Very High Temperature Gas-cooled Reactor Research Articles

A new method of fuel pebble temperature calculation based on neutronic and thermal coupling equivalent model

As one of the six reactor types in the fourth-generation nuclear energy systems, the Very High Temperature Gas-cooled Reactor (VHTR) possesses inherent safety features. One VHTR fuel pebble contains tens of thousands of randomly distributed TRISO fuel particles, displaying a double heterogeneity characteristic. Current research models employ the homogenized method for fuel pebble temperature calculation, which neglected the neutronic and thermal (N/T) coupling effect and double heterogeneity characteristic. This study proposes a fuel pebble coupled equivalent method (PCEM) for temperature calculation. The method is based on the effective thermal conductivity obtained by the fine structure fuel pebble N/T coupling method. The new method is evaluated on steady-state and transient VHTR multi-scale neutronic and thermal hydraulic (N/TH) coupling calculation. The results show that the traditional temperature calculation method is conservative. Under steady-state conditions, the maximum degree of conservatism of the fuel pebble average temperature and peak temperature are 0.6 K and 14.2 K respectively. For the sudden drop in coolant flow rate transient condition, the average temperature and peak temperature of the fuel pebble calculated by PCEM are smaller than the values calculated by traditional method at the same time. Due to the influence of the fuel Doppler effect, the transient response of the traditional calculation method is slower.

Corrosion behaviors of Inconel 617 and Incoloy 800H in impure helium with different CO contents at high temperatures

The helium coolant of the very high-temperature gas-cooled reactor (VHTR) is expected to contain trace impurities, which will cause corrosion to superalloys at high temperatures. The high-temperature corrosion behaviors of Inconel 617 and Incoloy 800H were investigated in the impure helium with various CO levels. For Inconel 617, the microclimate reaction occurred at high temperatures, releasing large amounts of CO and destroying the oxide layer. The corrosion model was established according to the chemical thermodynamics theory, showing that the reaction temperature is related to the CO content. However, this corrosion behavior was not observed for Incoloy 800H. The corrosion difference between these alloys was analyzed in this study, indicating that the silicon oxide interlayer of Incoloy 800H may inhibit the microclimate reaction. Although the interlayer could attenuate the chemical destruction of the oxide layer, the adhesion may be reduced leading to thermal damage, which was proved by the experiment in this study. This research clarified the different corrosion mechanisms of these two alloys and provided guidance for the control scheme of CO content in the primary circuit of HTGR.

Research on two novel hydrogen-electricity-heat polygeneration systems using very-high-temperature gas-cooled reactor and hybrid-sulfur cycle

The combination of the hybrid-sulfur (HyS) cycle with a very-high-temperature gas-cooled reactor (VHTR) presents a promising nuclear hydrogen production technology. This study proposes two novel hydrogen-electricity-heat polygeneration systems using VHTR and HyS cycle. A thermodynamic model is developed via energy and exergy analyses. Thermodynamic performance analysis and comparison are conducted on the two systems. For the two systems under design basis condition, the recommended variation range of the split ratio is 0–0.6395 and the hydrogen production rate is 0–55.7 mol/s. For systems 1 and 2, the energy utilization efficiencies are 65.91 % and 64.53 %, and the exergy efficiencies are 68.55 % and 67.36 %, at a hydrogen production rate of 55.7 mol/s. The process heat supply unit, steam generator (SG) and HyS cycle exhibit the top three largest exergy loss coefficients of 6.32 %, 6.26 % and 4.21 % at the split ratio of 0.5; therefore, they are key units for improving system performance. The intermediate heat exchanger's primary-side helium outlet temperature and SG's secondary-side main steam temperature both affect the variation range of split ratio. A slightly lower main steam temperature can increase the maximum hydrogen production rate and maximum energy utilization efficiency.

Study on oxidation kinetics of three kinds of candidate superalloys for VHTR under air ingress accident

Inconel 617 alloy, Incoloy 800H alloy and Hastelloy X alloy are three alternative superalloys for heat transfer tubes used in the steam generator (SG) in the very high temperature gas cooled reactors (VHTR). In this paper, the oxidation kinetics of isothermal oxidation of three superalloys in the circulating air at 950 ℃ during the air ingress accident is investigated. Oxidation kinetic parameters of superalloys, such as the oxidation rate indexes (np and nt), mass parabola rate constant (kp) and oxide layer thickness parabola rate constant (kt) are calculated by the experimental data. In addition, tested samples of superalloys were analyzed by Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS) and X-ray diffraction (XRD). The results show that these three superalloys exhibit parabolic oxidation behavior in air atmosphere. The oxidation behavior of Incoloy 800H alloy is closer to the cubic oxidation law as time increases. The mass gain of three alloys does not fully match with the growth law of the oxide layer, which should be attributed to the internal oxidation degree of the alloy. The arithmetic square root of kp and kt, L, are proposed to quantify the internal oxidation degree. And the L of Inconel 617 alloy is the largest compared to Inconel 800H alloy and Hastelloy X alloy. This work is helpful to explain the behavior of oxidation of superalloys.

Preparation of ZrC coating in TRISO fuel particles by precise transportation of solid precursor and its microstructure evolution

Very-high-temperature gas-cooled reactors (VHTRs) with outlet temperatures above 1273 K show higher thermal efficiencies, which require fuel elements to operate at higher temperatures. In such situations, the conventional silicon carbide (SiC) coatings in TRISO particles might not be able to adapt to such high temperatures, and zirconium carbide (ZrC) can function as a potential fission product barrier material. For ZrC coating, controlling the stoichiometric ratio is a key challenge in the preparation process in fluidized bed. In this paper, a novel solid precursor feeding mode was proposed, combining quantitative powder feeding with cold transport. By controlling the powder feeding rate of ZrCl4 and the flow rate of propylene, the C/Zr atomic ratio in ZrC coating was controlled accurately. ZrC coatings with different stoichiometric ratios were successfully prepared via fluidized-bed chemical vapor deposition (FB-CVD). The ZrC coatings were subjected to high temperature heat treatment for different times from 2473 to 2673 K under argon atmosphere and the microstructure and property evolutions after high-temperature heat treatment were investigated. The deposition mechanism of ZrC coating was proposed.

Study of the neutronic and thermal coupling effect on VHTR fuel pebble using OpenMC and OpenFOAM

Pebble bed very high-temperature gas-cooled reactor (VHTR) has the advantages of inherent safety and high outlet temperature. It adopts a pebble fuel element with a double heterogeneity structure. This paper presents the thermal evaluations of the VHTR fuel pebble using the OpenMC/OpenFOAM code system. Firstly, the neutronic model using Cell-Based Method (CBM) and the multiscale heat conduction model are established respectively. The multiscale heat conduction model decomposes the fuel pebble on two different scales: 1) the mesoscale, defined as the fuel pebble scale; and 2) the microscale, defined the TRISO fuel particle scale. The neutronic and thermal (N/T) calculations are coupled by Picard method. The results of the steady-state coupling study of the VHTR fuel pebble show that the spatial self-shielding effect has a greater impact on the temperature of the center of the fuel pebble compared with other locations. For different fuel pebble power and pebble boundary wall temperature, the maximum mesoscale temperature and microscale temperature calculated with N/T coupling are lower than those calculated without N/T coupling. Therefore, the traditional maximum temperature calculation method without N/T coupling is conservative. And the conservative degree increases with the operating parameters of the fuel pebble.

Study on corrosion behavior of two superalloys under two extreme atmospheres

Inconel 617 and Hastelloy X alloys are two candidate materials for very high temperature gas-cooled reactor (VHTR) intermediate heat exchanger and steam generator equipment. This work explored the corrosion behavior of two alloys in the atmosphere of extreme air ingress accidents and vacuum environment. After the experiment, the corrosion behaviors of the two alloys were characterized and analyzed by electronic balance, scanning electron microscope(SEM), energy dispersive spectroscopy(EDS) and X-ray diffraction(XRD) and carbon sulfur analyzer. The results showed that the mass change and oxide film thickness of Hastelloy X alloy and Inconel 617 alloy after corrosion in air met the oxidation parabola law, and the kp of Hastelloy is lower. The chromium migration of the two alloys was obvious after corrosion in air. The longer the corrosion time was, the more obvious the phenomenon was. The carbon content of the alloy did not change significantly with the corrosion time. Mass loss and decarburization occurred in both alloys after corrosion in vacuum, and volatile elements volatilized and migrated. The weight loss, decarburization and element volatilization of Hastelloy X were more obvious.

Parametric study of effective thermal conductivity for VHTR fuel pebbles based on a neutronic and thermal coupling method

Pebble bed very high-temperature gas-cooled reactor (VHTR) has many economic and safety advantages, such as high safety and high outlet temperature. It uses fuel pebbles with Tristructural-isotropic (TRISO) fuel particles. The effective thermal conductivity of fuel pebbles is a key parameter to evaluate the peak temperature and average temperature of the fuel pebbles. The currently available effective thermal conductivity models do not consider the internal heat sources and the complex neutronic and thermal (N/T) coupling effects in the fuel pebbles. In this study, a three-dimensional fuel pebble heat transfer and a Monte Carlo neutronic coupled model were developed. The model has a random distribution of TRISO fuel particles and an accurate geometric model. The results show that traditional multi-scale heat conduction methods based on the existing effective thermal conductivity model, such as those based on the Chiew and Glandt models, are conservative in predicting the temperatures of VHTR fuel pebbles. The degree of conservatism is affected by fuel pebble power and packing fraction. Under the N/T coupling condition, the effective thermal conductivity is not much affected by the power of the fuel pebble but decreases with the increase of the fuel pebble boundary temperature. Geometric parameters affect the internal temperature distribution and peak temperature by changing the uniformity of the heat source distribution.

Numerical investigation on thermal–hydraulic characteristics of the helically corrugated tube in intermediate heat exchangers for very-high-temperature gas-cooled reactors

A three-dimensional numerical simulation is conducted to investigate the flow and heat transfer characteristics of the helically corrugated tube (HCT) intermediate heat exchanger (IHX) for very-high-temperature gas-cooled reactors (VHTRs). Helium is the working fluid with Reynolds number (Re) ranging from 10,000 to 20,000 and Prandtl numbers of 0.66. The heat transfer enhancement mechanism is analyzed in detail with the use of the ANSYS Fluent 18.0 computational fluid dynamics program. The results show that the helical corrugation disrupts the boundary layer development, strengthens the secondary flow intensity, and improves the synergy between the velocity vector field and temperature gradient vector field, thus enhancing the heat transfer. Moreover, the effect of geometric parameters characterized by dimensionless corrugation height e/d (0.02, 0.04, and 0.06) and dimensionless corrugation pitch p/d (0.5, 1.0, and 1.5) on the thermal–hydraulic performance are examined. The results indicate that increasing e/d and reducing p/d can enhance heat transfer. The Nusselt number of HCTs is increased by 17.25 %-81.46 % compared to a smooth tube, while the friction factor is also increased by 38.01 %-361.69 %. The comprehensive performance represented by the performance evaluation criterion is improved by 3.26 %–11.84 %. In the present study, e/d = 0.04 and p/d = 0.5 are the optimal HCT structures to obtain better comprehensive performance for the IHX design.

Numerical study of thermal-hydraulic and dust-deposition of tube bundles in an intermediate heat exchanger

Very-high-temperature gas-cooled reactors (VHTRs) are promising for efficient, zero-carbon hydrogen production. The intermediate heat exchanger (IHX) is a key piece of equipment for VHTR-coupled hydrogen production. Constrained by the limited space within the VHTR, the IHX must be efficient and compact. Besides, radioactive graphite dust deposition will reduce the performance and reliability of IHXs. Therefore, understanding the thermal-hydraulic and dust-deposition characteristics of IHX tube bundles is crucial for the design and safe operation of IHXs. This study uses the unsteady k-kl-ω model to simulate flow and heat transfer in IHX tube bundles. The discrete particle model combined with a deposition model is used to predict the movement and deposition behavior of graphite dust. The deposition model is achieved using user-defined functions. The model and code are first validated by empirical correlations and experimental results. The transient flow fields show that, in the in-line arrangement, unsteady and asymmetric periodic flow occurs with a period of about 0.047 s. The flow is steadier and more symmetric in the staggered arrangement due to the restriction of the main flow to the separation vortices. The maximum circumferential heat-transfer coefficient for inner tubes occurs at the impact point, where the central angle θ is about 60° and 0° for the in-line and staggered arrangement, respectively. The graphite-dust deposition rate decreases with increasing Reynolds number and particle size and the deposition mechanism is also analyzed in detail. With the current design conditions, the Nusselt number of the staggered arrangement increases by 27.90%–29.17% compared with the in-line arrangement, and the deposition rate decreases by 1.52%–3.15%. Furthermore, new correlations for Nusselt number and friction factor are developed for the thermal-hydraulic design of IHX tube bundles.

Thermodynamic analysis of two novel very high temperature gas-cooled reactor-based hydrogen-electricity cogeneration systems using sulfur-iodine cycle and gas-steam combined cycle

In this paper, two novel system layouts are proposed to improve the thermodynamic performance of the conventional very high temperature gas-cooled reactor (VHTR)-based hydrogen-electricity cogeneration system using sulfur-iodine (S–I) cycle and gas-steam combined cycle (GSCC). The heat and electricity consumption data of the S–I cycle are obtained from our previous Aspen Plus simulation results, and the thermodynamic model of the entire hydrogen-electricity cogeneration system is established using energy and exergy analysis methods. The performance of the system under benchmark conditions is analyzed, and the effects of several key operating parameters on system performance are investigated. The results show that with these two new system layouts, the thermal efficiency and exergy efficiency of the conventional system are improved by 8.56%–10.27% and 9.01%–10.82%, respectively. The largest exergy loss of the system occurs in the VHTR, and the exergy efficiency of the S–I cycle is very low, only about 50.8%. Therefore, when the hydrogen production load is large, it is very important to optimize the process flow and operating parameters of the S–I cycle. Besides this, it is found that the S–I cycle-based nuclear hydrogen production efficiencies are lower than the GSCC-based nuclear power generation efficiencies.

Application of a new OpenFOAM-based neutron diffusion kinetics solver to pebble-type VHTRs

As an advanced reactor type, the pebble type very high temperature gas cooled reactor (VHTR) has the advantages of high safety and high outlet temperature. In this study, the OpenMC/OpenFOAM code system is developed for a neutronic analysis of VHTR. The workflow of OpenMC/OpenFOAM code system consists of two steps: the OpenMC code is used to generate few-group cross sections; then, the OpenFOAM code is used to perform core neutron diffusion kinetics analyses and a neutron diffusion kinetics simulation solver (DiFAOM V1.0) is developed. The finite volume method (FVM) is used to discretize the neutron diffusion kinetics equations. Two scale problems are selected to verified the DiFAOM V1.0 solver. Firstly, the PWR assembly model are compared by DiFAOM V1.0 and Monte Carlo results. The results show that for PWR assembly, the difference of keff is 41 pcm, and the maximum error of four group normalized flux is −0.0841%. Secondly, 2D-TWIGL benchmark is used to assess the accuracy of DiFAOM V1.0 at core scale, and the calculated results of keff and normalized power distribution calculated were in good agreement with the benchmark. The above results prove the correctness of DiFAOM V1.0 solver. And for application of VHTR BCC lattice model, the maximum error of four group normalized flux is only −0.826%.

Experimental and numerical investigation of turbulent heat transfer enhancement of an intermediate heat exchanger using corrugated tubes

The intermediate heat exchanger (IHX) in a very-high-temperature gas-cooled reactor (VHTR) can realize a variety of process heat applications including hydrogen production, which is beneficial to the green and sustainable development of global energy. In the present study, three corrugated tubes are adopted to enhance the heat transfer capabilities of the IHX. The flow and heat transfer characteristics of these tubes are comprehensively investigated and compared using experiments and three-dimensional numerical simulations. Air and helium are the working fluid of the experiment and numerical simulation, respectively, with Reynolds number (Re) ranging from 10,000 to 20,000 and Prandtl numbers of 0.70 and 0.66. An innovative corrugation parameter acquisition method is proposed by combing high-precision computed tomography imaging and computer vision techniques. Thermal-hydraulic experiments are conducted and the k-kl-ω transition model is applied and validates well with the experimental data obtained for Nusselt number (Nu) and friction factor (f). By analyzing the flow field, heat flux field and the synergy angle distribution, it is found that corrugation disrupts the boundary layer development, strengthens the turbulent mixing, reduces the temperature gradient in the boundary layer and improves the synergy between the velocity field and the heat flux field, thus enhancing the heat transfer. The effects of corrugation parameters on the thermal-hydraulic performance of the three tubes are also discussed. The transversely corrugated tube with dimensionless height e/d = 0.03, dimensionless pitch p/d = 0.6, and Re = 20,000 produces the maximum Nu obtained, which is about 60% larger than that of a smooth tube. The helically corrugated tube with e/d = 0.03, p/d = 0.6, and Re = 12,000 has the best comprehensive performance, with a performance evaluation criterion of about 1.09. Finally, empirical correlations with geometric parameters (e/d and p/d) and Re are proposed for Nu and f for all three corrugated tubes.

Turbulence model assessment and heat transfer phenomena inside a rectangular channel under forced and mixed convection

A very high-temperature gas-cooled reactor (VHTR) is an attractive option for hydrogen production due to its high operating temperature. The reactor cavity cooling system (RCCS), a passive safety system of the VHTR, surrounds the reactor vessel with rectangular riser channels and removes decay heat from it. Under emergency operation conditions such as a decrease in the flow rate of the RCCS, the flow condition inside the RCCS riser can become a mixed convection condition accompanying heat transfer deterioration. To design and operate the VHTR with reliable safety, accurate predictions for various convective heat transfer conditions in RCCS riser channels are required. In this study, by comparing the results of an air flow visualization experiment, the predictability of the turbulence models was assessed for the convective heat transfer and complex flow characteristics near the corner region of the rectangular duct confirmed from the experimental study. In this paper, the heat removal rate through the test section of the visualization experiment was quantified under various convective heat transfer conditions, and local temperature measurement data from the experiment were presented to describe the heat transfer deterioration phenomena inside a rectangular riser in turbulent mixed convection. Linear and nonlinear eddy viscosity models were evaluated by comparing the heat transfer coefficient, local temperature, local flow structure including secondary flow, and turbulence quantities obtained from the experiment. Furthermore, improvements to turbulence models for enhancing the prediction of mixed convection heat transfer in rectangular riser ducts were proposed.

Flow pattern analysis and multi-objective optimization of helically corrugated tubes used in the intermediate heat exchanger for nuclear hydrogen production

Using the heat of the very-high-temperature gas-cooled reactor (VHTR) for large-scale and carbon-free hydrogen production provide an excellent option for a new sustainable energy economy. The intermediate heat exchanger (IHX) is a crucial component to realize this new hydrogen production mode. To supply sufficient heat for hydrogen production, the heat transfer of IHX needs to be enhanced. In this study, helically corrugated tubes (HCTs) were used to enhance the heat transfer of IHX. The flow pattern of HCTs was analyzed and a multi-objective optimization was then performed. First, the numerical experiment design with two objectives and three factors was conducted by Box-Behnken design in the response surface methodology (RSM). Second, the significant regression models for Nusselt number, friction factor, and performance evaluation criteria were obtained by the analysis of variance. Combining the analysis of turbulence kinetic energy and flow dead zone, a more comprehensive mechanism for the effect of corrugation parameters on flow and heat transfer was proposed. Finally, the fast-elitist non-dominated sorting genetic algorithm (NSGA Ⅱ) was adopted to conduct the multi-objective optimization of HCTs. The technique for order of preference by similarity to ideal solution (TOPSIS) was adopted for multi-objective decision-making. The CFD validation and comparison with RSM experimental design confirmed that the obtained Pareto optimal set and Pareto front can provide a comprehensive and effective design strategy for the IHX used for nuclear hydrogen production.

Thermo-economic analysis and optimization of the very high temperature gas-cooled reactor-based nuclear hydrogen production system using copper-chlorine cycle

An improved very high temperature gas-cooled reactor (VHTR) and copper-chlorine (Cu–Cl) cycle-based nuclear hydrogen production system is proposed and investigated in this paper, in order to reveal the unknown thermo-economic characteristics of the system under variable operating conditions. Energy, exergy and economic analysis method and particle swarm optimization algorithm are used to model and optimize the system, respectively. Parametric analysis of the effects of several key operating parameters on the system performance is conducted, and energy loss, exergy loss, and investment cost distributions of the system are discussed under three typical production modes. Results show that increasing the reactor subsystem pressure ratio can enhance the system's thermo-economic performance, and the total efficiencies and cost of producing compressed hydrogen from nuclear energy are respectively lower and higher than that of generating electricity. When the system operates at the maximum hydrogen production rate of 403.1 mol/s, the system's net electrical power output, thermal efficiency, exergy efficiency, and specific energy cost are found to be 38.77 MW, 39.3%, 41.26%, and 0.0731 $/kW·h, respectively. And when the system's hydrogen production load equals to 0, these values are respectively calculated to be 177.25 MW, 50.64%, 53.29%, and 0.0268 $/kW·h. In addition, more than 90% of the system's total energy losses are caused by condenser and Cu–Cl cycle, and about 50–60% of the system's total exergy destructions occur in VHTR. About 60% and 30% of the system's specific energy cost are respectively caused by the equipment investment and the system operation & maintenance, and the investment costs of VHTR and Cu–Cl plant are the system's main capital investment sources.

A numerical study of heat transfer enhancement by helically corrugated tubes in the intermediate heat exchanger of a very-high-temperature gas-cooled reactor

We numerically investigate heat transfer enhancement by helically corrugated tubes (HCTs) in the intermediate heat exchanger (IHX) of a very-high-temperature gas-cooled reactor (VHTR). A comparison with heat transfer in a smooth tube shows that HCTs have better heat transfer over the full IHX temperature range. The helical corrugations strengthen the intensity of the turbulence and create rotational flow, which better mixes the fluid and improves the synergy between the velocity field and the temperature gradient field. The effects of tube structural parameters and fluid physical properties on the Nusselt number (Nu), friction factor, and performance evaluation criterion (PEC) were also analyzed. The corrugations have optimal heights for the maximum Nu and PEC. Nu decreases as the corrugation pitch increases, whereas the PEC shows the opposite trend. The best heat transfer enhancement is 1.69 for e/d = 0.03 and p/d = 0.4, and the best overall performance is 1.13 for e/d = 0.01 and p/d = 0.8, both of which are in the high-temperature region.

Applications of carbide ceramics in nuclear reactors

<p indent=0mm>After the Fukushima nuclear accident in Japan, the safety requirements of nuclear energy systems are constantly increasing. A series of new nuclear energy systems are under development, such as very high-temperature gas-cooled reactor (VHTR), lead-cooled fast reactor (LFR), gas-cooled fast reactor (GFR) and so on. In order to achieve the goals of higher safety, these nuclear systems have put forward higher requirements for nuclear materials. For example, materials working in VHTR and GFR are supposed to withstand higher temperatures, and LFR requires materials to be more corrosion-resistant. Nuclear materials have to meet these requirements under normal operating conditions, as well as in incidental and accidental conditions. Therefore, it is necessary to optimize the existing material system and develop new high-performance materials to meet the future development needs of advanced nuclear power systems. Compared with traditional nuclear materials, carbide ceramics have better mechanical properties under elevated temperatures, stronger irradiation resistance and better comprehensive thermophysical properties. Up to now, great breakthroughs have been made in the research of carbide ceramic materials and some key materials are going to be used in the new-generation nuclear systems. For example, uranium carbide (UC) is gradually replacing traditional uranium dioxide (UO₂), and silicon carbide (SiC) is acting as the main fission product barrier layer of coated fuel particles in HTGR. Other new carbide materials such as zirconium carbide (ZrC) and MAX phase carbide materials are also becoming a new research hotspot in the field of nuclear materials. Compared with other ceramic materials, the fabrication especially its densification of carbide is generally more difficult. In addition, high purity and good neutron properties are also the basic requirements of carbide ceramic materials for nuclear energy. The vapor deposition process and powder metallurgy process have been used to fabricate carbide ceramics for different components in nuclear reactors. Some new preparation methods have been developed to fabricate fuels and structural materials. For example, gelation progress is applied to prepare UCO microsphere fuel, and nano infiltration and transient eutectic (NITE) process is used in the preparation of SiC_f/SiC composites, which is potential for nuclear claddings. The irradiation-induced degradation of nuclear ceramic materials in the reactor is unavoidable. In order to evaluate the performance changes of materials under irradiation conditions, different irradiation methods such as electron irradiation, ion irradiation, proton irradiation and neutron irradiation are used. Due to its unique microstructure and properties, carbide ceramics exhibit different properties from other nuclear materials under irradiation. The performance changes of main nuclear carbide materials after irradiation are summarized, including microstructure evolution, macroscopic irradiation-induced swelling, changes in mechanical and thermal properties, etc. In general, based on the current research progress, the article reviews the application of carbide ceramics in new nuclear power systems and elaborates their service scenarios. Basic properties, preparation methods and irradiation properties of carbide ceramic materials are introduced in detail. The future development directions and the processing techniques are proposed. In the future, carbide ceramic materials should be further developed, and carbide ceramic materials will find diverse applications in the new generation nuclear systems.

Thermodynamic evaluation of hydrogen and electricity cogeneration coupled with very high temperature gas-cooled reactors

Hydrogen production using thermal energy, derived from nuclear reactor, can achieve large-scale hydrogen production and solve various energy problems. The concept of hydrogen and electricity cogeneration can realize the cascade and efficient utilization of high-temperature heat derive for very high temperature gas-cooled reactors (VHTRs). High-quality heat is used for the high-temperature processes of hydrogen production, and low-quality heat is used for the low-temperature processes of hydrogen production and power generation. In this study, two hydrogen and electricity cogeneration schemes (S1 and S2), based on the iodine-sulfur process, were proposed for a VHTR with the reactor outlet temperature of 950 °C. The thermodynamic analysis model was established for the hydrogen and electricity cogeneration. The energy and exergy analysis were conducted on two cogeneration systems. The energy analysis can reflect the overall performance of the systems, and the exergy analysis can reveal the weak parts of the systems. The analysis results show that the overall hydrogen and electricity efficiency of S1 is higher than that of S2, which are 43.6% and 39.2% at the hydrogen production rate of 100 mol/s, respectively. The steam generators is the components with the highest exergy loss coefficient, which are the key components for improving the system performance. This study presents a theoretical foundation for the subsequent optimization of hydrogen and electricity cogeneration coupled with VHTRs.

High-temperature mechanical behaviors of diffusion-welded Alloy 617

The development of a compact-type heat exchanger is one of the critical issues for realizing an efficient gas-to-gas heat exchanging system in a very high-temperature gas-cooled reactor (VHTR). The nickel-based superalloy Alloy 617 (Ni-Cr-Co-Mo, UNS N06617) is considered as the leading material for this application owing to the excellent thermal stability and strength in high temperature ranges. Sixty sheets having dimensions of 200 × 200 mm2 were diffusion-welded under uniaxial compressive pressure of 14.7 MPa at 1423 K (1150 °C) in a high vacuum condition. After 2 h of hot pressing, a post-weld heat treatment (PWHT) was applied to enhance the constituent atomic diffusion across the interface. To evaluate the high-temperature mechanical properties, tensile and stress-rupture tests were employed in this study. In tensile tests, the yield strength of the diffusion weldment is 318 ± 4 MPa at room temperature, and superior yield strengths over the ASME code requirement are maintained up to 1223 K (950 °C). Similarly, the tensile strength reaches 730 ± 15 MPa with ductility of 37.1 ± 1.4% at room temperature. However, both the tensile strength and ductility drop rapidly at elevated temperatures (≥973 K (700 °C)). During stress-rupture testing at a fixed temperature of 1173 K (900 °C) in an ambient air environment, the diffusion weldment shows lower creep properties. A plot of log time-to-rupture vs. log minimum creep rate is illustrated on the basis of the Monkman-Grant (M–G) relationship.