TakingITGlobal - Panorama - Is There a Role...

Published on: Sep 24, 2002

Topic:

Type: Opinions

https://www.tigweb.org/express/panorama/article.html?ContentID=648

While the HTGR has various safety advantages, it has never received the large development effort compared to water-cooled reactors. There were several reasons for focus on the water-cooled reactor. The first was that water-cooled reactors were much more practical for use in submarines, which was the principal driver for nuclear power development in the late 40's and early 50's. The second factor was simply that water and steam properties are well understood and have been studied for a longer time than helium gas. Another reason is that as fossil fueled plants (especially natural gas combustion turbines) are currently much cheaper to build than HTGRs. Of course, in a carbon-free, sustainable development context, fossil fuels would likely need to be phased out. Therefore, technologies such as the HTGR would be more competitive in a non-carbon energy market.

Below are a few of the significant HTGR prototypes that were operated:

The 13 MWe AVR (Arbeitsgemeinschaft Versuchsreaktor: translates in English to Jointly Developed Prototype Reactor) was operated in Germany from 1966 to 1988. This prototype was the first of the "pebble bed" reactors. Pebble bed reactors are essentially an assembly of balls (about the size of a tennis ball) that contains the TRISO fuel. Pebble bed reactors have the advantage of online refueling by pneumatic insertion and removal of pebbles from the core. Germany's largest pebble bed reactor was the 296 MWe Thorium High Temperature Reactor (THTR) which operated from 1985 to 1988. Technical difficulties from the larger design as well as political problems resulting from the Chernobyl accident forced the closure of this reactor.

In the US, HTGR design used fuel rods contained in hexagonal graphite blocks. While sacrificing online refueling, hexagonal blocks allow for enhanced fuel accountability (i.e., better for non-proliferation) and more certain core geometry control. The first prototype in the US was Peach Bottom 1 and it operated from 1967 to 1974. This 40 MWe design led to the 330 MWe Fort St. Vrain plant that operated from 1979 to 1989. To circulate the helium, special water-lubricated bearings were used for the helium system, which resulted in frequent water injection into the reactor and so caused significant down time. Despite the poor plant performance, Fort St. Vrain demonstrated the enhanced safety of the HTGR fuel and reactor core. In addition, workers at Fort St. Vrain received radiation doses only about 1% that of average water-cooled reactors.

The reader may be wondering why the HTGR is under consideration again after these mixed results from the various prototypes. For nuclear energy to contribute to sustainable development, smaller, safer reactors that do not need large quantities of water would be ideal in many developing countries that have higher population densities. With important lessons learned on helium circulation, using smaller reactors, and successful demonstration of the safety of HTGR fuel, a new generation of HTGR prototypes has arisen which may be able to fill the gaps in sustainable energy needs.

The main two prototypes operating currently are the HTTR in Japan and the HTR-10 in China. The HTTR, [http://www.jaeri.go.jp/english/temp/temp.html] is a 30 MW thermal reactor built in Japan that uses the hexagonal block fuel similar to that used in the previous US designs. HTTR has been operating since 1998. The purpose of the HTTR is to investigate the potential of hydrogen production using the higher temperatures provided by HTGR technology.

HTR-10 was built in China [http://www.inet.tsinghua.edu.cn/english/project/htr10.htm] and is a 10 MW thermal reactor that uses the pebble bed design. HTR-10 has been operating since 2000. The reactor will be used to perform experiments in the use of process heat and electric power production. These prototypes continue to provide valuable data for both hexagonal block and pebble bed HTGR development. Besides these small research devices, new designs that permit greater safety and modularity are under serious consideration in several countries. These programs represent the future potential of the HTGR.

Future Potential of the HTGR in Sustainable Development

Past designs of HTGR power plants used indirect steam cycles to produce power. The circuit with the high temperature Helium was passed through a heat exchanger to make steam for turbines. New designs take advantage of the advances in engineering materials that allow for turbine blades to be used in the main helium system. This will allow for both greater efficiency and cost savings from eliminating the steam system, as well as significant savings in water use to produce electricity. In addition, these newer HTGRs may be small and simple enough to permit factory construction and greater modularity. This is similar to current gas turbines that are factory built and quickly installed at the power plant site. Both pebble bed and hexagonal block designs are under study.

The Pebble Bed Modular Reactor (PBMR) project in South Africa is probably the most publicized of the current efforts [http://www.pbmr.co.za]. A decision is expected from the South African government by late 2002 or early 2003 whether to construct the prototype. The prototype would be approximately 150 MWe and would have online refueling capability. If built, this project would answer many questions on the feasibility of both helium turbines as well as the economics of the pebble bed reactor. In addition, the South African effort may provide impetus for other HTGR projects. In the US, a smaller scale effort is underway to design a pebble bed reactor [http://web.mit.edu/pebble-bed/]. This effort is part of the Generation IV project of the US Department of Energy.

In the Netherlands, a more unusual pebble bed reactor is being studied. Called NEREUS [http://www.romawa.nl], this design is both smaller (8 MWe) and would not have online refueling capability. Instead the fuel design would permit longer periods of operation between refuelings (three years). The main objective for the NEREUS design would be either for commercial ship propulsion or could be used for small remote villages. NEREUS has even been studied for desalination due to both its longer periods between refueling and greater portability.

Hexagonal block designs are also being considered. The main effort centers on the Gas Turbine - Modular Helium Reactor (GT-MHR). This effort is also part of the Generation IV program and is an evolution from the previous US designs [http://www.ga.com/gtmhr/]. The reactor would produce approximately 150 MWe. It would not have online refueling capability, however the hexagonal blocks would be arranged in an annular pattern, in other words a "donut" of fuel containing blocks with "empty" graphite blocks in the center and surrounding the outside. This annular configuration would serve as a further absorber of excess heat should a loss of coolant occur.
This article has presented a summary of the HTGR as an alternative to renewable energy sources when such sources are either unavailable or inadequate for the particular need. While the designs presented were not all inclusive, they represent the spectrum of efforts in progress to provide energy needed for sustainable development efforts. While no single energy technology can be a panacea for global energy needs, HTGRs represent an opportunity for nuclear energy to make a relevant contribution to a more sustainable world.
The recent World Summit on Sustainable Development (WSSD) as well as the debates on the Kyoto Treaty for the reduction of greenhouse gases (e.g., carbon dioxide, methane) has renewed discussion on energy sources that are carbon-free. Most of these discussions have concentrated on how to increase the fraction of electricity supplied by renewable sources such as solar or wind. These sources have strong advantages such as no-fuel cost, much lower greenhouse gas emission, and political and social popularity. However, questions remain as to whether such sources alone will be able to meet the diverse needs of the world’s growing population. Even now, over 2 billion people have no electricity at all. To ensure a reliable supply of electricity that does not produce greenhouse gases will likely require other technologies besides strictly renewable sources. Several technologies have been proposed that can both fill the voids left by the renewable technologies while still being relatively carbon free. Among the advanced technologies that have been proposed to fill some of the niches is that of the High Temperature Gas Cooled Reactor (HTGR). Most of this attention has centered on the recent efforts in South Africa, however other efforts are underway with HTGR technologies that offer promise as well.

This article will focus on the HTGR without any detailed discussion of nuclear energy in general. If the reader desires detailed information on the basics of nuclear energy and reactor types please see [http://www.uic.com.au/ne3.htm] and [http://www.uic.com.au/nip64.htm] respectively.

The important aspects of the HTGR design are its higher efficiency, greater safety than standard water-cooled reactors, and lesser water requirements due to the use of helium as the main coolant. The primary reason for the enhanced safety of the HTGR is the use of the TRISO fuel particles that contain the nuclear fuel inside multiple layers of special materials such as silicon carbide (used for inside coatings of high performance devices such as aircraft jet engines). A diagram of the TRISO particle can be seen at http://www.ga.com/prg/fuel.html. These particles are less than one millimeter across and are conglomerated into either small rodlets or spheres. As will be discussed later, the HTGRs that use spheres are pebble bed reactors and those that use rods are called hexagonal block reactors (i.e., the rodlets are contained within hexagonal blocks of nuclear grade graphite )

All of the HTGR designs discussed in the article can easily absorb the excess heat generated in an accident by virtue of the high temperature coatings and smaller size of the reactors. This essentially means that the HTGR designs currently being studied would avoid any of the consequences of accidents such as the Three Mile Island (water-cooled reactor) or Chernobyl (Russian RBMK design reactor). Smaller reactors that cannot meltdown or explode combined with a lower water requirement mean that the HTGR could supply carbon-free power, desalinated water, or process heat depending on the particular needs.

The HTGR fuel is also better designed for disposal. The TRISO particles would be very resistant to the geologic conditions that may be present in a repository . The safety, modularity, and simpler design make the HTGR a path by which nuclear power may make substantial contributions to future sustainable development. This article will briefly discuss the early projects, current activities, and future potential of the HTGR to contribute to sustainable development.

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by Robert Margolis
Published on: Sep 24, 2002
Topic:
Type: Opinions
https://www.tigweb.org/express/panorama/article.html?ContentID=648


While the HTGR has various safety advantages, it has never received the large development effort compared to water-cooled reactors. There were several reasons for focus on the water-cooled reactor. The first was that water-cooled reactors were much more practical for use in submarines, which was the principal driver for nuclear power development in the late 40's and early 50's. The second factor was simply that water and steam properties are well understood and have been studied for a longer time than helium gas. Another reason is that as fossil fueled plants (especially natural gas combustion turbines) are currently much cheaper to build than HTGRs. Of course, in a carbon-free, sustainable development context, fossil fuels would likely need to be phased out. Therefore, technologies such as the HTGR would be more competitive in a non-carbon energy market. Below are a few of the significant HTGR prototypes that were operated: The 13 MWe AVR (Arbeitsgemeinschaft Versuchsreaktor: translates in English to Jointly Developed Prototype Reactor) was operated in Germany from 1966 to 1988. This prototype was the first of the "pebble bed" reactors. Pebble bed reactors are essentially an assembly of balls (about the size of a tennis ball) that contains the TRISO fuel. Pebble bed reactors have the advantage of online refueling by pneumatic insertion and removal of pebbles from the core. Germany's largest pebble bed reactor was the 296 MWe Thorium High Temperature Reactor (THTR) which operated from 1985 to 1988. Technical difficulties from the larger design as well as political problems resulting from the Chernobyl accident forced the closure of this reactor. In the US, HTGR design used fuel rods contained in hexagonal graphite blocks. While sacrificing online refueling, hexagonal blocks allow for enhanced fuel accountability (i.e., better for non-proliferation) and more certain core geometry control. The first prototype in the US was Peach Bottom 1 and it operated from 1967 to 1974. This 40 MWe design led to the 330 MWe Fort St. Vrain plant that operated from 1979 to 1989. To circulate the helium, special water-lubricated bearings were used for the helium system, which resulted in frequent water injection into the reactor and so caused significant down time. Despite the poor plant performance, Fort St. Vrain demonstrated the enhanced safety of the HTGR fuel and reactor core. In addition, workers at Fort St. Vrain received radiation doses only about 1% that of average water-cooled reactors. The reader may be wondering why the HTGR is under consideration again after these mixed results from the various prototypes. For nuclear energy to contribute to sustainable development, smaller, safer reactors that do not need large quantities of water would be ideal in many developing countries that have higher population densities. With important lessons learned on helium circulation, using smaller reactors, and successful demonstration of the safety of HTGR fuel, a new generation of HTGR prototypes has arisen which may be able to fill the gaps in sustainable energy needs. The main two prototypes operating currently are the HTTR in Japan and the HTR-10 in China. The HTTR, [http://www.jaeri.go.jp/english/temp/temp.html] is a 30 MW thermal reactor built in Japan that uses the hexagonal block fuel similar to that used in the previous US designs. HTTR has been operating since 1998. The purpose of the HTTR is to investigate the potential of hydrogen production using the higher temperatures provided by HTGR technology. HTR-10 was built in China [http://www.inet.tsinghua.edu.cn/english/project/htr10.htm] and is a 10 MW thermal reactor that uses the pebble bed design. HTR-10 has been operating since 2000. The reactor will be used to perform experiments in the use of process heat and electric power production. These prototypes continue to provide valuable data for both hexagonal block and pebble bed HTGR development. Besides these small research devices, new designs that permit greater safety and modularity are under serious consideration in several countries. These programs represent the future potential of the HTGR. Future Potential of the HTGR in Sustainable Development Past designs of HTGR power plants used indirect steam cycles to produce power. The circuit with the high temperature Helium was passed through a heat exchanger to make steam for turbines. New designs take advantage of the advances in engineering materials that allow for turbine blades to be used in the main helium system. This will allow for both greater efficiency and cost savings from eliminating the steam system, as well as significant savings in water use to produce electricity. In addition, these newer HTGRs may be small and simple enough to permit factory construction and greater modularity. This is similar to current gas turbines that are factory built and quickly installed at the power plant site. Both pebble bed and hexagonal block designs are under study. The Pebble Bed Modular Reactor (PBMR) project in South Africa is probably the most publicized of the current efforts [http://www.pbmr.co.za]. A decision is expected from the South African government by late 2002 or early 2003 whether to construct the prototype. The prototype would be approximately 150 MWe and would have online refueling capability. If built, this project would answer many questions on the feasibility of both helium turbines as well as the economics of the pebble bed reactor. In addition, the South African effort may provide impetus for other HTGR projects. In the US, a smaller scale effort is underway to design a pebble bed reactor [http://web.mit.edu/pebble-bed/]. This effort is part of the Generation IV project of the US Department of Energy. In the Netherlands, a more unusual pebble bed reactor is being studied. Called NEREUS [http://www.romawa.nl], this design is both smaller (8 MWe) and would not have online refueling capability. Instead the fuel design would permit longer periods of operation between refuelings (three years). The main objective for the NEREUS design would be either for commercial ship propulsion or could be used for small remote villages. NEREUS has even been studied for desalination due to both its longer periods between refueling and greater portability. Hexagonal block designs are also being considered. The main effort centers on the Gas Turbine - Modular Helium Reactor (GT-MHR). This effort is also part of the Generation IV program and is an evolution from the previous US designs [http://www.ga.com/gtmhr/]. The reactor would produce approximately 150 MWe. It would not have online refueling capability, however the hexagonal blocks would be arranged in an annular pattern, in other words a "donut" of fuel containing blocks with "empty" graphite blocks in the center and surrounding the outside. This annular configuration would serve as a further absorber of excess heat should a loss of coolant occur. This article has presented a summary of the HTGR as an alternative to renewable energy sources when such sources are either unavailable or inadequate for the particular need. While the designs presented were not all inclusive, they represent the spectrum of efforts in progress to provide energy needed for sustainable development efforts. While no single energy technology can be a panacea for global energy needs, HTGRs represent an opportunity for nuclear energy to make a relevant contribution to a more sustainable world. The recent World Summit on Sustainable Development (WSSD) as well as the debates on the Kyoto Treaty for the reduction of greenhouse gases (e.g., carbon dioxide, methane) has renewed discussion on energy sources that are carbon-free. Most of these discussions have concentrated on how to increase the fraction of electricity supplied by renewable sources such as solar or wind. These sources have strong advantages such as no-fuel cost, much lower greenhouse gas emission, and political and social popularity. However, questions remain as to whether such sources alone will be able to meet the diverse needs of the world’s growing population. Even now, over 2 billion people have no electricity at all. To ensure a reliable supply of electricity that does not produce greenhouse gases will likely require other technologies besides strictly renewable sources. Several technologies have been proposed that can both fill the voids left by the renewable technologies while still being relatively carbon free. Among the advanced technologies that have been proposed to fill some of the niches is that of the High Temperature Gas Cooled Reactor (HTGR). Most of this attention has centered on the recent efforts in South Africa, however other efforts are underway with HTGR technologies that offer promise as well. This article will focus on the HTGR without any detailed discussion of nuclear energy in general. If the reader desires detailed information on the basics of nuclear energy and reactor types please see [http://www.uic.com.au/ne3.htm] and [http://www.uic.com.au/nip64.htm] respectively. The important aspects of the HTGR design are its higher efficiency, greater safety than standard water-cooled reactors, and lesser water requirements due to the use of helium as the main coolant. The primary reason for the enhanced safety of the HTGR is the use of the TRISO fuel particles that contain the nuclear fuel inside multiple layers of special materials such as silicon carbide (used for inside coatings of high performance devices such as aircraft jet engines). A diagram of the TRISO particle can be seen at http://www.ga.com/prg/fuel.html. These particles are less than one millimeter across and are conglomerated into either small rodlets or spheres. As will be discussed later, the HTGRs that use spheres are pebble bed reactors and those that use rods are called hexagonal block reactors (i.e., the rodlets are contained within hexagonal blocks of nuclear grade graphite ) All of the HTGR designs discussed in the article can easily absorb the excess heat generated in an accident by virtue of the high temperature coatings and smaller size of the reactors. This essentially means that the HTGR designs currently being studied would avoid any of the consequences of accidents such as the Three Mile Island (water-cooled reactor) or Chernobyl (Russian RBMK design reactor). Smaller reactors that cannot meltdown or explode combined with a lower water requirement mean that the HTGR could supply carbon-free power, desalinated water, or process heat depending on the particular needs. The HTGR fuel is also better designed for disposal. The TRISO particles would be very resistant to the geologic conditions that may be present in a repository . The safety, modularity, and simpler design make the HTGR a path by which nuclear power may make substantial contributions to future sustainable development. This article will briefly discuss the early projects, current activities, and future potential of the HTGR to contribute to sustainable development. « return.