Research and Development Program on HTTR Hydrogen Production System

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1 GENES4/ANP2003, Sep , 2003, Kyoto, JAPAN Paper 1062 Research and Development Program on HTTR Hydrogen Production System Yoshiyuki INAGAKI, Tetsuo NISHIHARA, Tetsuaki TAKEDA, Koji HAYASHI, Yoshitomo INABA and Hirofumi OHASHI Japan Atomic Energy Research Institute, Oarai, Ibaraki, , Japan Japan Atomic Energy Research Institute (JAERI) has been promoting R&D on the HTTR Hydrogen Production System (HTTR-H2) with a view to establishing and demonstrating the technology to couple a hydrogen production with a High Temperature gas-cooled Reactor (HTR). The coupling technology consists of control and safety-related ones. The control technology aims at preventing thermal mismatch between reactor and hydrogen production s. A simulation test with a mock-up model of the HTTR-H2 has been carried out to develop this technology. The safety-related technology includes countermeasures against explosion of combustible gas and tritium permeation from helium gas to process gas, and assurance of pressure boundary between the primary and secondary helium gases and that between secondary helium and process gases and a design and component tests are in progress. This report describes the R&D program of the HTTR-H2 and the present status. KEYWORDS: HTR, HTTR, Nuclear heat utilization, Hydrogen production, Coupling technology, Control technology, Safety-related technology I. Introduction Research and development (R&D) for clean, economical, stable, safe and abundant energy should be promoted from a viewpoint of technology as a potential measure to mitigate the global warming issue as well as for massive and stable energy supply and utilization. We have various options as alternative energy for fossil fuels: solar, geothermal, hydropower and nuclear energy and so on. While available natural energy is limited due to its stability, quality, quantity and density, it is sure that nuclear energy by High- Temperature gas-cooled Reactors (HTRs) has the potential to come up with a share as regards a satiable energy supply and utilization. Nuclear energy has been exclusively utilized for electric power generation, but the direct utilization of nuclear thermal energy is necessary and indispensable so that the energy efficiency can be increased and energy savings can be promoted in the near future. The hydrogen production is one of the key technologies for direct utilization of nuclear thermal energy. Japan Atomic Energy Research Institute (JAERI) has carried out the R&D on Hydrogen production High Temperature gas-cooled Reactor (H2HTR). There are three key technologies to commercialize H2HTR, that is, HTR technology, hydrogen production technology without CO 2 emission and coupling technology to connect a hydrogen production to the HTR. JAERI constructed the High Temperature Engineering Test Reactor (HTTR) to establish and upgrade the HTR technology 1). The full-power operation at thermal output of 30 MW and outlet coolant temperature of 850 o C was successfully achieved in December, The outlet coolant temperature will be * Corresponding author, Tel , Fax , inagaki@spa.oarai.jaeri.go.jp reached up to 950 o C in As for the hydrogen production technology without CO 2 emission, the R&D on the thermochemical water splitting method by IS process is on going 2,3). The R&D on the coupling technology has been carried out by the project of the HTTR Hydrogen Production System (HTTR-H2) including a design, simulation test with a mock-up model of the HTTR-H2 and some component tests as the contract research from Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan since The R&D program of the HTTR-H2 and the present status are reported in the following II. Feature of HTTR Hydrogen Production System The project of the HTTR Hydrogen Production System (HTTR-H2) aims at has been carried out to establish coupling technology which can be available to other hydrogen production s such as IS process, coal gasification and so on as well as the first demonstration of hydrogen production directly using heat supplied from HTR in the world. Figures 1 and 2 show a schematic flow diagram of the HTTR-H2 and detailed flow diagram of the hydrogen production downstream from the IHX, respectively 4). Main specifications of the HTTR-H2 are described in Table 1. The HTTR reactor supplies heat of 10MW with 950 o C to an intermediate heat exchanger (IHX) in the primary helium loop, and then the heat is transferred from the IHX to the secondary helium loop to be utilized for the hydrogen production by the steam reforming of methane; CH 4 +H 2 O=3H 2 +CO. It is the reason that the technology of the steam reforming matured in fossil-fired plant enables the coupling to the HTTR in the early 2000 s and technical solutions demonstrated can contribute to other hydrogen production s By a conceptual design, a steam generator (SG) is

2 installed downstream from a steam reformer (SR) in the secondary helium loop to mitigate fluctuation of helium temperature at SR outlet as mentioned later. A steam superheater which needs high temperature heat, helium cooler to adjust helium temperature at inlet of the IHX and isolation valves were also installed in the secondary helium loop. In fossil-fired plant, a SR has straight reaction tubes and the high temperature heat of process gas after steam reforming is used for steam generation. Since heat of helium is used for steam generation in HTTR-H2, the SR having bayonet-type reaction tubes was designed to reuse the high temperature heat of process gas for steam reforming as shown in Fig. 3. flows in a channel between guide and reaction tubes, and the process gas after passing downward in the catalyst layer flows upward in an inner tube heating process gas in the catalyst layer. Another advantage of the bayonet-type reaction tube is that it is not necessary to take a countermeasure against thermal expansion of the tube. The tube can be supported with only one tube plate, that is, one end of the tube is fixed at the tube plate and the other end is free. Wall thickness of the reaction tube should be designed considering both outer pressure of helium and inner pressure of process gas to assure the structural integrity in all conditions such as not only normal operation but also malfunction and accident. This design, however, makes the wall thickness too large. To realize the reasonable wall thickness, 10-mm level, the reaction tube should be designed considering the pressure difference between helium and process gas. Natural gas supplied from an underground LNG tank is mixed with steam and transported into the SR after passing through a raw gas preheater. The product gas is burned by a flare stack after passing through the raw gas preheater, water preheater, process gas cooler, separator and water drum. A nitrogen supply is installed to flow nitrogen in the SR instead of natural gas and steam at helium temperature below 700 o C at SR inlet in normal startup and shutdown operations. The process gas is controlled by a pressure valve installed downstream from the SR to keep the pressure difference of the reaction tube in the SR within an allowable value. Reactor containment vessel HTTR (30MW) 395 Core Pressurized water cooler (30MW) IHX (10MW) Hydrogen production Isolation valve Process gas (H 2, CO) Process gas (CH 4 ) circulator 585 Reaction tube reformer 880 Condenser generator Secondary helium superheater Feed water cooler Fig. 1 Schematic flow diagram of HTTR hydrogen production. Nitrogen supply LN 2 tank Evaporator Nitrogen compressor Flare stack Surge tank Surge tank Cutoff valve LNG tank Evaporator Desulfurizer Reactor containment vessel Raw gas supply (Intside) (Outside) Isolation valve reformer Pressure control valve Raw gas preheater Water preheater Product gas cooler Water drum Product gas combustion Coaxial pipe IHX circulator Filter Secondary helium loop Condenser superheater Separator Pressure control Purification cooler generator Water tank Deaerator Water tank Water supply Water purifier Fig. 2 Detailed flow diagram of hydrogen production downstream from IHX.

3 Table 1 Main design specifications of HTTR hydrogen production and mock-up model. Items HTTR-H2 Mock-up Pressure Process gas / helium Inlet temperature at steam reformer Process gas / helium gas Outlet temperature at steam reformer Process gas / helium 4.5 / 4.1 MPa 4.3 / 4.0 MPa 450 / 880 o C 580 / 585 o C 600 / 650 o C Natural gas feed 1296kg/h (81kmol/h) 43.2kg/h (2.7kmol/h) feed 8748kg/h kg/h -carbon ratio (S/C) Hydrogen product 4200Nm 3 /h 120Nm 3 /h Heat source Reactor (10MW) Electric heater (420kW) III. Coupling Technology The coupling technology consists of control and safety-related ones. The control technology aims at preventing thermal mismatch between reactor and hydrogen production s. The safety-related technology includes countermeasures against explosion of combustible gas and tritium permeation from helium gas to process gas, and assurance of pressure boundary between the primary and secondary helium and that between secondary helium and process gas. Prior to construction of the HTTR-H2, R&D has been carried out by FY2004 to solve the above technical problems as follows:. Tube plate Thermal insulator He flow pass Reaction tube Process gas Process gas He outlet gas 1. Control Technology The reactor and hydrogen production s are connected by the helium loop. A chemical reactor, namely SR, causes the temperature fluctuation of helium by the fluctuation of the reaction which can be occurred at normal start-up and shut-down operation as well as malfunction or accident of a process gas line. The reactor operation would be stopped by the temperature fluctuation. The SG was installed downstream from the SR in the secondary helium loop to mitigate the temperature fluctuation within 10 o C at the SG outlet, because the temperature rise above 15 o C compared with the normal temperature at the reactor inlet causes the HTTR reactor scram. The temperature fluctuation changes a generation rate of steam, but saturation temperature of water is constant in the SG. As the result, the helium temperature can be kept near at the saturation temperature. A simulation test with a mock-up model of the HTTR-H2 was planed to investigate performance of the SG for mitigation of the temperature fluctuation and transient behavior of the hydrogen production and to obtain experimental data for verification of a dynamic analysis code. The mock-up model test facility has an approximate hydrogen production capacity of 120Nm 3 /h and simulates Catalyst Guide tube Fig.3 Reaction tube tube Inner tube He flow pass He inlet Schematic view of steam reformer. key components downstream from the IHX. The SR has single full-scale reaction tube and an electric heater with 420kW is used as a heat source instead of the reactor in order to heat helium gas up to 880 o C at the SR inlet which is the same temperature as the HTTR-H2. Design specifications and a schematic flow diagram of the test facility are shown in Table 1 and Fig. 4, respectively 5,6). The fabrication of the test facility was completed in February, 2002 and the test is on going under conditions simulating normal start-up and shut-down operation and reaction fluctuation caused by malfunction or accident of the process gas line.

4 LN 2 tank LNG tank supply Water tank loop circulator Nitrogen supply Natural gas supply Condenser generator Electric heater superheater Flare stack Raw gas superheater Product gas combustion reformer Fig. 4 Schematic flow diagram of mock-up model Test facility. 2. Safety-related Technology (1) Countermeasure against explosion of combustible gas As for the countermeasure, the conceptual design has been carried out from a viewpoint of reduction of leakage amount by cutoff valves and coaxial pipe in the raw gas supply and product gas combustion s, mitigation of overpressure caused by explosion by separation of reactor and combustible gas components and so on. The coaxial pipe is composed of an inner pipe in which combustible gas flows and an outer pipe in which combustible gas flows. In the case of failure of the inner pipe, the outer pipe can protect leakage of combustible gas to atmosphere and the leakage can be easily detected. (2) Countermeasure against tritium permeation It is well known that hydrogen isotopes, hydrogen (H), deuterium (D), and tritium (T), permeate through solid metals. Thus, tritium produced in the core tends to permeate through heat transfer pipes of heat exchangers and catalyst pipes of a steam reformer in the hydrogen production. Further, it is probable that the tritium will mix with the product hydrogen. A component test was carried out to investigate the phenomenon of tritium permeation through the reaction tubes in the SR when a large amount of hydrogen is present in the reaction tubes 7). It is suppose that hydrogen and tritium will exists as molecule from a viewpoint of the dissociate energy of hydrogen molecule in the HTTR-H 2. For instance, the required energy to increase the temperature of hydrogen of 1 mole from 0 C to 1000 C is about 29 kj mol -1 and the dissociate energy of hydrogen is about 431 kj mol -1. Thus, excitation of molecular by an electromagnetic wave, electrolytic dissociation, and so on are necessary to dissociate hydrogen molecule. The thermal energy in the HTTR-H 2 is not really enough to dissociate hydrogen molecule. As far as hydrogen exists as molecule, it will dissociate and the H atom will adsorb on the reaction tube surface, and then H atoms will diffuse into the reaction tube. Therefore, the amount of permeated hydrogen in the HTTR-H 2 is limited by diffusion in the metal to be in proportion to the difference in square root of the hydrogen partial pressure, except for the case that the oxide film is formed on the surface. In the test, deuterium and hydrogen were used instead of tritium and hydrogen. Deuterium and hydrogen molecules will dissociate and the atoms will adsorb on the metal surface. The D and H atoms will then diffuse into the metal. Deuterium and hydrogen atoms diffuse in proportion to the concentration gradients of each. When these permeated atoms reach the opposite surface of the metal, both the D and H atoms will tend to recombine and desorb to the fluid phase. It is assumed that the rate of adsorption and desorption on the surface is faster than the rate of atomic diffusion in the metal and that the partial pressure of hydrogen is much higher than that of deuterium. As hydrogen adsorbs and desorbs repeatedly on the surface containing mostly H atoms, the D atoms permeating through the metal cannot reach the surface because most interstitial sites on the surface are occupied by H atoms. Thus, little deuterium can recombine and desorb to the fluid phase. From the results obtained in the experiment and analysis regarding the effectiveness of the hydrogen partial pressure on the amount of permeated deuterium, it is likely that the amount of permeated deuterium to counter permeated deuterium decreases with increasing hydrogen partial pressure. On the other hand, it also clear that the amount of permeated deuterium increases slightly when the deuterium partial pressure is near the hydrogen partial pressure. From this study, the amount of tritium permeating into the product hydrogen will be in allowable level without countermeasures against tritium permeation. (3) Assurance of pressure boundary between the primary and secondary helium The heat exchanger tube in the IHX, pressure boundary between the primary and secondary helium, is designed considering the pressure difference between the primary and secondary helium by the same reason as the reaction tube in the SR. The isolation valve is key component to assure the structure of the heat exchanger tube at an accident of rupture of the secondary helium loop. The HTIV used in the helium condition over 900 C, however, has been not made for practical use yet. JAERI has been conducting design and a component test on the HTIV. Figure 5 shows a schematic view of a high temperature isolation valve (HTIV) by the conceptual design. The structure was designed focusing on prevention of the valve seat deformation caused by thermal expansion. In addition to this, a new coating material was developed to keep face roughness of the seat within allowable level against open and close. A component test is in progress using a mock-up model on one-second scale.

5 tube, a component test has been carried out on characteristics of corrosion due to metal dusting and oxidation and strength reduction due to hydrogen embrittlement in the condition simulating the SR Fig. 5 Schematic view of high temperature isolation valve. (4) Assurance of pressure boundary between the secondary helium and process gas The reaction tube in the SR will be made of Hastelloy XR, which is a nickel-base, helium- corrosion and heat-resistance super alloy developed for the HTTR by JAERI, and its strength in high temperature is nearly equal to that of Alloy 800H. In order to design corrosion allowance of the reaction VI. Demonstration Test Plan of HTTR Hydrogen Production System Figure 6 shows a schedule of the HTTR-H2 project including the design, simulation and component tests, licensing, construction and demonstration test. The conceptual design, simulation test with the mock-up model and component tests will be completed by FY2004. The demonstration test of the HTTR-H2 will be separated into three items such as startup and shutdown, normal and abnormal operations aiming at demonstration of operability and controllability. Also verification of a dynamic simulation code including reactor and hydrogen production s will be carried out with obtained data. (1) First test: startup and shutdown The objective of the first test is demonstration of operation on normal startup and shutdown. The normal startup will be carried out in the following method. According to the helium temperature at SR inlet, nitrogen, steam and natural gas are supplied to the SR step by step. At first, nitrogen is only supplied up to 700 C of helium temperature at SR inlet in normal startup operation. and raw gas are gradually supplied with decrease of nitrogen feed over 700 C and feed of nitrogen is completely stopped in rated power operation. The normal shutdown operation has reverse step of the normal startup operation. The function of the SG to mitigate temperature fluctuation of helium is also demonstrated. (2) Second test: normal operation The objective of the second test is demonstration of long Fiscal Year Demonstration test 1 st 2 nd 3 rd Construction Conceptual design Simulation Test Component Tests - Tritium permeation - High temp. isolation valve - Reaction tube Safety Licensing Construction Coupling Construction Licensing Construction design Not founded Fig. 6 Time schedule of HTTR hydrogen production.

6 term hydrogen productivity in rated power operation and operability and controllability on reaction fluctuation at the SR against feed change of raw gas and steam. (3) Third test: abnormal operation The objective of the third test is demonstration on operability and controllability against reaction suspension at the SR simulating malfunction or accident of raw gas and/or steam supply s. From rated power operation condition, feed of raw gas and steam is cut off and nitrogen is supplied instead of raw gas and steam to keep the pressure difference of the reforming tube in the SR within allowable value. Since the cooling power of the SR and steam superheater is lost, helium is cooled by the SG only to maintain the reactor operation. At that time, generation rate of steam in the SG increases more than two times compared with that in rated power operation. is transported to the condenser installed above the SG and condensed water returns to the SG, that is, natural circulation of steam and condensed water occurs between the SG and condenser. V. Concluding Remarks In near future, a great deal of hydrogen will be necessary for fuel cells. The problem is how to supply energy to produce hydrogen without CO2 emission. The H2HTR can be considered as one of key technologies to solve the above problem. The HTTR-H2 is a very important milestone to commercialize the H2HTR from a viewpoint of demonstration of coupling technology and hydrogen production directly using thermal energy supplied from reactor. JAERI would like to contribute the solution of the environmental issue of CO 2 emission as well as a possible energy crisis which might happen in future. References 1) S. Saito, et al., "Design of High Temperature Engineering Test Reactor," JAERI-1332, Japan Atomic Energy Research Institute (JAERI), (1994). 2) S. Kubo, et al., Construction of Apparatus with Thermochemical Hydrogen Production Process, Proc. 11th Canadian Hydrogen Conf., Victoria, Canada (2001). 3) K. Onuki, et al., Thermochemical Hydrogen Production by Iodine-Sulfur Cycle, Proc. 14th World hydrogen Energy Conf., Montreal, Canada (2002). 4) T. Nishihara et al., A Conceptional Design Study on the Hydrogen Production Plant Coupled With the HTTR, Proc. 11th Int. Conf. on Nucl. Eng., Tokyo, Japan, April 20-23, 2003 ICONE (2003). 5) Y. Inagaki et al., Out-of-Pile Demonstration Test of Hydrogen Production System Coupling with HTTR, Proc. 7th Int. Conf. on Nucl. Eng., Tokyo, Japan, April 19-23, 1999, ICONE-7101 (1999). 6) H. Ohashi, et al., Performance Test Results of Mock-up Test Facility of HTTR Hydrogen Production System, Proc. 11th Int. Conf. on Nucl. Eng., Tokyo, Japan, April 20-23, 2003, ICONE (2003). 7) T. Takeda, et al., Study on Tritium/Hydrogen Permeation in the HTTR Hydrogen Production System, Proc. 7th Int. Conf. on Nucl. Eng., Tokyo, Japan, April 19-23, 1999, ICONE-7102 (1999).