The DBI Thorium Breeding/Breeder Reactor designs represent an evolutionary advance in nuclear reactor design. Under development for four decades, the reactor consists of a small number of robust, mechanically elegant and low-pressure core systems. This elegance reduces overall manufacturing, installation, operations and maintenance costs. The design simplicity includes extensive use of modern automated controls that also translates into greater operational safety.
The DBI designs comply with all safety requirements and specifically obviate nuclear accidents like those that occurred at Three Mile Island and Chernobyl.
The DBI Thorium Reactor designs provide for enhanced safety from a variety of perspectives including the mechanical design, fuel type usage and fuel waste.
DBI reactors are designed to comply with the prevailing “Defense in Depth” philosophy of nuclear reactor safety. Remote automated monitoring and operational control redundant computer systems and gravity driven fail-safe controls, etc. are just some of the safety features. These mechanically simple and robust engineered features have been used in the United States nuclear industry since first-generation nuclear reactors to eliminate the possibility of radiation release to the environment, catastrophic failure through operator error or core meltdown.
Unique among nuclear reactors designs, the DBI reactor operates at low pressure (such as 200 to 500 PSI) and uses simple inert gases as the heat transfer medium. By contract, conventional uranium light water reactors typically operate at 2000 PSI and require much more expensive process equipment. The low-pressure DBI designs allow the use of off the shelf commercial equipment, including pumps, piping and vessels, enhancing safety and further reducing costs.
The fuel that the DBI reactor uses also enhances safety. Most breeder reactors require extensive chemical reprocessing (currently the USA does not reprocess fuel) to retrieve bred fuel from used U02 fuel rods, used MOX (mixed U02 and PuO2) fuel or from ThO2 “blankets”. The high radiation level of used fuel makes chemical reprocessing expensive and presents a significant waste challenge.
The DBI reactor uses a different process. Over a period of years, neutrons from driver fuel elements transfer reactivity to geometrically interchangeable Breeding/Breeder fuel elements. The core is designed so that the reactivity in the Breeding/Breeder fuel grows faster than the accumulation of neutron absorbing fission products. Fuel is shuffled periodically to maintain balance between breeding and reactivity. DBI’s design goal provides for: a) the elimination of chemical reprocessing requirements, b) the retention of all fission products except perhaps gaseous xenon in the fuel, and c) the venting of accumulated fission gases for smaller reactors(if necessary) from within the fuel encapsulation.
Furthermore, when a DBI reactor core is decommissioned, the newly bred and other “unspent” useful fuel elements can be transferred with no processing into a second generation DBI reactor core for additional power generation, providing up to 100% reduced cost for the initial fuel load in a subsequent DBI reactor.
One other noteworthy safety point is that the thorium fuel cycle offers a significant reduction in the production of long-lived transuranic actinides compared to the uranium fuel cycle that is the standard today. Indeed, the International Atomic Energy Agency says that actinide toxicity of thorium-based fuel is 10-fold lower than that of the uranium/plutonium fuel cycle. The mass number of thorium is 232 compared to 238 for uranium, meaning that thorium must absorb six neutrons to reach the same atomic weight as uranium. Because of this, the production of transuranic long-lived actinides is orders of magnitude lower in the thorium cycle than in the uranium cycle.
