Research and development on nuclear hydrogen production using HTGR at JAERI

Progress in Nuclear Energy; g o l . Available online at www.sciencedirect.com

47, N o . 1-4, pp. 4 9 6 - 5 0 3 , 2005 © 2 0 0 5 E l s e v i e r Ltd. A l l r i g h t s r e s e r v e d

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P r i n t e d in G r e a t B r i t a i n 0 1 4 9 - 1 9 7 0 / $ - s e e front m a t t e r

doi: 10.1016/j.pnueene.2005.05.050

RESEARCH AND DEVELOPMENT ON NUCLEAR HYDROGEN PRODUCTION USING HTGR AT JAERI

KAORU ONUKI, YOSHIYUKI INAGAKI, RYUTARO HINO, and YUKIO TACHIBANA Oarai Research Establishment, Japan Atomic Energy Research Institute, Niihori 3667, Narita-cho, Oarai-machi, Ibaraki-ken, 311-1394 Japan ABSTRACT JAERI has been conducting R&D on HTGR and on hydrogen production using HTGR The reactor technology has been developed using HTTR installed at Oarai site of JAERI. HTTR reached its full power operation of 30MW in 2001 and demonstrated reactor outlet helium temperature of 950°C in April 2004. As for the hydrogen production technology, the thermo-chemical IS process is under study. The process control method for continuous hydrogen production has been examined using a bench-scale apparatus. Also, studies are underway on process improvement and on materials of construction to be used in the corrosive environment. As for the system integration of HTGR and the hydrogen production plant, R&D is underway aiming to develop technologies for safe and economical connection. It covers safety technology against explosion, safety technology against radioactive materials release, control technology to prevent the thermal disturbance from hydrogen production plant to reactor, etc. © 2005 Elsevier Ltd. All rights reserved KEYWORDS Hydrogen production; Thermo-chemical cycles; IS process; HTGR 1. INTRODUCTION Hydrogen has ideal characteristics as secondary energy carrier. It can be stored, transported with lower loss compared with electricity, and used as fuel. If necessary, chemical energy of hydrogen can be

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converted to electrical energy by means of fuel cells. Since it can be produced from water and, after combustion, it returns to water, hydrogen is quite "clean" from the viewpoint of environmental effects. Therefore, the realization of a "hydrogen energy system" where hydrogen and electricity serve complementary as secondary energy carriers has been considered for a long time. Recently, the hydrogen energy attracts growing interest due to the emerging concern on global environmental issues such as the greenhouse effect, and also owing to the rapid development of one of the promising hydrogen utilization technologies, fuel cells. It is expected that the hydrogen demand will steeply increase within a few decades (Buenger, 2004; Chalk et al., 2004; Fujita, 2004). One of the key technical issues for realizing the hydrogen energy system is hydrogen production technology to meet the anticipated enormous hydrogen demand. As for the primary energy for the massive hydrogen production, nuclear energy is one of the most suitable energies in the sense of low carbon dioxide emission and high energy density. The High Temperature Gas-cooled Reactor (HTGR) is the most suitable nuclear reactor for producing the secondary energy owing to its capability of producing high temperature heat of close to 1000°C, which assures an efficient energy conversion. As for the hydrogen production method utilizing nuclear energy, the thermo-chemical cycles are promising candidates mainly because of their anticipated scale merit. Based on these backgrounds, Japan Atomic Energy Research Institute (JAERI) has been conducting research and development (R&D) on HTGR and on nuclear hydrogen production using HTGR and thermo-chemical processes, as well. In this paper, the present status of the R&D is briefly described focusing on that of the hydrogen production process. 2. OUTLINE OF ACTIVITY AT JAERI

Figure 1 shows the framework of JAERI's R&D on HTGR and hydrogen production. It consists of the reactor technology, the hydrogen production technology, and the system integration technology that concerns the technology for realizing the safe and economical connection between the nuclear reactor and the chemical plant (Iyoku, 2004). The reactor technology is in the demonstration stage. JAERI has been conducting R&D on HTGR technology since 1969 as shown in Fig. 2. In 1987, based on the fruits of R&D accumulated so far, construction of the High Temperature Engineering Test Reactor (HTTR: 30 MWth) was decided. In 1990, construction of HTTR started at the Oarai Research Establishment of JAERI. HTTR reached its first criticality in November 1998 and attained its full power operation with reactor outlet helium temperature of 850°C in December 2001. Since then, the rated power operation, the safety demonstration test, etc. has been carried out, and, in April 2004, HTTR attained the reactor outlet helium temperature of 950°C. The objective of R&D on the system integration technology is to develop technology for safe and economical connection between nuclear reactor and hydrogen production plant. It covers the safety technologies against explosion and radioactive materials release, the system control technology to prevent the propagation of thermal disturbances from the chemical reactor to the nuclear reactor, and the plant simulation code, etc.

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Reactor Technology

System Integration Technology

Hydrogen Production Technology

Fig. 1o R&D on HTGR and HTGR-hydrogen production at JAERI

As for the explosion, following works are underway: design for protection and mitigation against combustible gas release, estimation of damage on nuclear plant by blast waves from explosion, etc As for the radioactive materials release, the estimation of tritium permeation from the primary helium gas to the secondary helium gas through the heat-exchanging wall of the intermediate heat exchanger (IHX) has been carried out. Also, the high temperature isolation valve is under development, the key function

ITEM~ FY

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1987 . . . . 1990 1991 . . . . 1998 1997 1998 1999 2000 2001 2002 2003 2004

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Fig. 2. History of the HTTR project

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Fig. 3. R&D on the control technology of HTGR hydrogen production system of which is to separate the nuclear reactor and the heat utilization plant in case of emergency. Up to now, it was completed the design of valve structure to mitigate the thermal deformation, the development of new coating material for valve seat and rod, etc. The study on "control technology" aims to develop technology that assures that the operation of nuclear reactor shall not be affected by transient behavior of hydrogen production plant, such as fluctuations of helium temperature. For this purpose, it was devised to use steam generator as the thermal absorber to prevent the thermal disturbance propagation from the hydrogen production plant to the nuclear reactor (Fig. 3). A simulation test has been carried out to verify its effectiveness with a pilot scale test apparatus featuring representative hydrogen production process, steam reforming of methane, and an electrically heated helium loop. As for the hydrogen production technology, the thermo-chemical iodine-sulfur (IS) process is under investigation. 3. THERMO-CHEMICAL IS PROCESS

Figure 4 shows the reaction scheme of the IS process. In the process, the raw material, water, reacts with iodine and sulfur dioxide to produce hydrogen iodide and sulfuric acid (Bunsen reaction). The product acids are then thermally decomposed to produce hydrogen and oxygen. Here, the thermal decomposition reactions are endothermic reactions and the Bunsen reaction is exothermic. The free energy required for the water decomposition with the heat of temperature lower than 1000°C is produced by the cycle of chemical reactions. The IS process has been studied in USA, Europe and Japan since

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1970's, and important breakthroughs have been attained by General Atomics (GA), which includes the separation of hydrogen iodide and sulfuric acid by liquid-liquid phase separation (LL separation) that occurs in the presence of excess amount of iodine (Norman et al., 1982). One of the specific and important characteristics of thermo-chemical cycles is that the reactants except water are cyclically used in the process. The closed-cycle hydrogen production by IS process

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featuring the LL separation has been examined at JAERI in lab-scale and also in bench-scale experiments (Kubo et al., 2004). Fig. 5 shows the simplified flowsheet of the test apparatus. The chemistry of sulfuric acid decomposition and that of hydrogen iodide decomposition are rather straightforward in terms of reaction and separation. However, in the Bunsen reaction section, occurrence of side reactions forming elemental sulfur and/or hydrogen sulfide should be suppressed while maintaining the LL separation. Recently, hydrogen production of about 30NL-Hz/h was demonstrated for 1 week with the bench-scale experiments, owing to a newly devised process control method. From the viewpoint of efficient hydrogen production, there exists challenging issues, especially in the hydrogen iodide decomposition section. In order to reduce the thermal burden in the process, it is important to reduce extra amounts of water and iodine than the stoichiometric amounts to be recycled in the process. The "best (=most concentrated)" composition for the HIx solution (HI-Iz-H20 system) that can be obtained in the Bunsen reaction section has been reported by General Atomics (Norman et al, 1982). Fig. 6 shows the schematic vapor-liquid equilibrium (VLE) of HIx solution under constant mole fraction of iodine and the composition of the "best" HIx solution. It can be seen from the figure that there exists pseudo-azeotrope in terms of HI-H20 system in the VLE, and that, even the "best" solution, it is leaner than the azeotropic solution. Therefore, the complete separation of HI and H20 is impossible by usual equilibrium separation process. For example, conventional distillation results in the distillate of azeotropic hydriodic acid with large reboiler duty for the vaporization of water (Fig. 7). On this problem, the researchers at General Atomics proposed to use phosphoric acid to break the azeotrope (Norman et al., 1982). Later, in the middle of 1980's, researchers at RWTH Aachen proposed a reactive distillation,

with which the separation of HI from HIx solution and the decomposition of HI into H2 11

and I2 be carried out in one column (Roth and Knoche, 1989). In JAERI, we are pursuing another

option

that

utilizes

membrane

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separation (Kasahara et al., 2003). Additional thermal burden arises from the unfavorable reaction equilibrium of thermal decomposition of HI, which limits the attainable conversion ratio of HI to a low level of about 20%, and

x-HI

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(Fig.

7).

On

this

problem,

researchers at General Atomics proposed to carry out the reaction in the liquid phase with

Fig. 6. Vapor-Liquid Equilibrium (VLE) of HI-I2-H20 mixture at constant mole fraction of I2 (schematic).

e.g. homogeneous Pd catalyst (Norman et al., 1982). In JAERI, gas phase membrane reactor featuring hydrogen permselective membrane is under investigation (Kasahara et al., 2003). Another technical issue on the IS process is

The ordinate and the abscissa denote molar ratio of HI/(HI+H20) in vapor and in liquid, respectively. The arrow indicates the composition of HIx solution obtained in the Bunsen reaction section

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corrosion. Since sulfuric acid and halogen are very corrosive, selection of the structural materials is important. So far, screening tests have been carried out of the corrosion resistant materials in the representative

/ large thermal burden for the distillation of azeotropic hydriodic acid (HI/H20: 1/5)

process environments by GA, JAERI etc., and the results might be summarized as follows (Kubo et al., 2003). As for the

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show

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The

severest

Fig. 7. Barriers against efficient hydrogen production by IS process

environment is the boiling condition of concentrated sulfuric acid under high pressure (e.g. 2 MPa), where ceramic materials containing Si such as SiSiC, SiC, and Si3N4 are the only materials that show corrosion resistance. Therefore, special consideration is required in designing the equipment to be used in the boiling and condensing conditions of the acids. 4. CONCLUDING REMARKS

The realization of nuclear hydrogen production using HTGR will offer an important basis for the hydrogen energy system. Although the state-of-art is in a basic stage and there exist a lot of technical issues, the development of thermo-chemical processes is worth challenging. JAERI plans to complete the bench-scale study of the IS process and proceed to the next stage of R&D from the next year, in which the hydrogen production under prototypical conditions will be examined to verify its technical feasibility. ACKNOWLEDGMENT R&D on the system integration technology and the bench-scale hydrogen production test of the IS process have been carried out under the contact of research between JAERI and the Ministry of Education, Sports, Culture, Science and Technology of Japan.

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REFERENCES Buenger U. (2004), On the path to hydrogen energy in Europe - HyNet and HyWays, Proc. 15th World Hydrogen Energy Conf., Yokohama, Japan, 27 June - 2 July. Chalk S., et al. (2004), The United States' Freedom CAR and Hydrogen Fuel Initiative, Proc. 15th World Hydrogen Energy Conf., Yokohama, Japan, 27 June - 2 July. Fujita M. (2004), Japan's approach to commercialization of fuel cell / hydrogen technology, Proc. 15th World Hydrogen Energy Conf., Yokohama, Japan, 27 June - 2 July. Iyoku T. (2004), Present status of HTTR project, HTTR Workshop on Hydrogen Production Technology, Oarai, Japan, 5-6 July. Kasahara, S., et al. (2003), Effects of process parameters of the IS process on total thermal efficiency to produce hydrogen from water, J. Chem. Eng. Jpn., 36, 887. Kubo S., et al. (2003), Corrosion test on structural materials for Iodine-Sulfur thermochemical water splitting cycle, Proc. 2003 AIChE Spring National Meeting, New Orleans, USA, 30 March - 3 April. Kubo S., et al. (2004), R&D on Water splitting hydrogen production using HTGR at JAERI, Proc. 2004 AIChE Spring National Meeting, New Orleans, USA, 25-29 April. Norman J.H., et al. (1982), Thermochemical Water-Splitting Cycle, Bench-Scale Investigations and Process Engineering, GA-A 16713, General Atomics (GA). Roth, M. and Knoche, K.F. (1989), Thermochemical water splitting through direct HI-decomposition from HzO/HI/I2 solutions, Int. J. Hydrogen Energy, 14, 545.