Pebble-bed High Temperature Gas-cooled Reactor Research Articles

The Stress and Reliability Analysis of HTR’s Graphite Component

The high temperature gas cooled reactor (HTR) is developing rapidly toward a modular, compact, and integral direction. As the main structure material, graphite plays a very important role in HTR engineering, and the reliability of graphite component has a close relationship with the integrity of reactor core. The graphite components are subjected to high temperature and fast neutron irradiation simultaneously during normal operation of the reactor. With the stress accumulation induced by high temperature and irradiation, the failure risk of graphite components increases constantly. Therefore it is necessary to study and simulate the mechanical behavior of graphite component under in-core working conditions and forecast the internal stress accumulation history and the variation of reliability. The work of this paper focuses on the mechanical analysis of pebble-bed type HTR's graphite brick. The analysis process is comprised of two procedures, stress analysis and reliability analysis. Three different creep models and two different reliability models are reviewed and taken into account in simulation. The stress and failure probability calculation results are obtained and discussed. The results gained with various models are highly consistent, and the discrepancies are acceptable.

Improvement of Electromagnetic Self-Locking Device for Cylinders in High Temperature Gas-Cooled Reactor

Several air cylinders are employed in complex equipment in pebble-bed high temperature gas-cooled reactor. They were designed to actuate remote movements in radioactive environment. An electromagnetic self-locking device was designed to lock them in safe position when the compressed air source is offline by accident. But the disadvantageous temporary standstill of armature in unlocking process still needed to be eliminated. Maxwell 2D transient analysis was performed to simulate the armature motion and compared with the experimental results. A comprehensive understanding was then achieved, and the parallel capacitor solution to the standstill problem was developed. With further numerical simulation and experimental verification, the self-locking device with capacitor connection was optimized. The armature motion was accelerated and the standstill was eliminated. The maximum temperature rise did not exceed the material limit value yet. The as-improved device would help guarantee the fail-safe feature of the cylinders.

Failure probability study of HTR graphite component using microstructure-based model

Nuclear graphite is widely used as in-core structural material in the high temperature gas-cooled reactors (HTRs). As mechanical properties of graphite change with neutron irradiation and temperature, the reliability evaluation of graphite internals of a HTR is commonly accomplished by the finite element analysis using various constitutive creep models and the following structural failure prediction with introduction of failure models that have essential impact on the final evaluation result. In this paper, a microstructure-based failure model proposed by Burchell in 1996 in conjunction with the finite element code INET-GRA3D of INET is used to study failure probability of a graphite side reflector design in the pebble-bed HTR during its entire service life from the microstructural view. The corresponding irradiation-induced creep stress analysis is carried out using both the UKAEA model and the Kennedy model. H-451 graphite is selected as material input since its irradiation material data from both macrostructure tests and microstructure observation is available. The results are compared with those using the macrostructure-based Weibull model commonly accepted in the engineering field and consistent trends are observed. Moreover, The Burchell model leads to the failure probability more sensitive to stress levels than the Weibull model.

Steam Cycle Modular Helium Reactor

The Next Generation Nuclear Plant (NGNP) project is being conducted by the U.S. Department of Energy (DOE) to demonstrate the technical and licensing viability of high-temperature gas-cooled reactor (HTGR) technology as a CO2 emission-free source of energy to displace the use of natural gas, petroleum, and coal for production of electricity and/or high-temperature process energy for a wide range of industrial applications. The DOE selected the HTGR as the reactor type for the NGNP project primarily because HTGRs can produce heat energy at much higher temperatures than other reactor types due to their use of ceramic, coated-particle fuel, helium coolant, and graphite as the core structural material. The DOE is considering a number of candidate HTGR designs for the NGNP demonstration plant; the DOE or a DOE-industry partnership will ultimately select the design to be licensed and constructed.The HTGR design option being advanced by General Atomics for the NGNP demonstration plant, and for follow-on commercial deployment, is the Steam Cycle Modular Helium Reactor (SC-MHR). The SC-MHR, which is the subject of this paper, uses fuel elements in the form of hexagonal blocks, which are stacked together to form the reactor core. This type of HTGR is referred to as a prismatic HTGR, as opposed to a pebble bed HTGR, which uses billiard ball-size spherical fuel elements. The above-noted generic features of HTGRs coupled with the modular helium reactor design features of the SC-MHR allow for adequate removal of residual heat from the reactor by completely passive means in the event of a loss of forced cooling or loss of coolant pressure. This ensures that the fuel remains below time-at-temperature limits at which fuel damage could occur during such events, thereby ensuring radionuclide retention within the fuel particles. Thus, the safety of the SC-MHR (as well as other modular HTGR designs) is inherent to the design, and the rare, but severe, accidents postulated for light water reactors and other advanced nuclear concepts are not possible with the SC-MHR.It is anticipated that design, licensing, and construction of the SC-MHR demonstration plant could potentially be completed to enable plant operations to begin in 2022.

Application of Time-Dependent Neutron Transport Theory to High-Temperature Reactors of Pebble Bed Type

We introduce a new coupled neutronics/thermal-hydraulics code system for analyzing transients of high-temperature gas-cooled reactors (HTGRs), based on a neutron transport theory approach. At the heart of the coupled code system resides the DORT-TD code, a time-dependent extension of the well-known DORT discrete ordinates code. DORT-TD uses a fully implicit time integration scheme and is coupled via its generalized thermal-hydraulics interface to the THERMIX-DIREKT code, an HTGR-specific heat conduction/convection code for pebble bed-type reactor cores. Feedback is accounted for by interpolating multigroup cross sections from libraries pregenerated with appropriate spectral codes. These libraries are structured for user-specified discrete sets of thermal-hydraulic parameters, e.g., fuel and moderator temperatures. The coupled code system is applied to a pebble bed HTGR model case, i.e., the PBMR 268 MW design. Steady-state studies are performed, and several design-basis and beyond-design-basis transients are simulated in an effort to assess the adequacy of using neutron diffusion theory against the more accurate but yet computationally more expensive neutron transport approach. Relatively small but significant differences arise from using either theoretical approach, from which it is concluded that transport theory as the more versatile tool should be used as reference to quantify the effects of the approximations inherent in diffusion and to gain confidence in its predictive power, especially in safety analyses. In an effort to validate the DORT-TD/THERMIX code system, the neutronics stand-alone solver is benchmarked against available transient benchmark exercises, and the coupled code system is applied to the Organisation for Economic Co-operation and Development/Nuclear Energy Agency/Nuclear Science Committee PBMR 400 MW Coupled Neutronics Thermal Hydraulics Transient Benchmark, demonstrating its remarkable viability for a wide range of safety cases. The final product is a high-fidelity, highly flexible, and well-validated state-of-the-art computer code system, with multiple capabilities to analyze HTGR safety-related transients in an accurate and efficient manner.

Analytical Solution of Fick's Law of the TRISO-Coated Fuel Particles and Fuel Elements in Pebble-Bed High Temperature Gas-Cooled Reactors

Two kinds of approaches are built to solve the fission products diffusion models (Fick's equation) based on sphere fuel particles and sphere fuel elements exactly. Two models for homogenous TRISO-coated fuel particles and fuel elements used in pebble-bed high temperature gas-cooled reactors are presented, respectively. The analytical solution of Fick's equation for fission products diffusion in fuel particles is derived by variables separation. In the fuel element system, a modification of the diffusion coefficient from D to D/r is made to characterize the difference of diffusion rates in distinct areas and it is shown that the Laplace and Hankel transformations are effective as the diffusion coefficient in Fick's equation is dependant on the radius of the fuel element. Both the solutions are useful for the prediction of the fission product behaviors and could be programmed in the corresponding engineering calculations.

Reactivity Accident in a High Temperature Gas-Cooled Reactor Due to Inadvertent Withdrawal of Control Rod

Abstract Reactivity accident due to inadvertent withdrawal of the control rod is one kind of the design basis accident for high temperature gas-cooled reactors, which should be analyzed carefully in order to validate the reactor inherent safety properties. Based on the preliminary design of the Chinese pebble-bed modular high temperature gas-cooled reactor (HTR-PM) with single module power of 250 MW, several cases of reactivity accident has been studied by the help of the software TINTE in the paper (e.g., the first scram signal works or not, the absorber balls (secondary shutdown units) drop or not) and the ATWS situation is also taken into account. The dynamic processes of the important parameters including reactor power, fuel temperature, and xenon concentration are studied and compared in detail between these different cases. The calculating results show that the decay heat during the reactivity accidents can be removed from the reactor core solely by means of physical processes in a passive way so that the temperature limits of the fuel element and other components are still obeyed, which can effectively keep the integrality of the fuel particles to avoid massive fission products release. This will be helpful to the further detail design of the HTR-PM demonstrating power plant project.

Experimental Investigation on Feasibility of Two-Region-Designed Pebble-Bed High-Temperature Gas-Cooled Reactor

Phenomenological experiments were performed on a 2-dimensional scaled model of the two-region designed pebble-bed high-temperature gas-cooled reactor core consisting of the distinct fuel pebble region and graphite pebble region. Issues with respect to the feasibility of the two-region design, including the establishment of the two-region arrangement, the mixing zone between the two regions, and the stagnant zone existence, were investigated. Three equilibrium conditions were proposed to evaluate the stable tworegion arrangement formation. The general characteristics of the flow of the pebble bed were analyzed on basis of the observed phenomenon. It was found that a stable two-region arrangement was formed under the experimental conditions: the pebbles' motion was to some extent random but also confined by the neighbors of pebbles so that the mixing zone is constrained to a reasonable size. Guide plates utilized to improve mixing are proved to be effective without noticeable effect on the two-region arrangement features. Stagnant zones were observed under the experimental conditions and they were expected to be avoided by improving the design of the experimental setup.

Experimental Investigation on Feasibility of Two-Region-Designed Pebble-Bed High-Temperature Gas-Cooled Reactor

Phenomenological experiments were performed on a 2-dimensional scaled model of the two-region designed pebble-bed high-temperature gas-cooled reactor core consisting of the distinct fuel pebble region and graphite pebble region. Issues with respect to the feasibility of the two-region design, including the establishment of the two-region arrangement, the mixing zone between the two regions, and the stagnant zone existence, were investigated. Three equilibrium conditions were proposed to evaluate the stable tworegion arrangement formation. The general characteristics of the flow of the pebble bed were analyzed on basis of the observed phenomenon. It was found that a stable two-region arrangement was formed under the experimental conditions: the pebbles' motion was to some extent random but also confined by the neighbors of pebbles so that the mixing zone is constrained to a reasonable size. Guide plates utilized to improve mixing are proved to be effective without noticeable effect on the two-region arrang...

Design aspects of the Chinese modular high-temperature gas-cooled reactor HTR-PM

The modular high-temperature gas-cooled reactor (MHTGR) has distinct advantages in terms of inherent safety, economics potential, high efficiency, potential usage for hydrogen production, etc. The Chinese design of the MHTGR, named as high-temperature gas-cooled reactor-pebble bed module (HTR-PM), based on the technology and experience of the HTR-10, is currently in the conceptual phase. The HTR-PM demonstration plant is planned to be finished by 2012. The main philosophy of the HTR-PM project can be pinned down as: (1) safety, (2) standardization, (3) economy, and (4) proven technology. The work in the categories of marketing, organization, project and technology is done in predefined order. The biggest challenge for the HTR-PM is to ensure its economical viability while maintaining its inherent safety. A design of a 450 MWth annular pebble bed core connected with steam turbine is aimed for and presented in this paper.

Feasibility of Burning First- and Second-Generation Plutonium in Pebble Bed High-Temperature Reactors

The core physics investigations at the Nuclear Research Consultancy Group in the Netherlands, as part of the activities within the HTR-N project of the European Fifth Framework Program, are focused on the incineration of pure (first- and second-generation) Pu fuels in the reference pebble bed high-temperature gas-cooled reactor (HTR) HTR-MODUL with a continuous reload [MEDUL, (MEhrfach DUrchLauf, multipass)] fueling strategy in which the spherical fuel elements, or pebbles, pass through the core a number of times before being permanently discharged. For pebbles fueled with different loadings of plutonium, the feasibility of a sustained fuel cycle under nominal reactor conditions was investigated by means of the reactivity and temperature coefficients of the reactor. The HTR-MODUL was found to be a very effective reactor to reduce the stockpile of first-generation plutonium. It reduces the amount of plutonium to about one-sixth of the original and reduces the risk of proliferation by denaturing the plutonium vector. For second-generation plutonium the incineration is less favorable, as the amount of plutonium is only halved.

Brayton Cycle for High-Temperature Gas-Cooled Reactors

This paper describes research on improving the Brayton cycle efficiency for a high-temperature gas-cooled reactor (HTGR). In this study, we are investigating the efficiency of an indirect helium Brayton cycle for the power conversion side of an HTGR power plant. A reference case based on a 250-MW(thermal) pebble bed HTGR was developed using helium gas as a working fluid in both the primary and power conversion sides. The commercial computer code HYSYS was used for process optimization. A numerical model using the Visual-Basic (V-B) computer language was also developed to assist in the evaluation of the Brayton cycle efficiency. Results from both the HYSYS simulation and the V-B model were compared with Japanese calculations based on the 300-MW(electric) Gas Turbine High-Temperature Reactor (GTHTR) that was developed by the Japan Atomic Energy Research Institute. After benchmarking our models, parametric investigations were performed to see the effect of important parameters on the cycle efficiency. We also investigated single-shaft versus multiple-shaft arrangements for the turbomachinery. The results from this study are applicable to other reactor concepts such as fast gas-cooled reactors, supercritical water reactors, and others.The ultimate goal of this study is to use other fluids such as supercritical carbon dioxide for the HTGR power conversion loop in order to improve the cycle efficiency over that of the helium Brayton cycle. This study is in progress, and the results will be published in a subsequent paper.

Assessments of Water Ingress Accidents in a Modular High-Temperature Gas-Cooled Reactor

Severe water ingress accidents in the 200-MW HTR-module were assessed to determine the safety margins of modular pebble-bed high-temperature gas-cooled reactors (HTR-module). The 200-MW HTR-module was designed by Siemens under the criteria that no active safety protection systems were necessary because of its inherent safe nature. For simulating the behavior of the HTR-module during severe water ingress accidents, a water, steam, and helium multiphase cavity model was developed and implemented in the dynamic simulator for nuclear power plants (DSNP) simulation system. Comparisons of the DSNP simulations incorporating these models with experiments and with calculations using the time-dependent neutronics and temperature dynamics code were made to validate the simulation. The analysis of the primary circuit showed that the maximum water concentration increase in the reactor core was <0.3 kg/(m3s). The water vaporization in the steam generator and characteristics of water transport from the steam generator to the reactor core would reduce the rate of water ingress into the reactor core. The analysis of a full cavitation of the feedwater pump showed that if the secondary circuit could be depressurized, the feedwater pump would be stopped by the full cavitation. This limits the water transported from the deaerator to the steam generator. A comprehensive simulation of the HTR-module power plant showed that the water inventory in the primary circuit was limited to ~3000 kg. The nuclear reactivity increase caused by the water ingress would lead to a fast power excursion, which would be inherently counterbalanced by negative feedback effects. The integrity of the fuel elements, because the safety-relevant temperature limit of 1600°C is not reached in any case, is not challenged.

03/02260 Prompt reactivity determination in a subcritical assembly through the response to a dirac pulse: Perdu, F. et al. Progress in Nuclear Energy, 2003, 42, (1), 107–120

Particle thermal radiation plays a significant role in many heat transport devices, such as pebble-bed high temperature gas-cooled nuclear reactors (HTGR). The effect of particle thermal radiation in packed pebble beds has been rarely investigated at particle scale. In this work, a complete CFD-DEM method coupled with particle-scale radiation is discussed for packed pebble beds, considering particle motion, fluid flow, particle-fluid interactions and heat convection, conduction and particle radiation. It is shown that, compared to the case without particle radiation, heat transfer is enhanced greatly by particle radiation at high temperatures. A particle radiation factor (PRF) is proposed as an independent non-dimensional parameter to qualify the effect of particle-scale radiation in packed pebble beds. The PRF increases significantly with temperature and decreases gradually as the heat storage capacity or the thermal conductivity of the fluid increases. A demonstrative utilization of the present model is performed for a benchmark problem based on the HTR-10 nuclear reactor, and the results in general are in agreement with the results predicted by other empirical codes. When the nuclear reactor is above full power, the power required by the fan will increase significantly. In the decay heat removal process, particle radiation is essential to keep the bed temperature below the allowable limit, which is significant for nuclear reactor safety.

Fuel management of the HTR-10 including the equilibrium state and the running-in phase

The mode of fuel management of the HTR-10 was studied, including the simulation of the fuel shuffling process and the measurement of the burnup of a fuel element. The prior consideration was the design of the equilibrium state. Based on this the fuel loading of the initial core and the fuel shuffling mode from the initial core through the running-in phase into the equilibrium state were studied. The code system VSOP was used for the physical layout of the HTR-10 at the equilibrium state and in the running-in phase. For the equilibrium state, in order to lessen the difference between the peak and the average burnup, 5-fuel-passage-through-the-core was chosen for the fuel management. The average burnup of the spent fuel for the equilibrium core is 80 000 MWd t −1, and the peak value of it is less than 100 000 MWd t −1 when the burnup of the recycled fuel element is under 72 000 MWd t −1. The mixture of fuel element and graphite element was used for the initial core loading, the volume fractions of the fuel and the graphite elements were 0.57 and 0.43, respectively. During the running-in phase, the volume fraction of graphite will decrease with the fresh fuel elements being loaded from the top of the core and the graphite elements discharged from the bottom of the core. The fuel shuffling mode is similar to that of the equilibrium state. The burnup limit of recycled fuel element is also 72 000 MWd t −1 and the peak burnup is less than 100 000 MWd t −1. Finally the core will be full of fuel elements with a certain profile of burnup and reaches the equilibrium state. According to the characteristics of the pebble-bed high temperature gas-cooled reactor, a calibrating method of concentration of 137Cs was proposed for the measurement of fuel burnup.

Prediction calculations and experiments for the first criticality of the 10 MW High Temperature Gas-cooled Reactor-Test Module

The 10 MW High Temperature Gas-cooled Reactor-Test Module (HTR-10) is a pebble bed experimental reactor built by the Institute of Nuclear Energy Technology (INET), Tsinghua University. This paper introduces the first critical prediction calculations and the experiments for the HTR-10. The German VSOP neutronics code is used for the prediction calculations of the first loading. The characteristics of pebble-bed high temperature gas-cooled reactors are taken into account, including the double heterogeneity of the fuel element, the buckling feedback of the spectrum calculation, the effect of the mixture of fuel elements and graphite balls, and the correction of the diffusion coefficients in the upper cavity based on transport theory. Also considered are the effects of impurities in the fuel elements, in the graphite balls and in the reflector graphite on the reactivity. The number of fuel elements and graphite balls in the initial core is predicted to provide reference for the first criticality experiment. The critical experiment adopts a method of extrapolating to approach criticality. The first criticality was attained on December 1, 2000. The first criticality experiment shows that the predicted critical number of the fuel elements and graphite balls is in close agreement with the experimental results. Their relative error is less than 1.0%, implying the physical predictions and the results of the criticality experiment are much beyond expectations.

The TINTE Modular Code System for Computational Simulation of Transient Processes in the Primary Circuit of a Pebble-Bed High-Temperature Gas-Cooled Reactor

The TINTE code deals with the nuclear and thermal transient behavior of a high-temperature reactor, taking into consideration the mutual feedback effects in two-dimensional r-z geometry, including the following: 1. time-dependent neutron flux calculation2. time-dependent heat source distribution (local and nonlocal fractions)3. time-dependent heat transport from the fuel to the fuel element surface4. time-dependent global temperature distribution5. gas flow distribution for a given mass flow or given pressure difference or even under natural circulation conditions6. convection and its feedback to the circulation. The TINTE code has been tested against transient experiments performed at the Arbeitsgemein-schaft Versuchsreaktor. It has also been used successfully for other reactor investigations and thermofluid problems including design studies for a small heating reactor of the peu à peu loading type.

Treatment of the Upper Cavity in a Pebble-Bed High-Temperature Gas-Cooled Reactor by Diffusion Theory

A simple method has been developed for treating large cylindrical empty and rodded void regions in high-temperature gas-cooled reactors with each existing diffusion code. The cavity is treated as a diffusion region with zero reaction cross sections. Only diffusion constants, found by an optimization process, are used. Verification of this model is done by comparing results with transport solutions for identical problems. Very good agreement is attained when anisotropic diffusion is foreseen in the diffusion code. Even with isotropic diffusion, however, eigenvalues and rod reactivities proved to be acceptable. This treatment yields usual diffusion running times and allows three-dimensional calculations.

Neutron Physical Investigations on the Shutdown Effect of Small Boronated Absorbing Spheres for Pebble-Bed High-Temperature Gas-Cooled Reactors

An emergency shutdown system for high-temperature gas-cooled pebble-bed reactors is proposed in addition to the common absorber rod shutdown system. This system is based on the strongly absorbing effect of small boronated graphite spheres (called KLAK), which trickle in case of emergency by gravity from the top reflector into the reactor core. The inner reflector of the Siemens-Argonaut reactor was substituted by an assembly of spherical Arbeitsgemeinschaft Versuchsreaktor fuel elements, and the shutdown effect was examined by installing well-defined KLAK nests inside this assembly. The purpose was to develop and prove a calculational procedure for determining criticality values for assemblies of large fuel spheres and small absorbing spheres.

The Critical HTGR Test Facility KAHTER–An Experimental Program for Verification of Theoretical Models, Codes, and Nuclear Data Bases

The critical facility KAHTER was used to verify and to adjust theoretical models describing the physics of pebble-bed high-temperature gas-cooled reactors. Using methods such as evaluating critical masses, flux mappings, and reaction rate measurements, more sophisticated experimental and theoretical methods have been developed to get exact information and interpretation of control rod efficiency, power determination, and fast neutron damage.