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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.
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Comments
Nuclear Waste and Graphite Flammability Robert Margolis | Sep 29th, 2002
Reviewing my article I realized that readers may have more questions regarding nuclear waste and graphite flammability. Here is some additional information links on these important subjects.
1) Attached an interesting website on the natural reactors at Oklo (contained high-level nuclear waste for two billion (two thousand million) years:
http://www.curtin.edu.au/curtin/centre/waisrc/OKLO/index.shtml
2) In addition, here is a description on graphite flammability:
http://www.ga.com/gtmhr/graphites_all.html
Feel free to post or contact me if you have any further questions.
Pebble bed reactor Robert Hargraves | Dec 28th, 2009
Nuclear power is key to global sustainable development. South Africa intended to export the PBMR (Pebble Bed Modular Reactor) within Africa (and also the US). The key criterion is "energy cheaper than coal", wich can not only stop global warming, but increase prosperity to achieve a lifestyle that includes sustainable populations.
You can visit my blog http://tigurl.org/qpt6md for a tutorial on the PBR.
I also advocate the Liquid Fluoride Thorium Reactor (LFTR), especially for the cost goal. Check the Jan 2010 Wired or visit http://tigurl.org/g60epg
Thanks Robert Margolis | Dec 28th, 2009
Glad to see that the article still sparks interest (even after seven years). :-)
I keep in contact with some colleagues in SA and they are keeping up the work. Also, China continues to work on the HTR-PM in Shidaowan. The HTGR continues to makes its way among the more common technologies.
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