High-temperature Reactor Pebble-bed Module Research Articles

Study on corrosion of T-22 alloy under high temperature condition of VHTR

ABSTRACT T-22 alloy is applied to the steam generator of the high-Temperature Reactor–Pebblebed Modules (HTR-PM). In this study, a corrosion experiment of T-22 alloy was carried out for 50 h under the environment of air, vacuum, and impure helium at ultra-high temperature (950°C). After the experiment, the corrosion behaviors of T-22 alloy were characterized and analyzed by electronic balance, scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), carbon sulfur analyzer, and gas chromatography. The results show that the T-22 alloy was seriously corroded and destroyed under the air ingress accident; in the vacuum environment, the alloy appeared mass loss and slight decarburization; in the impure helium environment, the microclimate reaction would occur on the surface of T-22 alloy, and the oxide on the surface of the alloy would be destroyed and the alloy decarburized obviously; CO2 in the ppm range will accelerate the oxidation and decarburization of the alloy.

Numerical study on heat transfer characteris tics of supercritical water in straight and helical tubes

The subcritical steam generators in high-temperature gas-cooled reactor pebble-bed modules can be replaced by supercritical steam generators to improve thermal efficiency. The axial and circumferential nonuniform flow and heat transfer characteristics of supercritical water in straight and helical tubes with varying inclinations (0–90°) are simulated to investigate the combined effects of tube structure, flow direction, thermophysical properties, property variations, buoyancy, and centrifugal forces. With heat fluxes of 233–930 kW/m2 and inlet mass fluxes of 225 or 1260 kg/(m2 s) under a working pressure of 24.5 MPa, the friction coefficients, turbulent kinetic energy, secondary flow, temperature, and heat transfer coefficients are studied in the cross section of variable structure tubes. The wall temperature and heat transfer coefficients of four regions are compared in straight and helical tubes; the maximum heat transfer coefficient appeared before the pseudo-critical point. The heat transfer coefficients are the greatest at the bottom of the straight tubes, where fluidity was high, and at the outer of the helical tubes, where the fluid velocity was high under centrifugal force. The secondary flow in the helical tubes causes a nonuniformity in the wall temperature, causing the circumferential maximum wall temperature to move.

Fracture behavior analysis of brittle graphite sphere using a special bond model

High-Temperature Reactor–Pebble-bed Module (HTR-PM) takes the spherical fuel element design. Failure or not of fuel sphere would highly affect system operation and nuclear safety. Crush strength of fuel sphere is an important quality control measure, which is mainly determined by the graphite matrix. A bonded particle model (BPM) from discrete element method (DEM) is disclosed, validated, and applied in the fracture analysis of brittle graphite sphere. On the basis of validated BPM, the breakage behavior of nuclear fuel sphere under high-speed impact, that is, dynamic loading, is revealed in detail by a time-marching numerical simulation. This meshfree particle method can be effectively extended to the failure computation of different composite structures (e.g., particle-reinforced metal matrix composite) by defining heterogeneous bonds inside distinct material interface.

Drop analysis of Active Magnetic Bearing with rolling-sliding integrated auxiliary bearing

The Active Magnetic Bearing (AMB) technology is introduced in the High Temperature Reactor-Pebble-bed Modules (HTR-PM) demonstration nuclear power plant, which is being constructed in Shandong province, China. The auxiliary bearing is one of the most important components guaranteeing the reliability of the AMB. It also has an important impact on the reliability of the whole reactor system. Compared with the traditional auxiliary bearing, a novel one proposed by the authors has a smaller impact force and the rotor center orbit is much more concentrated during the rotor drop. This paper establishes an analytical model of drop of rolling-sliding integrated auxiliary bearing to analyze the above phenomena. Based on the Hertz contact theory, the complex structure inside the rolling bearing is simplified through a spring damping model. The overall impact model of the rolling-sliding integrated auxiliary bearing is established. Then, according to the structural characteristics of the rolling-sliding integrated auxiliary bearing, the tangential force inside the rolling- sliding integrated auxiliary bearing can be obtained by applying the angular momentum theorem. Finally, a four-degree-of-freedom horizontal rotor drop model is established to analyze and calculate the center orbit and motion state of the rotor. The analytical model is helpful in the selection and design of auxiliary bearing for AMB. In further research this contact model can be used to calculate the center orbit and contact forces in the application of the rolling-sliding integrated auxiliary bearing.

Prediction calculations for the first criticality of the HTR-PM using the PANGU code

The high-temperature reactor pebble-bed module (HTR-PM) is a modular high-temperature gas-cooled reactor demonstration power plant. Its first criticality experiment is scheduled for the latter half of 2021. Before performing the first criticality experiment, a prediction calculation was performed using PANGU code. This paper presents the calculation details for predicting the HTR-PM first criticality using PANGU, including the input model and parameters, numerical results, and uncertainty analysis. The accuracy of the PANGU code was demonstrated by comparing it with the high-fidelity Monte Carlo solution, using the same input configurations. It should be noted that keff can be significantly affected by uncertainties in nuclear data and certain input parameters, making the criticality calculation challenge. Finally, the PANGU is used to predict the critical loading height of the HTR-PM first criticality under design conditions, which will be evaluated in the upcoming experiment later this year.

Structural health monitoring of metal-to-glass\u2013ceramics penetration during thermal cycling aging using femto-laser inscribed FBG sensors

Maintaining the mechanical strength and hermetic reliability of metal-to-glass–ceramics electrical penetration assembly (MTGC-EPA) is a key concern for ensuring the pressure boundaries of nuclear power plants. The transient temperature change caused by power adjusting or accidents in High Temperature Reactor Pebble-bed Modules may affect the structural health of sealing glass–ceramics, even leading to radiation leakage. To evaluate whether the function could survive temperature variations during the service life, thermal cycling aging experiments were imposed to MTGC-EPA. A grating length-mismatched sensing method to monitor the residual strain, an important factor of glass–ceramics structural health, was demonstrated in real-time by femto-laser inscribed fiber Bragg grating (FBG) sensor during the curing process and thermal cycling aging. Scanning electron microscope (SEM) and leakage rate tests were carried out to obtain the comparisons of microstructure and hermeticity before and after the thermal cycling. The residual strain showed a slight growth trend with thermal cycles repetition and it persisted a high value (~ 4,000 με) reflected by both Bragg wavelength shift and spectrum shape. The grating length mismatched single FBG embedded in glass–ceramics was feasible to demodulate the temperature and strain simultaneously, and the embedded FBG method achieved the structural health monitoring of MTGC-EPA during thermal cycling aging with good accuracy and reliability. Combining with the results of SEM and leakage rate detecting, the structural health of MTGC-EPA was demonstrated to be capable to endure the severe thermal conditions in nuclear reactors.

Characterization of crack propagation of Incoloy 800H by the combination of DIC and XFEM

In the steam generator (SG) of the high temperature reactor – pebble-bed module (HTR-PM), the outlet connection tubes are made of Incoloy 800H. The fatigue analysis of these tubes is necessary since there are periodic flow-induced forces by the high-speed helium gas. In this paper, we focus on the crack propagation of the outlet connection tubes at room temperature by means of digital image correlation (DIC) and extended finite element method (XFEM). First, a uniaxial tensile sample was fabricated from the 800H tube with a pre-crack in the middle edge. Then, the sample was painted with spackle pattern for DIC measurement. Afterwards, the displacement and strain fields of region around the crack were calculated by DIC as the sample was subjected to static tensile load. Two bi-telecentric lenses of distinct magnification levels were used to image the crack at two scales. Also, a method based on virtual strain gauge was proposed to locate the crack tip and to characterize the crack propagation process. Finally, the DIC results were compared with those by XFEM. It was found that the results by these two methods agreed with each other very well in general. The results are useful for investigating the fracture behavior of 800H at room temperature.

Use readings of thermocouples to reconstruct the power distribution of HTR-PM

There is no detector in the active zone of HTR-PM (High-Temperature-Reactor Pebble-bed Module). The main way to monitor the power distribution of HTR-PM is through ex-core neutron detectors. Besides neutron detectors, there are thermocouples which are arranged in the side-reflector of HTR-PM, and they might be helpful in the power distribution reconstruction. In this paper, reconstruction method which uses thermocouple readings was developed based on Harmonic Synthesis Method, Improved Harmonic Synthesis Method and Newton Method. And the method was tested in numerical simulations. The results show that the method is correct and the characteristic of convergence is not sensitive to the initial value. But the robustness of the method is not good enough and the reconstruction method couldn’t be used in practice before the robustness of the method being improved.

Experiments on helium breakdown at high pressure and temperature in uniform field and its simulation using COMSOL Multiphysics and FD-FCT

In this paper, experiments on breakdown voltages of helium gas in a uniform field have been carried out at a separation from 0.25 to 3.02 mm. The pressure and temperature respectively vary from 1 to 7 MPa, 25 °C to 180 °C, which is the working condition for the main helium blower and its Active Magnetic Bearings (AMBs) in the High Temperature Reactor-Pebble-bed Module (HTR-PM). COMSOL Multiphysics and a self-programmed FD-FCT (Finite Difference-Flux Corrected Transport) code with a two-dimensional drift–diffusion plasma model have been used to simulate the breakdown process and the results agree well with the experimental data. This kind of theoretical and experimental work provides valuable references for helium insulation design of electric devices in HTR-PM and other helium gas-cooled reactors. It also helps understand the mechanisms in helium discharge at high temperature and pressure which is quite different from its low counterpart.

Diffusion of hydrogen isotopes in 3C-SiC in HTR-PM: A first-principles study

Silicon carbide (SiC) is the main diffusion barrier of tri-isotropic particles in high-temperature gas-cooled reactor pebble-bed modules (HTR-PMs). When analyzing the source term of tritium in a HTR-PM primary circuit, it is essential to determine the amount of tritium released, which requires the diffusion coefficient of tritium in SiC. In the present work, the diffusion behavior of hydrogen in 3C-SiC is investigated using density functional theory-based first-principles calculations, which can be used to estimate the diffusivity of tritium while ignoring the neutron and mass effects. Different charge states of hydrogen (i.e., H0, H− and H+) and all possible low-energy configurations and diffusion paths are also considered in the calculations. The results indicate that the charge state of hydrogen changes from negative to positive with the increase in Fermi energy. In addition, the most stable position of H− in SiC is a tetrahedral site surrounded by four Si atoms. Furthermore, minimum diffusion barriers of 0.45, 0.46 and 1.52 eV are determined for H0, H− and H+, respectively. The calculated diffusion coefficients from the present work agree well with those computed in the literature. In addition, the experimental results are closer to the negative hydrogen values computed in this study, indicating that the most likely charge state of hydrogen is negative. Our calculations provide a good reference for nuclear safety evaluation in HTR-PMs using the diffusivity of hydrogen in SiC.

Experimental study on the novel rolling-sliding integrated auxiliary bearing in active magnetic bearing during rotor drop

The Active Magnetic Bearing (AMB) is used in the High Temperature Reactor- Pebble-bed Modules (HTR-PM) which is being constructed in Shandong province, China. The auxiliary bearing is one of the most important components guaranteeing the reliability of the AMB. A novel rolling-sliding integrated auxiliary bearing is studied through the drop experiment. For comparison, a traditional rolling auxiliary bearing in the same AMB is also studied experimentally. It was found that the rotor center orbit with the novel auxiliary bearing is much simpler and more focused. The impact force with the novel auxiliary bearing during the drop is much less. The drop process with the novel auxiliary bearing is more stable. All these phenomena are due to the fact that the contact point of the rotor with the novel auxiliary bearing is easy to stay at the lowest position of the circular contact surface during drop process.

Improved Harmonics Synthesis Method and its application in reconstructing power distribution of HTR-PM

It is difficult to arrange neutron detectors in the active zone of high temperature reactor. Therefore, the power distribution of HTR-PM (High-Temperature-Reactor Pebble-bed Module) could be monitored mainly by ex-core neutron detectors. Harmonics Synthesis Method (HSM) has been developed to monitor the power distribution of HTR-PM, and could reconstruct the power distribution with the limited neutron detectors in HTR-PM. However, it was found that the error of existing HSM might be relatively large in some conditions. In this work, the performance of HSM has been analyzed furtherly. It shows that the use of improper harmonics to reconstruct power distributions is the main reason which causes the large error. Because HSM uses fixed harmonics on all core status, the harmonics used in HSM hardly suit to all core status. Improved Harmonic Synthesis Method (IHSM) is proposed in this work to improve the accuracy of power distribution reconstruction. In IHSM, harmonics are selected based on core status to make sure the harmonics are appropriate for the reconstruction. Then, the reconstruction results of IHSM are presented and compared with the results of the original HSM. The results show that the performance of improved method is better than that of the original method on normal operation status of HTR-PM.

Study on impact of Cr and Mo on diffusion of H in 2.25Cr1Mo steel using first-principle calculations

Chrome-molybdenum steel (2.25Cr1Mo steel) is one of the main materials of a steam generator (SG) in high-temperature reactor-pebblebed modules (HTR-PM). It is essential to analyze the source term of tritium in this material, because the behavior of tritium in a SG is important for performing source term analysis in normal and accident conditions. In this article, the diffusion behavior of H atom in 2.25Cr1Mo steel was calculated to estimate the diffusivity of tritium using first-principles density functional theory. To develop and simplify the model of hydrogen diffusion in 2.25Cr1Mo steel, the impact of Cr and Mo on the diffusion of hydrogen in bcc-Fe were first calculated, all the possible diffusion paths were considered, and the minimum energy path was obtained. The diffusion activation energy and pre-exponential factor of the diffusion coefficient were obtained from Vienna Ab initio Simulation Package combined with the climbing image-nudged elastic band method. The results indicate that the minimum energy path for the impurity H atom is from one tetrahedral interstitial site to an adjacent tetrahedral interstitial site. The function of the diffusion coefficient of H in 2.25Cr1Mo steel with temperature T can be expressed as D=1.486×10−7×(−16.350kJ/molRT) (m2/s). The diffusion coefficient of our calculation and some of the previous experiments have an excellent quantitative agreement, which indicates the reliability of our crystalline model and the practicability of the present theoretical approach. More importantly, the computational results in this work can be treated as a good screening method to collect reasonable experimental data, which will provide a good reference for tritium source term evaluation in the SG of the HTR-PM.

Economic feasibility of flexible energy productions by small modular reactors from the perspective of integrated planning

Nuclear power plants are known as low carbon baseload electricity generating options, but can they become sufficiently flexible to follow the fluctuating demand economically? Small modular reactors, conceived to address the issues associated with large sized nuclear reactors such as safety, costs, and completion timeline, can offer some flexibility through modularity. However, there remain the questions of whether and how these advanced concepts could become commercially viable for load-following electricity production. From the perspective of integrated long-term energy planning and in a scenario-based approach, this study is conceived to evaluate the economic feasibility of flexible energy productions from two first-of-a-kind demonstration projects; namely a floating nuclear power plant KLT-40S in Russia and a high temperature reactor pebble-bed module or HTR-PM in China. Although both projects are solely designed for electric-application, this study proposes the evaluation of a load-following arrangement through the co-production of hydrogen and the integration of grid-connected battery energy storage systems. All scenarios are investigated using a conservative or a reasonable worst-case approach so as to contribute to setting the bottom line for future commercial deployment of small modular reactors. Findings suggest that small modular reactors can already be commercially deployed in the short-to mid-term, and there are opportunities for further cost reductions through technological innovation and economies of repeat project implementation.

An accident diagnosis algorithm for HTR-PM based on deep learning methods

The Chinese High Temperature Reactor Pebble-bed Module (HTR-PM) is the world first commercial nuclear power plant (NPP) with the characteristics of fourth generation. Accident diagnosis tasks for HTR-PM are directly associated with safe and efficient operation. Although different kinds of accident diagnosis methods have been studies on conventional NPPs, the research of accident diagnosis of HTR-PM is relatively lacking according to the different characteristics and new applications of HTR-PM. In this article, a new algorithm for HTR-PM accident diagnosis based on deep learning methods is proposed. By using the preprocessing, classification network and postprocessing techniques, the proposed algorithm could avoid over-reliance on the previous experiences and make fully use of the signals, also it can use only few number of training signal sequences to get high accuracy results, which is a significant advantage compared with traditional algorithms using deep learning methods. This algorithm is tested using the signals produced by the engineering simulator of HTR-PM for normal state and several accidents, including loss of feed water, large break depressurized loss of forced cooling, small break depressurized loss of forced cooling and inadvertent withdrawal of a single control rod. The results show the feasibility and effectiveness of the algorithm for HTR-PM accident diagnosis, and also the potentiality to use in other NPPs accident diagnosis tasks.

Radiological habitability evaluation of the control room of the high-temperature reactor pebble-bed module: post-accident dose assessment and sensitivity studies

ABSTRACT The radiological habitability of a control room is important for nuclear emergency response, which is also a licensing prerequisite for nuclear power plants. It is determined by both atmospheric relative concentrations and doses received via different pathways. However, most recent studies have focused only on the former, which may not be adequate. The present study therefore investigates the radiological habitability and its sensitivity to different parameters in the high-temperature gas-cooled reactor pebble-bed module power plant. For three typical accidents, the study estimates the body, thyroid and skin doses received via different pathways using the Nuclear Regulatory Commission recommended ARCON96 and dose calculation method in RG 1.195. To make a realistic evaluation, the latest design and site-specific information, including the unique accidental source term, are collected and used as input parameters. The evaluation results reveal that the total dose of different pathways in the control room is far below the limit, which demonstrates the effectiveness of the current design. The inhalation exposure is the dominant pathway, and iodine and caesium are the primary contributors of the inhalation dose. The particle filter removal fraction is the most influential parameter in an accident which the activities of metal radionuclides are high.

Helium blower test based on aerodynamic force simulation

In high temperature reactor pebble-bed modules (HTR-PM), the helium blower is the key equipment which is in the primary loop and drives the helium to exchange the energy. The usage of active magnetic bearing (AMB) is to avoid the lubricating oil leaked and keep the environment of helium pure. However, helium blower test needs to proceed under the condition of helium loop, which is difficult in early construction stage and will result in much more time and money cost. In this paper, we propose and produce a new device to simulate the dynamic force to build a force environment similar to the helium loop. The device called aerodynamic force simulator (AFS) derives from the idea of active magnetic bearing and utilizes electromagnetic force replacing aerodynamic force when helium blower works. Active magnetic bearing commissioning and dropping load test of helium proceed successfully with the utilization of aerodynamic force simulator. Through a series of tests, it is proved that the aerodynamic force simulator is effective and economic, and the design of helium blower and its components is reasonable and reliable.

HTR-PM fuel pebble irradiation qualification in the high flux reactor in Petten

NRG has performed a High Temperature Reactor fuel qualification test in the framework of the Chinese HTR development for INET – Tsinghua University, as a part of the qualification process of Chinese fuel for use in the HTR-PM (High Temperature Reactor-Pebble bed Module). Fuel qualification is an essential part of the licensing process of the HTR-PM, currently under construction in the Shandong province in the People’s Republic of China.The design of the INET irradiation setup for irradiation in the High Flux Reactor is performed by NRG. In the design of the irradiation rig, the pebbles are encased in graphite half shells to form a solid cylindrical stack. Within the graphite, thermocouples are located for temperature monitoring, and neutron fluence monitor sets for neutron fluence verification. Next to the temperature measurements, the online monitoring consisted of fission product release measurements. These measurements were performed with the Sweep Loop Facility, designed and built by JRC-IET.Post Irradiation Examinations are performed and consisted of gamma scanning, dimensional and weight measurements and visual inspection. In April 2016, the pebbles were transported from Petten to Karlsruhe, Germany where the second required step of fuel qualification will be performed, namely heating tests in the KÜFA facility.This paper describes the global lay-out and functionality of the irradiation facility. The online data generated and irradiation parameters are reported, with focus on the measured fission product release behavior of the fuel. The paper also presents non-destructive post irradiation examinations.

Research on the induced radioactivity of HTR-PM’s reactor pressure vessel: A comparative study between FLUKA, KORIGEN and QAD-CGA

The reactor pressure vessel (RPV), which is made of steel containing impurities, will be irradiated by a strong neutron field during the long-term operation of the reactor. Induced radioactivity due to neutron irradiation, which may affect the health of staffs during the maintenance and decommissioning, was studied in the case of the High Temperature Reactor – Pebble-bed Module (HTR-PM). The activation products and their corresponding specific activities were calculated by KORIGEN, which is based on deterministic method, and by the Monte Carlo code FLUKA. A comparison of these two programs shows good agreement, and the differences of specific activities are within one order of magnitude for 60% of the nuclides. Furthermore, the remanent dose rates at shutdown time and after 30 days, 1 year, 5 years and 10 years of cooling times were calculated by FLUKA and QAD-CGA, which is based on point-kernel integration technology. The comparison of dose results also shows good agreement. Meanwhile Monte Carlo method can give more accurate results. In the end, the ambient dose equivalent rates at typical points and times caused by the whole RPV were calculated by integration method. A preliminary radiation protection strategy was also concluded from these calculation results for HTR-PM’s RPV decommissioning.

Development of a thermal-hydraulic analysis code TAC-DS for spent fuel dry-storage system for high-temperature pebble bed reactor

ABSTRACT We report the development of a thermal-hydraulic analysis code (called TAC-DS: Thermal-hydraulic Analysis Code for Dry-storage System). The spent fuel dry-storage system of High-Temperature Reactor Pebble-bed Modules in China is simulated using the TAC-DS to confirm the design basis and to analyze the transient behavior following an accident involving blower failure. The TAC-DS includes mathematical models for the air-coolant system, heat conduction within spent fuel canisters, and thermal radiation between heat structures. The time-dependent hydrodynamic model of the TAC-DS is formulated using one-dimensional mass, momentum and energy equations, and solved using semi-implicit finite-difference scheme. The complicated heat transfer models of heat structure are incorporated into the hydrodynamic system implicitly with enclosure correlations. Code is written in Fortran 90. A validation calculation is performed by solving a simplified model. Thermal performance of the buffer storage region in the system under forced ventilation scenario is studied with TAC-DS to validate the design requirement, as well as to provide the initial condition for the transient analysis. Blower failure accident is studied to assess the performance of the safety features during the transient accident. Since the code is modular, TAC-DS can be easily modified and applied to other spent fuel dry-storage system in the future.