Key milestones have been passed after three years of planning, machining, wiring and welding, in the progress of the Advanced Graphite Capsule project. This six-phase project will test over 2,000 different samples of graphite in INL’s Advanced Test Reactor facility over a roughly 10-year period that will last until 2020.
Today, nuclear experts envision two different versions of gas cooled VHTRs for next-generation use. Both designs will require large amounts of high-quality graphite.
The “pebble-bed” style reactor uses billiard-ball-size “pebbles” of nuclear fuel particles coated with several layers of silicon-carbide and carbon. The pebbles enter the reactor from the top, work their way down through and exit the reactor from the bottom. There, they are monitored for remaining fuel to make another pass. Or, if the useable fuel is consumed by the time it reaches the bottom, it is collected for disposal. A second design utilizes a honeycomb block of graphite into which fuel rods would be inserted.
A critical step in developing new Very High Temperature Reactors (VHTR) is certifying the graphite that is used in many parts of the reactor’s core. Take the welding, for example. On the 14-foot section of the capsule that will be entered into the reactor core, 13 super precise welds cause less than twenty-thousandths of an inch in variation from one end of the capsule to the other.
With the capsule finished and inspected, it will enter INL’s Advanced Test Reactor in June. There, it will endure average temperatures of 600 degrees Celsius (six times the temperature of boiling water) for almost two years. Five similar Advanced Graphite Capsule experiments will follow the first one.
The capsules will be exposed to successively increased temperatures so the last capsule will experience temperatures of over 1,200 degrees Celsius. Experimenters will also expose the samples to varying levels of radiation, all several times what they would experience in a normal reactor. The higher radiation levels give researchers a sense of how the material will behave under the prolonged irradiation the graphite would experience over many years in a next-generation reactor.
After data from each test is gathered and each capsule is removed from the reactor, more work awaits the team. Post Irradiation Examination will involve removing the graphite samples and measuring and recording the differences in each one’s characteristics compared to before its trip to the reactor.
Every detail of the half-inch diameter samples will be considered. Researchers will construct a new database after measuring how the irradiation changed the physical dimensions of the pieces, examining their “thermal diffusivity” using lasers, and recording other specifications.
This information will allow those who build advanced nuclear reactors to be sure that communities will, for generations, reap the benefits of clean, safe, inexpensive and abundant energy to power their progress