FUNDAMENTAL SAFETY OVERVIEW VOLUME 2: DESIGN AND SAFETY CHAPTER F: CONTAINMENT AND SAFEGUARD SYSTEMS

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1 PAGE : 1 / 8 4. CONTRO OF COMBUSTIBE GASES (ETY) 4.0. SAFETY REQUIREMENTS Safety functions The ETY [containment hydrogen control] forms part of the "containment of radioactive substances" safety function. It ensures: - a reduction of the local and average hydrogen concentration in severe accidents, in order to maintain the containment integrity against the effects of: global deflagration of hydrogen formed during an accident, dynamic phenomena linked to the flame acceleration and deflagration-to-detonation transition (DDT). - reduction of the hydrogen concentration in OCA, to remove risks due to combustion in the containment Functional criteria The functional criteria associated with the ETY performance are based on predicted hydrogen concentrations in representative extreme severe accident scenarios (involving hydrogen generation in the containment). The requirements are: - the hydrogen (resulting mainly from oxidation of the zirconium in the core) must be mixed in the containment atmosphere, and the volume of hydrogen reduced, with the aim of reducing the overall hydrogen concentration to below the flammability limit within 12 hours; - the containment must be able to withstand (i.e. maintain its leaktightness) under the pressure load resulting from the complete adiabatic and isochoric combustion of the quantity of hydrogen which may be contained in the building regardless of the scenario selected, taking into account the means of limiting the hydrogen concentration. To avoid the occurrence of dynamic phenomena, the local hydrogen concentration must be maintained below 10% Requirements relating to the design Requirements from safety classifications - Safety classification The ETY system is classified in accordance with the classification given in Chapter C.2. - Single failure criterion

2 PAGE : 2 / 8 The single failure criterion does not apply to the ETY system. - Emergency power supplies The hydrogen catalytic recombiner units are passive and do not require electrical power. The mixing of the containment atmosphere is partially passive. - Qualification for operating conditions The ETY system equipment is qualified in the ambient conditions to which it is subjected when performing its task in meeting its safety role. - Mechanical, electrical and instrumentation and control classifications The ETY is classified in accordance with Chapter C.2. - Seismic classification The ETY [containment hydrogen control] is classified in accordance with the rules stated in Chapter C.2. - Periodic tests Periodic tests will be carried out on the ETY safety classified equipment to ensure their availability Other statutory requirements - Technical Guidelines The ETY system meets the requirements in A1.3, B1.4.1, E2.2.4 and E2.4 of the Technical guidelines (see Chapter C.1.2). - EPR specific documents Not applicable Hazards Chapter C.3 describes requirements with regard to external hazards and Chapter C.4 for the requirements for internal hazards ROE OF THE SYSTEM In the management of severe accident and long-term OCAs, the ETY system promotes mixing of hydrogen in the containment atmosphere, reduces the average hydrogen concentration and prevents formation of high local concentrations.

3 PAGE : 3 / DESIGN BASIS The system consists of: - Hydrogen catalytic recombiner units, - Rupture disks and shutters to promote mixing in the containment atmosphere. Because of the efficiency of the recombiner units, a hydrogen concentration measuring system is not required as concentration information is not needed for accident management System design General system requirement To limit loads due to combustion, the hydrogen concentration must be reduced: - the hydrogen concentration must be returned to below the flammability limit (4% by volume) within 12 hrs; - The AICC pressure (complete isochoric adiabatic combustion pressure) must remain lower than the containment design pressure for representative accident scenarios. Design studies are used to demonstrate that, for regions where the local hydrogen concentration cannot be maintained below 10%, flame acceleration is not likely. The following methods based on the evaluation of specific criteria are applied: - the sigma criterion is used for evaluating the potential for rapid deflagration (flame acceleration at sonic speed). The sigma value is defined as the ratio of the volume of gas involved in combustion to the volume of gas that is not burned at constant pressure. Rapid deflagration may be excluded if the sigma value is lower than unity. Sigma depends on the gas temperature, the composition of the gaseous mixtures (steam concentration, hydrogen concentration), the saturation temperature and the characteristic dimensions of the region. - The seven lambda criterion is used for evaluating the potential for a deflagration to detonation transition (DDT). This criterion states that the DDT transition will not take place if the characteristic length of the hydrogen cloud in which the flame may accelerate does not exceed seven times the width of the gas mixture detonating cell (lambda). ambda values are experimentally and depend on the composition of the gas. The lambda criterion is linked to the sigma criterion: A potential for detonation is identified in gas clouds with lambda > 1 that also meets the sigma > 1 condition. If these criteria are not met, a 3-dimensional combustion calculation is made to determine the loads on structures caused by the combustion. Recombiner unit design principles The design of the recombiner units and their positions in the containment are defined with the aim of limiting and reducing local and global hydrogen concentration. The recombiner units are thus installed taking into account:

4 PAGE : 4 / 8 - the hydrogen release locations, - their benefits in improving gaseous mixing in the containment atmosphere, - their benefits on overall convection, - their effect on reducing global hydrogen concentration and prevention of local high concentrations. The recombiner units are designed to operate in pressure, temperature, humidity, and radioactive environmental conditions corresponding to severe accident conditions. Equipment qualification takes into account the potential poisoning of catalysts due to aerosols from the molten core and boric acid from the spray system. The recombiner units are designed to withstand loads due to temperatures and pressures in accidents, and to remain intact following an earthquake so they do not present a hazard to other equipment. The recombiner units operate based on a hydrogen concentration of approximately 2% by volume. Shutter design rules and principles The shutters are designed to withstand loads due to the temperature, pressure, humidity and radiation environment expected during accidents. The differential pressure for opening the shutters is defined to ensure an adequate mixing function for specified accident conditions. The mechanical design ensures that, in the event of failures, the shutters do not present a hazard to adjacent safety equipment Fluid characteristics The fluids in question are mainly air, steam, hydrogen and the various products released during accidents DESCRIPTION OF SYSTEM - EQUIPMENT CHARACTERISTICS Hydrogen control concept The hydrogen control concept is based on the use of catalytic recombiner units and does not require igniters. Studies have been carried out with a standard distribution of recombiner units (see F.2.4 TAB 1 note that the number and distribution of units are likely to change during detailed design studies). The recombiner units are placed mainly in primary component rooms so that elimination of the hydrogen can be initiated as soon as possible and with a high degree of efficiency (higher concentration inside the primary component rooms). Some recombiner units are installed in the dome to promote global convection and to minimise stratification. There is no need to install recombiner units in the lower annulus compartments. The recombiner distribution is confirmed in a sensitivity analysis and the number of recombiner units is selected to meet the criteria listed in in Sub-chapter F.2. The configuration of the recombiner units also promotes global convection in the containment during the recombination.

5 PAGE : 5 / 8 Hydrogen control is strongly promoted by the RCP [RCS] depressurisation systems used for normal depressurisation and depressurisation via the system dedicated to severe accidents. These systems release coolant directly from the primary cooling system into the lower part of the containment. Two types of scenarios have been used as a basis for the design of the hydrogen control system: representative scenarios selected according to their probability of occurrence, and extreme scenarios which place the greatest demand on hydrogen control. The latter correspond to late depressurisation (of the primary circuit) or flooding (of the uncovered core), as these provide the most onerous hydrogen generation conditions in terms of quantity as well as flow rate. Extreme scenarios are used to confirm the robustness of the design and demonstrate strict compliance with criteria (e.g. AICC pressure always lower than the design pressure) in the representative scenarios. Justification of the hydrogen control system is carried out in four phases: 1) calculation of hydrogen production for a number of scenarios with the MAAP code. 2) calculation of the gas and temperature distribution in the containment for the scenarios selected, using a design code that takes into account the spatial distribution of the recombiner units. 3) evaluation of the hydrogen risk, based on the results from the design code. This consists of analysing the AICC pressure, evaluating the risk of deflagration and DDT using experimentally established criteria ("sigma" and "7 lambda" criteria as described in in Sub-chapter F.2). 4) If it is impossible to exclude the risk of rapid deflagration or DDT using the criteria, a direct calculation of the combustion process and the resulting pressure history is carried out. The results from the analyses carried out to justify the ETY hydrogen control system are presented in Chapter S General description of the system The recombiner units are mainly placed in primary component rooms to promote global convection and thus improve atmospheric mixing whilst preventing local hydrogen accumulation. However, some recombiner units are placed at higher elevations, for example, in the dome, to reduce stratification and to increase hydrogen recombination after improving the mixing in the atmosphere. The recombiner units are installed so that easy access is possible to facilitate their maintenance. They are positioned high enough off the ground to ensure sufficient venting. They are also positioned far enough away from equipment that has a safety function (in particular electrical equipment and most importantly the electrical cables) to prevent such equipment being damaged by hot gas emitted by the recombiner unit. Furthermore, the recombiner units are placed so as to avoid direct contact with water from the spray system (even if they are qualified to operate in the presence of water) to reduce the likelihood of catalyst poisoning. Due to the efficiency of the recombiner units, it is not necessary to measure the composition of reactor building atmosphere as this information is not needed by the operators for accident management.

6 PAGE : 6 / 8 In the event of an accident, the segregation between the equipment area and the reactor building service area may be removed, to create a single containment volume. This action may occur before an accident transitions into a severe accident. This function is provided by the containment atmospheric mixing system which consists of: - rupture disks placed on the upper part of the GV [SG] compartments, - a number of shutters placed on the upper part of the GV [SG] compartments, - shutters located in the lower part between the containment annulus and the IRWST. The shutters are held closed by an electric motor, which compresses a return spring. If a power failure occurs, the shutters are automatically opened by the return spring. Opening is controlled by differential pressure sensors. It may also be manually controlled. Dependent on the accidental scenario, in addition to shutter operation, rupture disks located above the GV [SG] compartments may passively create openings. These rupture disks limit the differential pressure (between the equipment area and service area) inside the containment and further improve mixing of the gases in the entire containment. The design of the rupture disks (differential pressure threshold) depends on load calculations for the containment structures (and the existing differential pressure in normal operation) and will be defined following detailed studies Equipment characteristics The recombiner units The choice of recombiner units has not yet been made Instrumentation and control The recombiner units operate without electrical power, without instrumentation and without control systems. The shutters are electrically controlled on closing and their position is monitored. The position of the rupture disks is not monitored. Their state may be deduced directly by monitoring the differential pressure of the containment (equipment and service area) Interfaces with other systems The ETY system has no direct interfaces with other systems. An operational interface exists between the ETY system used in the event of a hydrogen release and the RCP [RCS] depressurisation and spray systems OPERATING CONDITIONS The ETY hydrogen control system is not required in normal and transient operating conditions. The ETY hydrogen control system is operational in severe accident conditions and in long-term post-oca management. The system startup is passive.

7 PAGE : 7 / PREIMINARY SAFETY ANAYSIS Compliance with regulations To follow Compliance with the functional criteria The results of analyses carried out to justify the hydrogen control system, presented in Chapter S show that the design of the ETY hydrogen control system ensures that the safety requirements and functional criteria described in and in Sub-chapter F.2 are satisfactorily met Compliance with the design requirements Safety classification Compliance of the design and construction of materials and equipment with the requirements of the classification rules is described in detail in Chapter C SFC or redundancy All of the ETY system components are F2 classified, so the single failure criterion does not apply to ETY Qualification The equipment is qualified in accordance with the requirements described in Chapter C Instrumentation and control and emergency power supplies The hydrogen recombination carried out by the catalytic plates requires no electrical power or monitoring system, which gives it a completely passive role. The containment atmospheric mixing system consists of shutters and rupture disks. It starts to operate (opening) when the pressure threshold is exceeded (the design of differential pressure sensors for the shutters, mechanical dimensioning for the disks). The shutters open on loss of power Hazards The general provisions described in Chapter C.3 for the requirements in terms of external hazards and Chapter C.4 for the requirements in terms of internal hazards, are incorporated into the system design TEST, INSPECTION, MAINTENANCE Test, inspection and maintenance rules for the ETY hydrogen control system will be defined during the detailed studies phase.

8 TABE : 1 PAGE : 8 / 8 F.2.4 TAB.1: STANDARD RECOMBINER UNIT DISTRIBUTION Compartments Type Compartments Type Compartments Type 1. GV [SG] and GMPP [RCP] lower areas 3. GV [SG]upper areas 5. Top of the vessel GMPP1 lower area GV1 upper area Walkway of the polar crane GMPP2 lower area GV1 upper area Walkway of the polar crane GMPP3 lower area GV2 upper area Walkway of the polar crane GMPP4 lower area GV2 upper area Walkway of the polar crane GV1 lower area GV3 upper area Top of the polar crane GV2 lower area GV3 upper area Top of the polar crane GV3 lower area GV4 upper area GV4 lower area GV4 upper area 2. GV [SG] and GMPP [RCP] intermediate areas 4. Other rooms 6. Upper areas of the containment annulus GMPP1 intermediate area ower pressuriser area S Close to the pressurizer GMPP1 intermediate area Discharge tank room S Close to the reactor pool GV1 intermediate area Reactor compartment S GV2 intermediate area Intermediate pressurizer area S GMPP2 intermediate area Upper pressurizer area S 7. Operating floor GMPP2 intermediate area Upper pressurizer area S close to the reactor pool GMPP3 intermediate area close to the GMPP1 compartment GMPP3 intermediate area close to the GMPP4 compartment V3 intermediate area close to the GMPP3 compartment GV4 intermediate area close to the GMPP2 compartment GMPP4 intermediate area GMPP4 intermediate area : arge S: Small