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Hydrogen, nuclear energy, and the advanced high-temperature reactor
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International Journal of Hydrogen Energy 28 (2003) 1073 – 1081 www.elsevier.com/locate/ijhydene
Hydrogen, nuclear energy, and the advanced high-temperature reactor Charles W. Forsberg∗;1 Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6179, USA
Abstract Nuclear energy has been proposed as an energy source to produce hydrogen (H2 ) from water. An examination of systems issues in this paper indicates that the infrastructure of H2 consumption is now compatible with the production of H2 by nuclear reactors. Alternative H2 production processes were examined to de.ne the requirements such processes would impose on the nuclear reactor. These requirements include supplying heat at a near-constant high temperature, providing a low-pressure interface with the H2 production processes, isolating the nuclear plant from the chemical plant, and avoiding tritium contamination of the H2 product. A reactor concept—the advanced high-temperature reactor—was developed to match these requirements for H2 production. Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy. Keywords: Hydrogen; Nuclear energy; Advanced high-temperature reactor
1. Introduction The annual world consumption of H2 is ∼ 50 million tons [1], which is used primarily for ammonia production and conversion of heavier crude oils to clean liquid fuels [2]. The rapid growth in demand is a result of decreased availability of light crude oils that do not require extra H2 for conversion to gasoline, with a corresponding increased use of heavy crude oils that require massive amounts of H2 for conversion to gasoline. If the cost goals for automotive fuel cells are reached, the transportation sector may ultimately be fueled by H2 . This implies a growth in H2 consumption of one to two orders of magnitude over a period of several decades [3]. The current and projected H2 demands under either (1) a business-as-usual scenario or (2) a H2 transport economy are su9cient to justify massive investments in new methods to produce H2 . Because of these recent ∗ Corresponding author. Tel.: +1-865-574-6783; fax: +1-865-574-9512. E-mail address: [email protected] (C.W. Forsberg). 1 Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the US Department of Energy under contract DE-AC05-00OR22725.
changes, an examination of the use of nuclear energy to produce H2 was undertaken. The use of nuclear energy for H2 production raises three questions that are addressed herein. • Is nuclear energy compatible with H2 production? • What requirements does H2 production impose on the nuclear plant? • How should the reactor be designed?
2. Compatibility of nuclear energy with hydrogen production Each energy technology has a set of characteristics that determine what applications are potentially viable in terms of both technical feasibility and economics. For example, the characteristics of internal combustion engines (small size, high energy output per unit mass, etc.) make them suitable for automobiles. However, the high cost of liquid fuels makes such engines unsuitable for large-scale production of electricity. The viability of nuclear energy for H2 production depends upon the match between the intrinsic characteristics of H2 systems and nuclear energy systems. Four issues
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were examined: production scale, load factor, H2 transmission, and pipeline infrastructure. 2.1. Economics of scale Experience has demonstrated that nuclear energy production in small units on a small scale is not economically viable. If nuclear energy is to be used for economic H2 production, the H2 demand must match the scale of H2 production from a nuclear reactor. The rapid growth in H2 demand has resulted in the growth of the size of H2 production plants, pipelines, and associated facilities. The “world-class” H2 plants that are under construction [4] have production capacities of 200 million standard cubic feet per day. New plants have been announced with capacities of 300 million standard cubic feet per day (1200 MW of hydrogen energy based on the higher heating value). These plants use steam reforming of natural gas to produce H2 . A number of processes are being developed to produce H2 from water and high-temperature heat (see below). If such a process is 50% e9cient, ∼ 2400-MW(th) reactor would be required to produce 300 million standard cubic feet of H2 per day. In terms of energy Nows, the size of today’s H2 production plant is now equivalent to the size of a nuclear power plant. There is no longer a size mismatch between the scale of conventional H2 production facilities and nuclear energy facilities. 2.2. Demand versus time Nuclear power plants are characterized by high capital costs and low operating costs. The economics are strongly dependent upon maintaining base-load operations with continuous output. The characteristics of the H2 system decouple production from consumption [5]. Hydrogen transport is by pipeline, where packing (increasing the pressure) creates signi.cant storage capacity. Using the techniques developed by the natural gas pipeline industry, H2 storage in large volumes is expected to be relatively low cost. The production characteristics vs. time for H2 are compatible with nuclear energy. 2.3. Transmission Nuclear power plant sites are rare and expensive. The need for security, the advantages of using common facilities, and other factors encourage siting multiple reactors at each site. A large electrical transmission line carries about 2 GW. Large H2 pipelines, similar in size to the proposed Alaskan natural gas pipeline, would carry more than 20 GW. Transmission of large quantities of energy in the form of H2 in a few pipelines to urban areas is simpler than construction of large numbers of power lines. Hydrogen production is intrinsically more suitable than electricity for siting large numbers of reactors at a limited number of remote sites.
2.4. Infrastructure The economic viability of any energy system depends upon the delivered cost of energy. The costs include production, storage, and transportation. If one H2 production system has signi.cantly higher costs for transport or storage than another system, these system costs can determine the preferred methods of H2 production. For this reason, it is important to understand the system characteristics. Although insu9cient information exists for detailed analysis at the current time, it is possible to draw some general conclusions. The average long-term transport costs of H2 produced using nuclear energy will be less than those for H2 produced from natural gas. Large nuclear power stations are typically located a hundred to a few hundred kilometers from large urban areas. This de.nes the maximum necessary distance for H2 transport. Natural gas deposits are typically several thousand kilometers from large markets. Whether the natural gas is transported long distances and then converted to H2 or the natural gas is converted to H2 with transport of the H2 , the total transport system based on natural gas will have higher total energy transport costs and require signi.cantly longer pipelines. This same conclusion applies to any other energy source that is separated by large distances from the ultimate user. For a H2 economy based on nuclear energy, the total trunk pipeline system would be signi.cantly smaller and more secure than the current natural gas system. Pipelines would transport H2 over limited distances or serve to equalize demand over larger geographical areas. Newer technologies should enable the implementation of such a system within a relatively short period of time. Most of the population lives near the coasts [6] with 37% within 100 km of the coasts and 49% within 200 km of the coasts. Moreover, because of the availability of cooling water, many nuclear facilities are located on the coasts. The development of a H2 economy would likely see the construction of large H2 pipelines oQ each coast with smaller trunk lines into major urban centers and nuclear H2 production facilities. An increasing number of natural gas and oil pipelines are located oQshore on the seabed because of (1) the ease of .nding rights-of-way and (2) the advances in automated pipeline assembly aboard pipeline-laying ships. The same technology can be applied to H2 pipelines. 3. Hydrogen production requirements Nuclear energy provides a source of heat to produce H2 . Multiple processes are being investigated to produce H2 from water and heat. If nuclear energy is to be used for H2 production, the nuclear reactor must deliver the heat at conditions that match the requirements imposed by the H2 production process. At this stage of development, it is unclear which chemical processes will be the most economic;
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thus the major candidate technologies were examined to determine if they imposed similar requirements on the reactor. 3.1. Hydrogen production methods The viability of H2 production from nuclear power ultimately depends upon the economics, which, in turn, depend upon both the proposed methods of H2 production and the available reactors. Four methods have been proposed to produce H2 from nuclear power. 3.1.1. Electrolysis The electrolysis of water [5] to produce H2 is an old technology that is used today to produce ultrapure H2 and to produce H2 in small quantities at dispersed sites. Electrolysis is not currently competitive for the large-scale production of H2 , except where low-cost electricity is available. The long-term viability of electrolysis for large-scale H2 production depends upon the evolution of the electric grid, the capital costs of electrolysis, and other factors. Current capital costs [7] are estimated to be near $600/kW, with future capital costs that may approach $300/kW. Conventional alkaline electrolyzers have e9ciencies of 70 –85%, with proton-exchange-membrane electrolyzers projected to have e9ciencies of 80 –90%. There is a signi.cant tradeoQ between capital costs and e9ciency. In many industrialized countries, the peak electrical demand is twice the minimum demand. Consequently, low-cost oQ-peak electricity is available (e.g., in the middle of the night). Electrolysis may be viable provided there is successful development of e9cient, low-cost electrolysis systems and associated local H2 storage systems. 3.1.2. Steam reforming Today, H2 is produced primarily from the steam reforming of natural gas (net reaction: CH4 + 2H2 O → CO2 + 4H2 ). Steam reforming is an energy-intensive endothermic low-pressure process requiring high-temperature heat input. The natural gas [1] is (1) used as the reduced chemical source of H2 and (2) burned to produce heat to drive the process at temperatures of up to 900◦ C. The amount of natural gas required for steam reforming can be signi.cantly reduced when heat is provided by a nuclear reactor. The Japan Atomic Energy Research Institute [8,9] is currently preparing to demonstrate the production of H2 by steam reforming of natural gas with the heat input provided by its High-Temperature Engineering Test Reactor (HTTR). The nuclear power plant provides heat that replaces heat from a gas Name. Because this system uses standard H2 production technology, it represents the near-term nuclear-H2 technology. Only nuclear reactor issues must be addressed. For Japan and other countries with high-cost natural gas, economic analysis [8] indicates that H2 from nuclear-assisted steam reforming of natural gas will have lower costs than H2 from natural gas alone. This is the basis of the near-term Japanese development program for
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H2 production. The economics would be expected to apply to any country where a signi.cant fraction of the natural gas is supplied in the form of lique.ed natural gas. 3.1.3. Hot electrolysis Electrolysis [10–12] can be operated at high temperatures (700 –900◦ C) and low pressures to replace some of the electrical input with thermal energy. Because heat is cheaper than electricity, the H2 costs via this production method could ultimately be lower than those for traditional electrolysis. Equally important, the high temperature results in better chemical kinetics within the electrolyzer that reduces (1) equipment size and (2) ine9ciencies. However, the technology [7] is in an early state of development although it derives much of its technology from solid-oxide fuel cells. Hot electrolysis requires collocation of H2 production with the nuclear reactor to provide the heat. 3.1.4. Thermochemical hydrogen production Hydrogen can be produced by direct thermochemical processes [9,13] in which the net reaction is heat plus water yields H2 and oxygen. These are the leading long-term options for production of H2 using nuclear energy. For low production costs, however, high temperatures (¿ 750◦ C) are required to ensure rapid chemical kinetics (i.e., small plant size with low capital costs) and high conversion e9ciencies. Many types of thermochemical processes for H2 production exist. The sulfuric acid processes (hydrogen sul.de, iodine–sulfur, sulfuric acid–methanol) and the Br–Ca–Fe cycle are currently the leading candidates. In the sulfuric acid processes, the high-temperature, low-pressure endothermic (heat-absorbing) reaction is the thermal decomposition of sulfuric acid to produce oxygen: H2 SO4 → H2 O + SO2 + 1=2O2 (high temperature):
(1)
◦
Typically temperatures in the range of 800 C are needed for e9cient H2 production. The high-temperature steps operate at low pressures. After oxygen separation, additional chemical reactions are required to produce H2 . The leading candidate for thermochemical H2 generation is the iodine– sulfur process (Fig. 1), which has two additional chemical reactions: I2 + SO2 + 2H2 O → 2HI + H2 SO4 (low temperature) (2) and the H2 -producing step 2HI → H2 + I2 (intermediate temperature):
(3)
Of the advanced methods for hydrogen generation using nuclear power, thermochemical cycles have received the most attention because current estimates [8] indicate that thermochemical H2 production costs could be as low as 60% of those from room-temperature electrolysis. These Japanese estimates assume dedicated nuclear reactors for the thermochemical processes (no electricity production) and electricity from the electrical grid for room-temperature electrolysis. The cost advantage is a consequence of process e9ciencies.
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C.W. Forsberg / International Journal of Hydrogen Energy 28 (2003) 1073 – 1081 ORNL DWG 2001-102R
Water
Oxygen
Hydrogen 2H2O
O2
H2 SO2
I2
Heat 800-1000°C
I2 + SO2 + 2H2O
H2SO4
2HI
2HI + H2SO4
H2O + SO2 + ½O2
H2SO4
H2+I2
HI
Fig. 1. Iodine–sulfur process for thermochemical production of H2 .
Production e9ciency can be de.ned as the energy content of the resulting H2 divided by the energy expended to produce the H2 . Although H2 production by electrolysis [5] can be relatively e9cient (∼ 80%), there is a signi.cant tradeoQ between capital cost and e9ciency with this technology. However, when this factor is combined with the electrical conversion e9ciency, which ranges from approximately 34% (in current light-water reactors) to 50% (for advanced systems), the overall heat-to-H2 e9ciency would be approximately 25 – 40%. For thermochemical approaches such as the sulfur–iodine process described previously, an overall e9ciency of ¿ 50% has been projected. Combined-cycle (H2 and electricity) plants may have e9ciencies of ∼ 60%. The diQerences in e9ciency reNect the penalties of converting thermal energy to electricity and then to chemical energy (H2 ) vs. converting thermal energy directly to chemical energy (H2 ). Parallel to the e9ciency considerations, the capital costs for thermochemical processes are expected to be lower than for room-temperature electrolysis because less equipment is required. However, large-scale pilot plants will be required before de.nitive cost .gures will be available. The sulfur–iodine process is being developed in Japan with the ultimate goal of connecting it to the HTTR. Research on this process is also under way in the United States. Thermochemical process will require the development of better high-temperature materials of construction for the corrosive environments. Signi.cant development work on H2 thermochemical cycles is required, with the technology being applicable to both nuclear and solar-power tower heat sources. 3.2. Requirements The process requirements for H2 production via nuclear steam reforming of methane, hot electrolysis, and
thermochemical cycles are similar. All three technologies impose similar requirements on the nuclear reactor. • Reactor power. Reactor powers in the range of typical nuclear applications (100 –1000 MW(e) match well with the scale of H2 production facilities. Economic considerations for speci.c applications will determine actual plant size. • Peak temperature. All the low-cost methods require high temperatures (750 –900◦ C) • Temperature range of delivered heat. All of the endothermic high-temperature chemical reactions are dissociation reactions that operate at nearly a constant temperature. Heat should therefore be delivered over a small temperature range. • Pressure. The chemical reactions go to completion at low pressures. High pressures reverse the desired chemical reactions. The H2 -nuclear interface should be at low pressure to (1) minimize the risk of pressurization of the chemical plant and (2) minimize high-temperature materials strength requirements. • Isolation. The nuclear and chemical facilities should be isolated from each other so that upsets in one facility do not impact the other. The system must also minimize tritium (radioactive hydrogen) production and transport from the reactor to the H2 production facility.
4. The advanced high-temperature reactor There are two approaches to developing a nuclear reactor for H2 production. An existing reactor system can be modi.ed to meet the H2 production requirements, or a new reactor system can be developed. At the current time, only one
C.W. Forsberg / International Journal of Hydrogen Energy 28 (2003) 1073 – 1081
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02-080
Passive Decay Heat Removal
Reactor
Hot Air Out
Hydrogen Production
750° to1000°C Control Rods
Air Inlet
Nuclear-assisted steam reforming of natural gas Hot electrolysis using water Thermochemical using water
Fuel (Coated-particle Graphite-matrix)
Cooling Water
Reactor Vessel Guard Vessel
Fig. 2. Advanced high-temperature reactor.
nuclear reactor system, the gas-cooled (helium) reactor, has the high-temperature capabilities to provide the heat at suf.cient temperatures to drive a H2 production system. This reactor has historically [8] been considered the one reactor that would be used to provide high-temperature heat for H2 production. The gas-cooled (helium) reactor was developed for electricity production and uses a coated-particle fuel (see below) and high-pressure helium as a coolant. Several prototype reactors have been built in systems where the reactor was connected to a steam power plant to produce electricity. An advanced version of the reactor is being developed for electricity production using a closed Brayton helium cycle where the helium coolant directly drives a helium gas turbine to produce electricity. Alternatively, a reactor can be designed speci.cally for H2 production. The Advanced High-Temperature Reactor (AHTR) has been proposed [14] to match H2 production requirements. The AHTR is described herein, as well as the basis for selection of particular technical characteristics of the reactor. This discussion provides a basis for understanding some of the key interface issues between any type of nuclear plant and a H2 production plant. The AHTR is based on several earlier technological developments: • High-temperature, low-pressure molten-salt reactor coolants from the aircraft nuclear propulsion program [15] of the 1950s and the molten-salt breeder reactor program [16] of the 1960s • Coated-particle graphite-matrix fuel developed in the 1970s for gas-cooled reactors • Passive safety systems for gas-cooled and liquid-metal reactors developed in the 1980s
4.1. Concept description The AHTR reactor core consists of coated-particle graphite-matrix fuel cooled with a molten Nuoride salt. The molten salt (Fig. 2) Nows through the reactor core to an external heat exchanger (to provide the interface for the H2 production system), dumps the heat load, and returns to the reactor core. The molten salt can be circulated by natural or forced circulation. The fuel (Fig. 3) is essentially the same as that used for the gas-cooled (helium) reactor. The important characteristic of these fuels is that they can operate at very high temperatures with peak fuel operating temperatures of ∼ 1200◦ C. Significant fuel failure does not occur below 1600◦ C. These fuels are the only demonstrated nuclear fuels capable of producing heat at temperatures su9cient for H2 production. The fuel consists of small particulates of uranium dioxide coated with layers of carbon and silicon carbide. The multiple layers isolate the fuel and .ssion products (produced by the nuclear reactions) from the coolant. The microspheres are embedded in a compact made of graphite. The fuel compact is embedded in graphite blocks. The hexagonal blocks are assembled into a reactor core. Fig. 3 shows the speci.c design of the Japanese HTTR fuel, a helium-cooled reactor with a helium exit temperature of up to 950◦ C. The AHTR fuel would be similar. Molten Nuoride salts are the only high-temperature liquids that have been fully demonstrated to be chemically compatible with graphite fuels [17]. The atmospheric boiling points for molten Nuoride salts are near 1400◦ C. This results in a reactor system that operates at low pressures and matches the low pressures of H2 production systems. There is a century of industrial experience with the
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Fuel Kernel High Density PyC SiC Low Density PyC
Coated Fuel Particle
Fuel Rod Annular Coolant Channel
Fuel Handling Hole Dowel Pin
Plug Fuel Compact
Graphite Block
Graphite Sleeve
8 mm 0.92 mm
580 mm
39 mm
34 mm
Dowel Socket
26 mm
Fuel Compact
360 m m
Fuel Rod
Fuel Block
Fig. 3. Coated-particle graphite-matrix fuel (high-temperature engineering test reactor fuel).
compatibility of graphite and molten Nuoride salt— aluminum is electrolytically produced from cryolite (3NaF– AlF3 ) in very large graphite baths at ∼ 1000◦ C. Molten salts are leading candidates for cooling the .rst wall of fusion reactors [18]. At operating conditions, molten-salt properties are similar to those of water. Although they do not react with air or carbon dioxide, molten salts will slowly react with water. The physics of the AHTR reactor core, the general core design, and the fuel cycle are similar to those of the proposed General Atomics gas-turbine modular helium reactor [19] (GT-MHR). The low-power-density graphite-moderated core also has the long neutron lifetime, slow kinetics, and thermal neutron spectrum characteristic of the proposed GT-MHR. There is reactor experience with molten Nuoride salts. The Aircraft Reactor Experiment, a 2.5-MW(th) reactor, operated in the 1950s with a NaF=ZrF4 molten salt at 815◦ C, while the Molten Salt Reactor Experiment, an 8-MW(th) reactor, operated in the 1960s with a 7 LiF=BeF2 molten salt. The Aircraft Reactor Experiment was part of the large Aircraft Nuclear Propulsion Program [15], which had as its goal a nuclear-powered military jet aircraft with unlimited range. The reactor would produce high-temperature heat to heat the air going through the jet engines. A low-pressure system was required to avoid the weight and technical dif.culties associated with the use of a high-pressure reactor coolant. While nuclear-powered jet aircraft are impractical because of the weight of radiation shielding and risk of accidents, the reactor requirements—that is, to produce high-temperature heat at low pressure—match the requirements for production of H2 . This earlier work provides
much of the technical basis for an advanced reactor for H2 production. In these early reactors, the fuel was dissolved in the Nuoride salt. However, the AHTR uses solid fuel with a clean molten salt. The use of solid fuel, which contains the radioactivity, provides the .rst of several barriers to separate the radioactivity in the reactor from the H2 production facilities. 4.2. Matching reactor characteristics to hydrogen plant characteristics For H2 production, the critical requirement is to deliver the heat from the reactor core to the H2 thermochemical system under appropriate conditions. DiQerent characteristics of the reactor are used to meet these conditions. 4.2.1. Delivering heat to match H2 production requirements All the nuclear H2 production methods require large quantities of high-temperature heat at near-constant temperatures to drive the desired chemical disassociation reactions. From an engineering perspective, these are very high temperatures. Peak temperatures should be minimized to reduce thermal and pressure stresses. This can be accomplished by using a liquid reactor coolant. Liquid coolants have good heat transfer capabilities and low pumping power costs in comparison with gas coolants. As a direct consequence, liquid-cooled reactors can deliver most of their heat at near-constant temperatures while gas-cooled reactors generally deliver their heat over a wide range of temperatures to reduce pumping power costs. Some examples (Table 1, Fig. 4) demonstrate
C.W. Forsberg / International Journal of Hydrogen Energy 28 (2003) 1073 – 1081
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Table 1 Temperature drops for diQerent reactor coolants System
ST Inlet to outlet (◦ C)
Inlet T (◦ C)
Outlet T (◦ C)
Coolant
GT-MHR AGR (Hinkely) PWR (Point Beach) LMR (Super Phenix)
359 355 20 150
491 310 299 395
850 665 319 545
Gas (helium) Gas (CO2 ) Liquid (water) Liquid (sodium)
01-050
1000
Temperature (°C)
800
AHTR
1000°C 850°C
925°C
750°C AHTR
665°C
600
675°C 545°C
GT-MHR (General Atomics)
LMFBR
400
(Super Phenix)
AGR
319°C
310°C
(Hinkley Point B)
PWR
200 0 Inlet
491°C 395°C
299°C
(Point Beach)
Liquid Gas
Outlet Delivered Heat
Fig. 4. Temperatures of delivered heat from diQerent types of reactors.
these diQerences between gas-cooled and liquid-cooled nuclear reactors. The gas-cooled GT-MHR (General Atomics) has a ST across the reactor core of 359◦ C, while the Advanced Gas-Cooled Reactor (Hinkley Point B) has a ST of 355◦ C. Liquid-cooled reactors typically have muchsmaller temperature increases across the reactor core. The Point Beach pressurized-water reactor has a ST across the reactor core of 20◦ C, while a liquid-metal fast reactor (Super Phenix) has a ST of 150◦ C. The AHTR, as a liquid-cooled reactor, can deliver its heat with small temperature drops (20 –100◦ C) with low pumping power. If heat is needed at 750◦ C, the maximum temperature of the gas coolant in a gas-cooled reactor may exceed 1000◦ C whereas that of the liquid coolant in a liquid-cooled reactor will not exceed 850◦ C. Liquid coolants are preferred because they minimize materials requirements by lowering the peak temperatures and avoiding the need to withstand high stresses from a pressurized gas coolant. 4.2.2. Pressure The H2 production facility will contain signi.cant inventories of hazardous chemicals. A low-pressure, non-chemically reactive coolant minimizes safety risks by minimizing the consequence of heat-exchanger failures between the chemical and nuclear facilities. High-pressure reactor coolants create the potential for pressurization of
the chemical plant and releases of toxic gases. At high temperatures, high-pressure coolants also place much greater demands on the materials of construction. Low pressure liquid coolants can match the low pressures of the hydrogen production systems. Molten Nuoride salts have boiling points near 1400◦ C (and thus avoid the potential for chemical plant pressurization), do not react with air, and only slowly react with water. 4.2.3. Isolation The nuclear facility will be separated from the chemical plant by some distance. Because a high-heat-capacity low-pressure molten-salt coolant is used, the AHTR heat losses between the two facilities will be minimal. German chemical industry studies indicated that molten salts (low pressure, high heat capacity, low pumping costs) would be the preferred method to transfer heat from a reactor to a high-temperature chemical system. Molten salts are traditionally used in the chemical industry to transfer heat at high temperatures. It is unclear at this time whether an intermediate molten-salt heat transfer loop would be required to provide more isolation between the nuclear and chemical facilities. Nuclear reactors generate tritium (3 H), the radioactive form of hydrogen. Hydrogen can diQuse through hot metals; thus, the reactor design must be chosen to minimize the
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amount of tritium entering the H2 production facility. The reactor can be designed to minimize tritium production, tritium can be trapped in the coolant, or heat exchangers can be specially designed to minimize tritium transport. Minimizing the tritium inventory is likely to be the preferred strategy if a reactor is designed speci.cally for H2 production. For reactors that produce electricity, tritium is not a major issue because the tritium can be trapped in the power cycle. Studies of high-temperature gas-cooled (helium) reactors [20] indicated three sources of tritium production in these reactors: neutron bombardment of 3 He in the coolant, neutron bombardment of the 6 Li impurity in the graphite fuel, and ternary .ssion in the fuel. Tritium generation in the coolant can be minimized by choosing a molten Nuoride salt coolant such as NaF=ZrF4 that does not produce tritium. High-temperature bake-out of the graphite during the manufacturing process can be used to minimize the lithium impurities in the graphite. Appropriate fuel design can minimize the tritium that is released by the fuel. While early coated-particle fuels used two (biso) coatings, newer fuels use three coatings (triso). There has been signi.cant testing and a rapidly developing understanding of what is required to prevent the release of tritium from components within coated-particle fuels [21]. Coated-particulate fuels with three coatings will be required for tritium control. 4.3. Safety systems If a reactor shuts down, heat continues to be generated from the decay of short-lived radionuclides in the fuel. The decay heat decreases with time. If a method to remove decay heat is not provided, the reactor core will overheat, with potential damage to the core—such as occurred at the Three Mile Island reactor. Several passive safety options are available to ensure that overheating of the reactor core will not occur. A unique feature of a H2 production reactor is that the high temperatures required for H2 production also enable the use of more eQective safety systems in large reactors whose operations bene.t from the higher temperatures. For the AHTR, several options exist to ensure reactor safety against overheating. One option (Fig. 2) is a pool-type reactor with passive safety, similar to the proposed General Electric S-PRISM liquid-metal-cooled reactor [22,23]. The size of the S-PRISM reactor is limited by passive decay-heat cooling to ∼ 1000 MW(th). In this pool reactor, decay heat is conducted through the reactor vessel wall, transferred across an argon gap by radiation to a guard vessel, conducted through the guard vessel, and then removed from the second wall by natural circulation of air. The radiation heat transfer from the reactor vessel to the guard vessel increases by T 4 ; thus, a small rise in the reactor vessel temperature greatly increases heat transfer out of the system. The argon gap acts as a thermal switch to limit heat losses during normal operation but allows radiation heat transfer to increase heat losses if the reactor overheats.
If the same type of passive cooling system is applied to the AHTR (Fig. 2), the reactor size limits could potentially exceed 2000 MW(th)—matching the energy requirements for a world-class H2 production facility. The AHTR operates 200 –500◦ C hotter than the S-PRISM (500 –550◦ C for the S-PRISM versus 750 –1000 +◦ C for the AHTR). Because natural circulation of cooling air increases with temperature and because heat transfer across the argon gap varies with T 4 , the higher temperatures allow for more e9cient removal of decay heat, with heat removal rates adjusted by design of the decay-heat removal system. 5. Conclusions The viability of H2 production using nuclear energy involves infrastructure-, process-, and reactor-speci.c issues. In recent years, the size of conventional H2 production facilities using steam reforming of natural gas has grown. The size of H2 production units is now compatible with the scale of nuclear operations. Equally important, the characteristics of nuclear energy systems are compatible with proposed global H2 systems. In some areas, such as H2 transmission, a nuclear-H2 system has major advantages in that it minimizes the size of the H2 pipeline systems compared with systems that produce H2 using natural gas or other energy sources far from the large urban markets. Several methods to produce H2 using high temperature heat are under development. Signi.cant development work is required before any of these chemical processes can be commercialized. The potentially viable H2 production methods impose a set of di9cult requirements on the reactor, including delivery of heat at a near-constant high temperature, isolation of the reactor from the H2 production facility, and low-pressure-coolant interfaces between the reactor and the H2 production facility. Current reactor technology could be modi.ed to meet these requirements, but the severity of the requirements suggests that the nuclear reactor should be designed to match chemical plant requirements. One reactor, the AHTR, with the potential capabilities to match H2 production requirements has been identi.ed. Signi.cant development will be required of both the H2 production methods and the corresponding reactor systems. References [1] Stoll RE, Von Linde F. Hydrogen—what are the costs? Hydrocarbon Process 2000;79(12):42–6. [2] Forsberg CW, Peddicord KL. Hydrogen production as a major nuclear energy application. Nucl News 2001;44(10):41. [3] Forsberg CW. Hydrogen, electricity, and nuclear power. Nucl News 2002;45(10):30–1. [4] Parkinson G. The utility of hydrogen. Chem Eng 2001;108(10):29–37. [5] Ogden JM. Prospects for building a hydrogen energy infrastructure. Annu Rev Energy Environ 1999;24:227–79.
C.W. Forsberg / International Journal of Hydrogen Energy 28 (2003) 1073 – 1081 [6] Cohen JE, et al. Estimates of coastal populations. Science 1997;278:1211–2. [7] She9eld J et al. A study of options for the development of large fusion power plants, JIEE2000-06. Knoxville, TN: Joint Institute for Energy and Environment, University of Tennessee, June 2000. [8] OECD-NEA Nuclear Science Committee. First International Exchange Meeting on Nuclear Production of Hydrogen, Paris, France, October 2–3, 2000. [9] Japan Atomic Energy Research Institute. Present status of HTGR research and development. Oarai-machi, Japan: Japan Atomic Energy Research Institute, March 2000. [10] Dutta S. Technology assessment of advanced electrolytic hydrogen production. Int J Hydrogen Energy 1990;15(6): 379–86. [11] Quandt KH, Streicher R. Concept and design of a 3:5 MW pilot plant for high temperature electrolysis. Int J Hydrogen Energy 1986;11(5):309–15. [12] Hino R et al. Study on hydrogen production by high temperature electrolysis of steam, 97-064. Oarai-machi, Japan: Japan Atomic Energy Research Institute, September 1997. [13] Brown CL, Funk JF, Showalter SK. High e9ciency generation of hydrogen fuels using nuclear power, GA-A23451. San Diego, CA: General Atomics Corp., July 2000. [14] Forsberg CW, Pickard PS, Peterson PF. Molten-salt-cooled advanced high-temperature reactor for production of hydrogen and electricity. Nucl Technol, in press.
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[15] Fraas AP, Savolainen AW. Design report on the aircraft reactor test. ORNL-2095 Oak Ridge National Laboratory, December 7, 1956. [16] Nucl. Appl. Technol. 1970; 8(2) [Entire issue] [17] Grimes WR. Molten salt reactor chemistry. Nucl Appl Technol 1970;8(2):137–55. [18] Sagara A, et al. Design and development of the FLIBE blanket for helical-type fusion reactor FFHR. Fusion Eng Des 2000;49 –50:661–6. [19] LaBar MP. The gas turbine—modular helium reactor: a promising option for near term deployment. International Congress on Advanced Nuclear Power Plants, Hollywood, FL, June 9 –13, 2002, GA-A23952. San Diego, CA: General Atomics Corp., 2002. [20] Wichner RP, Dyer FF. Distribution and transport of tritium in the Peach Bottom HTGR, ORNL-5497. Oak Ridge National Laboratory, Oak Ridge, TN, 1979. [21] Petti DA, Interim assessment of tritium release database, EDF: NPR-MHTGR-0412. Idaho National Engineering Laboratory, Idaho Falls, ID, 1992. [22] Boardman CE et al. Optimizing the size of the super-prism reactor, ICONE8003. Proceedings of the Eighth International Conference on Nuclear Engineering, Baltimore, MD, April 2– 6, 2000. [23] Boardman CE et al. A description of the S-PRISM plant, ICONE-8168. Proceedings of the Eighth International Conference on Nuclear Engineering, Baltimore, MD, April 2– 6, 2000.
International Journal of Hydrogen Energy 28 (2003) 1073 – 1081 www.elsevier.com/locate/ijhydene
Hydrogen, nuclear energy, and the advanced high-temperature reactor Charles W. Forsberg∗;1 Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6179, USA
Abstract Nuclear energy has been proposed as an energy source to produce hydrogen (H2 ) from water. An examination of systems issues in this paper indicates that the infrastructure of H2 consumption is now compatible with the production of H2 by nuclear reactors. Alternative H2 production processes were examined to de.ne the requirements such processes would impose on the nuclear reactor. These requirements include supplying heat at a near-constant high temperature, providing a low-pressure interface with the H2 production processes, isolating the nuclear plant from the chemical plant, and avoiding tritium contamination of the H2 product. A reactor concept—the advanced high-temperature reactor—was developed to match these requirements for H2 production. Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy. Keywords: Hydrogen; Nuclear energy; Advanced high-temperature reactor
1. Introduction The annual world consumption of H2 is ∼ 50 million tons [1], which is used primarily for ammonia production and conversion of heavier crude oils to clean liquid fuels [2]. The rapid growth in demand is a result of decreased availability of light crude oils that do not require extra H2 for conversion to gasoline, with a corresponding increased use of heavy crude oils that require massive amounts of H2 for conversion to gasoline. If the cost goals for automotive fuel cells are reached, the transportation sector may ultimately be fueled by H2 . This implies a growth in H2 consumption of one to two orders of magnitude over a period of several decades [3]. The current and projected H2 demands under either (1) a business-as-usual scenario or (2) a H2 transport economy are su9cient to justify massive investments in new methods to produce H2 . Because of these recent ∗ Corresponding author. Tel.: +1-865-574-6783; fax: +1-865-574-9512. E-mail address: [email protected] (C.W. Forsberg). 1 Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the US Department of Energy under contract DE-AC05-00OR22725.
changes, an examination of the use of nuclear energy to produce H2 was undertaken. The use of nuclear energy for H2 production raises three questions that are addressed herein. • Is nuclear energy compatible with H2 production? • What requirements does H2 production impose on the nuclear plant? • How should the reactor be designed?
2. Compatibility of nuclear energy with hydrogen production Each energy technology has a set of characteristics that determine what applications are potentially viable in terms of both technical feasibility and economics. For example, the characteristics of internal combustion engines (small size, high energy output per unit mass, etc.) make them suitable for automobiles. However, the high cost of liquid fuels makes such engines unsuitable for large-scale production of electricity. The viability of nuclear energy for H2 production depends upon the match between the intrinsic characteristics of H2 systems and nuclear energy systems. Four issues
0360-3199/03/$ 30.00 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy. PII: S 0 3 6 0 - 3 1 9 9 ( 0 2 ) 0 0 2 3 2 - X
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were examined: production scale, load factor, H2 transmission, and pipeline infrastructure. 2.1. Economics of scale Experience has demonstrated that nuclear energy production in small units on a small scale is not economically viable. If nuclear energy is to be used for economic H2 production, the H2 demand must match the scale of H2 production from a nuclear reactor. The rapid growth in H2 demand has resulted in the growth of the size of H2 production plants, pipelines, and associated facilities. The “world-class” H2 plants that are under construction [4] have production capacities of 200 million standard cubic feet per day. New plants have been announced with capacities of 300 million standard cubic feet per day (1200 MW of hydrogen energy based on the higher heating value). These plants use steam reforming of natural gas to produce H2 . A number of processes are being developed to produce H2 from water and high-temperature heat (see below). If such a process is 50% e9cient, ∼ 2400-MW(th) reactor would be required to produce 300 million standard cubic feet of H2 per day. In terms of energy Nows, the size of today’s H2 production plant is now equivalent to the size of a nuclear power plant. There is no longer a size mismatch between the scale of conventional H2 production facilities and nuclear energy facilities. 2.2. Demand versus time Nuclear power plants are characterized by high capital costs and low operating costs. The economics are strongly dependent upon maintaining base-load operations with continuous output. The characteristics of the H2 system decouple production from consumption [5]. Hydrogen transport is by pipeline, where packing (increasing the pressure) creates signi.cant storage capacity. Using the techniques developed by the natural gas pipeline industry, H2 storage in large volumes is expected to be relatively low cost. The production characteristics vs. time for H2 are compatible with nuclear energy. 2.3. Transmission Nuclear power plant sites are rare and expensive. The need for security, the advantages of using common facilities, and other factors encourage siting multiple reactors at each site. A large electrical transmission line carries about 2 GW. Large H2 pipelines, similar in size to the proposed Alaskan natural gas pipeline, would carry more than 20 GW. Transmission of large quantities of energy in the form of H2 in a few pipelines to urban areas is simpler than construction of large numbers of power lines. Hydrogen production is intrinsically more suitable than electricity for siting large numbers of reactors at a limited number of remote sites.
2.4. Infrastructure The economic viability of any energy system depends upon the delivered cost of energy. The costs include production, storage, and transportation. If one H2 production system has signi.cantly higher costs for transport or storage than another system, these system costs can determine the preferred methods of H2 production. For this reason, it is important to understand the system characteristics. Although insu9cient information exists for detailed analysis at the current time, it is possible to draw some general conclusions. The average long-term transport costs of H2 produced using nuclear energy will be less than those for H2 produced from natural gas. Large nuclear power stations are typically located a hundred to a few hundred kilometers from large urban areas. This de.nes the maximum necessary distance for H2 transport. Natural gas deposits are typically several thousand kilometers from large markets. Whether the natural gas is transported long distances and then converted to H2 or the natural gas is converted to H2 with transport of the H2 , the total transport system based on natural gas will have higher total energy transport costs and require signi.cantly longer pipelines. This same conclusion applies to any other energy source that is separated by large distances from the ultimate user. For a H2 economy based on nuclear energy, the total trunk pipeline system would be signi.cantly smaller and more secure than the current natural gas system. Pipelines would transport H2 over limited distances or serve to equalize demand over larger geographical areas. Newer technologies should enable the implementation of such a system within a relatively short period of time. Most of the population lives near the coasts [6] with 37% within 100 km of the coasts and 49% within 200 km of the coasts. Moreover, because of the availability of cooling water, many nuclear facilities are located on the coasts. The development of a H2 economy would likely see the construction of large H2 pipelines oQ each coast with smaller trunk lines into major urban centers and nuclear H2 production facilities. An increasing number of natural gas and oil pipelines are located oQshore on the seabed because of (1) the ease of .nding rights-of-way and (2) the advances in automated pipeline assembly aboard pipeline-laying ships. The same technology can be applied to H2 pipelines. 3. Hydrogen production requirements Nuclear energy provides a source of heat to produce H2 . Multiple processes are being investigated to produce H2 from water and heat. If nuclear energy is to be used for H2 production, the nuclear reactor must deliver the heat at conditions that match the requirements imposed by the H2 production process. At this stage of development, it is unclear which chemical processes will be the most economic;
C.W. Forsberg / International Journal of Hydrogen Energy 28 (2003) 1073 – 1081
thus the major candidate technologies were examined to determine if they imposed similar requirements on the reactor. 3.1. Hydrogen production methods The viability of H2 production from nuclear power ultimately depends upon the economics, which, in turn, depend upon both the proposed methods of H2 production and the available reactors. Four methods have been proposed to produce H2 from nuclear power. 3.1.1. Electrolysis The electrolysis of water [5] to produce H2 is an old technology that is used today to produce ultrapure H2 and to produce H2 in small quantities at dispersed sites. Electrolysis is not currently competitive for the large-scale production of H2 , except where low-cost electricity is available. The long-term viability of electrolysis for large-scale H2 production depends upon the evolution of the electric grid, the capital costs of electrolysis, and other factors. Current capital costs [7] are estimated to be near $600/kW, with future capital costs that may approach $300/kW. Conventional alkaline electrolyzers have e9ciencies of 70 –85%, with proton-exchange-membrane electrolyzers projected to have e9ciencies of 80 –90%. There is a signi.cant tradeoQ between capital costs and e9ciency. In many industrialized countries, the peak electrical demand is twice the minimum demand. Consequently, low-cost oQ-peak electricity is available (e.g., in the middle of the night). Electrolysis may be viable provided there is successful development of e9cient, low-cost electrolysis systems and associated local H2 storage systems. 3.1.2. Steam reforming Today, H2 is produced primarily from the steam reforming of natural gas (net reaction: CH4 + 2H2 O → CO2 + 4H2 ). Steam reforming is an energy-intensive endothermic low-pressure process requiring high-temperature heat input. The natural gas [1] is (1) used as the reduced chemical source of H2 and (2) burned to produce heat to drive the process at temperatures of up to 900◦ C. The amount of natural gas required for steam reforming can be signi.cantly reduced when heat is provided by a nuclear reactor. The Japan Atomic Energy Research Institute [8,9] is currently preparing to demonstrate the production of H2 by steam reforming of natural gas with the heat input provided by its High-Temperature Engineering Test Reactor (HTTR). The nuclear power plant provides heat that replaces heat from a gas Name. Because this system uses standard H2 production technology, it represents the near-term nuclear-H2 technology. Only nuclear reactor issues must be addressed. For Japan and other countries with high-cost natural gas, economic analysis [8] indicates that H2 from nuclear-assisted steam reforming of natural gas will have lower costs than H2 from natural gas alone. This is the basis of the near-term Japanese development program for
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H2 production. The economics would be expected to apply to any country where a signi.cant fraction of the natural gas is supplied in the form of lique.ed natural gas. 3.1.3. Hot electrolysis Electrolysis [10–12] can be operated at high temperatures (700 –900◦ C) and low pressures to replace some of the electrical input with thermal energy. Because heat is cheaper than electricity, the H2 costs via this production method could ultimately be lower than those for traditional electrolysis. Equally important, the high temperature results in better chemical kinetics within the electrolyzer that reduces (1) equipment size and (2) ine9ciencies. However, the technology [7] is in an early state of development although it derives much of its technology from solid-oxide fuel cells. Hot electrolysis requires collocation of H2 production with the nuclear reactor to provide the heat. 3.1.4. Thermochemical hydrogen production Hydrogen can be produced by direct thermochemical processes [9,13] in which the net reaction is heat plus water yields H2 and oxygen. These are the leading long-term options for production of H2 using nuclear energy. For low production costs, however, high temperatures (¿ 750◦ C) are required to ensure rapid chemical kinetics (i.e., small plant size with low capital costs) and high conversion e9ciencies. Many types of thermochemical processes for H2 production exist. The sulfuric acid processes (hydrogen sul.de, iodine–sulfur, sulfuric acid–methanol) and the Br–Ca–Fe cycle are currently the leading candidates. In the sulfuric acid processes, the high-temperature, low-pressure endothermic (heat-absorbing) reaction is the thermal decomposition of sulfuric acid to produce oxygen: H2 SO4 → H2 O + SO2 + 1=2O2 (high temperature):
(1)
◦
Typically temperatures in the range of 800 C are needed for e9cient H2 production. The high-temperature steps operate at low pressures. After oxygen separation, additional chemical reactions are required to produce H2 . The leading candidate for thermochemical H2 generation is the iodine– sulfur process (Fig. 1), which has two additional chemical reactions: I2 + SO2 + 2H2 O → 2HI + H2 SO4 (low temperature) (2) and the H2 -producing step 2HI → H2 + I2 (intermediate temperature):
(3)
Of the advanced methods for hydrogen generation using nuclear power, thermochemical cycles have received the most attention because current estimates [8] indicate that thermochemical H2 production costs could be as low as 60% of those from room-temperature electrolysis. These Japanese estimates assume dedicated nuclear reactors for the thermochemical processes (no electricity production) and electricity from the electrical grid for room-temperature electrolysis. The cost advantage is a consequence of process e9ciencies.
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C.W. Forsberg / International Journal of Hydrogen Energy 28 (2003) 1073 – 1081 ORNL DWG 2001-102R
Water
Oxygen
Hydrogen 2H2O
O2
H2 SO2
I2
Heat 800-1000°C
I2 + SO2 + 2H2O
H2SO4
2HI
2HI + H2SO4
H2O + SO2 + ½O2
H2SO4
H2+I2
HI
Fig. 1. Iodine–sulfur process for thermochemical production of H2 .
Production e9ciency can be de.ned as the energy content of the resulting H2 divided by the energy expended to produce the H2 . Although H2 production by electrolysis [5] can be relatively e9cient (∼ 80%), there is a signi.cant tradeoQ between capital cost and e9ciency with this technology. However, when this factor is combined with the electrical conversion e9ciency, which ranges from approximately 34% (in current light-water reactors) to 50% (for advanced systems), the overall heat-to-H2 e9ciency would be approximately 25 – 40%. For thermochemical approaches such as the sulfur–iodine process described previously, an overall e9ciency of ¿ 50% has been projected. Combined-cycle (H2 and electricity) plants may have e9ciencies of ∼ 60%. The diQerences in e9ciency reNect the penalties of converting thermal energy to electricity and then to chemical energy (H2 ) vs. converting thermal energy directly to chemical energy (H2 ). Parallel to the e9ciency considerations, the capital costs for thermochemical processes are expected to be lower than for room-temperature electrolysis because less equipment is required. However, large-scale pilot plants will be required before de.nitive cost .gures will be available. The sulfur–iodine process is being developed in Japan with the ultimate goal of connecting it to the HTTR. Research on this process is also under way in the United States. Thermochemical process will require the development of better high-temperature materials of construction for the corrosive environments. Signi.cant development work on H2 thermochemical cycles is required, with the technology being applicable to both nuclear and solar-power tower heat sources. 3.2. Requirements The process requirements for H2 production via nuclear steam reforming of methane, hot electrolysis, and
thermochemical cycles are similar. All three technologies impose similar requirements on the nuclear reactor. • Reactor power. Reactor powers in the range of typical nuclear applications (100 –1000 MW(e) match well with the scale of H2 production facilities. Economic considerations for speci.c applications will determine actual plant size. • Peak temperature. All the low-cost methods require high temperatures (750 –900◦ C) • Temperature range of delivered heat. All of the endothermic high-temperature chemical reactions are dissociation reactions that operate at nearly a constant temperature. Heat should therefore be delivered over a small temperature range. • Pressure. The chemical reactions go to completion at low pressures. High pressures reverse the desired chemical reactions. The H2 -nuclear interface should be at low pressure to (1) minimize the risk of pressurization of the chemical plant and (2) minimize high-temperature materials strength requirements. • Isolation. The nuclear and chemical facilities should be isolated from each other so that upsets in one facility do not impact the other. The system must also minimize tritium (radioactive hydrogen) production and transport from the reactor to the H2 production facility.
4. The advanced high-temperature reactor There are two approaches to developing a nuclear reactor for H2 production. An existing reactor system can be modi.ed to meet the H2 production requirements, or a new reactor system can be developed. At the current time, only one
C.W. Forsberg / International Journal of Hydrogen Energy 28 (2003) 1073 – 1081
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02-080
Passive Decay Heat Removal
Reactor
Hot Air Out
Hydrogen Production
750° to1000°C Control Rods
Air Inlet
Nuclear-assisted steam reforming of natural gas Hot electrolysis using water Thermochemical using water
Fuel (Coated-particle Graphite-matrix)
Cooling Water
Reactor Vessel Guard Vessel
Fig. 2. Advanced high-temperature reactor.
nuclear reactor system, the gas-cooled (helium) reactor, has the high-temperature capabilities to provide the heat at suf.cient temperatures to drive a H2 production system. This reactor has historically [8] been considered the one reactor that would be used to provide high-temperature heat for H2 production. The gas-cooled (helium) reactor was developed for electricity production and uses a coated-particle fuel (see below) and high-pressure helium as a coolant. Several prototype reactors have been built in systems where the reactor was connected to a steam power plant to produce electricity. An advanced version of the reactor is being developed for electricity production using a closed Brayton helium cycle where the helium coolant directly drives a helium gas turbine to produce electricity. Alternatively, a reactor can be designed speci.cally for H2 production. The Advanced High-Temperature Reactor (AHTR) has been proposed [14] to match H2 production requirements. The AHTR is described herein, as well as the basis for selection of particular technical characteristics of the reactor. This discussion provides a basis for understanding some of the key interface issues between any type of nuclear plant and a H2 production plant. The AHTR is based on several earlier technological developments: • High-temperature, low-pressure molten-salt reactor coolants from the aircraft nuclear propulsion program [15] of the 1950s and the molten-salt breeder reactor program [16] of the 1960s • Coated-particle graphite-matrix fuel developed in the 1970s for gas-cooled reactors • Passive safety systems for gas-cooled and liquid-metal reactors developed in the 1980s
4.1. Concept description The AHTR reactor core consists of coated-particle graphite-matrix fuel cooled with a molten Nuoride salt. The molten salt (Fig. 2) Nows through the reactor core to an external heat exchanger (to provide the interface for the H2 production system), dumps the heat load, and returns to the reactor core. The molten salt can be circulated by natural or forced circulation. The fuel (Fig. 3) is essentially the same as that used for the gas-cooled (helium) reactor. The important characteristic of these fuels is that they can operate at very high temperatures with peak fuel operating temperatures of ∼ 1200◦ C. Significant fuel failure does not occur below 1600◦ C. These fuels are the only demonstrated nuclear fuels capable of producing heat at temperatures su9cient for H2 production. The fuel consists of small particulates of uranium dioxide coated with layers of carbon and silicon carbide. The multiple layers isolate the fuel and .ssion products (produced by the nuclear reactions) from the coolant. The microspheres are embedded in a compact made of graphite. The fuel compact is embedded in graphite blocks. The hexagonal blocks are assembled into a reactor core. Fig. 3 shows the speci.c design of the Japanese HTTR fuel, a helium-cooled reactor with a helium exit temperature of up to 950◦ C. The AHTR fuel would be similar. Molten Nuoride salts are the only high-temperature liquids that have been fully demonstrated to be chemically compatible with graphite fuels [17]. The atmospheric boiling points for molten Nuoride salts are near 1400◦ C. This results in a reactor system that operates at low pressures and matches the low pressures of H2 production systems. There is a century of industrial experience with the
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C.W. Forsberg / International Journal of Hydrogen Energy 28 (2003) 1073 – 1081 ORNL DWG 2001-45
Fuel Kernel High Density PyC SiC Low Density PyC
Coated Fuel Particle
Fuel Rod Annular Coolant Channel
Fuel Handling Hole Dowel Pin
Plug Fuel Compact
Graphite Block
Graphite Sleeve
8 mm 0.92 mm
580 mm
39 mm
34 mm
Dowel Socket
26 mm
Fuel Compact
360 m m
Fuel Rod
Fuel Block
Fig. 3. Coated-particle graphite-matrix fuel (high-temperature engineering test reactor fuel).
compatibility of graphite and molten Nuoride salt— aluminum is electrolytically produced from cryolite (3NaF– AlF3 ) in very large graphite baths at ∼ 1000◦ C. Molten salts are leading candidates for cooling the .rst wall of fusion reactors [18]. At operating conditions, molten-salt properties are similar to those of water. Although they do not react with air or carbon dioxide, molten salts will slowly react with water. The physics of the AHTR reactor core, the general core design, and the fuel cycle are similar to those of the proposed General Atomics gas-turbine modular helium reactor [19] (GT-MHR). The low-power-density graphite-moderated core also has the long neutron lifetime, slow kinetics, and thermal neutron spectrum characteristic of the proposed GT-MHR. There is reactor experience with molten Nuoride salts. The Aircraft Reactor Experiment, a 2.5-MW(th) reactor, operated in the 1950s with a NaF=ZrF4 molten salt at 815◦ C, while the Molten Salt Reactor Experiment, an 8-MW(th) reactor, operated in the 1960s with a 7 LiF=BeF2 molten salt. The Aircraft Reactor Experiment was part of the large Aircraft Nuclear Propulsion Program [15], which had as its goal a nuclear-powered military jet aircraft with unlimited range. The reactor would produce high-temperature heat to heat the air going through the jet engines. A low-pressure system was required to avoid the weight and technical dif.culties associated with the use of a high-pressure reactor coolant. While nuclear-powered jet aircraft are impractical because of the weight of radiation shielding and risk of accidents, the reactor requirements—that is, to produce high-temperature heat at low pressure—match the requirements for production of H2 . This earlier work provides
much of the technical basis for an advanced reactor for H2 production. In these early reactors, the fuel was dissolved in the Nuoride salt. However, the AHTR uses solid fuel with a clean molten salt. The use of solid fuel, which contains the radioactivity, provides the .rst of several barriers to separate the radioactivity in the reactor from the H2 production facilities. 4.2. Matching reactor characteristics to hydrogen plant characteristics For H2 production, the critical requirement is to deliver the heat from the reactor core to the H2 thermochemical system under appropriate conditions. DiQerent characteristics of the reactor are used to meet these conditions. 4.2.1. Delivering heat to match H2 production requirements All the nuclear H2 production methods require large quantities of high-temperature heat at near-constant temperatures to drive the desired chemical disassociation reactions. From an engineering perspective, these are very high temperatures. Peak temperatures should be minimized to reduce thermal and pressure stresses. This can be accomplished by using a liquid reactor coolant. Liquid coolants have good heat transfer capabilities and low pumping power costs in comparison with gas coolants. As a direct consequence, liquid-cooled reactors can deliver most of their heat at near-constant temperatures while gas-cooled reactors generally deliver their heat over a wide range of temperatures to reduce pumping power costs. Some examples (Table 1, Fig. 4) demonstrate
C.W. Forsberg / International Journal of Hydrogen Energy 28 (2003) 1073 – 1081
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Table 1 Temperature drops for diQerent reactor coolants System
ST Inlet to outlet (◦ C)
Inlet T (◦ C)
Outlet T (◦ C)
Coolant
GT-MHR AGR (Hinkely) PWR (Point Beach) LMR (Super Phenix)
359 355 20 150
491 310 299 395
850 665 319 545
Gas (helium) Gas (CO2 ) Liquid (water) Liquid (sodium)
01-050
1000
Temperature (°C)
800
AHTR
1000°C 850°C
925°C
750°C AHTR
665°C
600
675°C 545°C
GT-MHR (General Atomics)
LMFBR
400
(Super Phenix)
AGR
319°C
310°C
(Hinkley Point B)
PWR
200 0 Inlet
491°C 395°C
299°C
(Point Beach)
Liquid Gas
Outlet Delivered Heat
Fig. 4. Temperatures of delivered heat from diQerent types of reactors.
these diQerences between gas-cooled and liquid-cooled nuclear reactors. The gas-cooled GT-MHR (General Atomics) has a ST across the reactor core of 359◦ C, while the Advanced Gas-Cooled Reactor (Hinkley Point B) has a ST of 355◦ C. Liquid-cooled reactors typically have muchsmaller temperature increases across the reactor core. The Point Beach pressurized-water reactor has a ST across the reactor core of 20◦ C, while a liquid-metal fast reactor (Super Phenix) has a ST of 150◦ C. The AHTR, as a liquid-cooled reactor, can deliver its heat with small temperature drops (20 –100◦ C) with low pumping power. If heat is needed at 750◦ C, the maximum temperature of the gas coolant in a gas-cooled reactor may exceed 1000◦ C whereas that of the liquid coolant in a liquid-cooled reactor will not exceed 850◦ C. Liquid coolants are preferred because they minimize materials requirements by lowering the peak temperatures and avoiding the need to withstand high stresses from a pressurized gas coolant. 4.2.2. Pressure The H2 production facility will contain signi.cant inventories of hazardous chemicals. A low-pressure, non-chemically reactive coolant minimizes safety risks by minimizing the consequence of heat-exchanger failures between the chemical and nuclear facilities. High-pressure reactor coolants create the potential for pressurization of
the chemical plant and releases of toxic gases. At high temperatures, high-pressure coolants also place much greater demands on the materials of construction. Low pressure liquid coolants can match the low pressures of the hydrogen production systems. Molten Nuoride salts have boiling points near 1400◦ C (and thus avoid the potential for chemical plant pressurization), do not react with air, and only slowly react with water. 4.2.3. Isolation The nuclear facility will be separated from the chemical plant by some distance. Because a high-heat-capacity low-pressure molten-salt coolant is used, the AHTR heat losses between the two facilities will be minimal. German chemical industry studies indicated that molten salts (low pressure, high heat capacity, low pumping costs) would be the preferred method to transfer heat from a reactor to a high-temperature chemical system. Molten salts are traditionally used in the chemical industry to transfer heat at high temperatures. It is unclear at this time whether an intermediate molten-salt heat transfer loop would be required to provide more isolation between the nuclear and chemical facilities. Nuclear reactors generate tritium (3 H), the radioactive form of hydrogen. Hydrogen can diQuse through hot metals; thus, the reactor design must be chosen to minimize the
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amount of tritium entering the H2 production facility. The reactor can be designed to minimize tritium production, tritium can be trapped in the coolant, or heat exchangers can be specially designed to minimize tritium transport. Minimizing the tritium inventory is likely to be the preferred strategy if a reactor is designed speci.cally for H2 production. For reactors that produce electricity, tritium is not a major issue because the tritium can be trapped in the power cycle. Studies of high-temperature gas-cooled (helium) reactors [20] indicated three sources of tritium production in these reactors: neutron bombardment of 3 He in the coolant, neutron bombardment of the 6 Li impurity in the graphite fuel, and ternary .ssion in the fuel. Tritium generation in the coolant can be minimized by choosing a molten Nuoride salt coolant such as NaF=ZrF4 that does not produce tritium. High-temperature bake-out of the graphite during the manufacturing process can be used to minimize the lithium impurities in the graphite. Appropriate fuel design can minimize the tritium that is released by the fuel. While early coated-particle fuels used two (biso) coatings, newer fuels use three coatings (triso). There has been signi.cant testing and a rapidly developing understanding of what is required to prevent the release of tritium from components within coated-particle fuels [21]. Coated-particulate fuels with three coatings will be required for tritium control. 4.3. Safety systems If a reactor shuts down, heat continues to be generated from the decay of short-lived radionuclides in the fuel. The decay heat decreases with time. If a method to remove decay heat is not provided, the reactor core will overheat, with potential damage to the core—such as occurred at the Three Mile Island reactor. Several passive safety options are available to ensure that overheating of the reactor core will not occur. A unique feature of a H2 production reactor is that the high temperatures required for H2 production also enable the use of more eQective safety systems in large reactors whose operations bene.t from the higher temperatures. For the AHTR, several options exist to ensure reactor safety against overheating. One option (Fig. 2) is a pool-type reactor with passive safety, similar to the proposed General Electric S-PRISM liquid-metal-cooled reactor [22,23]. The size of the S-PRISM reactor is limited by passive decay-heat cooling to ∼ 1000 MW(th). In this pool reactor, decay heat is conducted through the reactor vessel wall, transferred across an argon gap by radiation to a guard vessel, conducted through the guard vessel, and then removed from the second wall by natural circulation of air. The radiation heat transfer from the reactor vessel to the guard vessel increases by T 4 ; thus, a small rise in the reactor vessel temperature greatly increases heat transfer out of the system. The argon gap acts as a thermal switch to limit heat losses during normal operation but allows radiation heat transfer to increase heat losses if the reactor overheats.
If the same type of passive cooling system is applied to the AHTR (Fig. 2), the reactor size limits could potentially exceed 2000 MW(th)—matching the energy requirements for a world-class H2 production facility. The AHTR operates 200 –500◦ C hotter than the S-PRISM (500 –550◦ C for the S-PRISM versus 750 –1000 +◦ C for the AHTR). Because natural circulation of cooling air increases with temperature and because heat transfer across the argon gap varies with T 4 , the higher temperatures allow for more e9cient removal of decay heat, with heat removal rates adjusted by design of the decay-heat removal system. 5. Conclusions The viability of H2 production using nuclear energy involves infrastructure-, process-, and reactor-speci.c issues. In recent years, the size of conventional H2 production facilities using steam reforming of natural gas has grown. The size of H2 production units is now compatible with the scale of nuclear operations. Equally important, the characteristics of nuclear energy systems are compatible with proposed global H2 systems. In some areas, such as H2 transmission, a nuclear-H2 system has major advantages in that it minimizes the size of the H2 pipeline systems compared with systems that produce H2 using natural gas or other energy sources far from the large urban markets. Several methods to produce H2 using high temperature heat are under development. Signi.cant development work is required before any of these chemical processes can be commercialized. The potentially viable H2 production methods impose a set of di9cult requirements on the reactor, including delivery of heat at a near-constant high temperature, isolation of the reactor from the H2 production facility, and low-pressure-coolant interfaces between the reactor and the H2 production facility. Current reactor technology could be modi.ed to meet these requirements, but the severity of the requirements suggests that the nuclear reactor should be designed to match chemical plant requirements. One reactor, the AHTR, with the potential capabilities to match H2 production requirements has been identi.ed. Signi.cant development will be required of both the H2 production methods and the corresponding reactor systems. References [1] Stoll RE, Von Linde F. Hydrogen—what are the costs? Hydrocarbon Process 2000;79(12):42–6. [2] Forsberg CW, Peddicord KL. Hydrogen production as a major nuclear energy application. Nucl News 2001;44(10):41. [3] Forsberg CW. Hydrogen, electricity, and nuclear power. Nucl News 2002;45(10):30–1. [4] Parkinson G. The utility of hydrogen. Chem Eng 2001;108(10):29–37. [5] Ogden JM. Prospects for building a hydrogen energy infrastructure. Annu Rev Energy Environ 1999;24:227–79.
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