5. The Nuclear Island Vent and Drain System (RPE)
The purpose of this chapter is to describe the system and the processes that has been the subject of this study.
The RPE system (Nuclear Venting and Drain System) is used to collect and route primary gaseous and liquid effluents to systems in charge for their treatment.
A non exhaustive list of the tasks performed by this system is the following:
̶ Leakage recovery from primary circuit (pump seals, valve casings, RPV, etc); ̶ Floor draining;
̶ Venting of the Pressurizer Relief Tank;
̶ Venting the steam phase of the PRZ during plant shutdown.
Two other functions are performed by a dedicated sub-system of the RPE: the RPE vacuum unit (see §5.1):
̶ RCP nitrogen/air sweeping;
̶ RCP vacuum pulling and RCP vacuum maintaining during RCP filling by RCV. Following paragraph will deal with these two processes, more in detail §5.1 will describe the RPE vacuum unit that has been the main subject of the study; and §5.2 and 5.3 provide information about the two aforementioned processes carried out by the RPE vacuum unit and the I&C functions involved in these processes. It is worthy to remark that the final scope of the study is to validate/optimize these I&C functions.
5.1.
Nuclear Island Vent and Drain (RPE) vacuum unit
The RPE vacuum unit is composed by the following components/pipelines (see Figure 15): ̶ A water ring vacuum pump (VP) and its ancillary heat exchanger;
̶ The supply pipeline of the vacuum pump;
̶ A by pass line from the discharge line to the supply line with a control valve (BP) on its half;
̶ A water separator and its drain pot;
̶ A discharge line that can be lined to TEG or EBA by opening the TEG admission valve (CV2) and closing the EBA isolation valve (IV) and vice versa respectively;
̶ Pressure transducer (PT1) upstream the vacuum pump; pressure transducer (PT2) downstream the vacuum pump on the outlet line of the phase separator; temperature transducer (TT) measuring the temperature into the phase separator; volumetric flow rate transducer (composed of two pressure transducers and an orifice plate), (FT) on the discharge pipeline to the TEG system.
The RPE vacuum unit is connected to the RCP by means of two venting lines: ̶ One on the top of the reactor pressure vessel head;
̶ And the second one on the top of the pressurizer.
Figure 15: RPE Vacuum unit
Pump extracts a wet mixture (gas and water steam) from the RCP via the RVH and/or the PRZ top according to needs of the process. After an isothermal compression, the mixture is discharged to the water separator to separate the liquid phase from the gaseous one. The mixture entrains some water also during compression in the pump.
At the outlet of the separator the gas can be routed to one of the interfacing systems of the RPE vacuum unit according to the characteristics of the gas: TEG or EBA.
The by pass line is used when the gas flow must be discharged to the TEG: this system can accept a maximum volumetric flow rate of 100 Nm3/h while the working point of the pump is of 400 Nm3/h. this constraint is due to the presence of active charcoal filters in the TEG system: the maximum flow rate must not be exceeded in order to assure a minimum delay time to reduce activity of the gas.
5.2.
Reactor Coolant System (RCP) Sweeping
RCP sweeping is performed after reactor shutdown in the transition between operational state C and D. Main purpose of the sweeping is the elimination of hydrogen and fission gases from the primary volume before vessel opening.
5.2.1. The process
During plant shut-down after RCP cool down, water level is brought and kept constant at ¾ loop level by means of the RCV. Moreover RCP temperature is controlled by RIS/RRA system (it must be lower than 60°C).
Sweeping is performed by flushing the whole primary volume firstly with a flow of nitrogen because of the potential presence of hydrogen, and secondly with a flow of air. During nitrogen sweeping a pressure of 0.8 bara is maintained in the RCP to prevent from the risk of anoxia.
Air sweeping is performed to eliminate the RCP atmosphere rich in nitrogen before vessel opening. During this phase a pressure of 1 bara is maintained in the RCP.
In both cases the gas is provided by the SGN system and it is injected through the four primary pumps seal number one and reactor vessel head; a control valve is installed before the branching off of the line for purpose of pressure control into the RCP. Gas is evacuated from the top of the pressurizer by means of the RPE vacuum pump.
When sweeping by nitrogen, gas extracted by RPE is routed to TEG, while when sweeping by air gas is routed to EBA (no constraints on volumetric flow rate delivered to it).
The by pass line is used during nitrogen sweeping because the maximum volumetric flow rate to TEG is 100 Nm3/h (consequently this is the injection flow rate from SGN) while working point of the pump is at 400 Nm3/h.
Inlet gas flow rate Flow rate to TEG Flow rate to EBA Pump flow rate RCP pressure set point N2 sweeping 100 Nm 3 /h 100 Nm3/h - 400 Nm3/h 0.8 bara Air sweeping 400 Nm3/h - 400 Nm3/h 400 Nm3/h 1 bara
5.2.2. I&C Functions
To assure previous conditions in the systems involved in the sweeping process, some operational I&C functions are implemented. Some of these are closed loop automatic regulations and some are manual regulation performed by the operator from the MCR.
The fundamental aspects of this chapter apply either to N2 sweeping either to air sweeping; when there will be a difference among the two procedures, conditions referring to air sweeping will be enclosed in [square brackets].
There are two closed loop regulations to implement:
̶ The pressure control upstream the vacuum pump (representative of the pressure inside the primary volume);
̶ Pressure regulation downstream the vacuum pump (representative of the pressure upstream of the TEG admission valve).
These two regulations will be verified and if needed optimized.
There is one manual regulation to control the volumetric flow rate to TEG.
All these regulations are performed by means of operational I&C systems based on SPPA-T2000 platform.
Transducers and actuators are managed by acquisition and control modules respectively and software functions implemented in the SPPA-T2000 platform. For the purpose of this study it is possible to group the totality of these functions into a “complex block” that has some input signals, then performs some processing and then outputs something. According to the structure of the control loop there will be two types of complex block:
̶ Complex block for measurement acquisition; ̶ Complex block for valve control;
̶ Complex block for electrical motor driving.
More details about these blocks and the way they have been implemented into the model are provided in §6.3.
5.2.2.a. Pressure upstream vacuum pump (VP)
The objective of this regulation is to maintain a pressure of 0.8 [~1] bara in the RCP during sweeping.
It involves a closed loop control where the process deviation signal, generated by means of the measure acquisition transducer PT1 and the set point window on PICS, is sent to a step PI controller that acts the regulation valve CV1 (see Figure 18).
Moreover following invalidity signals are provided to the complex block associated to the control of CV1 (see Figure 16):
̶ PT1 measurement invalid: measurement provided by PT1 is invalid and CV1 is forced in manual mode. -+ Set point P PT1 Measurement Measurement invalid CV1 Process deviation Force manual mode
ACQ CTRL
Binary signal
Analogue digitalized signal
Figure 16: Logic for the regulation "Pressure upstream VP"
5.2.2.b. Pressure downstream vacuum pump (N2 sweeping only)
This regulation is required in order to assure a constant pressure upstream the valve CV2 that is operated in manual mode and controls the volumetric flow rate to TEG. When gas flow rate is routed toward EBA, valve BP is placed in manual mode and closed since there is no need to control the volumetric flow rate that flows towards the EBA system.
When the vacuum pump VP is started up, the valve BP is manually switched to automatic mode in order to control the pressure measured downstream of the vacuum pump by the transducer PT2.
The regulation is performed by a closed loop that involves a process deviation signal generated by means of the measure acquisition transducer PT2 and the pressure set point
(specified by the operator on a window of the HMI). This difference forms the input for a step PI controller, which operates control valve BP (see Figure 18).
Moreover following invalidity signals are provided to the complex block associated to the control of BP (see Figure 17):
̶ Minimum pressure at pump suction: it is provided by the complex block that manages PT2 to the complex block that controls BP. On appearance of this signal BP opens with priority over all other commands to restore optimal pressure condition at pump inlet and to assure pump integrity. An alarm is switched on in the MCR too on PICS. ̶ PT2 measure invalid: it is provided by the complex block that manages PT2 to the
complex block that controls BP. On appearance of this signal BP opens with priority over all other commands to safeguard the pump from working at wrong pressure condition.
̶ Maximum temperature in the phase separator: it is provided by the complex block that manages TT to the complex block that controls BP. On appearance of this signal BP opens with priority over all other commands. The temperature in the phase separator is greater than a threshold and so BP is opened to prevent further inlet of hot gas into the pump and to reduce the temperature of the pump by means of gas recirculation: in fact an amount of heat is removed from the gas flow at each passage of it through the pump. Before this maximum is reached an alarm alerts the operator in the MCR. The vacuum pump is protected from the effects of ingesting water during sweeping operations due to a high level (96% of the Measuring Range - MR - of cold calibrated PRZ wide range level transducers) in the PZR. In this case the signal that drives the “organ protection – close” (not shown in Figure 17) input of the complex block that manage the vacuum pump, is switched to TRUE (=1 – high logic level).
Figure 17: Logic for the regulation "Pressure downstream VP"
5.2.2.c. Volumetric flow rate to TEG (N2 sweeping only)
This regulation is performed manually by the operator in MCR in order to keep volumetric flow rate evacuated to TEG constant at 100 Nm3/h. Flow rate to TEG measurement is available on the Human Machine Interface (min=0 Nm3/h max=110 Nm3/h) so that the operator can adjust the opening of the valve CV2 (see Figure 18).
Figure 18: Closed loop regulations during sweeping phase (simplified)
5.3.
Vacuum pulling and maintaining
This procedure is implemented after refuelling during the transition from operational state D to operational state C. It is divided in two phases: the vacuum pulling in the RCP and the vacuum maintaining during RCP filling.
Main purpose is to evacuate the air in the RCP in order to avoid air bubble formation during filling and avoid oxygen solution into water.
5.3.1. The process
After refuelling during plant start-up, RPE system is lined to the RCP. A pressure of 0.2 bara is reached. At this point filling of the RCP by the RCV is implemented.
A pressure of 0.2 bara is maintained during the filling of the RCP from ¾ loop level to 90% of the pressurizer MR level. At a first time pump suction is either from PRZ and RPV head; when water level reaches the PRZ surge line, RPV suction line is isolated from the RPE to prevent the vacuum pump from water ingestion and venting to RPE is performed manually by
the operator: a manual valve is closed when no more air is seen flowing through a probe by the operator.
Either during vacuum pulling, either during vacuum maintaining, the wet mixture (air and water steam) is routed to EBA.
5.3.2. I&C Functions
For this regulation no frequency study is going to be carried on since it involves an ON/OFF type control. Only time domain data will be used to optimize the system.
Once the sub atmospheric pressure is established (~0.2 bara), the operator will begin filling the RCP and will place the vacuum pump in automatic mode.
When the vacuum pump is in automatic mode during filling operation, it will maintain primary circuit pressure (as measured in the PZR by a pressure transducer on the spray line) within a predetermined range. A restriction on the time between an OFF and a successive ON order is placed in order to assure a time to the water ring to settle. This interval will be named rest time and at the moment it is set to 300 seconds.
To this aim the following logic signals are provided to the complex block that drives the vacuum pump (see Figure 19):
̶ MIN1: it is provided by the complex block that manages the pressure transducer to the complex block that controls VP. The pressure in the RCP is lower than MIN1 (0.2 bara) consequently pump requires to be switched OFF. This signal locks the starting of the pump too.
̶ MAX1: it is provided by the complex block that manages the pressure transducer to the complex block that controls VP. The pressure in the RCP is higher than MAX1 (0.25 bara) consequently pump requires to be switched ON. This signal locks the shutting off of the pump too.
Figure 19: Logic for the pulling/maintaining phase
