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how the PBMR fuel works
PBMR fuel is based on a proven, high-quality German fuel design consisting of low enriched uranium triplecoated isotropic (LEU-TRISO) particles contained in a moulded graphite sphere.  A coated particle consists of a kernel of uranium dioxide surrounded by four coating layers as shown in the diagram.
In the fabrication process, a solution of uranyl nitrate is dropped from small nozzles to form microspheres, which are then gelled and calcined (baked at high temperature) to produce uranium dioxide fuel "kernels".  The kernels are then run through a Chemical Vapour Deposition (CVD) furnace in an argon environment at a temperature of 1 000 0C (1 832 F), in which layers of specific chemicals can be added with extreme precision.
For PBMR fuel, the first layer deposited on the kernels is porous carbon.  This is followed by a thin coating of pyrolitic carbon (a very dense form of carbon), a layer of silicon carbide (a strong refractory materisl), and finally, another layer of pyrolitic carbon.
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 pyrolytic carbon and silicon carbide layers provide an impenetrable barrier designed to contain the fuel and fission products.
Some 12 000 of these coated particles, now about a millimetre in diameter, are then mixed with graphite powder and a phenolic resin into 50 mm diameter spheres. A 5 mm thick layer of pure carbon is then added to form a "non-fuel" zone, and the resulting spheres are sintered and annealed to make them hard and durable.
Finally, the spherical fuel "pebbles" are machined to a uniform diameter of 60 mm.  Each fuel pebble contains about 9 g of uranium.  The total uranium in one fuel load is 2.5 metric tons, and the total mass of a fuel pebble is 210 g.
During normal operation, the PBMR contains a load of approximately 360 000 fuel pebbles.
Graphite is used in the reactor core because of its structural characteristics and its ability to slow down neutrons to the speed required for the nuclear fission reaction to take place.The core and core structures geometry used in the PBMR provide inherent characteristics which limit the peak temperature in the fuel following an accidental loss of active cooling.
The uranium-235 isotope occurs in natural uranium at a concentration of approximately 0.7 percent. In order to have a self-sustaining or "chain" reaction, the uranium in the PBMR fuel is enriched to about 9.6 percent in uranium-235, which is the isotope of uranium which mainly undergoes fission in the core.
The reactor is continuously replenished with fresh or re-usable fuel from the top, while used fuel is removed from the bottom.  After each pass through the reactor core, the fuel pebbles are measured to determine the amount of fissionable material left.  If a pebble still contains a usable amount of the fissile material, it is returned to the reactor at the top for a further cycle.  Each cycle takes just over three months.
Each pebble passes through the reactor about six times and lasts about 1000 days before it is spent, which means that a reactor will used 13 total fuel loads in its design lifetime of 40 years.
The extent to which the enriched uranium is consumed during the lifetime of a fuel pebble (called the "burn-up"), is much greater in the PBMR than in conventional power reactors. The quality and quantity of the fissile material that is left in a used fuel sphere at discharge from the core, makes the fuel material very unattractive for proliferation purposes. In addition, over 100 000 fuel spheres have to be diverted to accumulate sufficient material for covert use. This should be promptly detected by the installed surveillance measures of the International Atomic Energy Agency.
The fuel is transported to the spent fuel storage facility in the reactor building by means of a pneumatic fuel handling system.  The spent fuel storage consists of 10 tanks, each with a diameter of 3.2 m (10.4 ft) and a height of 18 m (58.5 ft).  One tank can store 600 000 pebbles.
The PBMR fuel was planned to be manufactured at the Necsa site at Pelindaba near Pretoria, using the technology established in Germany.  The facility was planned to have an initial capacity of 270 000 fuel spheres per year.
Last Updated: 6 February 2017
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