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What the reactor coolant system?

The loop, the fundamental element of the reactor coolant system

circuit primaire

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Every "loop" of the reactor coolant system is connected with the reactor vessel and consists of a reactor coolant pump, a steam generator and piping which connect these equipments. A pressurizer comes to add to these equipments, connected with one of the reactor coolant loops and has for role to maintain automatically the pressure to avoid that the water enters boiling. The water is circulation by the reactor coolant pump and an electric engine, and walks through the reactor coolant loop since the reactor core where it is warmed by the fission of the fuel inserted into the reactor vessel. Once taken out of the reactor vessel, this water crosses all the piping which lead it towards the steam generator where it sells its heat, then it is managed towards the reactor coolant pump, this last one renewing the water again in the core.
The reactors which equip the current power plants contain 2 in 4 loops following their design.

  • The reactor vessel

    The reactor pressure vessel and  the internal structures

    reactor vessel

    Technical characteristics of the RPV:

    • Weight: 420 tons
    • Height: 10.6 m / 34.4 ft
    • Diameter (internal): 5,3 m / 16.1 ft
    • Temperature in operation: > 300°C

    The reactor vessel is the main component of the reactor coolant system. It completely seals the core and the fuel assemblies contained therein.

    The thickness of the vessel’s carbon steel wall (at 20 to 30cm) and the stainless steel cladding guarantee the containment of the internal reactions.

    The RPV internals support the fuel assemblies and keep them properly aligned and spaced to ensure free motion of the control rods and core cooling by the primary coolant in any circumstances, including postulated accident circumstances.

    cuve ol3

    Main parts of the RPV internals:

    • Upper internals:
      • Rod Cluster Control Assembly (RCCA)
      • guides, RCCA
      • guide support plate,
      • upper core plate
    • Lower internals:
      • core barrel
      • the core support plate 
    • Heavy reflector.

  • Vessel heads

    the RPV closure head

    couvercle de cuve

    Technical characteristics of the RPV closure head:

    • Weight: 116 tons unassembled / 200 tons totally assembled
    • Height: around 2.2 m / 7.2 ft

    Weighing several metric tons and clad in stainless steel, vessel heads form a further protective barrier between the fuel inside the vessel and the outside atmosphere. Precision-machining permits integration of control rod mechanisms into the vessel head to remotely regulate the nuclear reaction.

    Vessel heads must effectively and safely guarantee both the integrity of the vessel (by sealing it) and the ability to remotely regulate the nuclear reaction inside it.
    The reactor pressure vessel and its closure head were manufactured by MHI at Kobe, Japan.

  • Steam generators

    The steam generators

    generateur de vapeur

    Technical characteristics:

    • Weight: 550 tons
    • Height: 25 m / 82 ft
    • Diameter (external): 5.2 m / 15.7 ft

    The four steam generators (SG) – one in each loop – are the interface between the primary water heated by the nuclear fuel and the secondary water which provides steam to the turbine generator.

    The reactor coolant – which remains in the liquid state as a result of the pressure – enters the SG through the channel head at an approximate temperature of 320°C/608°F, then flows through the tubes, where its heat is transferred to the secondary coolant, then flows out the other end of the channel head at a temperature of around 280°C/536°F. The heat transferred serves to convert the liquid coolant, which comes from the feedwater rings and flows downwards by gravity, into steam, which is exhausted through the top of the equipment.

    The 4 SG were manufactured by AREVA Chalon/St Marcel.

  • Coolant pumps

    The reactor coolant pumps

    The reactor coolant pump

    Technical characteristics:

    • Weight: 112 tons
    • Height: 9.3 m / 30.5 ft

    The reactor coolant pumps (RCP) provide forced circulation of water through the reactor coolant system. This circulation removes heat from the reactor core to the steam generators, where it is transferred to the secondary system.

    A RCP is located between the steam generator outlet and the reactor vessel inlet of each of the four primary loops.

    The RCP consists of three major components:

    • The pump hydraulic cell
    • The shaft seal system
    • The motor.

    The reactor coolant pumps were manufactured by AREVA JSPM at Jeumont.

  • Pressurizer

    The pressurizer

    pressuriseur circuit primaire

    Technical characteristics:

    • Weight: 155 tons
    • Height: 14 m / 46 ft
    • Diameter: 3.1 m / 10.2 ft

    The pressurizer role is to maintain the pressure of the primary cicuit inside prescribed limits. It is a part of this circuit, and is connected through a surge line to the hot leg of one of the four primary loops.

    The pressurizer was manufactured by AREVA Chalon/St Marcel.

  • The piping of the primary loops

    The piping of the primary loops

    The piping of the primary loops

    Technical characteristics:

    • Weight: 112 tons
    • Height: 9.3 m / 30.5 ft

    The piping of the four primary loops and the pressurizer surge line are part of the Reactor Coolant System. The reactor main coolant lines convey the reactor coolant from the reactor pressure vessel to the steam generators and then to the reactor coolant pumps, which discharge it back to the reactor pressure vessel. The surge line connects one of the four primary loops with the pressurizer.

    Each of the four reactor coolant loops comprises:

    • a hot leg, from the RPV to a steam generator
    • a cross-over leg, from the steam generator to a RCP 
    • a cold leg, from the RCP to the RPV.

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