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| how the PBMR fuel works |
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| 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. |
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| 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° 0°C (1 832° F), in which layers of specific chemicals can be added
with extreme precision. |
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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| During normal operation, the PBMR contains a load of approximately 360 000 fuel pebbles. |
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| 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. |
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| 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.
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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| 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. |
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