Evaluation of alloys for advanced high-temperature reactor systems

The qualification of metallic materials for advanced HTR applications is based on creep behaviour, fatigue properties, structural stability and corrosion resistance. A brief state of the art is provided for the materials for heat exchanging components. Of specific interest are the possible effects of helium on the mechanical properties. Helium, which serves as primary coolant, contains traces of reactive impurities such as hydrogen, methane, carbon monoxide and water vapor. Alloy improvement and the progress in development of new alloys are reviewed. In addition, the influence of neutron irradiation on the hot tensile and creep properties of absorber rod materials are investigated.The evaluation of the HTR materials programme provide the basis for structural design rules of components with operation temperatures above 800°C.     

Layout of an internally heated gas generator for the steam gasification of coal

Industrial-scale steam gasification of coal using heat from high temperature reactors requires research and development on allothermal gas generators. Bergbau-Forschung GmbH, Essen, does theoretical and experimental work in this field. The experiments deal with reaction kinetics, heat transfer and material tests. Their significance for the layout of a full-scale gas generator is shown. Including material specifications, the feasibility of a gasifier, characterized by a fluid bed volume of 318 m³ and a heat transferring area of 4000 m², results. The data, now available, are used to determine the gasification throughput from the heat balance, i.e. the equality of heat consumed and heat transferred. Throughputs of about 50 t/hr of coal are possible for a single gas generator, the helium outlet temperature of the HTR being 950°C. Bergbau-Forschung has commissioned a medium-scale pilot plant (200 kg/hr).     

Coal gasification with heat from high temperature reactors: Objectives and status of the project “Prototype Plant for Nuclear Process Heat (PNP)”

It is by now fairly widely known that the high temperature reactor (HTR) is a unique nuclear energy source which can supply heat at temperatures up to 1000°C for application in chemical processes, for which previously exclusively combustion heat sources have been used. With the HTR, it is possible to apply nuclear energy not only for electrical power production but also for the synthesis of liquid or gaseous energy carriers. Nuclear coal gasification appears most promising as the first step for the demonstration and industrial application of nuclear process heat technology in the Federal Republic of Germany. Reactor manufacturers and coal mining companies in co-operation with the Nuclear Research Center at Jülich established a joint project in 1975 for the development of an HTR with a coolant outlet temperature of 950°C, for the development and testing of nuclear coal gasification, for the detailed engineering of a prototype plant consisting of an HTR and gasification plant and finally for the construction and operation of this prototype plant for nuclear process heat (PNP). This contribution describes the status of the PNP-Project and the scope for future development.     

Gasification of coal with steam using heat from HTRs

In existing coal gasification processes a substantial part of the coal is used to provide the heat for the reaction, for the generation and superheating of steam and for the production of oxygen. By using heat from HTRs to substitute this part, the coal is then completely used as raw material for gas production. This offers the following advantages compared with the existing processes: a saving of coal, less CO₂ emission and, in countries with high coal costs, lower gas production costs. A survey is given of the state of the project, discussing the first design of a commercial gasifier, the influence of the helium outlet temperature of the HTR, kinds of products, and the overall efficiency of the plant. The aim of the development is to demonstrate the use of heat from an HTR for full scale coal gasification, starting in 1985.     

The HTR for process heat application according to the BBC/HRB-concept

The development of the High-Temperature Reactor (HTR) for the generation of nuclear process heat for coal gasification applying temperatures up to 950°C is one of the most important long-term HTR-development objectives pursued within the German PNP-project. The HTR for nuclear process heat generation according to the concept of BBC/HRB is part of the commercialization strategy of the HTR-line, which is based on the preceding AVR experimental reactor, the THTR-300 MW_c prototype plant and the HTR-500 MW_c plant. This strategy permits an optimum utilization of the development of the nuclear heat supply system of the THTR-300 and HTR-500 and represents a consequent continuation of the German HTR development pursued up to now. It will result in the lowest possible cost and time expenditure on the commercialization of the HTR for all applications. A new reactor development is not required with this concept. The earliest possible realization of a first-of-its-kind nuclear process heat plant will be determined by the development of the gasification processes.     

Nuclear Process heat—application to fuels and chemical production

Synthesis gas, a mixture of CO and H₂, produced from coal and a HTR nuclear source can be an economic feedstock for synthetic fuel and chemicals production. Such chemicals as hydrogen, ammonia, methanol, steel, can be readily produced from synthesis gas and other raw feedstocks by standard chemical engineering practice. Direct coal liquefaction is accomplished by adding H₂ to a pressurized coal slurry or solution. The use of the HTR to provide both the synthesis gas (using its high temperature capability) and steam or electricity for chemical process application (using its steam bottoming cycle capability) gives substantial conservation advantages in the use of coal compared to the non-nuclear equivalent processes. The desirability of efficiently using both the high and low temperature sources of the HTR requires a coupling between two or more chemical processes and the HTR (in a ‘Chemplex’) if a match is to be made between the high temperature and steam cycles of the HTR and the needs of the chemical processes.     

Thermal response of a modular high temperature reactor during passive cooldown under pressurized and depressurized conditions

The concept of inherent safety features of the modular HTR design with respect to passive decay heat removal through conduction, radiation and natural convection was first introduced in the German HTR-module (pebble fuel) design and subsequently extended to other modular HTR design in recent years, e.g. PBMR (pebble fuel), GT-MHR (prismatic fuel) and the new generation reactor V/HTR (prismatic fuel).This paper presents the numerical simulations of the V/HTR using the thermal-hydraulic code THERMIX which was initially developed for the analysis of HTRs with pebble fuels, verified by experiments, subsequently adopted for applications in the HTRs with prismatic fuels and checked against the results of CRP-3 benchmark problem analyzed by various countries with diverse codes.In this paper, the thermal response of the V/HTR (operating inlet/outlet temperatures 490/1000 °C) during post shutdown passive cooling under pressurized and depressurized primary system conditions has been investigated. Additional investigations have also been carried out to determine the influence of other inlet/outlet operating temperatures (e.g. 490/850, 350/850 or 350/1000 °C) on the maximum fuel and pressure vessel temperature during depressurized cooldown condition. In addition, some sensitivity analyses have also been performed to evaluate the effect of varying the parameters, i.e. decay heat, graphite conductivity, surface emissivity, etc., on the maximum fuel and pressure vessel temperature. The results show that the nominal peak fuel temperatures remain below 1600 °C for all these cases, which is the limiting temperature relating to radioactivity release from the fuel. The analyses presented in this paper demonstrate that the code THERMIX can be successfully applied for the thermal calculation of HTRs with prismatic fuel. The results also provide some fundamental information for the design optimization of V/HTR with respect to its maximum thermal power, operating temperatures, etc.     

The feasibility of cooling heavy-water reactors with supercritical fluids

It appears technically feasible to use supercritical carbon dioxide as a coolant for a CANDU-type reactor. A new supercritical loop is proposed in which the reactor is cooled by a single-phase fluid pumped in a high density liquid-like state. The supercritical fluid-cooled reactor has the advantage of gas-cooled reactors of avoiding dryout, and of liquid-cooled reactors of low coolant-circulation power. By eliminating dryout, the maximum operating temperature of the fuel sheath can be increased to 1021°F (550°C) for existing Canadian fuel bundles, with a coolant exit temperature of 855°F (458°C) producing steam comparable to that of conventional fossil-fuel plants. Since the reactor coolant exit temperature from the steam generator may be as high as 280°F (138°C) low-pressure steam may also be produced. A new dual-reheat cycle is proposed with an ideal overall plant efficiency of 33%, comparable to the Pickering generating station.     

An evaluation of reactor cooling and coupled hydrogen production processes using the modular helium reactor

The high-temperature characteristics of the modular helium reactor (MHR) make it a strong candidate for producing hydrogen using either thermochemical or high-temperature electrolysis (HTE) processes. Using heat from the MHR to drive a sulfur-iodine (SI) thermochemical hydrogen production process has been the subject of a U.S. Department of Energy sponsored Nuclear Engineering Research Initiative (NERI) project led by General Atomics, with participation from the Idaho National Laboratory (INL) and Texas A&M University. While the focus of much of the initial work was on the SI thermochemical production of hydrogen, recent activities included development of a preconceptual design for an integral HTE hydrogen production plant driven by the process heat and electricity produced by a 600 MW MHR.This paper describes ATHENA analyses performed to evaluate alternative primary system cooling configurations for the MHR to minimize peak reactor vessel and core temperatures while achieving core helium outlet temperatures in the range of 900–1000 °C that are needed for the efficient production of hydrogen using either the SI or HTE process. The cooling schemes investigated are intended to ensure peak fuel temperatures do not exceed specified limits under normal or transient upset conditions, and that reactor vessel temperatures do not exceed American Society of Mechanical Engineers (ASME) code limits for steady-state or transient conditions using standard light water reactor vessel materials. Preconceptual designs for SI and HTE hydrogen production plants driven by one or more 600 MW MHRs at helium outlet temperatures in the range of 900–1000 °C are described and compared. An initial SAPHIRE model to evaluate the reliability, maintainability, and availability of the SI hydrogen production plant is also described. Finally, a preliminary flowsheet for a conceptual design of an HTE hydrogen production plant coupled to a 600 MW modular helium reactor is presented and discussed.     

Carbon dioxide zero-emission hydrogen system based on nuclear power

The possibility of a hydrogen production system for Fuel Cell (FC) vehicles, which was zero carbon dioxide emission based on nuclear power, was investigated. The reactivity of calcium oxide to carbon dioxide was used for the carbon dioxide fixation and also for heat source of fuel reforming in experimental discussion. Methane was chosen as the first candidate reactant for steam reforming. Simultaneous reaction of methane reforming and carbon dioxide fixation by calcium oxide was demonstrated in a reactor packed with a reforming catalyst and calcium oxide. High-purity hydrogen, of which the concentration was higher than one at reaction equilibrium of conventional reforming, was generated from the reactor under mild operation conditions at temperature of 500–600°C and under pressure of 101 MPa. The efficiency of the fuel reforming system was estimated from the experimental results. The proposed system was expected to be applicable as a hydrogen carrier system in carbon dioxide zero-emission FC vehicles based on nuclear power.     

Synergistic hydrogen production by nuclear-heated steam reforming of fossil fuels

Processes and technologies to produce hydrogen synergistically by the nuclear-heated steam reforming reaction of fossil fuels are reviewed. Formulas of chemical reactions, required heats for reactions, saving of fuel consumption, reduction of carbon dioxide emission, and possible processes are investigated for such fossil fuels as natural gas, petroleum and coal.

In this investigation, examined are the steam reforming processes using the “membrane reformer” and adopting the recirculation of reaction products in a closed loop configuration. The recirculation-type membrane reformer process is considered to be the most advantageous among various synergistic hydrogen production processes. Typical merits of this process are; nuclear heat supply at medium temperature around 550°C, compact plant size and membrane area for hydrogen production, efficient conversion of a feed fossil fuel, appreciable reduction of carbon dioxide emission, high purity hydrogen without any additional process, and ease of separating carbon dioxide for future sequestration requirements.

The synergistic hydrogen production using fossil fuels and nuclear energy can be an effective solution in this century for the world which has to use fossil fuels to some extent, according to various estimates of global energy supply, while reducing carbon dioxide emission.     

38. R&D on hydrogen production by high-temperature electrolysis of steam

One of the objectives of the high-temperature engineering test reactor (HTTR) is to demonstrate the effectiveness of high-temperature nuclear heat utilization, which aims to extend the application of nuclear heat to non-electric fields, especially to hydrogen production. As part of the development of the hydrogen production processes, laboratory-scale experiments of a high-temperature electrolysis of steam (HTES) had been carried out with a practical electrolysis tube with 12 solid-oxide cells connected in series. Using this electrolysis tube, hydrogen was produced at the maximum density of 44 N cm³/cm² h at a electrolysis temperature of 950 °C. Thereafter, to improve hydrogen production performance, a self-supporting planar electrolysis cell with a practical size (80 mm × 80 mm of electrolysis area) was fabricated. In the preliminary electrolysis experiment carried out at 850 °C, the planar cell produced hydrogen at the maximum density of 38 N cm³/cm² h, and the energy efficiency was almost as high as that obtained with the electrolysis tube at 950 °C. However, both electrolysis tubes and planar cells did not keep their integrity in one thermal cycle. Durability of the solid-oxide cell against the thermal cycle is one of the key issues of HTES.     

Application of the ASME-Code-Case N 47 to a typical thick walled incoloy 800 HTR-component

In HTR-plants various components are exposed to temperatures above 500°C i.e. in the high temperature range. The service life of such components is limited not only by fatigue damage but in particular by creep damage and accumulated inelastic strains. These may be conservatively evaluated according to ASME-Code (High-Temperature Section CC N47) based on the results of elastic calculations, but this simplified procedure is often too conservative, as has been shown in the present case of the main steam header of the HTR steam generator.In such cases the verification that the stresses to which the component will be subjected are within permissible limits, requires a time-consuming and expensive inelastic analysis.The two-dimensional inelastic analysis, which will be discussed in detail, shows that the creep and fatigue damage as well as the inelastic strains of the main steam header accumulated over the service life remain below the permissible limits indicated in the ASME-Code. Thus failure of these components over their service life in the reactor can be excluded.     

Steam reformers heated by helium from high temperature reactors

The manifold possibilities of the application of helium-heated steam reformers combined with high temperature nuclear reactors are elucidated in this article. It is shown that the thermodynamic interpretation of the processes does not cause difficulties because of the good heat transfer in helium at high pressure and that helium peak temperatures of 950°C are sufficient for carrying out the process. The mechanical design of the reformer tube does not lead to problems because the helium and process pressures are so chosen as to be approximately equal. The problems of hydrogen and tritium permeation as well as the contamination of the reformer tube with solid fission products seem to be solvable using the knowledge available at present. Furthermore, the various possibilities for the design arrangements of helium-heated reformer tube furnaces are shown. The status of development attained to date is outlined and in conclusion there is a survey regarding the next steps to be taken in steam reformer technology.     

Direct-contact condensation heat transfer model in RELAP5/MOD3.2 with/without noncondensable gases for horizontally stratified flow

Direct-contact condensation experiments on atmospheric steam and steam/air mixture on subcooled water flowing co-currently in nearly-horizontal channels are carried out and the local heat transfer coefficients are obtained. Inlet air mass fraction in the mixture is varied by up to 50%. Based on previous and present experimental data, a direct-contact condensation database is constructed. The horizontally stratified condensation model of RELAP5/MOD3.2 overpredicts both co-current and counter-current experimental data. The correlation proposed by Kim predicts the database relatively well compared with that of RELAP5/MOD3.2. In the presence of noncondensable gases, RELAP5/MOD3.2 overpredicts the database with percentage errors of 55.1 and 61.1% for pipe inclination angles of θ=2.1° and θ=5.0°, respectively. The UCB correction factor is modified to consider the effects of noncondensable gases on heat transfer coefficients. When Kim's correlation is substituted with the Dittus–Boelter type correlation in RELAP5/MOD3.2 and a modified correction factor is used, the prediction errors are greatly reduced to 20.7 and 28.8% for inclination angles of θ=2.1° and θ=5.0°, respectively.     

Engineering design elements of a two-phase thermosyphon for the purpose of transferring NGNP thermal energy to a hydrogen plant

Two hydrogen production processes, both powered by Next Generation Nuclear Plant (NGNP), are currently under investigation at the Idaho National Laboratory and University of Idaho. The first is high temperature steam electrolysis utilizing both heat and electricity, and the second is thermo-chemical production through the sulfur iodine process which primarily utilizes heat. Both processes require high temperature (>850 °C) for enhanced efficiency; temperatures indicative of NGNP. Safety and licensing mandates prudently dictate that the NGNP and the hydrogen production facility be physically isolated, perhaps requiring separation of over 100 m. There are several options to transferring multi-megawatt thermal power over such a distance. One of the options is two-phase heat transfer utilizing a high temperature thermosyphon. Heat transport occurs via evaporation and condensation, and the heat transport fluid is re-circulated by gravitational force. A thermosyphon has the capability to transport heat at high rates over appreciable distances, virtually isothermally and without any requirement for external pumping devices. This paper addresses the engineering design elements of an industrial-scale (50 MW), high temperature controllable thermosyphon for NGNP process heat transfer. Although several different working fluids are under consideration, alkali metals are used herein as reference fluids to illustrate elements of design.     

Advanced pebble bed high temperature reactor with central graphite column for future applications

Design evaluations of the advanced pebble bed high temperature reactor, AHTR, with central graphite column are given. This reactor, as a nuclear heat source, is suitable for coal refinement as well as for electricity generation with closed gas turbine primary helium circuit. With this design of the central graphite column, it is possible to limit the core temperatures under the required value of about 1600°C in case of accident conditions, even with higher thermal power and higher core inlet and outlet temperatures. The designs of core internals are described. The after heat removal system is integrated in the prestressed concrete reactor pressure vessel, which is based on the principals of natural convection.Research work is being carried out, whereby the spherical fuel elements are coated with a layer of silicon carbide, to improve the corrosion resistance as well as the effectiveness of the fission products barrier.     

Preliminary operating experiences with the AVR at an average hot-gas temperature of 950°C

The two-loop system with a high temperature reactor, which is operated by the Arbeitsgemeinschaft Versuchsreaktor (AVR) GmbH and which was built by the BBC/Krupp consortium (today HRB), has been in operation for more than seven years. In that time more than 635 × 10⁶ kWhr have been produced and more than 6.5 × 10⁵ spherical fuel elements have been circulated under operation. The fully integrated design, and above all the ceramic gas duct, permit very high gas temperatures although no high alloyed, heat resistant steels were used in the reactor. In February 1974 the average hot-gas temperature at the outlet of the core could thus be increased from its original design value of 850°C to 950°C. Peak temperatures of above 500°C are thereby confined to a small region between the middle of the core and the beginning of the steam generator. Carbon protects the steel structures against high temperatures. Unplanned interruptions and reductions of operation due to the increase of the hot-gas temperature have not occurred so far. Some thermocouples in the hot-gas region failed. All other components functioned satisfactorily and, one year after the increase in the hot-gas temperature, there are no misgivings as to their future functioning. These satis-factory but short operating experiences at 950°C will have to be supplemented in the next few years by experiences over a longer period.     

The He/He heat exchanger — Design and semitechnical testing

This is a report on the development of the He/He heat exchanger which is used for high-temperature reactors (HTR) combined with the steam gasification of coal. A concept has been agreed on the basis of the requirements resulting from the application of the HTR. Subsequently those steps, which are required for the development of this component up to construction maturity are described. Simultaneously, questions dealing with material, construction, design, manufacture and related experimental development are taken into consideration.