Areva EPR

■ EPR NUCLEAR ISLAND
Instrumentation (level 0)
A number of instrumentation channels supply measured data for
control, surveillance and protection systems and for information of the
control room staff. Multiple-channel acquisition is used for important
controls such as control of pressure and temperature of the primary
coolant, liquid level in the reactor pressure vessel. Multiple-channel
and diversified data acquisition means are implemented.
Aeroball system
Carrier gas
Concerning the protection of the reactor, a major aspect is the
capacity to predict and measure the nuclear power (or neutron flux)
level and the three dimensional distribution of power in the core.
Ball guide tube
The measurement of the power level is performed using ex-core
instrumentation which also provides signals to monitor the core
criticality. Relying on temperature measurements in the cold and hot
legs of the four primary loops, a quadruple-redundant primary heat
balance is achieved and complemented by neutron flux measurements
with very short response time.
Prediction and measurement of the three-dimensional power
distribution relies on two types of in-core instrumentation:
• “movable” reference instrumentation to validate the core design
and to calibrate the other sensors utilized for core surveillance and
protection purposes,
• “fixed” instrumentation to deliver online information to the
surveillance and protection systems which actuate appropriate
actions and countermeasures in case of anomalies or exceeding
of predefined limits.
The movable reference instrumentation for power distribution
assessment is an “aeroball” system. Stacks of vanadium-alloy balls,
inserted from the top of the pressure vessel, are pneumatically
transported into the reactor core (inside guide thimbles of fuel
assemblies), then, after three minutes in the core, to a bench where the
activation of each probe is measured at 30 positions in five minutes.
This gives values of the local neutron flux in the core, which are
processed to construct the three-dimensional power distribution map.
The fixed in-core instrumentation consists of neutron detectors
and thermocouples to measure the neutron flux radial and axial
distribution in the core and temperature radial distribution at the core
outlet. The neutron flux signals are utilized to control the axial power
distribution, and for core surveillance and protection. The core outlet
thermocouples continuously measure the fuel assembly outlet
temperature and provide signals for core monitoring in case of loss
of coolant event. They also provide information on radial power
distribution and thermal-hydraulic local conditions.
A
Shroud
Steel balls
Ball stop
Active core height
EPR in-core instrumentation
B C D E F G H J K L M N P R S T
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241 Fuel assemblies
89 Control rods
12 In-core detectors
40 Aeroball probes
12 Instrumentation
lance yokes
42 I