Solid Radioactive Waste Strategy Report.pdf - UK EPR

EPR UK
N° NESH-G/2008/en/0123
REV. A PAGE 120 / 257
9 SPENT FUEL
9.1 EPR Spent fuel Characteristics
9.1.1 Summary Description of the Core and Fuel Assemblies
The EPR reactor core contains the nuclear fuel in which the fission reaction occurs. The
remainder of the active core structure serves either to support the fuel, control the chain
reaction or to channel the coolant.
The reactor core consists of fuel rods held in bundles by spacer grids and top and bottom
fittings. The fuel rods consist of uranium oxide pellets stacked in a cladding tube, plugged and
seal-welded to encapsulate the fuel (see Figure 34). The fuel rods are arranged in fuel
assemblies (see Figure 35 and Table 38) which are formed by a 17 x 17 array of 265 fuel rods
and 24 guide thimbles. The 24 guide thimbles are joined to the grids and to the top and bottom
nozzles. The guide thimbles may also hold rod cluster control assemblies, neutron source rods
or the in-core instrumentation. Guide thimbles that do not contain one of these components are
fitted with plugs to limit the bypass flow. The grid assemblies consist of an arrangement of
interlocked straps. The straps contain spring fingers and dimples for fuel rod support, as well as
coolant mixing vanes. The reactor core consists of 241 assemblies.
The fuel assemblies are made from a variety of fuel formulations. These include different
enrichments of uranium and some fuel assemblies also contain a neutron poison, gadolinium
oxide (Gd 2 O 3 ). The gadolinium oxide is mixed with the fuel and depletes slowly with burn up
which helps to control the fission process. For core refuelling, the number and the
characteristics of the fresh assemblies depend on reactor operating parameters and the fuel
management strategies e.g. cycle length, type of loading, fuel management regime, fuel type,
etc. Typically, around one third of the fuel assemblies are replaced at each refuelling.