PAGE : 1 / 16 7. CONTAINMENT HEAT REMOVAL SYSTEM (EVU [CHRS]) 7.0. SAFETY REQUIREMENTS 7.0.1. Safety functions The main functions of the EVU system [CHRS] are to limit the pressure inside the containment and to ensure decay heat removal from the containment in severe accidents (RRC-B). - Control of reactivity The system does not directly contribute to performing the safety function of reactivity control. - Decay heat removal a) The EVU system [CHRS] transfers the decay heat from the IRWST to the ultimate cooling water system using a dedicated cooling system: during severe accidents (RRC-B), in RRC-A pipe breaks with loss of the ISBP [LHSI], transiently in certain PCC-4 pipe breaks with the RIS [SIS] in RRA [RHR] mode, a ) The intermediate cooling system of the EVU [CHRS] train 1 also provides cooling of the third PTR [FPCS] train when the latter is operating (PCC or RRC-A). - Containment of radioactive substances b) The EVU system [CHRS] transfers decay heat from the containment atmosphere to the IRWST during a severe accident (RRC-B) in order to maintain the containment pressure at values that are compatible with maintaining its integrity. c) The EVU system [CHRS] carries out flooding of the corium spreading compartment with water from the IRWST during a severe accident (RRC-B). d) The EVU system [CHRS] ensures cooling of the reactor building foundation raft during a severe accident (RRC-B). e) The EVU system [CHRS] participates in containment isolation during accidents that do not require its operation. f) The part of the EVU [CHRS] (main system) located outside the containment constitutes a containment barrier (to maintain the containment integrity and to contain radioactive substances) during an accident that requires its operation. 7.0.2. Functional criteria The EVU system [CHRS] meets the following functional criteria:
PAGE : 2 / 16 Not applicable. - Control of reactivity - Decay heat removal a) The EVU system [CHRS] cooling capacity must be sufficient to ensure, in all system operating situations, decay heat transfer from the IRWST to the SRU [ultimate heat sink] (see Chapter I.1.6). The risk of EVU [CHRS] filter clogging must be taken into account. a ) The intermediate cooling system of the EVU [CHRS] train 1 must be in service when the third PTR [FPCS] train is started up (see Chapter I.1.3). - Containment of radioactive substances b) The EVU system [CHRS] suction capacity must be sufficient to ensure the heat transfer from the containment atmosphere to the IRWST, so that the containment pressure limits are met (see pressure curve). c) The EVU system [CHRS] flooding capacity must be sufficient to ensure the passive flooding function of the corium spreading compartment in order to cool the corium. d) The EVU system [CHRS] cooling capacity must be sufficient to ensure the foundation raft cooling function in order to protect the latter. e) It must be possible to isolate the part of the EVU system [CHRS] located outside the containment from the containment in the event of an accident that does not require its operation. f) Provisions must be made to prevent any leak on the main system outside the containment. Note 1: The EVU system [CHRS] must enable an RCV [CVCS] pump suction line to be connected to the IRWST, in the event of an accident that does not require its operation (see Chapter I.3.2). Given the containment characteristics (volume, design, thermal inertia of the containment structures) and those of the IRWST, a grace period of at least 12 hours must be available after the start of a severe accident (during this period, no system is to be required for maintaining the containment pressure at a level below the design pressure; see note 2). Note 2: This 12-hour time period is not an EVU system [CHRS] start-up criterion, but only a design value for designing the system capacities.
PAGE : 3 / 16 Short-term functional criterion: - Starting up two EVU [CHRS] trains after a 12-hour grace period must be able to reduce the containment pressure below 2 bar within 24 hours, - Starting up an EVU [CHRS] train after a 12-hour grace period must enable the containment pressure to be maintained below the design pressure (5.5 bar). Long-term functional criterion: Over the long term, one EVU [CHRS] train must enable the containment pressure to be maintained below 2 bar. Pressure curve: If 2 trains are started up 12 hrs after the severe accident. The containment pressure limit curve Absolute pressure in MPa 0,55 0.2 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 Time in hours 7.0.3. Requirements relating to the design 7.0.3.1. Requirements from safety classifications - Safety classification The EVU [CHRS] is classified in accordance with the classification presented in Chapter C.2. - Single failure criterion (active and passive) For components that provide F1 functions (containment isolation), the single failure criterion is applied to ensure a sufficient degree of redundancy. - Emergency power supplies The EVU system [CHRS] has an emergency power supply so that the operation is ensured in the event of station blackout.
PAGE : 4 / 16 - Qualification for operating conditions The requirements relating to the EVU system [CHRS] qualification are presented in Chapter C.7. - Mechanical, electrical, instrumentation and control classifications The EVU system [CHRS] mechanical, electrical and instrumentation and control classifications are defined in accordance with the classifications presented in Chapter C.2. - Seismic classification The EVU system [CHRS] seismic classification is defined in accordance with the classification rules presented in Chapter C.2. - Periodic tests Periodic tests are performed regularly on the EVU system [CHRS] to ensure its availability. The classified systems must be easily accessible to allow periodic tests to be carried out. 7.0.3.2. Other statutory requirements - Official documents, laws, orders and decrees To follow. - Basic Safety Rules Not applicable. - Technical guidelines EPR Technical Guidelines are presented in Chapter C.1.2. Specific requirements for the EVU system [CHRS] are: - Section A.1.3: General strategy relating to severe accidents - Section B.1.4.2: Prevention of containment bypasses - Section B.2.3.5: The containment heat removal function - Section E.2.3.1: Corium cooling capacity outside the vessel - Section E.2.3.2: Removal of containment heat without venting - Specific EPR documents Not applicable. 7.0.3.3. Hazards Internal and external hazards taken into account in the EVU [CHRS] design are presented in Chapters C.3 and C.4.
PAGE : 5 / 16 7.1. SYSTEM FUNCTION The EVU [CHRS] has no function during normal operation. The main functions of the EVU system [CHRS] are to limit the pressure inside the containment and to ensure decay heat removal from the containment during severe accidents (RRC-B). The EVU system [CHRS] also ensures cooling of the 3 rd PTR [FPCS] train and decay heat transfer from the IRWST to the ultimate cooling water using a dedicated cooling system in RRC- A pipe breaks with loss of the ISBP [LHSI] and transiently in certain PCC-4 pipe breaks outside the containment with the RIS [SIS] in RRA [RHR] mode. 7.2. DESIGN BASIS The EVU [CHRS] design is based on an assumed thermal power of 4,500 MWth. The EVU [CHRS] is designed for the most conservative conditions of decay heat and IRWST temperature, irrespective of the system operating conditions (RRC-A, RRC-B and PCC-4) and the SRU [ultimate heat sink] (see Chapter I.1.6). Redundancy Application of the single failure criterion is not required as the EVU [CHRS] is an F2 classified system. However, the EVU [CHRS] comprises two separate trains so that the long-term failure of a train does not prevent the system from fulfilling its function. Long-term reparability It is possible to carry out long-term maintenance operations on the main system pumps and heat exchangers following a severe accident. Additional design requirements - Spray nozzles must be protected from blocking. - The main pumps must be able to operate with water containing small particles; they must remain operational over long periods of time and be leaktight. - All sensitive parts (seals, flanges, etc.) must be able to withstand high irradiation (Chapter C.7). 7.3. EQUIPMENT DESCRIPTION AND CHARACTERISTICS 7.3.1. Description The EVU [CHRS] consists of two trains (see F.2.7 FIG 1), each one including: 1) A main system with: a suction line from the IRWST using a dedicated filter for the EVU [CHRS] belonging to the RIS [SIS],
PAGE : 6 / 16 a pump and a heat exchanger. The heat exchanger, used to remove the decay heat from the containment, is supplied by a dedicated intermediate cooling system, a spray system (consisting of a ring equipped with spray nozzles) to reduce the pressure and the temperature inside the containment, a passive flooding system located in a compartment separate from the spreading compartment and the IRWST. This system includes a flooding valve maintained closed by a system of cables. During a core meltdown, the corium melts the cable(s) and the valve opens under the pressure of the water, a corium spreading compartment active flooding line (for long-term severe accident management), a cooling system for the foundation raft and the corium located below the spreading compartment layer of sacrificial concrete, ensuring that the water is poured from above into the spreading compartment. The cooling system for the foundation raft structure and corium is connected to the IRWST and to the active flooding lines via the flooding valves, a line dedicated to unclogging the sump filters. This line from an EVU [CHRS] train returns water into the sumps of the two EVU [CHRS] trains. It therefore enables the suction filter from the other EVU [CHRS] train to be unclogged in "crossed" unclogging mode ("direct" unclogging is not effective in this configuration see Chapter F.3.2). The line is also used as a test line. a second unclogging line which is connected between the RIS [SIS] and EVU [CHRS] systems. The water is drawn from an RIS [SIS] sump and discharged into an EVU [CHRS] sump. This line allows an EVU [CHRS] filter to be unclogged in "direct" mode (diversification from the "crossed" unclogging mode see Chapter F.3.2). A connection to the RCV [CVCS] on the suction line of the two trains, and to the discharge nozzle, or on the unclogging line of train 2.
PAGE : 7 / 16 2) An intermediate cooling system consisting of: a pump used to supply the EVU [CHRS] main system heat exchanger and, for one train only, to supply the heat exchanger of the third PTR [FPCS] train, a heat exchanger supplied by the EVU [CHRS] dedicated cooling system (SRU [ultimate heat sink]), an expansion tank maintained pressurised by dedicated means and an intermediate system feedwater supply system to meet makeup water requirements, 3) A dedicated cooling system, the SRU [ultimate heat sink] (see Chapter I.2.6). 7.4. OPERATING CONDITIONS 7.4.1. Normal operation When the unit is in normal operation, the EVU system [CHRS] is not in service: - the pumps are shut down, - the flooding valves are closed, - the containment isolation valves are closed, - the intermediate cooling systems are pressurised. However, the third train of the PTR [FPCS] may be started up in the case of non-availability of a main PTR [FPCS] train during preventive maintenance of the PTR [FPCS] or its support systems. The SRU [ultimate heat sink] and the intermediate system for the EVU [CHRS] train 1 are in service when the third PTR [FPCS] train is started up. 7.4.2. Severe accidents The system is designed to operate in the event of severe accidents with core meltdown (RRC-B). The safety injection system is (or was) out of service. Emergency plant cooldown diesel generator sets are available in the event of station blackout. Other emergency plant auxiliaries (RRI [CCWS] and SEC [ESWS] Systems, emergency power supply) are not necessary. Passive flooding of the corium
PAGE : 8 / 16 The arrival of corium in the spreading compartment melts the cables connected to the flooding valves. The cables therefore no longer maintain the flooding valves in their closed position. The flooding valves are located in a separate compartment from the spreading compartment and the IRWST. These valves open under the static pressure of the water. The water from the IRWST circulates through the discharge channel, which is now open, into the cooling channels in the foundation raft. When these channels are full, the water pours from above onto the corium and the latter is flooded (see Chapter F.2.6). Spraying On contact with the corium, the water flowing from the IRWST boils and the corium is cooled. The steam production causes the containment pressure and temperature to increase. 12 hours after the start of a severe accident, if necessary, the operator starts up the one or both trains to maintain the containment pressure and temperature within design limits. The EVU [CHRS] may be started up before the end of the 12-hour grace period. Startup is manual and the decision to start the EVU [CHRS] is based mainly on a pressure criterion. Water is drawn from the IRWST and is cooled in the heat exchangers before being sprayed into the containment from the reactor building dome. Once the pressure has dropped below its long-term nominal limit (2 bar) and after at least 15 days if MOX fuel is used (10 days with UO 2 fuel), a single train is enough to maintain the pressure below 2 bar. Furthermore, when spray is no longer needed to maintain a low containment pressure, corium cooling may also be provided by the active flooding line: the water heated by the corium returns to the IRWST by overflowing from the spreading compartment. 7.4.3. RRC-A accident The EVU [CHRS] is also required to operate in RRC-A pipe breaks with loss of the ISBP [LHSI]. In this case, the EVU [CHRS] is used to ensure decay heat removal via the IRWST. This is the only RRC-A accident requiring the EVU [CHRS] to be operated. 7.4.4. PCC-4 accident The EVU system [CHRS] is also required to operate during certain PCC-4 pipe breaks with the RIS [SIS] in RRA [RHR] mode. In this case, the EVU [CHRS] is used to remove the decay heat via the IRWST. 7.5. PRELIMINARY SAFETY ANALYSIS 7.5.1. Compliance with the regulations To follow. - Official documents, laws, orders and decrees
PAGE : 9 / 16 - Basic Safety Rules Not applicable. - Technical guidelines To meet the requirements of Technical Guidelines listed in section E.2.3.2: Removal of the containment heat without venting, the EVU [CHRS] design features are as follows: Potential leaks from the system During a severe accident, contaminated fluid circulates in the EVU [CHRS] main systems. The following precautions are taken to limit the doses outside containment: 1) The EVU [CHRS] main system components outside containment are classified in accordance with the classification presented in Chapter C.2. 2) The main system equipment located outside containment is leaktight: - bellows seal valves or leakoff connection valves, - tubular heat exchangers, - pump with double mechanical seals with an injection system at the seals to ensure the leaktightness. 3) Sensors are placed on the EVU [CHRS] main system to rapidly detect leaks and reduce the risk of leaks. 4) Appropriate instrumentation is installed (activity, dose level, water level). 5) The EVU [CHRS] main system components located outside containment (valves, penetrations, pumps, heat exchangers, etc.) are installed in dedicated compartments with specific protection (air lock, thick walls, etc.) 6) Filtered ventilation before discharge into the stack enables dedicated rooms to be isolated: At the same time as starting up the EVU [CHRS], the air supply to the EVU [CHRS] compartments is isolated by leaktight dampers. If the EVU [CHRS] is started up, air from the EVU [CHRS] rooms is extracted and filtered via DWL [CSBVS] iodine filtering system. This maintains a constant negative pressure and prevents the contamination from spreading. The exhausted air passes through high efficiency filters and iodine traps before being released into the stack. 7) Each EVU [CHRS] train may be isolated if a leak is detected in its room by activity sensors or by water level sensors in the room.
PAGE : 10 / 16 8) The pressure of the EVU [CHRS] dedicated intermediate cooling system is greater than the operating pressure of the EVU [CHRS] main system. This ensures the absence of leaks from the main system into the cooling system, via the main heat exchanger in the event of a tube rupture in the latter. This pressurisation is maintained during normal operation to ensure the availability of the EVU [CHRS] system if demanded. Also, a dedicated system enables the water inventory to be maintained (small leaks) in the cooling system without having to depressurise the system. 9) Detection means are provided to enable the dedicated intermediate cooling system to be isolated in the event of a leak in the main heat exchanger (frequent water makeup, low level in the tank). 10) Given that a single isolation valve exists on each of the lines between the IRWST and a main EVU [CHRS] pump, each isolation valve (outside containment) is designed with a special leaktight device and the section of the pipe between the IRWST and the valve is contained in a leaktight sheath (with double seals) thus offering a double anti-leak penetration barrier. Common causes of failure of the EVU [CHRS] with other systems The common causes of potential failure for the EVU [CHRS] and RIS [SIS] functions may include: - loss of the RIS [SIS] IRWST (following a loss of water or blockage of the water intake channels), - loss of common auxiliary functions (cooling water, power supply), The methods implemented to eliminate or limit the consequences of these common causes are described below: - Loss of water from the IRWST: The IRWST is lined to protect the concrete against the permanent presence of water during the reactor life. The liner is not required to ensure the leaktightness and even if it is damaged, there are no consequences on the correct operation of the IRWST. To prevent the possible loss of water due to the presence of a leak in a pipe connected to the IRWST, any train from the safety injection system in which water circulates outside the containment is isolated (with an F1A classified isolation) if a leak indication appears outside the containment (via water level and/or pressure measurement in the RIS [SIS] pump rooms). - Clogging of the water intake channels in the IRWST: Provisions are made to prevent the sumps from blocking: - provision of suction protection via filters, - use of suitable thermal insulation (type of components), - use of suitable devices for retaining insulation and other materials. Also, additional provisions are made to prevent clogging; in particular: - separate suction lines are used for the RIS [SIS] and the EVU [CHRS]
PAGE : 11 / 16 - there is geographical separation of the sumps used by the RIS [SIS] and the EVU [CHRS] - an EVU [CHRS] filter back-flushing system (counter-current water injection inside the filters) is employed. This system enables these filters to be unclogged if blocked. A dedicated unclogging line exists for each EVU [CHRS] train (see Chapters C.1 and F.3.2 ). - Loss of common auxiliary functions: The dedicated cooling systems and the power supply by the emergency plant diesel generator sets have been implemented to improve the system efficiency and to meet the objectives in respect of probabilistic safety analysis. Long-term corium stabilisation in the spreading compartment After the flooding valves have passively opened, the heat extracted by natural circulation in the cooling channels is sufficient to ensure corium stabilisation, even over the long-term. In the long term, the EVU [CHRS] also facilitates active cooling by supplying the recovery tank with cold water Prevention of spray nozzle blockage Screen filters with adequate diameters are located upstream of the IRWST suction lines. - Specific EPR documents Not applicable. 7.5.2. Compliance with the functional criteria Safety function a) is carried out by cooling the water from the IRWST via the EVU [CHRS] main heat exchanger which is cooled using a dedicated intermediate cooling system. The intermediate cooling system is cooled in turn by the SRU [ultimate heat sink]. An unclogging system reduces the risk of the filters blocking. Safety function a ) is carried out by using the PTR/EVU [FPCS/[CHRS] heat exchanger which is cooled by the EVU [CHRS] intermediate cooling system (train 1), itself cooled by the SRU system [ultimate heat sink]. Safety function b) is carried out by drawing cold water from the IRWST and spraying it through spray rings at the top of the containment. The spray water causes the containment atmospheric steam to be cooled and condensed. The condensate falls by gravity into the IRWST. The condensed water is recirculated through heat exchangers and returned to the spray nozzles. The decay heat is transferred to the containment atmosphere by boiling in the corium recovery area. The boiled water is replaced by water from the IRWST. Safety function c) is carried out by opening the dedicated flooding valves which connect the IRWST and the spreading compartment through the foundation raft cooling system. The flooding valves are opened passively when the corium melts the cables in the spreading compartment, and the water flows in under gravity. Over the long term, when spraying is no longer necessary, the EVU [CHRS] can continue to provide active corium cooling (subcooled water heat removal)
PAGE : 12 / 16 Safety function d) is carried out by the foundation raft cooling system located below the spreading compartment layer of sacrificial concrete. The foundation raft cooling system is supplied passively by the natural circulation of water from the IRWST when the flooding valves are opened. It may be supplied actively during long-term accident management. Safety function e) is carried out by the EVU [CHRS] containment isolation valves and the IRWST double seal valves (see Chapter F.3.2) during accidents which do not require the EVU [CHRS] to operate. Safety function f) is carried out by the robust mechanical design of the EVU [CHRS] main system. During an accident requiring its operation, the containment function is provided by the parts of the EVU [CHRS] located outside containment. 7.5.3. Compliance with the design requirements See 7.0.2 within Sub-chapter F.2. 7.5.3.1. Safety classifications The EVU system [CHRS] is designed in accordance with the safety classification principles presented in Chapter C.2. 7.5.3.2. Single failure criterion Two containment isolation valves are installed on each line crossing the containment to meet the single failure criterion (for isolating the containment during an accident that does not require the EVU [CHRS] to be operated). Although application of the single failure criterion is not required for the EVU [CHRS], the EVU [CHRS] system consists of two separate trains, so that long-term failure of one train does not prevent the system from operating. 7.5.3.3. Qualification The part of the EVU [CHRS] that performs F1 or F2 classified functions is designed in accordance with that presented in chapter C.7. 7.5.3.4. Instrumentation and control The EVU system [CHRS] mechanical, electrical and instrumentation and control classifications are defined in accordance with the classification principles presented in chapter C.2. 7.5.3.5. Emergency power supplies Although not required for non-f1 function elements, the EVU system [CHRS] may be powered by emergency power supplies (emergency plant cooldown diesel generator sets).
PAGE : 13 / 16 7.5.3.6. Hazards Internal hazards Summary tables of the hazards taken into account Protection required in principle General protection Specific protection introduced in the system design Pipe ruptures - - For the internal hazards Tank, pump and valve - - that may result from an ruptures RRC-A accident (small Internal projectiles - - break APRP [LOCA] with Dropped load - - ISBP [LHSI] loss) and from Internal explosion - - an RRC-B accident. Fire - - Internal flooding - - External hazards Protection required in principle General protection Specific protection introduced in the system design Earthquake Yes Location in the BAS [safeguard buildings] and BR [reactor building] Aircraft crash No - - External No - - explosion External Yes Location in the BAS [safeguard - flooding buildings] and BR [reactor building] Snow and Yes Location in the BAS [safeguard - wind buildings] and BR [reactor building] Extreme cold Yes Location in the BAS [safeguard - buildings] and BR [reactor building] Seismic design for the entire system 7.5.3.7. Other requirements This system is taken into account in the demonstration of the practical elimination of the containment bypass risk (see Chapter R.1).
PAGE : 14 / 16 Tests 7.6. TESTS, INSPECTION AND MAINTENANCE The EVU [CHRS] is designed to allow periodic tests to be carried out that aim to ensure: - the structural integrity and leaktightness of equipment - the availability of the systems and active components - the availability of the entire system in conditions that are as close as possible to design conditions; all the operational sequences to activate the system are carried out, including switching the normal power supplies to emergency power supplies and operation of the dedicated cooling system. Preventive maintenance Preventive maintenance is possible during plant operation. Long-term maintenance Long-term maintenance is possible following a severe accident. If maintenance takes place on the main system pumps and heat exchangers, then additional equipment may be connected to the system, allowing the volume of water contained in the system to be directly injected into the BR [reactor building] and the part of the system affected by the maintenance to be decontaminated. 7.7. FLOW DIAGRAMS See F.2.7 FIG 1.
FIGURE : 1 PAGE :15 / 16 F.2.7 FIG 1: EVU [CHRS] FUNCTIONAL FLOW DIAGRAM
FIGURE : 1 PAGE :16 / 16 6.2.7 FIG 1: EVU [CHRS] FUNCTIONAL FLOW DIAGRAM