Locally developed pebble-bed reactor fuel
 spheres have been shipped to, and 
 will undergo irradiation testing at, the Institute of Nuclear Materials (INM), in Zarechny, near Ekaterinburg, in Russia, where similar tests have previously been carried out. The fuel spheres were manu-
factured by South Africa’s Pebble Bed Modular Reactor (PBMR) Company in collaboration with the South African Nuclear Energy Corporation (Necsa), at Pelindaba, near Tshwane.

In September, sixteen high-temperature reactor (HTR) fuel spheres, or pebbles, containing 9,6% enriched uranium particles, were shipped to Russia for irradiation tests to demonstrate the fuel’s integrity under reactor
conditions. Four of the 16 fuel spheres will be irradiated as part of the tests in Russia. During irradiation, the release of gaseous fission products will be measured to establish if there are any defective particles in the fuel.

“It is the first time that HTR fuel has been manufactured in the southern hemisphere and this achievement is the culmination of a number of years of intensive development work at the PBMR [Company’s] fuel development laboratories on the Necsa site, at Pelindaba, near Tshwane,” says PBMR Company CEO Jaco Kriek.

The testing, which will start in January 2010 and take about two years to complete, will involve the measurement of the release of gaseous fission products, as well as any defective particles in the fuel. After the first round of tests, the fuel spheres will be subjected to further examinations to measure the fission product content of each sphere by gamma spectrometry. During this pro-
cess, one of the irradiated fuel spheres will be deconsolidated and the fission product inventory of the matrix graphite will be measured by gamma spectrometry to determine the fission product retention capability of the coating.

According to the PBMR Company chief technology officer Dr Johan Slabber, the INM will be able to provide independent verification of the company’s fuel manufacturing capability, as the test pebbles are similar to the fuel that will be used in the future pebble-bed modular reactor. The irradiation tests will determine whether 
the fission product retention capability of the PBMR fuel spheres is comparable with that of the German HTR fuel on which the PBMR technology is based.

The 60-mm-diameter graphite spheres contain 14 000 uranium-enriched granules, explains Slabber. 
Each granule receives four ceramic coatings to form a particle about 1 cm in diameter, 
a measure to prevent leakage of fission products into the environment. 
While 14 000 particles are used in one sphere, Slabber points out that each sphere contains only 9 g of uranium and that the temperature of the reactor will never go high enough to compromise the integrity of the fuel.

“The silicon carbide coating applied to the granules and the final layer on the graphite sphere will last for over one-million years and ensure that the fission products are kept 
inside the sphere. 
“Our fuel is, therefore, ideally prepackaged for final disposal purposes, which is at the heart of the PBMR [Company’s] safety case,” he adds.

In emphasising the capability of a single pebble, Slabber points out that one pebble is able to generate the energy equivalent of 5 t of coal, which is enough energy to gener-
ate electricity for a family of four for an 
entire year. A total of 360 000 pebbles will go into a 200-MW pebble-bed reactor.

The manufacture of the fuel spheres follows the PBMR Company’s successful manufacture of enriched uranium coated particles in December 2008 at the PBMR Company’s fuel laboratories at Pelindaba.

A sample of coated granules used in the pebbles was sent to the Oak Ridge National Laboratory, in January 2009. The samples are to be compacted into specimens and inserted in irradiation test samples for testing at the Idaho National Laboratory early next year. This testing is part of the PBMR Company’s involvement in the US Department of Energy’s Next Generation Nuclear Plant project.

According to Slabber, fuel spheres will also be sent to the Institute for Energy of the Joint Research Centre of the European Commission, in the Netherlands, for irradiation testing. 
The company reports that the intention in the long term is to develop the capability to test and irradiate nuclear fuel in South Africa.

“These tests are the final step in the 
development of the fuel for the PBMR 
demonstration unit and are an important part of the licensing procedure. We are looking forward to the results, which we are confident will show that we have a high-quality product,” concludes Slabber.