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SUB-CHAPTER: J.4 SECTION : -

PAGE : 1 / 17 UK-EPR

FUNDAMENTAL SAFETY OVERVIEW VOLUME 2: DESIGN AND SAFETY

CHAPTER J: MAIN STEAM LINE SYSTEMS

SUB CHAPTER J.4 OTHER FEATURES OF STEAM AND POWER CONVERSION SYSTEMS

The secondary cooling system is described in Chapter J.1 (role, design basis, safety analysis). The secondary cooling system consists principally of the feed water and steam systems and the turbine generator unit.

Sub-chapter J.4 contains a brief description of the steam-water system. The main systems covered are:

o The condenser and the condensate extraction system (CEX),

o The turbine bypass to the condenser (GCT) [MSB],

o the feedwater plant (ABP-ADG-APA-AHP-AAD) [MFWPS-SSS],

o the cooling water system (CRF),

o the turbine gland system (CET),

o the steam generator blowdown system (APG) [SGBS].

1. CONDENSER (CEX)

To follow.

2. CONDENSER EXTRACTION SYSTEM (CEX)

To follow.

3. TURBINE BYPASS SYSTEM (GCT) [MSB]

3.1. ROLE AND DESCRIPTION

The role of the GCT [MSB] is to discharge the steam flow to the condenser when the turbine is unavailable. During normal operation, the power provided by the nuclear steam supply system is matched by the power consumed by the turbine.

However, during rapid transient conditions or power variations at low load, the power imbalance between the turbine and the reactor is compensated for by the opening of the GCT [MSB].





3.2. DESIGN BASIS

The turbine bypass to the condenser accommodates the discharge of excess steam from the nuclear steam supply system during transient conditions (house load operation, large load decrease, turbine trip, etc.). It is accordingly designed to handle approximately 60% of the nominal steam flow produced by the reactor.

o Following reactor trip, it prevents primary circuit heat-up and a demand on the atmospheric bypass (VDA) [Main Steam Atmospheric Dump System];

o it also allows, during start-up and shutdown phases, control of either the SG temperature rise or of SG cooling for the switch to RIS [SIS] in RHR mode;

o it controls the secondary pressure during turbine start-up and at synchronisation.

3.3. PRELIMINARY SAFETY ANALYSIS

The GCT [MSB] system is not safety classified.

The pressure reducing valves close on loss of fluid or the absence of a control signal.

Failure to open of a bypass valve can cause the VDA [Main Steam Atmospheric Dump System] to open.

Should a GCT [MSB] valve be left open, causing cooling of the primary circuit, the reactor protection system initiates a demand for steam isolation.

4. FEEDWATER PLANT (ABP-ADG-APA-AHP-AAD) [MFWPS-SSS]

To follow

5. COOLING WATER SYSTEM (CRF – FOR A COASTAL SITE)

To follow

6. TURBINE GLAND SYSTEM (CET)

To follow





7. STEAM GENERATOR BLOWDOWN SYSTEM (APG) [SGBS]

7.0. SAFETY REQUIREMENTS

7.0.1. Safety functions

The steam generator blowdown system (APG) [SGBS] performs the basic safety function of activity containment in the event of a steam generator tube rupture SGTR.

7.0.2. Functional requirements

The functional requirements linked to the safety function of the APG [SGBS] are as follows:

o retention of radioactive materials in the affected SG in the event of SGTR,

o transfer of a part of the water inventory of the affected SG in the event of SGTR (PCC-3 and PC-4) to another SG in order to prevent the SG from overfilling, which would cause contaminated water to be discharged to the environment.

o isolation of the containment

The APG [SGBS] must also meet the following requirements, indirectly linked to a safety function:

o Isolation of the secondary system, i.e.:

prevention of excessive mass flow rate during a leak or an APG [SGBS] pipe break, to prevent an unacceptable increase in containment pressure,

prevention of steam flashing in the unaffected SGs in the event of a non-isolatable pipework break in the main feedwater system in the containment.

o prevention of excessive mass flow rate from the feedwater tank into the containment (mitigating internal hazards).

o prevention of excessive mass flow rate of feedwater into the containment in the event of regenerative heat exchanger rupture (APG[SGBS]/CEX) to prevent the containment from flooding and, as a result, boron dilution in the IRWST.

7.0.3. Design Requirements

7.0.3.1. Requirements that result from safety classification

o 1: Safety classification

The APG [SGBS] system is safety classified as defined in Chapter C.2.

o 2: Single failure criterion





The single failure criterion applies to the active elements of the APG [SGBS] system that perform an F1 function.

o 3: backed-up power supplies

The F1 classified elements are supplied from the backed-up power supply switchboards.

o 4: Qualification for operating conditions

The APG [SGBS] is qualified to perform its safety function safety under the conditions which prevail when the safety function is demanded (see Chapter C.7).

o 5: Mechanical, electrical and I&C classification

The APG [SGBS] system is safety classified as defined in Chapter C.2.

o Seismic classification

The APG [SGBS] system is seismically classified as defined in Chapter C.2.

o 7: Periodic tests

Periodic tests will be carried out on the safety classified elements of the APG [SGBS] (F1 and F2) to confirm their availability with a sufficient degree of confidence.

7.0.3.2. Other regulatory requirements

o Basic Safety Rules

To follow.

o Technical Directives

There are no specific requirements in the Technical Directives regarding the APG [SGBS].

o Specific EPR texts

Not applicable.

7.0.3.3. Hazards

The requirements for protection against external and internal hazards are set down in Chapters C.3 and C.4 respectively.

7.0.4. TEST AND COMMISSIONING

a) Commissioning tests before operation

The system will be put through a commissioning test programme after installation, to confirm the capability to perform its safety functions.

b) Periodic tests





The system is designed for the performance of periodic tests.

7.1. ROLE OF THE SYSTEM

In conjunction with the REN [NSS], the APG [SGBS] steam generator blowdown circuit is used to maintain the radioactive and chemical properties of the feedwater within defined limits in all operating conditions of the plant, by continuously sampling the feedwater. The feedwater may be contaminated by corrosion products, by an internal leak of the condenser or by a primary-secondary system leak.

The APG [SGBS] is also used for:

o partially or completely draining secondary water from the SGs,

o mixing the secondary side of the SG, with the support of the nuclear island nitrogen distribution system, in the event that a chemical reagent is injected during cold shutdown,

o reprocessing of secondary water samples from the REN [NSS],

o participating in the isolation of the SGs in the event of SGTR, and possibly transferring the excess water from the affected SG to another.

After processing, the blowdown water is usually recycled and sent to the condenser or, exceptionally, to the KER [LRMDS].

7.2. DESIGN BASIS

7.2.1. Blowdown flow rate

In normal operation, the nominal blowdown rate for all the SGs is 1% of the total feedwater flow rate.

In the special case of operation, with three isolated SGs, the maximum continuous blowdown flow rate of the fourth SG is approximately 2% of the total water flow rate.

7.2.2. Blowdown water collection

Steam flashing is prevented by minimising pressure losses and by providing a continuous descending gradient between the SG nozzles and the flash vessel.

Each blowdown line is equipped with:

o two isolation valves in the Reactor Building, inside the missile protection shield (one on the blowdown line on the hot leg of the SG and the other on the blowdown line on the cold leg of the SG),

o a secondary system isolation valve, located outside the missile protection shield in the Reactor Building.





These valves guarantee the containment of radioactivity in the event of an SGTR, while preventing a partial and simultaneous blowdown of two SGs. They also allow for the isolation of three of the four SGs in order to achieve a higher blowdown flow rate on the fourth SG.

7.2.3. Blowdown expansion and cooling

Each SG blowdown is directed through its flash valve to reduce the pressure of the blowdown and regulate its flow rate. The four flash valves are connected to the flash vessel which performs the separation of the liquid/gas phases.

The SG blowdown water is then cooled to a temperature that enables the demineralisers to be maintained in a satisfactory state. A regenerative heat exchanger, located downstream from the flash vessel, cooled by the Condensate Extraction System (CEX), cools the water to a temperature lower than 55°C.

7.2.4. Treatment chain

After pressure reduction and cooling, the GV [SG] blowdown water is directed outside the containment to the treatment chain where it is firstly filtered and then decontaminated before being recycled to the condenser.

7.2.5. Sampling lines

The radioactive and chemical properties of the SG blowdown water are controlled through the REN/KRT [NSS/RPMS] lines.

As a result, connections with the REN [NSS] are devised:

o at the outlet of the SG, on the blowdown lines SG on the hot and cold legs (as the concentration of impurities can differ between the hot legs and cold legs),

o downstream from the treatment chain equipment.

All feedwater samples taken from the REN [NSS] are re-injected downstream of the regenerative heat exchanger.

The sample lines connected at the SG outlet are located upstream from the SG isolation valves, so as to be able to sample the secondary SG water and measure its chemical and radioactive properties even if the SGs are isolated.

7.2.6. SG transfer lines

To prevent the excessive filling of a SG affected by an SGTR and the discharge of contaminated water, transfer lines, one connecting SG1 to SG2 and another connecting SG3 to SG4 are installed between the APG [SGBS] blowdown lines on the hot leg of the SGs and upstream of the first isolation valves. They enable transfer of excess water from the affected SG to the unaffected neighbouring SG.

7.2.7. Fluid Properties

The APG [SGBS] controls and enables maintenance of SG feedwater quality.





The REN [NSS] monitors the following main control parameters:

o the cationic conductivity,

o the sodium content.

The other parameters monitored are the pH, the total conductivity and the various contents (ammonia, morpholine, calcium, suspended solids, ionised silica, chlorides, etc.).

The measured values are used:

o to adjust the blowdown flow rate of each SG (up to the maximum flow rate, so that the feedwater properties remain acceptable),

o to control the efficiency of the treatment chain to meet secondary side water quality requirements

7.3. SYSTEM OPERATION

7.3.1. System Description

7.3.1.1. Functional interfaces

APG [SGBS] interfaces with the following systems:

o primary cooling system: interface with the secondary part of the SGs,

o ADG feedwater tank: for the discharge of steam at reduced pressure from the APG [SGBS] flash vessel,

o CEX Condensate Extraction System:

for the return of blowdown water to the condenser after treatment,

for the cooling of the regenerative heat exchanger.

o Nuclear sampling system (REN [NSS]):

for the sampling of the blowdown water and chemical control of the treatment chain,

for the recycling of uncontaminated blowdown samples.

o Solid waste treatment system:

for the discharge of the spent resin from the demineraliser.

o Liquid waste discharge system:

for the discharge of blowdown water, if recycling the blowdown water to the condenser is not possible.

o Nuclear island nitrogen distribution system:





for the injection of nitrogen into the SGs (for agitating of the SG feedwater during shutdown, for mixing before sampling or after the injection of the chemical reagent).

o Demineralised water distribution system:

for blowing down the demineralisers,

for the makeup needed for the maintenance of the liquid seal at the treatment chain outlet.

o Vent and blowdown system of the nuclear island:

for the collection of waste from the APG [SGBS] venting and blowdown.

7.3.1.2. Blowdown collection

Each EPR steam generator is equipped with blowdown collection lines made up of two drains on the hot leg side and one drain on the cold leg. These three drains are located at the level of the SG tubesheet.

Two REN [NSS] lines are connected to these lines;

o one on the cold leg blowdown collection line,

o the other on the common hot leg blowdown collection line.

The cold leg and common leg blowdown collection lines are each fitted with one SG secondary side automatic isolation valve (mptor-operated valve).

An SGN (nitrogen distribution) line is connected to each blowdown line for the mixing of the secondary-side water.

The transfer lines which enable connection of SG1 to SG2, and SG3 to SG4, are connected to the blowdown collection lines on the hot leg in order to transfer the excess water volume of the affected SG to an unaffected SG in the event of SGTR.

7.3.1.3. Blowdown expansion and cooling

Each SG blowdown is directed through a dedicated flash valve.

Subsequently the pressure of the blowdown water is reduced in the flash vessel.

The blowdown flow rate of each SG is regulated by the four flash valves located upstream of and as close as possible to the flash vessel.

The pressure in the flash vessel is regulated by a control valve located on the line that removes the flashed steam to the feedwater tank (ADG).

The water level in the flash vessel is regulated by a control valve located downstream of the regenerative heat exchanger.





Subsequently the blowdown water is cooled by the regenerative heat exchanger (located in the Reactor Building), itself cooled by the CEX water.

After passing through the heat exchanger, the CEX water returns to the feedwater tank. The system comprising the flash vessel and regenerative heat exchanger is protected against overpressure by a pressure relief valve. The uncontaminated secondary-side samples from the REN [NSS] are handled by the APG [SGBS] by being re-injected downstream of the regenerative heat exchanger and upstream of the first filter of the treatment chain.

7.3.1.4. Blowdown treatment

The treatment chain can be isolated and protected against excessive temperatures by the containment isolation valves.

The blowdown water is decontaminated in the treatment chain after being cooled by the regenerative heat exchanger.

After initially being filtered, the blowdown water can be directed to:

o the liquid waste discharge system if recycling via the normal route to the condenser is impossible

o the normal treatment chain.

At the normal treatment chain outlet, the decontaminated blowdown water is discharged to the condenser, via a liquid seal that consists of a demineralised water supply nozzle.

7.3.2. System Materials

(to be completed and confirmed during the detailed design phase).

The pipework and equipment located upstream of the isolation valve of the initial filters (including this valve and excluding the flash vessel) are made of ferritic steel, the flash vessel is made of austenitic steel.

As the blowdown water has a pH between 6 and 8 after treatment, the pipework and equipment downstream of the demineralisers are made of austenitic stainless steel.

The chemical treatment of the blowdown water relies on two 50% treatment lines, with filters (2 x 100%) with cartridges and non-regenerative demineralisers.

7.4. OPERATING CONDITIONS

7.4.1. Definition of normal operating conditions

P > 15% PN

The normal operating conditions of the APG [SGBS] (P > 15% PN) are defined below:

o the four SGs are permanently blown down only on the hot leg,





o the normal blowdown flow rate is adjustable up to 23 t/hr per SG at full power,

o the blowdown water is flashed by the flash valves, then separated into its liquid and gas phases in the flash vessel (the flashing pressure depends on the conventional island conditions and is linked to the feedwater tank pressure). The water is then cooled by water from the CEX [Condensate Extraction System] in the regenerative heat exchanger,

o finally the blowdown water is filtered and treated before being recycled to the condenser,

o the flashed steam is returned to the feedwater tank.

P < 15% PN

In normal operating conditions with P < 15% PN, the four SGs are permanently blown down, simultaneously on the hot and cold leg, with the same operating conditions as mentioned above.

In addition, the use of the regenerative heat exchanger for the operations described above requires the availability of the condenser, a CEX pump and the feedwater tank.

7.4.2. Normal start-up

The ADG feedwater tank and a CEX pump are available.

The regenerative heat exchanger is available.

After the opening of the containment internal and external isolation valves, the required temperature and pressure are reached by opening the SG secondary isolation valves.

The flash valves are then opened until the total required blowdown flow rate is reached.

A level control valve adjusts the flow rate to obtain the operating water level in the flash vessel.

A pressure control valve adjusts the pressure in the flash vessel to its operating value.

The blowdown water is directed to the regenerative heat exchanger, then to the treatment chain or the KER [LRMDS] liquid waste discharge system.

7.4.3. Normal shutdown

Shutdown of the APG [SGBS] is performed as follows:

o gradual closure of the flash valves,

o isolation of the treatment chain to maintain a minimum volume of water in the flash vessel and thus a sufficient blowdown pressure for sub-cooling in the regenerative heat exchanger (to prevent water hammer),

o closure of the containment internal and external isolation valves,

o closure of the SG secondary isolation valves.





7.4.4. Other steady-state operating conditions

Other reactor steady-state conditions, such as standby or hot shutdown, or transient conditions after or before cold shutdown, can require the following:

o A blowdown collection alignment change: only one SG can be blown down with the required high blowdown flow rate, while the other three SGs are isolated.

o This configuration can occur in significant transient power conditions that lead to the large-scale suspension of solids in the SGs and in the feedwater plant.

o A blowdown discharge alignment change: the blowdown water can be discharged to the liquid waste discharge system rather than to the condenser.

This configuration can occur:

during start-up or cold shutdown of the plant,

during an internal condenser leak,

during an SGTR.

In each of these operating states blowdown water can have chemical or radioactive properties that may make it unsuitable for recycling to the condenser.

Preferential blowdown of a SG

Only one SG is blown down while the three others are isolated.

The maximum blowdown flow rate of an concerned SG is approximately 46t/hr.

Discharge without treatment

If the blowdown water cannot be recycled to the condenser after the first filtration, it is not treated in the demineralisation line, but is discharged in the liquid waste discharge system.

The maximum blowdown flow rate that can be discharged is 92t/hr for a duration to be specified at a later stage during the Detailed Design Phase.

The various chemical and radioactive limits applicable for this discharge will be defined at a later stage in specific reports.

SG draining

SGs can be drained through the APG [SGBS] when wet and cold storage or dry storage is required.

The draining, by gravity, can be partial or complete.

The drain water is directed to the condenser, or if this is not possible, via the blowdown system and RPE [NVDS] vent and drain system of the nuclear island.





7.4.5. Special transient operating conditions

Isolation of SG blowdown after start-up of the ASG [EFWS]

During ASG [EFWS] start-up, the APG [SGBS] is isolated as close as possible to the SG, to limit the feedwater volume lost for cooling.

Isolation of SG blowdown after activation of the containment isolation signal

The APG [SGBS] lines run through the containment. As a result, they are involved in containment isolation.

In the event of an SIS signal, all the APG [SGBS] containment isolation valves are closed.

7.4.6. APG [SGBS] protection devices – detection of activity in blowdowns

This event can cause contamination of the secondary side.

If activity is detected in the samples on the hot or cold legs, the affected SG is isolated by closing the associated isolation valves and the containment internal isolation valve.

7.4.7. Transfer of SG water in the event of SGTR

To prevent any liquid discharge due to an increase in the level of the affected SG in the event of SGTR, the APG [SGBS] is used to transfer water from the affected SG to the neighbouring unaffected SG.

One APG [SGBS] transfer line at the most must be used between the two SGs, and the blowdown lines of the affected SG must be isolated from the flash vessel. The transfer line isolation valve is then opened.

7.5. PRELIMINARY SAFETY ANALYSIS

7.5.1. Compliance with the regulations

To follow.

7.5.2. Achievement of safety functional requirements

The safety function performed by the APG [SGBS] is the containment of radioactivity released into the secondary system during an SGTR. Containment isolation is achieved by closing isolation valves on each SG. These isolation valves also prevent excessive mass flow rate during a leak or an APG [SGBS] pipe break, so as to prevent an unacceptable increase in containment pressure. They also prevent flashing in the unaffected SGs in the event of a non-isolatable pipework rupture in the main feedwater system.

In addition, in the event of an SGTR, part of the water inventory of the affected SG is transferred to another SG in order to prevent the SG from overflowing, which would cause contaminated water to be discharged to the environment.





Containment isolation of the APG [SGBS] system is achieved at three levels:

• at the blowdown outlet (liquid phase) of the APG[SGBS]/CEX heat exchanger,

• at the flash vessel blowdown outlet (steam phase) to the ADG feedwater tank,

• at the inlet and outlet of the heat exchanger CEX cooling water.

The following measures have been taken to limit the risks of flooding the containment (and diluting the IRWST) by water from the APG [SGBS]:

• a non-return valve is installed on the steam line to the ADG feedwater tank, preventing ADG feedwater from entering the enclosure should the line break;

• at the CEX water inlet to the containment, prevention of excessive feedwater flow into the containment in the event of APG[SGBS]/CEX heat exchanger break is achieved by a non-return valve on the CEX outlet line and isolation of the CEX with two motor-operated valves at the CEX inlet and outlet.

7.5.3. Compliance with design requirements

7.5.3.1. Safety classification

The compliance of the design and manufacture of the system with the design requirements, in line with the system safety classification, is set out in Chapter C.2.

7.5.3.2. Single Failure Criterion

The single failure criterion is applied to the active elements of the APG [SGBS] system that perform the F1 function. Thus:

• all the SG isolation valves are redundant: the valves at the blowdown collection on the cold and hot leg of the SG that are duplicated by a valve on the common collection line. This also applies to the REN [NSS] sample collection valves on the cold and hot leg (the second valve belongs to the REN [NSS] system);

• all the containment isolation devices are duplicated (see 7.5.1 within this Sub-chapter);

• there are redundant transfer lines between the SGs.

7.5.3.3. Qualification

The equipment is qualified in accordance with the requirements described in Chapter C.7.

7.5.3.4. Control and Instrumentation

The compliance of the design and manufacture of the control and instrumentation system with the requirements defined by its safety classification is set out in Chapter C.2.





7.5.3.5. Backed-up power supply

All the F1 safety classified valves have backed-up power supplies. The power supply circuits of these devices will be established at the detail design stage.

7.5.3.6. Hazards

The following tables summarise the hazards that are taken into account for the F1 safety classified part of APG [SGBS] system:


UK-

PAGE : 15 / 17 EPR



INTERNAL HAZARDS Protection required General protection Specific protection

introduced in the design of the system

Pipework ruptures - Ruptures of tanks, pumps and valves -

Internal missiles - Dropped loads -

Internal explosion - Fire -

Internal flood

No loss of more than one train

Reactor Building compartmentalisation +

fire segregation

Geographic separation

EXTERNAL HAZARDS Protection required General protection Specific protection

introduced in the design of the system

Earthquake √ Installation in the BAS and the BR Seismic design

Aircraft crash √ Installation in the BAS and the BR Seismic design

External explosion √ Installation in the BAS and the BR -

External flood √ Installation in the BAS and the BR -

Snow and wind √ Installation in the BAS and the BR -

Extreme cold √ Installation in the BAS and the BR -

Electromagnetic interference √ Installation in the BAS

and the BR -

BR: Reactor Building

BAS: Safeguard Building

7.6. TESTING, INSPECTION, MAINTENANCE

Testing, inspection and maintenance requirements for the APG [SGBS] system will be defined during the detailed study phase.

SUB-CHAPTER J.4 SECTION : -

FIGURE : 1 UK-EPR


PAGE : 16 / 17 CHAPTER J: MAIN STEAM LINE SYSTEMS

J.4 FIG 1 (FOLIO 1/2): FUNCTIONAL FLOW DIAGRAM OF THE APG [SGBS]

FUNCTIONAL MECHANICAL DIAGRAMSYSTEM :

SUB-CHAPTER J.4 SECTION : -

FIGURE : 1 UK-EPR


PAGE : 17 / 17 CHAPTER J: MAIN STEAM LINE SYSTEMS

J.4 FIG 1 (Folio 2/2): FUNCTIONAL FLOW DIAGRAM OF THE APG [SGBS]

FUNCTIONAL MECHANICAL DIAGRAMSYSTEM :

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