISON analysis demonstrating the excellent performance of TRISO fuel during PLOFC and DLOFC transients.3. Fuel behavior showing negligible fission product release.