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The high temperature gas-cooled reactor as a source of high temperature process heat
NUCLEAR ENGINEERING AND DESIGN 26 (1974) 179-186. © NORTH-HOLLAND PUBLISHING COMPANY
THE HIGH TEMPERATURE GAS-COOLED REACTOR AS A SOURCE OF HIGH TEMPERATURE PROCESS HEAT R.N. QUADE Gulf GeneralA tomic Company, San Diego, California 92138, USA Received 10 July 1973
The nuclear reactor has established itself as a future major supplier of electrical energy. The industrial market for other forms of energy, however, is almost as large and represents a new potential for the use of nuclear reactors. The high temperature gas-cooled reactor (HTGR) has been developed for commercial application in the electric power generation field. Since the HTGR is capable of delivering process heat in the temperature range of 10001500 ° F, it has many applications that would not be possible at the lower operating temperatures of water-cooled reactors. This paper briefly summarizes the development of the HTGR and outlines its salient technical features. Modifications to the reactor that enable it to be used as a process heat source are discussed. Specific applications are developed for coal gasification, steelmaking, and hydrogen production.
1. Introduction Nuclear reactors are in a field o f rapidly developing technology. Only a little more than 30 years ago, the first sustaining nuclear reaction was demonstrated. Today, nuclear power represents about 1% of US electric generating capacity. By 1985 this is expected to increase to 50%. The place o f the nuclear reactor in the electric power industry is established, but other areas remain where nuclear power, as an energy source may make significant contributions. Foremost o f these areas is industry, representing about 25% of the energy market. All current nuclear reactors are capable o f providing saturated steam in the range 4 5 0 - 5 0 0 ° F , either as a total output or in combination with electric power production. This steam can be useful in some process applications, such as the application at the Midland, Michigan Nuclear Energy Center o f the Consumers Power Company, where steam will be generated and sold to the Dow Chemi, cal Company. Various methods have been proposed for combining nuclear power plants with desalination projects. There is, however, a whole series o f processheat applications in which temperatures o f 1 0 0 0 1800OF are required. Major industrial applications include metallic ore reduction and roasting, the pro-
duction o f hydrogen, and the production o f substitute pipeline gas (SPG) from coal or oil shale. In the temperature range 1 0 0 0 - 1 8 0 0 ° F , only the high temperature gas-cooled reactor (HTGR) is currently a candidate for providing the required energy. In the United States, the H T G R has been developed and brought to commercial status by Gulf General Atomic. The first HTGR plant went on-line in Peach Bottom, Pennsylvania, in June 1967. This plant produces 40 MW o f electricity with steam conductions o f 1450 psi at 1000°F. The next HTGR plant to go on-line will be at Fort St. Vrain, located near Platteville, Colorado. This plant will produce 330 MW(e) with steam conditions of 2500 psi and 1005/1001 °F. Start-up is scheduled for 1974. Several orders have been received for additional HTGR plants in much larger sizes - 770 and 1160 MW(e).
2. H T G R as a h e a t s o u r c e
2.1. Principal features o f the HTGR Helium is used as the reactor coolant. Gases do not have the inherent temperature limitations that liquids have, e.g. phase changes. They offer great
180
R.N. Quade, High temperature process heat from the HTGR
flexibility in terms of operating temperatures and pressures. They have low macroscopic neutron cross sections, and they neither poison nor moderate the core. An inert gas, such as helium, is more compatible than other coolants with fuel elements and structural materials at high temperatures. Graphite is used as the moderator, reflector, and matrix for uranium-thorium fuel. The use of graphite benefits heat transfer and makes it possible to achieve high core outlet temperatures. (Graphite also provides an inherent safety feature, i.e. its high heat capacity provides an excellent heat sink to minimize the effects of temperature transients.) The HTGR is an 'advanced converter'. This means that a significant amount of fuel is produced and used within the reactor. The reactivity lifetime of the core is increased significantly, and the fissile product remaining at the end of fuel life can be recycled. 2.2. Nuclear steam supply system components
HTGR operating conditions have been developed and optimized for electric power production and are shown in table 1. Major components of the nuclear steam supply system are discussed below. 2. 2.1. Reactor core
The individual fuel moderator elements are hexagonal in cross section. They measure 14.2 in. across the flats, are 31.2 in. high, and are made of nucleargrade graphite. Internal coolant channels within each element are aligned with coolant channels in elements above and below. The active fuel is contained in an array of small diameter blind holes, which are parallel to the coolant channels and occupy alternating positions in a triangular array within the graphite structure. The coated-particle fuel, in the form of bonded rods, is placed into the fuel holes. The reactor core consists of vertical columns of the fuel-moderator elements and graphite reflector blocks grouped in a cylindrical array and supported by a graphite core support structure. The active core has the approximate shape of a right circular cylinder, and has unfueled graphite reflector elements located above, below, and around it. Each region rests on a single, large, hexagonal graphite core support block supported by three graphite posts. The core support blocks are arranged on a uniform
Table 1. Typical HTGR operating conditions. Helium circuit
Core inlet temperature (° F) Core outlet temperature (°F) Pressure level (psia) Pressure drop (psid)
760 1400 700 ~ 20
Steam generator
Outlet temperature (° F) Outlet pressure (psia)
955 2515
Reactor core
Power density (kW/~) Fuel life (yr)
8 4
triangular pitch and are keyed together to provide a uniform pitch spacing at the core support plane. The active core is completely surrounded by a graphite reflector structure. The side reflector consists of a cylindrical graphite shell formed by a ring of hexagonal elements adjacent to the active core and an outer ring of larger, irregularly shaped blocks. These larger blocks, which surround the entire hexagonal element core and reflector structure, remain in the reactor permanently. The top and bottom reflectors are made up of hexagonal graphite elements that are similar to the graphite fuel elements but do not contain fuel. The top, bottom, and side hexagonal reflector elements, immediately adjacent to the active core, can be replaced. The heat output of the core is controlled by cable-operated control rods. Each control rod consists of a series of canned neutron absorber sections held together by a metal spine. 2.2.2. Prestressed concrete reactor vessel (PCR V)
The PCRV contains the reactor core and the entire primary coolant system, including the steam generators, the helium circulators, the auxiliary cooling system, and the helium purification systems. It functions as the pressure boundary for the primary coolant system and as biological shielding for the reactor. The complete PCRV is composed of: (1) an internal steel liner acting as a sealing membrane, (2) a thermal barrier and water cooling system to limit concrete temperature, (3) penetrations and closures to provide access to the vessel cavities, and (4) high-
R.N. Quade, High temperature process heat from the HTGR
181
i~¸~ii •i¸II! ilia!¸II!I•~~~•'~::¸ i iiii~iii:•iii•i
Fig. 1. Nuclear reactor plant.
strength concrete with bonded reinforcing steel and prestressing systems to give strength to the pressure vessel. The entire assembly is mounted on a support structure. The liner, concrete, reinforcing steel, and prestressing system function as a composite structural system.
2.2.3. Steam generating system Identical once-through steam generators are located in cavities symmetrically oriented in the PCRV around the reactor core. The heat transfer section of the steam generator consists of helically-coiled tube bundies. Feedwater enters this main steam section, is evaporated and superheated, and exist at approximate-
ly 955°F and 2500 psig. The steam leaves the section through the superheat tubesheet to the steam pipe headers on the steam generators. For the s t e a m electric cycle, a single steam reheat to lO00°F is used. The materials used in the steam generators range from carbon steel to nickel-base alloys. The specific material selected depends upon the generators' particular design conditions of temperature, pressure, corrosion resistance, weldability, and long-term creep properties.
2.2.4. Helium circulation The primary coolant system is equipped with
182
R.N. Quade, High temperature process heat from the HTGR
COAL
STEAM
~ ~
coolant system by continuously purifying a side stream of gas. The purified helium is used for purging helium circulator seals, control rod drives, instruments and valves, and PCRV penetrations.
p PIPELINE GAS
HEAT 2 C +2H20÷ HEAT-'-CH4÷ C02 (a) COAL- - ~ STEAM
•
't
l
=
4H2 + 2C--~2CH4
HEAT CH4+ 2H20+ HEAT-"-4H2+ C02 (b) Fig. 2: (a) Carbon plus steam reaction; (b) methane reforming
plus hydrogenation. identical helium circulators. Each circulator unit consists of a single stage axial flow compressor and a single stage steam turbine main drive. The circulator is mounted to the PCRV. The compressor and the drive turbine are mounted integrally on a single vertical shaft and are overhung from a central bearing and a seal housing that contains the water-lubricated bearings and seals. A primary coolant shutoff valve is installed in the diffuser section of each main circulator. The purpose of the valve is to prevent backflow of the helium coolant when the associated circulator is shut down. 2.2.5. Auxiliary cooling system The auxiliary cooling system comprises redundant cooling loops that circulate and cool primary helium during plant shutdown to remove reactor decay heat. This system is used if the main loops are out of service for maintenance or as a result of an equipment failure. Each cooling loop includes a heat exchanger, an auxiliary circulator, and a helium shutoff valve, all located within a PCRV penetration. 2.2. 6. Helium purification system The helium purification system removes gaseous activity and chemical impurities from the primary
2.2. 7. Reactor containment building The reactor containment building provides a leakage barrier to prevent significant fission product release to the atmosphere following any accident. The containment also allows maintenance of a proper atmosphere in the PCRV for adequate core cooling. Fig. 1 is a cutaway of the reactor containment building showing the major components of the reactor system.
3. HTGR appfication to coal gasification With the HTGR well developed for electric power production, attention has been directed to other applications. A prime possibility is the use of the HTGR in a coal gasification process. The impending gap between the supply and demand of natural gas in the United States has stimulated a vigorous search for substitutes. Coal, which is a relatively abundant natural resource, can be converted to a substitute pipeline gas (SPG). Two major means of conversion are available. One is the steam plus carbon reaction, while the other is the hydrogen plus carbon method. These two methods are shown in fig. 2. The significant difference between the two processing methods, insofar as the reactor is concerned, is that the hydrogen plus carbon set of reactions involves the addition of heat for steam-methane reforming at 1200-1600°F, a range well suited to today's HTGR technology. This method also appears to offer several chemical processing advantages, such as a higher processing efficiency and greater ease in cleaning up the gas. With these basic processing schemes in mind, the basic HTGR was modified to perform the coal conversion task. Fig. 3 shows the general arrangement of the HTGR modified as a process heat source. Helium flows downward through the core, where it is heated to 1620°F. The helium passes through one of the radial ducts going to the reformer and then passes upward through the reformer cavity. Heat is transferred through the reformer tube walls to the steam-methane mixture. The helium then flows through the cir-
R.N. Quade, High temperature process heat from the HTGR
COREAUXILIARYCOOLING~
,CONTROLRODDRIVE ANDREFUELING PENETRATIONS
/
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CORE~
REFORMER~
183
1
3 3 3
3 II ]
,
3
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3 3 3
3 3 3 3
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Fig. 3. HTGR as a process heat source. cumferential duct (not shown) to the adjacent steam generator cavity, where it passes downward over the steam generator coils. It passes upward around the steam generator and into the helium circulator, where it is compressed. It then passes through the upper horizontal duct back into the core. Most of the features o f the process heat HTGR are identical to those of the HTGR that is used for electric power. Significant differences are: (a) the addition of a helium-heated steam-methane reformer in a wall cavity of the PCRV (this reformer is similar in size to the existing steam generators and can be installed and removed much as they are);
(b) the inclusion of a circumferential helium duct between the reformer and steam generator so that the helium flows in series from the core, to the reformer, and then to the steam generator; (c) the elimination of the reheat portion of the steam generator, since a high efficiency steam cycle is not required; and (d) an increase in helium core outlet temperature from 1400 to 1620°F. A process flow diagram is shown in fig. 4. The steam-methane mixture is preheated before it enters the reformer. It passes through the reformer, where H2, CO, and CO2 are formed in the presence of a
184
R.N. Quade, High temperature process heat from the HTGR H20 CH
~
1800
1350°F STEAM 2500 PSIG
H2, CO, CO2
CIRCULATOR
1600 TEMPERATURE°F
~>i113000 MWt .r RTURO.E
1400
EO.T R REFORMER HELIUM 1230°F
1200
STEAM GmERATOR
Fig. 4. Helium heat transfer loop. nickel catalyst with the addition of heat from the helium. Reformer outlet conditions are 1350°F at 300 psig. Downstream processing of this mixture will shift the CO to H 2 and CO 2 and scrub out the CO2, leaving 87% H2, the remainder being CH4. For coal gasification this purity is acceptable. The oncethrough steam generator takes feedwater and produces 2500 psig steam at 955°F, which is the same as in the conventional HTGR. The operating parameters have not yet been optimized. Fig. 5 shows equilibrium reforming conditions. The reformer outlet temperature is plotted against the steam/methane ratio and the reforming pressure is a parameter. There is an incentive to raise the reforming temperature and gain a benefit from a lower steam/methane ratio. Higher pressure reforming has an added advantage with the nuclear reactor, since the reformer tube wall becomes pressure-balanced as the process pressure approaches helium pressure (700 psi). (This effect contrasts with that of a conventional reformer, where higher pressure means heavier tube walls.) The coal gasification process requires hydrogen pressures of 1000-1500 psi, so higher reforming pressures can provide the advantage of reducing the number of compressors required. The helium temperature of 1620°F is about 200°F higher than that of the conventional HTGR. This level is obtained without increasing the maximum fuel temperature of approximately 2300°F. Several methods, such as more refined core orificing or revised fuel management, can be used to achieve this higher average helium outlet temperature. Additional increases in helium temperature are achievable without exceeding current design values of fuel temperature. It is anticipated, however, that problems of thermal insulation materials and reformer tube metals may be encountered as the temperature is raised.
4 STEAM/METHANE
Fig. 5. Equilibrium reforming temperatures, 70% methane conversion. The reformer itself represents the one new piece of equipment in the HTGR. Although it is expected that the design will differ significantly from that of the steam generator, the experience and technical information gained in developing the HTGR steam generator will be most valuable. The safety implications of coupling a nuclear reactor with a chemical processing plant will require careful technical analysis. Operational requirements and limitations of the two plants must be identified, and a suitable control and safety system must be developed. The present HTGRs have core thermal outputs of 3000 MW(th) yielding 1160 MW(e), and 2000 MW(th), yielding 770 MW(e). Typical values for a coal gasification plant using the larger core are as follows: Reactor heat Coal feed Synthetic pipeline gas
3000 MW(th) 12 million ton/yr 630 million scf/day
4. HTGR application to other processes 4.1. Hydrogen production
Many industrial processes in addition to coal gasification require large quantities of hydrogen. The fertilizer industry and the petroleum industry are prime examples. Fig. 6 shows H 2 consumption in the United States since 1955 and gives an estimate of future requirements. These data do not include coal gasification. The basic process shown in fig. 4 is applicable to an HTGR producing hydrogen as an end product. The feedstock material could be natural gas, if this is available. However, indications are
185
R.N. Quade, High temperature process heat from the HTGR STEAM METHANE REFORMINGOF NATURAL GAS UTILITY FINANCING
6000 5000
[
•SYNTHETIC ] AliONIA
n
X~OROC,~CKmG
90 80 70
MISCELLANEOUS
4000
HYDROGEN 50 60 PRICE (¢/MSCFI 40 3O 2O 10
3000 Do,
~NU
FOSSIL
2000
0/
1000
o
I 20
i 40
I 60
t
I
80
100
CLEAR
I 120
140
PRICE OF NATURAL GAS (o/MSCF)
0 1955
1960
1965
1970
1975
1980
Fig. 6. Annual hydrogen consumption in the United States.
Fig. 8. Comparative costs of hydrogen manufacture using fossil fuel versus an HTGR heat source. NERAIOR
"
I
~/~
' "
CO2
I
CULATOR o l
I
I
I
,
I
,,
-
-~:E)" . . . .
',- - ' - - . . . .
,20
l
CH4
~ CO+H2
H2
H~O
PCRV
Fig. 7. HTGR hydrogen plant.
Fig. 9. HTGR reducing gas plant.
that it will become increasingly more difficult to obtain natural gas, particularly for industrial applications. Heavier petroleum feedstocks or coal can be used as a source for the light, steam-reformable hydrocarbon. In either case, the gas mixture from the primary reformer is cooled down, the CO converted to H2, and CO 2 scrubbed out by using one of the physical or chemical absorption processes. The chemical processing steps are well-known and are commercially available. Steam consumed in the reaction, as well as steam used in the steam turbine drives, is produced by the HTGR. Even though the plant output may be 100% hydrogen, i.e. no net electric power is produced, substantial power is required for compressing the hydrogen as well for other in-plant uses. A process flow diagram is shown in fig. 7. Hydrogen, as a product, provides a good base for considering the economics of nuclear process heat. The most common method of producing hydrogen today is by steam-methane reforming with a fossil-
fired furnace suplying the heat. An HTGR can be substituted for this heat source. A comparison of the cost of hydrogen produced by the two methods is shown in fig. 8. Using the utility type of financing, the 'break point cost',.i.e, the cost of methane fuel material at which hydrogen production costs are equal under the two methods, is about US $ 0.33• MSCF. This value has already been surpassed for intrastate gas contracts as well as new interstate contracts. 4.2. Steelmaking
One of the promising methods o f steel production that has attractive environmental advantages is the direct reduction of iron oxide followed by the use of an electric-arc furnace. Several plants for the direct reduction of iron oxide have been built and are operating. Additional plants are under construction.
186
R.N. Quade, High temperature process heat from the HTGR
Although the details of several specific processes [1 ] differ, the overall procedure is as follows: (a) prepare the iron ore by producing small pieces, e.g. pellets; (b) reduce the iron oxide pellets in a furnace to a high percentage of metallized iron (reduction is accomplished by H 2 or by a mixture of H 2 and CO); and (c) use the metallized iron as a feedstock for openhearth or electric-furnace production of steel. The HTGR would provide the reducing gas by steam reforming of natural gas or methane, derived from petroleum feedstock or coal. If an electric-arc furnace and, perhaps, a rolling mill were used at the same site, large quantities of electrical energy would be required. A process flow diagram for the combination plant is shown in fig. 9. Typical quantities of electricity required and steel produced using a 3000 MW(th) reactor are 710 000kW and 5 million ton/yr, respectively. 4.3. Combination plants
Since the HTGR is a highly competitive producer of electricity, a nuplex (nuclear complex) using the HTGR as an energy center is a good possibility. A combination plant might produce: (1) hydrogen for converting coal to synthetic crude oil or for hydrotreating petroleum feedstocks; (2) process steam for an industrial process plant; and (3) electricity to use
within the nuplex or to feed into the grid. Economics of size for nuclear reactors, and the trend to larger and larger electric power plants, indicate that the size of commercial nuclear plants will continue to increase.
5. Conclusions A growing number of process heat applications can be foreseen in which nuclear power can make a real contribution. Many of these applications require a heat source capable of supplying heat in the 1 0 0 0 1500°F range. The HTGR is well suited to this application and can offer the major advantages of an environmentally clean source of energy for the future; an economical source of energy today, and one whose price can be expected to escalate at a slower rate than the prices of fossil fuel, thereby increasing this advantage; and also an opportunity to extend the uselife of fossil fuel resources 30--40% by using nuclear heat to produce environmentally acceptable fuels. Finally, it offers an opportunity to develop a new technology well-suited to world-wide markets.
Reference [1 ] J.R. Miller, Direct reduction of iron comes of age in the 70s, Eng. Mining J. 173 (5) (1973) 68-76.
THE HIGH TEMPERATURE GAS-COOLED REACTOR AS A SOURCE OF HIGH TEMPERATURE PROCESS HEAT R.N. QUADE Gulf GeneralA tomic Company, San Diego, California 92138, USA Received 10 July 1973
The nuclear reactor has established itself as a future major supplier of electrical energy. The industrial market for other forms of energy, however, is almost as large and represents a new potential for the use of nuclear reactors. The high temperature gas-cooled reactor (HTGR) has been developed for commercial application in the electric power generation field. Since the HTGR is capable of delivering process heat in the temperature range of 10001500 ° F, it has many applications that would not be possible at the lower operating temperatures of water-cooled reactors. This paper briefly summarizes the development of the HTGR and outlines its salient technical features. Modifications to the reactor that enable it to be used as a process heat source are discussed. Specific applications are developed for coal gasification, steelmaking, and hydrogen production.
1. Introduction Nuclear reactors are in a field o f rapidly developing technology. Only a little more than 30 years ago, the first sustaining nuclear reaction was demonstrated. Today, nuclear power represents about 1% of US electric generating capacity. By 1985 this is expected to increase to 50%. The place o f the nuclear reactor in the electric power industry is established, but other areas remain where nuclear power, as an energy source may make significant contributions. Foremost o f these areas is industry, representing about 25% of the energy market. All current nuclear reactors are capable o f providing saturated steam in the range 4 5 0 - 5 0 0 ° F , either as a total output or in combination with electric power production. This steam can be useful in some process applications, such as the application at the Midland, Michigan Nuclear Energy Center o f the Consumers Power Company, where steam will be generated and sold to the Dow Chemi, cal Company. Various methods have been proposed for combining nuclear power plants with desalination projects. There is, however, a whole series o f processheat applications in which temperatures o f 1 0 0 0 1800OF are required. Major industrial applications include metallic ore reduction and roasting, the pro-
duction o f hydrogen, and the production o f substitute pipeline gas (SPG) from coal or oil shale. In the temperature range 1 0 0 0 - 1 8 0 0 ° F , only the high temperature gas-cooled reactor (HTGR) is currently a candidate for providing the required energy. In the United States, the H T G R has been developed and brought to commercial status by Gulf General Atomic. The first HTGR plant went on-line in Peach Bottom, Pennsylvania, in June 1967. This plant produces 40 MW o f electricity with steam conductions o f 1450 psi at 1000°F. The next HTGR plant to go on-line will be at Fort St. Vrain, located near Platteville, Colorado. This plant will produce 330 MW(e) with steam conditions of 2500 psi and 1005/1001 °F. Start-up is scheduled for 1974. Several orders have been received for additional HTGR plants in much larger sizes - 770 and 1160 MW(e).
2. H T G R as a h e a t s o u r c e
2.1. Principal features o f the HTGR Helium is used as the reactor coolant. Gases do not have the inherent temperature limitations that liquids have, e.g. phase changes. They offer great
180
R.N. Quade, High temperature process heat from the HTGR
flexibility in terms of operating temperatures and pressures. They have low macroscopic neutron cross sections, and they neither poison nor moderate the core. An inert gas, such as helium, is more compatible than other coolants with fuel elements and structural materials at high temperatures. Graphite is used as the moderator, reflector, and matrix for uranium-thorium fuel. The use of graphite benefits heat transfer and makes it possible to achieve high core outlet temperatures. (Graphite also provides an inherent safety feature, i.e. its high heat capacity provides an excellent heat sink to minimize the effects of temperature transients.) The HTGR is an 'advanced converter'. This means that a significant amount of fuel is produced and used within the reactor. The reactivity lifetime of the core is increased significantly, and the fissile product remaining at the end of fuel life can be recycled. 2.2. Nuclear steam supply system components
HTGR operating conditions have been developed and optimized for electric power production and are shown in table 1. Major components of the nuclear steam supply system are discussed below. 2. 2.1. Reactor core
The individual fuel moderator elements are hexagonal in cross section. They measure 14.2 in. across the flats, are 31.2 in. high, and are made of nucleargrade graphite. Internal coolant channels within each element are aligned with coolant channels in elements above and below. The active fuel is contained in an array of small diameter blind holes, which are parallel to the coolant channels and occupy alternating positions in a triangular array within the graphite structure. The coated-particle fuel, in the form of bonded rods, is placed into the fuel holes. The reactor core consists of vertical columns of the fuel-moderator elements and graphite reflector blocks grouped in a cylindrical array and supported by a graphite core support structure. The active core has the approximate shape of a right circular cylinder, and has unfueled graphite reflector elements located above, below, and around it. Each region rests on a single, large, hexagonal graphite core support block supported by three graphite posts. The core support blocks are arranged on a uniform
Table 1. Typical HTGR operating conditions. Helium circuit
Core inlet temperature (° F) Core outlet temperature (°F) Pressure level (psia) Pressure drop (psid)
760 1400 700 ~ 20
Steam generator
Outlet temperature (° F) Outlet pressure (psia)
955 2515
Reactor core
Power density (kW/~) Fuel life (yr)
8 4
triangular pitch and are keyed together to provide a uniform pitch spacing at the core support plane. The active core is completely surrounded by a graphite reflector structure. The side reflector consists of a cylindrical graphite shell formed by a ring of hexagonal elements adjacent to the active core and an outer ring of larger, irregularly shaped blocks. These larger blocks, which surround the entire hexagonal element core and reflector structure, remain in the reactor permanently. The top and bottom reflectors are made up of hexagonal graphite elements that are similar to the graphite fuel elements but do not contain fuel. The top, bottom, and side hexagonal reflector elements, immediately adjacent to the active core, can be replaced. The heat output of the core is controlled by cable-operated control rods. Each control rod consists of a series of canned neutron absorber sections held together by a metal spine. 2.2.2. Prestressed concrete reactor vessel (PCR V)
The PCRV contains the reactor core and the entire primary coolant system, including the steam generators, the helium circulators, the auxiliary cooling system, and the helium purification systems. It functions as the pressure boundary for the primary coolant system and as biological shielding for the reactor. The complete PCRV is composed of: (1) an internal steel liner acting as a sealing membrane, (2) a thermal barrier and water cooling system to limit concrete temperature, (3) penetrations and closures to provide access to the vessel cavities, and (4) high-
R.N. Quade, High temperature process heat from the HTGR
181
i~¸~ii •i¸II! ilia!¸II!I•~~~•'~::¸ i iiii~iii:•iii•i
Fig. 1. Nuclear reactor plant.
strength concrete with bonded reinforcing steel and prestressing systems to give strength to the pressure vessel. The entire assembly is mounted on a support structure. The liner, concrete, reinforcing steel, and prestressing system function as a composite structural system.
2.2.3. Steam generating system Identical once-through steam generators are located in cavities symmetrically oriented in the PCRV around the reactor core. The heat transfer section of the steam generator consists of helically-coiled tube bundies. Feedwater enters this main steam section, is evaporated and superheated, and exist at approximate-
ly 955°F and 2500 psig. The steam leaves the section through the superheat tubesheet to the steam pipe headers on the steam generators. For the s t e a m electric cycle, a single steam reheat to lO00°F is used. The materials used in the steam generators range from carbon steel to nickel-base alloys. The specific material selected depends upon the generators' particular design conditions of temperature, pressure, corrosion resistance, weldability, and long-term creep properties.
2.2.4. Helium circulation The primary coolant system is equipped with
182
R.N. Quade, High temperature process heat from the HTGR
COAL
STEAM
~ ~
coolant system by continuously purifying a side stream of gas. The purified helium is used for purging helium circulator seals, control rod drives, instruments and valves, and PCRV penetrations.
p PIPELINE GAS
HEAT 2 C +2H20÷ HEAT-'-CH4÷ C02 (a) COAL- - ~ STEAM
•
't
l
=
4H2 + 2C--~2CH4
HEAT CH4+ 2H20+ HEAT-"-4H2+ C02 (b) Fig. 2: (a) Carbon plus steam reaction; (b) methane reforming
plus hydrogenation. identical helium circulators. Each circulator unit consists of a single stage axial flow compressor and a single stage steam turbine main drive. The circulator is mounted to the PCRV. The compressor and the drive turbine are mounted integrally on a single vertical shaft and are overhung from a central bearing and a seal housing that contains the water-lubricated bearings and seals. A primary coolant shutoff valve is installed in the diffuser section of each main circulator. The purpose of the valve is to prevent backflow of the helium coolant when the associated circulator is shut down. 2.2.5. Auxiliary cooling system The auxiliary cooling system comprises redundant cooling loops that circulate and cool primary helium during plant shutdown to remove reactor decay heat. This system is used if the main loops are out of service for maintenance or as a result of an equipment failure. Each cooling loop includes a heat exchanger, an auxiliary circulator, and a helium shutoff valve, all located within a PCRV penetration. 2.2. 6. Helium purification system The helium purification system removes gaseous activity and chemical impurities from the primary
2.2. 7. Reactor containment building The reactor containment building provides a leakage barrier to prevent significant fission product release to the atmosphere following any accident. The containment also allows maintenance of a proper atmosphere in the PCRV for adequate core cooling. Fig. 1 is a cutaway of the reactor containment building showing the major components of the reactor system.
3. HTGR appfication to coal gasification With the HTGR well developed for electric power production, attention has been directed to other applications. A prime possibility is the use of the HTGR in a coal gasification process. The impending gap between the supply and demand of natural gas in the United States has stimulated a vigorous search for substitutes. Coal, which is a relatively abundant natural resource, can be converted to a substitute pipeline gas (SPG). Two major means of conversion are available. One is the steam plus carbon reaction, while the other is the hydrogen plus carbon method. These two methods are shown in fig. 2. The significant difference between the two processing methods, insofar as the reactor is concerned, is that the hydrogen plus carbon set of reactions involves the addition of heat for steam-methane reforming at 1200-1600°F, a range well suited to today's HTGR technology. This method also appears to offer several chemical processing advantages, such as a higher processing efficiency and greater ease in cleaning up the gas. With these basic processing schemes in mind, the basic HTGR was modified to perform the coal conversion task. Fig. 3 shows the general arrangement of the HTGR modified as a process heat source. Helium flows downward through the core, where it is heated to 1620°F. The helium passes through one of the radial ducts going to the reformer and then passes upward through the reformer cavity. Heat is transferred through the reformer tube walls to the steam-methane mixture. The helium then flows through the cir-
R.N. Quade, High temperature process heat from the HTGR
COREAUXILIARYCOOLING~
,CONTROLRODDRIVE ANDREFUELING PENETRATIONS
/
HELIUM PURIFICATIONWELLS'-----
CORE~
REFORMER~
183
1
3 3 3
3 II ]
,
3
II
GENERATOR
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3 3 3 3
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3 ] l
II
Fig. 3. HTGR as a process heat source. cumferential duct (not shown) to the adjacent steam generator cavity, where it passes downward over the steam generator coils. It passes upward around the steam generator and into the helium circulator, where it is compressed. It then passes through the upper horizontal duct back into the core. Most of the features o f the process heat HTGR are identical to those of the HTGR that is used for electric power. Significant differences are: (a) the addition of a helium-heated steam-methane reformer in a wall cavity of the PCRV (this reformer is similar in size to the existing steam generators and can be installed and removed much as they are);
(b) the inclusion of a circumferential helium duct between the reformer and steam generator so that the helium flows in series from the core, to the reformer, and then to the steam generator; (c) the elimination of the reheat portion of the steam generator, since a high efficiency steam cycle is not required; and (d) an increase in helium core outlet temperature from 1400 to 1620°F. A process flow diagram is shown in fig. 4. The steam-methane mixture is preheated before it enters the reformer. It passes through the reformer, where H2, CO, and CO2 are formed in the presence of a
184
R.N. Quade, High temperature process heat from the HTGR H20 CH
~
1800
1350°F STEAM 2500 PSIG
H2, CO, CO2
CIRCULATOR
1600 TEMPERATURE°F
~>i113000 MWt .r RTURO.E
1400
EO.T R REFORMER HELIUM 1230°F
1200
STEAM GmERATOR
Fig. 4. Helium heat transfer loop. nickel catalyst with the addition of heat from the helium. Reformer outlet conditions are 1350°F at 300 psig. Downstream processing of this mixture will shift the CO to H 2 and CO 2 and scrub out the CO2, leaving 87% H2, the remainder being CH4. For coal gasification this purity is acceptable. The oncethrough steam generator takes feedwater and produces 2500 psig steam at 955°F, which is the same as in the conventional HTGR. The operating parameters have not yet been optimized. Fig. 5 shows equilibrium reforming conditions. The reformer outlet temperature is plotted against the steam/methane ratio and the reforming pressure is a parameter. There is an incentive to raise the reforming temperature and gain a benefit from a lower steam/methane ratio. Higher pressure reforming has an added advantage with the nuclear reactor, since the reformer tube wall becomes pressure-balanced as the process pressure approaches helium pressure (700 psi). (This effect contrasts with that of a conventional reformer, where higher pressure means heavier tube walls.) The coal gasification process requires hydrogen pressures of 1000-1500 psi, so higher reforming pressures can provide the advantage of reducing the number of compressors required. The helium temperature of 1620°F is about 200°F higher than that of the conventional HTGR. This level is obtained without increasing the maximum fuel temperature of approximately 2300°F. Several methods, such as more refined core orificing or revised fuel management, can be used to achieve this higher average helium outlet temperature. Additional increases in helium temperature are achievable without exceeding current design values of fuel temperature. It is anticipated, however, that problems of thermal insulation materials and reformer tube metals may be encountered as the temperature is raised.
4 STEAM/METHANE
Fig. 5. Equilibrium reforming temperatures, 70% methane conversion. The reformer itself represents the one new piece of equipment in the HTGR. Although it is expected that the design will differ significantly from that of the steam generator, the experience and technical information gained in developing the HTGR steam generator will be most valuable. The safety implications of coupling a nuclear reactor with a chemical processing plant will require careful technical analysis. Operational requirements and limitations of the two plants must be identified, and a suitable control and safety system must be developed. The present HTGRs have core thermal outputs of 3000 MW(th) yielding 1160 MW(e), and 2000 MW(th), yielding 770 MW(e). Typical values for a coal gasification plant using the larger core are as follows: Reactor heat Coal feed Synthetic pipeline gas
3000 MW(th) 12 million ton/yr 630 million scf/day
4. HTGR application to other processes 4.1. Hydrogen production
Many industrial processes in addition to coal gasification require large quantities of hydrogen. The fertilizer industry and the petroleum industry are prime examples. Fig. 6 shows H 2 consumption in the United States since 1955 and gives an estimate of future requirements. These data do not include coal gasification. The basic process shown in fig. 4 is applicable to an HTGR producing hydrogen as an end product. The feedstock material could be natural gas, if this is available. However, indications are
185
R.N. Quade, High temperature process heat from the HTGR STEAM METHANE REFORMINGOF NATURAL GAS UTILITY FINANCING
6000 5000
[
•SYNTHETIC ] AliONIA
n
X~OROC,~CKmG
90 80 70
MISCELLANEOUS
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HYDROGEN 50 60 PRICE (¢/MSCFI 40 3O 2O 10
3000 Do,
~NU
FOSSIL
2000
0/
1000
o
I 20
i 40
I 60
t
I
80
100
CLEAR
I 120
140
PRICE OF NATURAL GAS (o/MSCF)
0 1955
1960
1965
1970
1975
1980
Fig. 6. Annual hydrogen consumption in the United States.
Fig. 8. Comparative costs of hydrogen manufacture using fossil fuel versus an HTGR heat source. NERAIOR
"
I
~/~
' "
CO2
I
CULATOR o l
I
I
I
,
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l
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Fig. 7. HTGR hydrogen plant.
Fig. 9. HTGR reducing gas plant.
that it will become increasingly more difficult to obtain natural gas, particularly for industrial applications. Heavier petroleum feedstocks or coal can be used as a source for the light, steam-reformable hydrocarbon. In either case, the gas mixture from the primary reformer is cooled down, the CO converted to H2, and CO 2 scrubbed out by using one of the physical or chemical absorption processes. The chemical processing steps are well-known and are commercially available. Steam consumed in the reaction, as well as steam used in the steam turbine drives, is produced by the HTGR. Even though the plant output may be 100% hydrogen, i.e. no net electric power is produced, substantial power is required for compressing the hydrogen as well for other in-plant uses. A process flow diagram is shown in fig. 7. Hydrogen, as a product, provides a good base for considering the economics of nuclear process heat. The most common method of producing hydrogen today is by steam-methane reforming with a fossil-
fired furnace suplying the heat. An HTGR can be substituted for this heat source. A comparison of the cost of hydrogen produced by the two methods is shown in fig. 8. Using the utility type of financing, the 'break point cost',.i.e, the cost of methane fuel material at which hydrogen production costs are equal under the two methods, is about US $ 0.33• MSCF. This value has already been surpassed for intrastate gas contracts as well as new interstate contracts. 4.2. Steelmaking
One of the promising methods o f steel production that has attractive environmental advantages is the direct reduction of iron oxide followed by the use of an electric-arc furnace. Several plants for the direct reduction of iron oxide have been built and are operating. Additional plants are under construction.
186
R.N. Quade, High temperature process heat from the HTGR
Although the details of several specific processes [1 ] differ, the overall procedure is as follows: (a) prepare the iron ore by producing small pieces, e.g. pellets; (b) reduce the iron oxide pellets in a furnace to a high percentage of metallized iron (reduction is accomplished by H 2 or by a mixture of H 2 and CO); and (c) use the metallized iron as a feedstock for openhearth or electric-furnace production of steel. The HTGR would provide the reducing gas by steam reforming of natural gas or methane, derived from petroleum feedstock or coal. If an electric-arc furnace and, perhaps, a rolling mill were used at the same site, large quantities of electrical energy would be required. A process flow diagram for the combination plant is shown in fig. 9. Typical quantities of electricity required and steel produced using a 3000 MW(th) reactor are 710 000kW and 5 million ton/yr, respectively. 4.3. Combination plants
Since the HTGR is a highly competitive producer of electricity, a nuplex (nuclear complex) using the HTGR as an energy center is a good possibility. A combination plant might produce: (1) hydrogen for converting coal to synthetic crude oil or for hydrotreating petroleum feedstocks; (2) process steam for an industrial process plant; and (3) electricity to use
within the nuplex or to feed into the grid. Economics of size for nuclear reactors, and the trend to larger and larger electric power plants, indicate that the size of commercial nuclear plants will continue to increase.
5. Conclusions A growing number of process heat applications can be foreseen in which nuclear power can make a real contribution. Many of these applications require a heat source capable of supplying heat in the 1 0 0 0 1500°F range. The HTGR is well suited to this application and can offer the major advantages of an environmentally clean source of energy for the future; an economical source of energy today, and one whose price can be expected to escalate at a slower rate than the prices of fossil fuel, thereby increasing this advantage; and also an opportunity to extend the uselife of fossil fuel resources 30--40% by using nuclear heat to produce environmentally acceptable fuels. Finally, it offers an opportunity to develop a new technology well-suited to world-wide markets.
Reference [1 ] J.R. Miller, Direct reduction of iron comes of age in the 70s, Eng. Mining J. 173 (5) (1973) 68-76.