Areva EPR

■ EPR NUCLEAR ISLAND
PRIMARY SYSTEM
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PRIMARY SYSTEM CONFIGURATION
The EPR primary system is of a well proven 4-loop design.
French 1,300 MWe and 1,500 MWe N4 reactors as well as German
KONVOI reactors are also of 4-loop design.
In each of the four loops, the primary coolant leaving the reactor
pressure vessel through an outlet nozzle goes to a steam generator
– the steam generator transfers heat to the secondary circuit –, then
the coolant goes to a reactor coolant pump before returning to the
reactor pressure vessel through an inlet nozzle. Inside the reactor
pressure vessel, the primary coolant is first guided downward outside
the core periphery, then it is channeled upward through the core,
where it receives heat generated by the nuclear fuel.
A pressurizer, part of the primary system, is connected to one of the
four loops. In normal operation, its main role is to automatically
maintain the primary pressure within a specified range.
The EPR main reactor components: reactor pressure vessel,
pressurizer and steam generators feature larger volumes than similar
components from previous designs to provide additional benefit in
terms of operation and safety margins.
The increased free volume in the reactor pressure vessel, between
the nozzles of the reactor coolant lines and the top of the core,
provides a higher water volume above the core and thus additional
margin with regard to the core “dewatering” time in the event of a
postulated loss of coolant accident. Therefore, more time would be
available to counteract such a situation.
This increased volume would also be beneficial in shutdown
conditions in case of loss of the Residual Heat Removal System
function.
Larger water and steam phase volumes in the pressurizer smooth
the response of the plant to normal and abnormal operating
transients allowing extended time to counteract accident situations
and extended equipment lifetime.
The larger volume of the steam generator secondary side results in
increasing the secondary water inventory and the steam volume,
which offers several advantages.
• During normal operation, smooth transients are obtained and thus
the potential for unplanned reactor trips is reduced.
• Regarding the management of steam generator tube rupture
scenarios, the large steam volume, in conjunction with a setpoint of
the safety valves of the steam generators above the safety injection
pressure, prevents liquid release outside the reactor containment.
• Due to the increased mass of secondary side water, in case of an
assumed total loss of the steam generator feedwater supply, the
dry-out time would be at least 30 minutes, sufficient time to recover
a feedwater supply or to decide on other countermeasures.
In addition, the primary system design pressure has been increased
in order to reduce the actuation frequency of the safety valves which
is also an enhancement in terms of safety.
Cattenom, France (4 X 1,300 MWe): inside a reactor building.
14 I