Pebble Bed Modular Reactor Research Articles

Investigating the possibility of using a mixture of thorium with different fissile materials as a fuel in TRISO particles for the PBMR-400 reactor

The increasing demand for nuclear reactors as a source of clean energy forced nuclear researchers to search for ways to enhance the safety operation of nuclear reactors. This paper discusses the viability of using different thorium compounds ((233U, Th)O2, (235U, Th)O2, (RGPu, Th)O2) as an accident tolerant fuel/advanced technology fuel (ATF) for Pebble Bed Modular Reactor 400 (PBMR-400). A comprehensive analysis of thorium-based fuel performance was carried out and compared with UO2 to develop a novel nuclear fuel with the capability to tolerate severe accident conditions. The neutronic characteristics such as the infinity multiplication factor, initial heavy metal concentration, RGPu concentration and fission products concentration were analyzed. In addition to, the main safety parameters have been calculated to distinguish the effect of the proposed fuels on them. The thermal spectrum and thermal output power have been visualized to give a good insight view inside the core about the distribution of the heat. In general, the neutronic analysis demonstrated good behavior of the examined thorium-based fuel in the PBMR-400 and especially in the case of (RGPu, Th)O2. PBMR-400 showed excellent performance in incinerating a large amount of reactor-grade plutonium when it was used in a kernel fuel bed.

A proposed refueling scheme for the modular pebble bed reactor

In this paper, a suggested design of the fuel pebble and loading pattern in the MIT Modular Pebble Bed HTGR (MPBR) using thorium (232Th) is presented. Using the MCNP5 code model for the MPBR reactor, different compositions of Thorium Low Enriched Uranium (ThLEU) with different Heavy Metal (HM) loading and 235U enrichment were investigated to achieve the maximum thorium utilization with relatively minimum reactivity insertion in case of water ingress accident. Using pure Low Enriched Uranium (LEU) pebbles and ThLEU pebbles with the same HM loading and 235U enrichment, different pebbles loading patterns were studied. One loading pattern was selected to achieve the criticality with enough excess reactivity compensating for the poisoning effect of xenon build-up at the startup of the reactor operation and the negative effect of fuel and reflector temperature coefficients of reactivity. This loading pattern aims also to reduce the radial power peaking in the core and to minimize the axial Power Peaking Factor (PPF) in the central hot channel, with a minimum number of pure LEU fuel batches. A calculation procedure was developed that combines fuel pebble depletion and full core calculations to investigate the impact of a proposed online refueling scheme for the MPBR reactor and to simulate the multicycle refueling mechanism. Using SERPENT 1.1.7, the nuclide concentrations were calculated as a function of pebble burnup for both LEU and ThLEU pebbles. A multicycle online refueling scheme was proposed and the average number of passes required for LEU and ThLEU pebbles to reach their maximum burnup was estimated.

Analysis of forced convection in the HTTU experiment using numerical codes

The High Temperature Test Unit (HTTU) was an experimental set-up to conduct separate and integral effects tests of the Pebble Bed Modular Reactor (PBMR) core. The annular core consisted of a randomly packed bed of uniform spheres. Natural convection tests using both nitrogen and helium, and forced convection tests using nitrogen, were conducted. The maximum material temperature achieved during forced convection testing was 1200 °C. This paper presents the numerical analysis of the flow and temperature distribution for a forced convection test using 3D CFD as well as a 1D systems-CFD computer code. Several modelling approaches are possible, ranging from a fully explicit to a semi-implicit method that relies on correlations of their associated phenomena. For the comparison between codes, the analysis was performed using a porous media approach, where the conduction and radiative heat transfer were lumped together as an effective thermal conductivity and the convective heat transfer was correlated between the solid and gas phases. The results from both codes were validated against the experimental measurements. Favourable results were obtained, in particular by the systems-CFD code with minimal computational and time requirements.

High uranium loading TRISO particle for microreactor applications

Tristructural isotropic (TRISO) fuel is currently being considered for novel high-temperature reactor (HTR) designs that deviate significantly from the prismatic or pebble bed modular high-temperature gas-cooled reactors (MHTGRs) that have been the focus of development efforts for the last four decades. These new designs include microreactors with compact cores that can benefit from increasing the core fissile density beyond that conventionally required in an MHTGR using TRISO fuel to enable longer fuel in-core residence time or higher power density. One means of increasing the fissile density in an HTR core is to adjust the coated particle design to increase the ratio of kernel volume to total particle volume. This would increase the volume of fissile phase in a fuel compact with no increase in particle packing fraction beyond that used in fuel compacts tested by the AGR program. Results show that the end-of-life failure probabilities of these high uranium loading particles and those of the validated reference particles are similarly low (∼2 × 10-5) for burnups of at least 9 % FIMA.

Technical and economic analysis of a pebble bed modular reactor with nitrogen coolant

High-temperature nuclear reactors (HTR) could benefit from using the gas turbine in tandem with a steam turbine; this is the principle of combined cycles using natural gas. HTRs have the characteristic that the turbine works in high-temperature conditions. The flue gas could feed a steam generator to produce additional electricity improving system efficiency. The current study assesses the likeliness of having a combined cycle from a Pebble Bed Modular Reactor using nitrogen as a coolant. The balance of the plant and the corresponding configuration to meet this goal are given; the results show an efficiency of 53.56% of electrical generation compared to 42% reported by the HTR-PM. The share cost for turbine plant equipment represents 8.9%–14.0% of the total cost of the power plant. Three scenarios are contemplated to assess the economic impact of the additional generation where an increase in the overnight cost is considered, 9.99%, 12.41%, and 15.73% because of the additional cost of the steam turbine and associated equipment. Assuming a 4500 US$/kW cost for installed capacity for the HTR-PM reactor, for a 10% discount rate, the electricity total levelized cost is 95.56 US$/MWh. For the proposed combined cycle using nitrogen as a coolant, the electricity total levelized cost is 89.78 US$/MWh, assuming a 10% discount rate when the higher overnight cost is taken into account (5208 US$/kW). Using nitrogen under this alternative will produce an additional electricity production of 425,742 MWh annually. It leads to an electricity cost reduction of 5.78 US$/MWh even in the higher cost for the nitrogen combined cycle here considered, and a 10% discount rate compared with the HTR-PM steam cycle reported data, showing the technical and economic feasibility of using nitrogen as a coolant gas.

Evaluation of the Basic Neutronics and Thermal-Hydraulics for the Safety Evaluation of the Advanced Micro Reactor (AMR)

South Africa requires safe affordable distributed base load energy, one way to achieve this is to use nuclear power integrated with renewable energy sources on a decentralized basis. This suggests the development of its own micro modular nuclear reactor, to supply energy to towns, small communities, mines and processing plants. Large Light Water Reactors (LWRs) are expensive and require a large infrastructure development. A High Temperature Reactor (HTR) called the Advanced Micro Reactor (AMR) is in the process of being developed and the design philosophy is to design for inherent safety, maximally using technology that has been developed and validated in previous HTR programs albeit in a completely different and unique configuration. The concept is based on existing knowhow and experience/expertise in South Africa during the time of the Pebble Bed Modular reactor (PBMR) project. These AMR reactors are to be factory built to obtain good quality control and rolled out to various sites. Once the reactor has reached its end of life, it would be returned to a licensed organisation for refuelling. The AMR produces 10MW of thermal power. The reactor configuration uses hexagonal graphite blocks for structural and moderator material, which are arranged to form a cylindrical core layout. The fuel assemblies are silicon carbide tubes that house coated particle fuel, immersed in a lead-bismuth eutectic alloy (LBE). Each fuel assembly is contained in a boring within the graphite moderator that allows an annulus for cooling. There are 420 fuel assemblies in the core. Low enriched fuel in the form of UO₂ or UCO is used. Helium gas is used as coolant. The coolant enters the core at 450°C and exits at 750°C. The mechanical, neutronic and thermal-hydraulic design of the AMR, is being evaluated with assistance from STL Nuclear (Pty) Ltd., the University of Pretoria (UP), the North-West University and the South African Nuclear Energy Corporation (NECSA). The OSCAR-5 code package, together with the Serpent neutronic code were used to perform the basic neutronic studies while the Flownex package was used to determine the thermal-hydraulic and safety evaluation for the Design Base Accident (DBA) specifically the Depressurized Loss of Forced Cooling (DLOFC) event.

University of Pretoria – A Centre for Pioneering Research on Pebble Bed Modular Reactor (PBMR): 4thGeneration Nuclear Reactor

The University of Pretoria - South Africa, one of the most prestigious universities of the African continent, has a sprawling campus with a profound aristocracy similar to that of Oxford University. The Opera House of the University carries a great legacy of the Royal ‘Concertgebouw’ (concert hall) in Amsterdam. Research on materials for constructing the appropriate fuel particles in a “Pebble Bed Modular Reactor” (PBMR), initiated at the Physics Department of this university, can be considered fundamental and somewhat revolutionary. The article will in a nutshell touch upon the essence of PBMR and the enriched academic ambiance of the University of Pretoria, reflected by author’s own experiences of working at this university.

Techno-Economic Assessment of Fuel Cycle Facility of System Integrated Modular Advanced Reactor (SMART)

The economic assessment of advanced nuclear power reactors is very important, specifically during the early stages of concept design. Therefore, a study was performed to calculate the total cost estimation of fuel cycle supply for a system modular advanced reactor (SMART) by using the Generation-IV economic program called G4-ECONS (Generation 4 Excel-based Calculation of Nuclear Systems). In this study, the detailed description of each model and methodology are presented including facility, operations, construction matrix, post-production model, and fuel cycle cost estimation model. Based on these models, six Generation-III+ and Generation-IV nuclear reactors were simulated, namely System 80+ with benchmark data, System 80+ with uranium oxide (UOx) and mixed oxide (MOx) fuel assemblies, fast reactor, PBMR (Pebble Bed Modular Reactor), and PWR (Pressurized Water Reactor), with partially closed and benchmarked cases. The total levelized costs of these reactors were obtained, and it was observed that PBMR showed the lowest cost. The research was extended to work on the SMART reactor to calculate the total levelized fuel cycle cost, capital cost, capital component cost, fraction of capital spent, and sine curve spent pattern. To date, no work is being reported to calculate these parameters for the SMART reactor. It was observed that SMART is the most cost-effective reactor system among other proven and advanced pressurized water-based reactor systems. The main objective of the research is to verify and validate the G4-ECONS model to be used for other innovative nuclear reactors.

Study on the representativeness of airborne effluent sampling in the stack of a high-temperature gas-cooled pebble-bed modular reactor

Monitoring the airborne radioactive effluent from the stacks of nuclear power plants is very important. In order to obtain reliable and accurate measurements, the samples acquired from the stack must be representative of the whole. Thus, sampling should be performed at a location where the airflow is well mixed. We studied the transport behavior of airborne radioactive substances in the stack of a high-temperature gas-cooled reactor pebble-bed module (HTR-PM) demonstration power plant using a computational fluid dynamics method. The distributions of effluent flow velocity and concentration have been determined and compared to the mixing uniformity criteria of the ISO-2889 standard, then the source of uniformity is analyzed from vorticity distribution, particle trajectory, cyclone angle and coefficient of variation. We have determined the well-mixed location within the HTR-PM stack where a representative sample can be obtained. We discussed methods for the improvement of flow mixing and analyze the key factors involved.

Improved natural convection heat transfer correlations for reactor cavity cooling systems of high-temperature gas-cooled reactors: From computational fluid dynamics to Pronghorn

The reactor cavity cooling system (RCCS) is a common reactor safety system in high-temperature gas-cooled Reactors (HTGR) that removes heat from the reactor pressure vessel (RPV) by radiation (∼80%) and natural convection (∼20%). For the simulation of accident scenarios of HTGRs, intermediate fidelity and system codes models must be employed to limit the models’ execution time. While an accurate quantification of the radiative heat transfer is available in these models, the quantification of natural convection must rely on correlations of questionable accuracy for the Nusselt number. Commonly used correlations are based in experiments performed at low Rayleigh numbers and/or using isothermal walls in simplified geometries. This work improves on the accuracy of natural convection heat transfer correlations to support HTGR designs.These correlations include both local and average Nusselt numbers as a function of the global Rayleigh number, the local Rayleigh number, and the temperature profile at the hot wall of the RCCS. In the absence of dedicated experiments and the difficulty of performing high-fidelity simulations at realistic Rayleigh numbers, the data to fit the correlations are generated with computational fluid dynamics (CFD) using Reynolds Averaged Navier-Stokes (RANS) models. First, a careful selection of the RANS turbulence model is performed by comparing the results obtained with different RANS turbulence models against high-fidelity simulations of natural convection at Ra 1×1011 in a rectangular cavity. Next, the selected model is used to perform simulations of an HTGR cavity at different high Rayleigh numbers ∈[6.1×1011,2.9×1013] to encompass several HTGR designs, assuming an isothermal RPV wall. The results obtained are used to fit a correlation for the average and space-varying Nusselt number as a function of the global and local Rayleigh numbers via a sparsity-promoting, least-squares method. The selected RANS model is then used to perform simulations of a PBMR-400 (Pebble Bed Modular Reactor) HTGR cavity with the temperature profiles at the RPV wall obtained during a PLOFC (Pressurized loss of forced cooling) transient. We use the results obtained to fit a temperature-dependent correction to the space-varying Nusselt number with the sparsity-promoting, least-squares method. The results obtained in this work enable system-level codes, such as Pronghorn, to perform higher-fidelity simulations of the heat exchange process in the RCCS while still maintaining a low computational cost.

Pronghorn: A Multidimensional Coarse-Mesh Application for Advanced Reactor Thermal Hydraulics

This paper presents an overview of Pronghorn, a multiscale thermal-hydraulic (T/H) application developed by Idaho National Laboratory and the University of California, Berkeley. Pronghorn, built on the open-source finite element Multiphysics Object-Oriented Simulation Environment (MOOSE), leverages state-of-the-art physical models, numerical methods, and nonlinear solvers to deliver fast-running advanced reactor T/H simulation capabilities within a modern software engineering environment. This work summarizes the physical models, multiphysics and multiscale coupling, and numerical discretization in Pronghorn with emphasis on our initial target application to pebble bed reactors (PBRs). A diverse set of applications are shown to depressurized natural circulation in the SANA experiments, forced convection in the Pebble Bed Modular Reactor, three-dimensional (3-D)/one-dimensional coupling of Pronghorn and RELAP-7 systems T/H for loop analysis in the High Temperature Reactor Power Module, and forced convection in the Mark-1 Pebble Bed Fluoride-Salt-Cooled High-Temperature Reactor. A multiphysics coupling of Pronghorn, RELAP-7, and Griffin deterministic neutronics for a gas-cooled PBR demonstrates the capability of the MOOSE framework for reactor design calculations. These applications highlight the verification and validation underlying Pronghorn’s software development while emphasizing features that improve upon capabilities offered by legacy tools in areas such as 3-D unstructured meshing, physics modeling, and multiphysics coupling.

The RCCS Thermal Analysis during the Station Blackout Accident of RDE

RDE is designed based on HTGR technologies; it has an active, passive and inherent safety system. Defence in depth philosophy is conducted. The heat transferred from the reactor vessel to the structure of the containment is equipment in the RDE has a name is a reactor cavity cooling system (RCCS). During normal operation and protecting the reactor containment structure in the event of failure of all active cooling systems, the RCCS designed to take fulfill this role by preserving the vessel reactor under the maximum allowable temperature. Concerning heat dissipation design, RCCS assessment and resolution are considered as major factors in determining the maximum power level of heat dissipation. Active and passive mode is the equipment on RDE RCCS. This paper discusses the calculation of RCCS thermal analysis during the station blackout (SBO) accident. SBO occurred and the heat removal is conducted by RCCS when all of the active cooling systems were failed. Conditions will be safe if the RPV temperature must be maintained below 65°C. There is no electricity from diesel generator supplied to the primer compressor, it will the SBO accident. The methodology used is based on the calculation mathematical model of the pebble bed modular reactor heat transfer capability of the RCCS in the passive mode. The passive mode works during the failure of active mode, and the heat is released through the cavity by natural convection. The RCCS is capable to withdraw the heat at 50.54 kW per hour and keep the reactor pressure vessel temperature below 65°C at normal operation and below 125°C during the station blackout.

Technology, policy and politics: critical success factors in high-technology infrastructure projects

ABSTRACT The article examines critical success factors in high technology projects by comparing the Square Kilometre Array, the Pebble Bed Modular Reactor and the Reactor Conversion Project in the context of a historical view of South African technology policy. These three complex endeavours illustrate rather well what mistakes can be made in the conceptualising and in the execution of such projects. The article argues that the confluence of a range of diverse factors such as sufficient prototyping, understanding and balancing political stakeholders, getting the organisational culture right and managing ambitions and aspirations are necessary for success to be achieved.

Monte Carlo simulation of activity concentration measurement of primary coolant of high-temperature gas-cooled pebble-bed modular reactor

The real-time and prompt determination of the radioactive level in the primary coolant of the first demonstration high-temperature gas-cooled pebble-bed modular reactor (HTR-PM) in China is essential for establishing its radiation safety since it can reflect the radiological status of the fuel elements in the core. This paper presents the design of the activity concentration measurement of the primary coolant of HTR-PM and focuses on the detection efficiency simulation of the on-line gross γ monitoring instrument. The purpose of this instrument, which was set in place at the entrance of the helium purification system of HTR-PM, is to provide continuous, real-time, prompt, and reliable activity concentration monitoring of the gaseous fission products in the primary coolant. The Monte Carlo method is used to simulate the detection efficiency of the instrument. A sensitivity analysis with the variations in the source terms of the primary coolant of HTR-PM is presented and discussed.

EcSOD in the Health Benefits of Exercise

The coupling of the Qeshm island desalination plant with different energy sources was economically evaluated by using a Desalination Economic Evaluation Program (DEEP) package. The capacity of this plant was 18,000 m3/day and its energy source was 250 MW (th) gas power plant. Six energy source plants, namely, coal, oil, gas, combined cycle (CC), pressurized water reactor (PWR), and pebble-bed modular reactor (PBMR), were coupled with this desalination plant. The estimated annual electrical costs indicated that the optimum fossil power plant was CC with 97.72 $/kWh, which was 30% higher than 67.42 and 64.63 $/kWh in PWR and PBMR power plants, respectively. The total annual water costs and the total annual costs of coupling with the desalination plant of CC, PWR, and PMMR plants were approximately 1.78, 1.49, and 0.77 $/m3, and 94.74, 49.23, and 49.1 m$, respectively. The construction costs of these power plants are approximately 131, 510 and 553 M$, respectively, On the basis of the contraction times, costs, and lifetimes of these power plants 25, 40 and 60 years for CC, PBMR and PWR, we concluded that CC, PBMR and PWR are short-term, medium-term and long-term strategy to generate electricity and couple with desalination plants, respectively.

Study on the DLOFC and PLOFC accidents of the 200 MWe pebble-bed modular high temperature gas-cooled reactor with TINTE and SPECTRA codes

Validation and verification (V&V) is important and receives significant attention during the design and analysis of the nuclear power plant. As the first commercial modular high temperature gas-cooled reactor (HTR) demonstration plant, during the regulatory review of the preliminary safety analysis report (PSAR) and the final safety analysis report (FSAR) of the 200 MWe Pebble-bed Modular High Temperature gas-cooled Reactor (HTR-PM), the authorities also pay high attention to the code validation and verification.Within the cooperation between the Nuclear Research Group (NRG) of Netherland and Institute of Nuclear and new Energy Technology (INET), Tsinghua University of China, a code to code verification was carried out between the TINTE code, a thermal-hydraulic design and accident analysis tool for the Pebble-bed High Temperature Gas-cooled Reactor (HTGR), and the SPECTRA code, a thermal-hydraulic analysis code developed at the NRG. Three typical accidents scenarios, two depressurized loss of forced cooling (DLOFC) accidents scenarios and one pressurized loss of forced cooling (PLOFC) accident scenario, have been analyzed both with the TINTE code and the SPECTRA code.The simulation results show good agreement between TINTE code and SPECTRA code. The results also indicate that, due to the inherent safety feature of the HTR-PM design, the maximal fuel temperature during above accidents would never exceed the limitation of 1620 °C, below which there is no additional damage for the TRISO particles, and the SiC layer of each particle can guarantee the fission-product-retention capacity. This work will be helpful to the further study of the HTGR, and more code to code verification is planned.

Economic evaluation of Qeshm island MED-desalination plant coupling with different energy sources including fossils and nuclear power plants

The coupling of the Qeshm island desalination plant with different energy sources was economically evaluated by using a Desalination Economic Evaluation Program (DEEP) package. The capacity of this plant was 18,000m3/day and its energy source was 250MW (th) gas power plant. Six energy source plants, namely, coal, oil, gas, combined cycle (CC), pressurized water reactor (PWR), and pebble-bed modular reactor (PBMR), were coupled with this desalination plant. The estimated annual electrical costs indicated that the optimum fossil power plant was CC with 97.72$/kWh, which was 30% higher than 67.42 and 64.63$/kWh in PWR and PBMR power plants, respectively. The total annual water costs and the total annual costs of coupling with the desalination plant of CC, PWR, and PMMR plants were approximately 1.78, 1.49, and 0.77$/m3, and 94.74, 49.23, and 49.1m$, respectively. The construction costs of these power plants are approximately 131, 510 and 553M$, respectively, On the basis of the contraction times, costs, and lifetimes of these power plants 25, 40 and 60years for CC, PBMR and PWR, we concluded that CC, PBMR and PWR are short-term, medium-term and long-term strategy to generate electricity and couple with desalination plants, respectively.

Air ingress analysis of chimney effect in the 200 MWe pebble-bed modular high temperature gas-cooled reactor

In a high temperature gas-cooled reactor (HTGR), one can postulate simultaneous ruptures of the pressure boundary at points near the top and bottom of the reactor vessel. In this extremely low probability event, depressurization and heat-up of the core can lead to ingress of air into the vessel driven by the buoyancy of the hot helium relative to the colder, cavity air (‘chimney effect’). Although this is considered a ‘beyond design basis accidents’ (BDBA), it receives high attention because it may lead to graphite oxidation of reflectors and fuel elements so as to weaken the structural strength and lessen the ability of the coated particles to retain fission products.A study of the rate and severity of the air ingress and potential for oxidation in the 200MWe Pebble-bed Modular High Temperature gas-cooled Reactor (HTR-PM) is described in this paper. The TINTE code was used to simulate the air ingress accident caused by the simultaneous rupture of the fuel charging tube and fuel discharging tube. A mitigation measure, the injection of nitrogen into the vessel, has also been proposed and evaluated with the TINTE model.The results indicate that, due to the high flow resistance of the pebble-bed core, the air ingress flow rate is limited. The oxygen content in the reactor cavity is also limited due to the design of the Ventilated Low Pressure Containment (VLPC) that prevents the corrosion of the fuel elements and reflectors by limiting the amount of air that can enter the vessel. The results also indicate that the injection of the nitrogen in a suitable location and with a suitable mass flow rate can effectively prevent significant corrosion of the graphite.

Assessment of performing experimental investigation on a pebble bed modular reactor (PBMR) as a static packed bed approximation

Moving bed reactors have a wide range of applications in the chemical, petrochemical, and alternative energy industries in a special situation where there is a need to either continuously recirculate or replace solid particles catalysts. They are also under consideration as one of the 4th generation nuclear reactors known as pebble bed modular reactor (PBMR). In this work a continuous cold flow pebble recirculation experimental set-up, mimicking the flow of pebbles in a PBMR, is designed, developed, and tested at Missouri S&T. A unique experimental work has been performed on assessment of the possibility of using pebble bed modular reactor (PBMR) as static packed bed approximation for the needed experimental investigation. For this purpose, a radioactive particle tracking (RPT) technique is implemented around the continuous pebble recirculation experimental set-up, to compare the packing characteristics of static and moving pebble beds. The photopeak counts during RPT calibration were collected by placing radioactive particle (tracer) at different positions in the test reactor under different operating conditions of moving and static conditions. The photopeak counts between moving and static conditions show that attenuation characteristics of the medium in between tracer and detectors are not changing significantly due to movement of pebbles. This confirms that PBMR could be well approximated by static packed beds for experimental investigation which simplify the needed experimentation to advance PBMR technology and commercialization. Obtained results are serving as a unique benchmark data for an evaluation of the contact force model that used in the discrete element method (DEM) based simulations.

Study on air ingress of the 200 MWe pebble-bed modular high temperature gas-cooled reactor

Air ingress, one of the beyond design basis accidents (BDBA) for high temperature gas-cooled reactors (HTR), receives high attention during the design of the 200MWe pebble-bed modular high temperature gas-cooled reactor (HTR-PM), because it may result in severe consequence including the corrosion of the fuel elements and graphite reflector. A hypothetical air ingress accident, caused by the double-ended guillotine break (DEGB) of the horizontal coaxial hot-gas duct, is studied in this paper.After the rupture of the hot-gas duct, the following accident developing progress can be divided into three stages: (1) depressurization, (2) slow air ingress via diffusion and local convection, (3) onset of the overall, stable natural circulation and the consequent continuous air ingress. Based on the design of the HTR-PM, the time scale of diffusion and convection in the second stage is studied to evaluate the onset time of the stable natural circulation. Furthermore, the TINTE model of the HTR-PM is then established to simulate the hot-gas duct DEGB accident, including the flow rate of natural circulation, the corrosion of the fuel elements and the graphite reflector, the feasibility of the mitigation measures. The results show that due to the high pebble bed core with large flow resistance, as well as the complicated flow structure of the HTR-PM, it would take long time to obtain stable natural circulation. The results also indicate that, the bottom reflector graphite could consume most of the oxygen before it reaches the core and the injection of the nitrogen can effectively mitigate the corrosion of the graphite.