Currently nuclear energy provides electricity to nearly 1 billion people in the world. It is a safe and environmentally friendly way to produce the ever growing energy demands for the next generations. These demands lead to great advances in the already efficient nuclear fuel cycle. The next generations of nuclear power plants broaden further the capabilities of these plants. This newest generation is called “Generation IV” or the Very High Temperature Reactors. However this technology is still in the research stage but is expected to be available for commercial use by 2025. One problem that still needs worked out is how the exit coolant is going to reach the very high temperatures. Because of these very high temperatures in the coolant, research is also being conducted on the heat coatings for the parts in contact with this 900 degree Celsius coolant, especially the fuel pellets themselves. The fuel in these plants also needs to last 200,000 MWD/MTU and 1300 degrees Celsius, which is slightly more than normal.
Most of the research and development will be based on observations on future test reactors of this type. Some of these features that need to be assessed are the effects of the direct cycle on existing system codes required, the reactivity insertion events, the thermal mixing at core outlet, and the transient models need qualification.
The Very High Temperature Reactor, or VHTR, has a once through uranium fuel cycle which is graphite-moderated and has a helium cooled reactor. What makes this reactor unique is that it allows for an outlet temperature of 1000 degrees Celsius. This enables hydrogen production by means of an indirect cycle. This indirect cycle would use a heat exchanger allowing the exhaust heat to be used in a hydrogen production facility. Also, the exhaust heat can be used for other purposes in the industries that require heat but not necessarily electricity. If not used for these purposes, the high temperature exhaust heat could then be put through a second steam turbine drawing more electricity and making the cycle even more efficient. These processes can be seen in figure 1.
This system can adapt to use both U and PU fuel cycles with very little waste comparatively. The core can be made as either a prismatic block or a pebble-bed core. This system offers the same characteristics as most modular high temperature gas-cooled reactors but has a more energy efficient concept with the ability to use the very high exhaust temperature in practical applications.
Figure 1. Reactor systems using He gas to produce electricity
Rennhack, Michael D. “VHTR ? Very-High-Temperature Reactor System” 04/15/2006 http://www.nukeworker.com/pictures/thumbnails-475.html
Idaho National Laboratory “Very High Temperature Reactor (VHTR)” 10/22/2007 http://nuclear.inl.gov/gen4/vhtr.shtml
Ogawa, Masuro. “Very Hight Temperature Reactor(VHTR)” 6/25/2002 http://gif.inel.gov/roadmap/pdfs/p_grns_june_25-27_presentation_gp32-00.pdf