Areva EPR

■ EPR NUCLEAR ISLAND
CONTROL ASSEMBLIES
†
The control assemblies, inserted in the core through the guidethimbles
of fuel assemblies, provide reactor power control and
reactor trip.
Rod Cluster Control Assemblies
The core has a fast shutdown control system comprising 89 Rod
Cluster Control Assemblies (RCCAs). All RCCAs are of the same
type and consist of 24 identical absorber rods, fastened to a
common head assembly. These rods contain neutron absorbing
materials. When they are totally inserted in the core, they cover
almost the whole active length of the fuel assemblies.
The EPR is equipped with RCCAs of the HARMONI type, a proven
Framatome ANP design. The neutron absorbing components are
bars made of an Ag, In, Cd alloy and sintered pellets of Boron
carbide (B4C). Each rod is composed of a stack of Ag, In, Cd bars
and B4C pellets contained in a stainless steel cladding under a
Helium atmosphere (for efficient cooling of the absorbing materials).
Because mechanical wear of the rod claddings happens to be a
limiting factor for the operating life of RCCAs, the HARMONI
claddings benefit from a specific treatment (ion-nitriding) that makes
their external surface extremely wear-resistant and eliminates the
cladding wear issue.
The RCCAs are assigned to different control bank groups.
37 RCCAs are assigned to control average moderator temperature
and axial offset, and 52 RCCAs constitute the shutdown-bank. The
first set is divided into five groups split into quadruplets. These
quadruplets are combined to form four different insertion sequences
depending on cycle depletion. This sequence can be changed at
any time during operation, even at full power. A changeover is
performed at regular intervals, approximately every 30 equivalent
full power days, to rule out any significant localized burnup delay.
At rated power the control banks are nearly withdrawn. At
intermediate power level, the first quadruplet of a sequence can be
deeply inserted and the second may be also inserted. Shutdown
margins are preserved by the RCCA insertion limits.
† The EPR is equipped with RCCAs of the
proven HARMONI design that guarantees
a long operating life whatever the
operating mode of the reactor.
RCCA manufacturing at the FBFC Pierrelatte (France) fuel fabrication plant.
20 I