One of the big reasons that I continued to be fascinated by coated particle (TRISO) in graphite matrix fuel is the fact that it is possible to adjust the fuel to moderator ratios to obtain a variety of neutron spectra. By choosing to add less moderator, it is possible to harden the available neutron spectrum to provide high conversion ratios and the ability to convert nearly all transuranic isotopes into fission products. Instead of long lived waste, you get a lot of heat energy and short lived waste.
In other words, what people now call “waste” become a valuable raw material that lasts and lasts in reactors.
This concept is not widely known, but it is understood and well documented in nuclear engineering text books dating to the early 1960s.
The U. S. DOE is working to recover the technical capability of manufacturing reliable fuel that can handle the large burn-ups and subsequent stresses on the particle coatings. On July 25, 2008, World Nuclear News published a brief article titled Potential burner role for Next Generation Nuclear Plant about the most recently awarded research contract in this area. According to that article, the target burn-up is 65%. Apparently, there are some sodium cooled reactor boosters who are a bit surprised that they will be competing against a gas cooled VHTR in the burner reactor market.
Earlier this year, the DOE announced the achievement of a milestone of 9% burn-up which is almost exactly 2 times what a conventional light water reactor obtains. You can read about that achievement and the near term goals for the testing in the ScienceDaily.com article titled Nuclear Fuel Performance Milestone Achieved
One of the big challenges with this kind of research is that it requires patience and sustained support. Though it is possible to use high neutron flux reactors to speed up the process by a factor of 2 or 3, it simply takes a very long exposure time to perform the test. After all, the 5% burn-up fuel in current light water reactors stays in the reactor for about 4.5 years to achieve that level. Imagine how long it will take to get to the point where 65% of the initial heavy metal has broken into lighter isotopes!
One comment for the record – I think that the idea of achieving large burn-ups without recycling is very interesting. However, I am an advocate for arranging the core so that the burning in one portion of the core is happening as other portions are being converted from fertile materials like Th-232 and U-238 to fissile materials like U-233 and Pu-239. As that conversion process continues in the core, the heat production will also continue for a very long time.
