RCP steady state operating configurations are given in the following of the paragraph.

3. Reactor Coolant System (RCP) operating conditions

In this chapter the operating conditions of RCP in normal configuration (plant normal operation) will be briefly described together with the basic phases of transitory between these configurations.

The purpose of this description is to provide the framework in which process exposed in chapter 5 are performed and to motivate and clarify some of the assumptions taken in the modelling process.

RCP steady state operating configurations are given in the following of the paragraph.

̶ State A

This configuration characterizes the following standard states:

Normal power;

Reduced power;

Hot standby;

Subcritical reactor and primary coolant temperature is ****;

Intermediate shutdown on Steam Generators (decay power removed via SGs).

When RCP is in State A, control rods and boron injection via RCV control core reactivity.

Thermal power produced by fission or by nuclear decay reactions is removed via SGs, water circulation being assured by the operation of all four primary pumps.

Main RCP parameters, that are temperature, pressure and level in the PRZ, are controlled by I&C in order to keep primary coolant in the ranges defined by the plant operational diagram.

RCP mean temperature is controlled by means of rods motion and depends on the load according to a “reactor follows turbine strategy”. PRZ pressure is fixed at **** and it is controlled by means of proportional heaters, and the spray line. PRZ water level set point is changed accordingly to RCP temperature set point which in turn depends on the electrical load. PRZ water level set point varies to take into account water expansion and shrinkage with temperature. The level regulation is performed by the CVCS.

̶ State B (RCP in biphasic condition)

This configuration characterizes the following standard states:

Normal shutdown on steam generators.

In this state reactor core is subcritical (all rods are inserted); and heat is removed by steam generators. In this phase all four primary pumps are in operation.

PRZ pressure is controlled by means of spray line and heaters and its level is adjusted at the no load set point.

Water level in the steam generators is fixed at its no load set point.

̶ State C (Normal shutdown on RIS/RRA)

This configuration characterizes the following standard states:

Intermediate shutdown on RIS/RRA divided as follows:

• Two-phase intermediate shutdown on RIS/RRA;

• Solid phase intermediate shutdown on RIS/RRA;

Normal cold shutdown.

In this state reactor core is subcritical (all rods are inserted); and heat removal is assured by the (RIS/RRA)/RRI/SEC chain.

Number of primary pumps in operation changes according to primary coolant temperature:

when temperature is lower than **** some of the RIS/RRA trains are called in operation and correspondingly reactor primary pumps **** are shutdown; when temperature is lower than

**** other RIS/RRA trains are called in operation and at **** primary pumps are shutdown.

During the cool-down phase primary pumps are shutdown in sequence in order to minimize the heat amount that must be evacuated by the RIS/RRA.

Once all four primary pumps are shut down RCP depressurization (up to 1 bara) is implemented by means of the CVCS.

Once depressurized, RCP is drained manually at ¾ loop level while injecting N

for compensating for the volume creation. Level regulation is provided by the RCV at ¾ loop level.

̶ State D (cold shutdown)

This configuration characterizes the following standard states:

Reactor in cold shutdown;

Reactor in cold shutdown for maintenance.

In this configuration RCP is opened; reactivity is monitored by boron concentration meter;

water level is kept at ¾ level loop and regulated by CVCS.

Heat sink is provided by the four RIS/RRA operating trains that assure temperature does not exceed 60 °C.

With reactor in these conditions it is then possible to perform refuelling operations.

Safety analyses

domain

RHR connected to RCP

RCP status

Primary coolant level

RCP mean temperature

[°C]

RCP pressure [bar abs]

Primary pumps in operation A

B

D Open P=P

C

Full

3/4 loop Closed

No

Yes

4

Table 1: RCP States Synoptic table

3.1. Normal transients

In this paragraph normal transients between operational states exposed in §3, will be investigated.

̶ Plant shutdown: cooling/depressurization from State C to State D

It is in passing from State C to State D that RCP undergoes the second cooling phase. Cool down gradient is assured by automatic control acting on RIS/RRA heat exchangers control valves.

Before entering State D, RCP sweeping is performed (refer to chapter 5.2 for this phase)..

̶ Plant start-up: from State D to State C

This transition encompasses various process and procedures exposed hereunder.

1. Vacuum of the reactor before plant start-up

Reactor level is at ¾ loop (State D) an temperature is kept lower than 50 °C by RIS/RRA.

RVH and PRZ top venting line are lined to the RPE vacuum pump. The aim of this procedure

is to bring RCP under vacuum (0.2 bara) in order to avoid incondensable accumulation,

oxygen build up and air bubble formation under RVH, PRZ top, SG tubes during the filling phase. (See chapter 5.3 for further and more detailed information).

2. Filling of reactor coolant system and vacuum maintaining

The RCP is filled by the REA via RCV, while RPE vacuum pump keeps RCP pressure between 0.2 and 0.25 bara for the same reasons exposed in pt. 1 and RIS/RRA assures decay heat removal.

Reactor is in State C – Normal Cold Shutdown. (See chapter 5.3 for further and more detailed information).

3. Pressurizer pressure increase

RCP is isolated from RPE and pressure increase is started by RCV and further start-up

operations are performed.

References

[1] Areva’s internal documentation.