Eskom Recycles Nuke Concept Technology
By John C. Zink, Ph.D., P.gif.,
Eskom, the big South African national utility, recently announced that it is studying the possibility of reviving the pebble bed nuclear reactor concept. Reactor designers in both Germany and the United States developed and analyzed this concept as long as 40 years ago. The U.S. Atomic Energy Commission considered a 120 MW plant in 1958, and Brown Boveri-Krupp worked on a 15 MW version in 1960. Neither pursued commercialization, however, as the pebble bed reactor had to compete with the light water reactor designs that evolved from military research and development, and which came to the civilian world complete with a manufacturing and fuel infrastructure.
Eskom finds that the major appeal of the pebble bed reactor rests in its potential as an inherently safe design that can be built in small modules. In addition, it can be refueled on-line. In contemporary nuclear politics, these characteristics seem more important than the infrastructure that grew up around modified submarine reactor designs.
The pebble bed reactor contains a bed of spherical ceramic fuel elements of uranium oxide coated with silicon carbide then embedded in a large(tennis-ball size) graphite sphere, or “pebble.” The specific heat of the ceramic materials, along with their high melting temperatures, allow the fuel to fully contain the decay heat from the nuclear reaction without being damaged. In other words, in the contemplated 100 MW size, the pebble bed reactor can withstand a total loss of coolant without sustaining any damage and without requiring the intervention of any active safety systems.
The Eskom 100 MW plant module will require a graphite lined reactor vessel about 20 feet in diameter (inside) and 30 feet high, operating at a maximum pressure of approximately 1,000 psia. The pebble bed inside this vessel generates power via its helium coolant, which is the working fluid for a closed-cycle gas turbine (Brayton cycle) that is expected to achieve a thermal efficiency of nearly 50 percent with a maximum helium temperature of approximately 1,600 F. This is decidedly different than the Fort St. Vrain high temperature gas-cooled reactor that operated in the United States from 1976 to 1989. The Fort St. Vrain reactor also used helium coolant, but it circulated through a steam generator, with the power coming from a steam turbine (Rankine cycle). The Eskom design avoids the problems associated with having to seal the helium coolant from the high pressure steam working fluid.
During plant operation, fresh fuel spheres are continuously added to the top of the pebble bed, and old ones are removed from the bottom. As each sphere is removed, it is tested for burnup and, depending on the test results, it is either recycled into the top of the reactor or sent to a spent fuel storage tank. At full power, 198 fuel elements a day would be added to the reactor. Most fuel elements will be cycled through the reactor approximately 10 times, leading to efficient fuel utilization. Furthermore, because of the online refueling, Eskom expects to schedule major plant outages only every six years.
Besides its inherent safety features, Eskom feels the pebble bed concept also has economic advantages. The company currently estimates that the capital cost of each 100 MW module (when produced in blocks of 10 modules) will be between $70 million and $90 million, or $700 to $900 per kW. Fuel cost is expected to be less than 1.5 mills per kWh, assuming no fuel reprocessing.
Eskom hopes to have a pebble bed reactor pilot plant on-line by 2003. The company plans to market the plants worldwide. p