Areva EPR

■ EPR NUCLEAR ISLAND
REACTOR CORE
†
The reactor core contains the fuel material in which the fission
reaction takes place, releasing energy. The reactor internal
structures serve to physically support this fissile material,
control the fission reaction and channel the coolant.
The core is cooled and moderated by light water at a pressure of
155 bar and a temperature in the range of 300 °C. The coolant
contains soluble Boron as a neutron absorber. The Boron
concentration in the coolant is varied as required to control relatively
slow reactivity changes, including the effects of fuel burnup.
Additional neutron absorbers (Gadolinium), in the form of burnable
absorber-bearing fuel rods, are used to adjust the initial reactivity
and power distribution. Instrumentation is located inside and outside
the core to monitor its nuclear and thermal-hydraulic performance
and to provide input for control functions.
The EPR core consists of 241 fuel assemblies. For the first core,
assemblies are split into four groups with different enrichments (two
groups with the highest enrichment, one of them with Gadolinium).
For reload cores, the number and characteristics of the fresh
assemblies depend on the type of fuel management scheme
selected, notably cycle length and type of loading patterns. Fuel
cycle lengths up to 24 months, IN-OUT and OUT-IN fuel
management are possible. The EPR is designed for flexible operation
with UO 2 fuel and/or MOX fuel. The main features of the core and its
operating conditions have been selected to obtain not only high
thermal efficiency of the plant and low fuel cycle costs, but also
extended flexibility for different fuel cycle lengths and a high level of
maneuverability.
Core instrumentation
The core power is measured using the ex-core instrumentation, also
utilized to monitor the process to criticality.
The reference instrumentation to monitor the power distribution in
the core is an “aeroball” system. Vanadium balls are periodically
inserted in the core. Their activation level is measured, giving values
of the local neutron flux to construct the three-dimensional power
map of the core.
The fixed in-core instrumentation consists of neutron detectors and
thermocouples to measure the neutron flux distribution in the core
and temperature distribution at the core outlet.
The whole in-core instrumentation package is introduced from the
top of the reactor pressure vessel head. Therefore, the bottom of
the reactor pressure vessel is free from any penetration.
For additional information see the “Instrumentation and Control
systems” chapter, page 42.
The core design analyses demonstrate the feasibility of different
types of fuel management schemes to meet the requirements
expressed by the utility companies in terms of cycle length and fuel
cycle economy (reload fraction, burnup), and to provide the core
characteristics needed for sizing of the reactor systems. The nuclear
analyses establish physical locations for control rods, burnable
poison rods, and physical parameters such as fuel enrichments and
Boron concentration in the coolant. The thermal-hydraulic analyses
establish coolant flow parameters to ensure that adequate heat is
transferred from the fuel to the reactor coolant.
Isar 2 unit, Germany (KONVOI, 1,300 MWe): fuel loading operation.
16 I