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Aug 10, 2007

Fuel for thought - PBMR fuel set for year-end production

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The first pebble-bed modular reactor (PBMR) fuel containing uranium is expected to be produced toward the end of the year at the company’s fuel development laboratories at Pelindaba. The South African nuclear energy corporation (Necsa) will assist PBMR in certain phases of the manu- facturing.

PBMR chief technology officer Dr Johan Slabber states that the first uranium-containing batch will be irradiated in test reactors in Russia and Holland, to provide early proof of performance. He points out that the PBMR fuel is based on a proven, high-quality German fuel design, consisting of low-enriched uranium triple-coated isotropic particles contained in a moulded graphite sphere.

“Our objective is to emulate the German fuel,” says Slabber, “because its reliability was tested and proven in Germany over 21 years.” He says the results from the test reactors will provide confirmation that a fuel equivalent to the German fuel can be manufactured in South Africa, and that German high- temperature reactor fuel manufactur- ing and quality control technology has been successfully transferred to South Africa.

Slabber states that a number of successful runs have been com- pleted on the coater of the PBMR fuel development laboratories, using simulated kernel materials instead of uranium. The coater was commissioned early this year. “The next step is to obtain a licence from the national nuclear regulator for the manufacturing of fuel containing uranium.”

Under the umbrella of development laboratories falls a host of laboratory-scale facilities with the prime purpose of developing the expertise required to manufacture the PBMR fuel, namely the kernel laboratory for uranium dioxide kernels, the coating laboratory for coating kernels, the fuel sphere laboratory for PBMR spherical fuel elements, and the quality control laboratory to perform the prescribed chemical, physical, and dimensional tests.

While the kernel laboratory has produced kernels with depleted and natural uranium, the coater facility has only produced zirconia- and alumina-coated particles. The graphite laboratory has produced fuel spheres containing alumina kernels, alumina- coated particles, zirconia-coated particles, and depleted uraniumcoated particles.

The initial laboratory work for the laboratory fuel started in 2002, with preparations and planning for the actual fuel plant at Necsa starting in early 2006. This means that the layout and design for the fuel laboratories went hand in hand with the sourcing of equipment and the establishment of protocols to ensure the orderly development of quality controls.

Slabber explains that a solution of uranyl nitrate is sprayed to form microspheres, which are then gelled and calcined to produce uranium dioxide kernels. The kernels are run through a chemical vapour deposition oven, in which layers of specific chemicals can be added with extreme precision.

First to be deposited on the kernel is a porous carbon layer, which allows fission products to collect without overpressurising the coated fuel particles. This is followed by a thin coating of pyrolitic carbon, which is a very dense form of heat-treated carbon. A thin layer of silicon carbide, which is a strong refractory material, and another layer of pyrolitic carbon, follow.

The porous carbon accommodates any mechanical deformation that the uranium dioxide kernel may undergo during the lifetime of the fuel, as well as gaseous fission products diffusing out of the kernel. The pyrolitic carbon and silicon carbide layers provide an impene-trable barrier, designed to contain the fuel and radioactive fission products resulting from nuclear reactions in the kernel.

About 15 000 of these coated kernels, each of which is now about 1 mm in diameter, are mixed with graphite powder and a phenolic resin, which is then pressed into the shape of a 50-mm-diameter ball. A 5-mm layer of pure carbon is then added to form a nonfuel zone, and the resulting spheres are carbonised and annealed to make them hard and durable.

The spherical fuel pebbles are machined to a diameter of 60 mm, about the size of a billiard ball. Each fuel pebble contains 9 g of uranium, and Slabber says this holds enough generation capacity to sustain a family of four, for a year. “Five tons of coal and up to 23 000 8467 of water will be required to generate one pebble’s energy.”

During normal operation, the PBMR core contains a load of 456 000 pebbles and can generate about 165 MW of electricity. A graphite column is located at the centre of the core, and the fuel pebbles in the annulus around it. Graphite is used owing to its structural characteristics, and its ability to slow down neutrons to the speed required for the nuclear reaction to take place. This geometry also limits the peak temperature in the fuel, in the unlikely event of a loss of active cooling.

The reactor is continuously replenished with fresh or reusable fuel from the top, while used fuel is removed from the bottom of the reactor. After each pass through the reactor core, the fuel pebbles are measured to determine the amount of fissionable material left. If the pebble still contains a usable amount, it is returned to the top of the reactor for a further cycle.

Each cycle takes about six months, and each pebble passes through the reactor about six times, and lasts about three years before it is spent. This means that a reactor will use 12 total fuel loads in its 40-year design lifetime.

The extent to which the enriched uranium is consumed during the lifetime of a fuel pebble, is much greater in the PBMR than in con-ventional power reactors, states Slabber. There is, therefore, minimal fissionable material that could be extracted from spent PBMR fuel. This, coupled with the level of technology required to break down the barriers surrounding the spent fuel particles, protects the PBMR fuel against the possibility of nuclear proliferation, or other covert uses.

A 165-MW PMBR will generate around 32 t/y of spent pebbles, of which about 1 t will be uranium. The spent fuel storage consists of ten tanks, each of which can store up to 600 000 pebbles. After the 40-year life cycle of the fuel plant has ended, the spent fuel can be safely stored on site, for another 40 years, before being sent to a final repository.

Edited by: Laura Tyrer
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