0. EU-APR1400 Design Concepts for Mitigation of Severe Accidents
-RISC, INPRO, IAEA-
Dr. Lim, Hak Kyu,
Mr. Yoon, Sun Hong

1. I
Overview of EU-APR1400 II
Design Concept of Safety Systems
III
Severe Accident Mitigation Systems

2. 1 Overview of EU-APR1400 Part 1. 1 Nuclear Steam Supply System
1. 2 General Arrangement
1. 3 Reactor Containment
1. 4 Auxiliary Building
This slide shows the descriptions of the N+2 design concept.
N+2 design provides~
In EUR section 2.8, the N+2 design concept is described as followings. ~~

3. I. Overview of EU-APR1400 1.1 Nuclear Steam Supply System
Thermal Power: 4,000 MW
Two-Loop Configuration 2 hot legs, 4 cold legs
Increased volume for better accommodation of transients
Steam Generator
Reactor Vessel
Integrated Head Assembly
Pressurizer
Reactor Coolant Pump
This slide shows the descriptions of the N+2 design concept.
N+2 design provides~
In EUR section 2.8, the N+2 design concept is described as followings. ~~

4. I. Overview of EU-APR1400 1.2 General Arrangement
This slide shows the descriptions of the N+2 design concept.
N+2 design provides~
In EUR section 2.8, the N+2 design concept is described as followings. ~~

5. Secondary Containment
I. Overview of EU-APR1400
1.3 Reactor Containment
Accessible to the inside of containment in order to do something for overhaul activities during a operation mode
Thereby,
it is possible to
reduce the period
of OH
Tendon Gallery
RDS Valve Room
Removal Cover
Polar Crane
Secondary Containment
Building
Primary Containment
Core Catcher
Equipment Hatch
Personnel Airlock
Letdown Hx
RDT
Valve Room
IRWST
This slide shows the descriptions of the N+2 design concept.
N+2 design provides~
In EUR section 2.8, the N+2 design concept is described as followings. ~~

6. I. Overview of EU-APR1400 1.4 Auxiliary Building
Quadrant separation for four train safety systems
To ensure redundancy of safety related systems
Enhance plant safety by eliminating the chance of simultaneous failure of both trains due to flooding, fire or other events
No opening and door between quadrants below ground level (El. 0.0 m)
This slide shows the descriptions of the N+2 design concept.
N+2 design provides~
In EUR section 2.8, the N+2 design concept is described as followings. ~~

7. 2 Design Concept of Safety Systems Part
2. 1 Characteristics of Safety Design
2. 2 N+2 Design Concept
2. 3 DECs Considered in the Design
2. 4 Functional Diversity Considered in the DEC 7

8. II. Design Concept of Safety Systems
2.1 Characteristics of Safety Design (1/3)
Functional Defense-in-Depth 8

9. II. Design Concept of Safety Systems
2.1 Characteristics of Safety Design (2/3)
Increased Redundancy
Application of “N+2” concept to front line safety systems
Provision of on-line maintenance
Extended Diversity
Safety measures against extended Design Extension Conditions
Functional back-up against common cause failures of front line safety systems
Severe Accident Mitigation
Dedicated Severe Accident Mitigation systems
Separate and independent from the safety systems for Level-3 accidents 9

10. II. Design Concept of Safety Systems
2.1 Characteristics of Safety Design (3/3)
Protection Against External Hazards
Provision for site-specific natural external hazards
Protection against man-made hazard (Airplane Crash)
Post-Fukushima Safety Improvements
Elevation of ground level (site-specific)
Protection against flooding and tsunami
Securing mobile power supply and external cooling water injection
Extension of DC battery mission time 10

11. II. Design Concept of Safety Systems
2.2 N+2 Design Concept (1/2)
Providing optimized Site Plot Plan for following concerns
To enhance the safety, reliability and availability, the EU-APR1400 safety-related systems are designed to perform their functions even though an individual component in any system fails to operate (single failure concern) and any component affecting the safety function is simultaneously inoperable due to repair or maintenance. (so-called N+2 design). This N+2 design concept will enhance the plant safety, make the on-line maintenance possible, and reduce the planned overhaul period.
EUR 2.8 As Preventive Maintenance for some specific equipment performing safety functions is foreseen during plant Normal Operation conditions, the N+2 concept (unavailability due to maintenance plus SFC) shall be applied on a case by case basis depending on the level of function performed and on the design conditions for which the function may be required.
This slide shows the descriptions of the N+2 design concept.
N+2 design provides~
In EUR section 2.8, the N+2 design concept is described as followings. ~~

12. II. Design Concept of Safety Systems
2.2 N+2 Design Concept (2/2)
Systems with N+2 Design in EU-APR1400
NSSS Systems : Safety Injection System (SI), Shutdown Cooling System (SC/CS), Auxiliary Feedwater System (AF)
Containment Systems : Containment Spray System (SC/CS)
Cooling Systems : Component Cooling Water System (CC)
Essential Service Water System (ESW)
HVAC Systems : Control Room HVAC System
Electrical and I&C Equipment Area HVAC system
Aux. Building Controlled Area HVAC System
Essential Chilled Water System
Electrical Systems : Alternating Current Power System (AP)
Direct Current/Instrumentation and Control Power System (DC/IP)
I&C Systems : Reactor Protection System (RPS)
ESFAS(SIAS, CSAS, CIAS, PAFAS, MSIS, etc
This slide shows the system lists that apply the N+2 design concept.

13. II. Design Concept of Safety Systems
2.3 DECs Considered in the Design
Anticipated transient without scram (ATWS)
The loss of safety injection system with small break LOCA
The loss of external power supply and normal emergency power supply system (diesel generators)
The loss of component cooling water system
The loss of ultimate heat sink
The loss of programmable automation system / systems
Total loss of feedwater (normal and emergency feedwater)
Common cause failure of similar safety valves
The loss of residual heat removal system of the reactor
The loss of fuel pool cooling system
Multiple tube ruptures in steam generator 13

14. II. Design Concept of Safety Systems
2.4 Functional Diversity Considered in the DEC 14

15. 3 Severe Accident Mitigation System Part 2. 1 SAM Strategy Concept
2. 2 ERDS (Emergency Reactor Depressurization System)
2. 3 Hydrogen Mitigation System
2. 4 DCH Mitigation Design
2. 5 Severe Accident Containment Spray System
2. 6 Severe Accident Dedicated Component Cooling Water System
2. 7 Severe Accident Dedicated Essential Service Water System
2. 8 Passive Ex-vessel corium retaining and Cooling System
2. 9 Containment Filtered Vent System
Per EUR requirements, the dedicated system for a severe accident mitigation is designed as like that you can see, safety class3, dedicated and N+1 failure criteria. Such a general design criteria would be helpful to improve their functionality and reliability.
Basically, EU-APR1400 plant has the redundant and diverse engineered safety features to safely control a accident and the consequence. And also for control of a severe accident, a dedicated system to each phenomenon along the progress of severe accident is provided.
PAR for H2 control
RDS and a convoluted path for DCH
PECS for ex-vessel corium cooling
SACSS for containment pressure control
CFVS for containment integrity without uncontrolled release of fission products
More detailed description will be provided next slides in a gradual manner

16. III. Severe Accident Mitigation System
3.1 SAM Strategy Concept (1/2)
The general design requirements for SAMS have characteristics as follows:
Safety Class 3
Dedicated for severe accident
N+1 failure criteria
Dedicated SAM System and Actions would be implemented, according to the SAM strategy
Recovery a steady state from a beyond DBA
Entry condition of SA is defined by CET of C
Prevention of High Pressure Melt Ejection and Direct Containment Heating by the RDS & the convoluted path of reactor cavity
Hydrogen control by PARs
Prevention of MCCI in the reactor cavity by the PECS
Keeping the integrity of containment by the SACSS & CFVS
Per EUR requirements, the dedicated system for a severe accident mitigation is designed as like that you can see, safety class3, dedicated and N+1 failure criteria. Such a general design criteria would be helpful to improve their functionality and reliability.
Basically, EU-APR1400 plant has the redundant and diverse engineered safety features to safely control a accident and the consequence. And also for control of a severe accident, a dedicated system to each phenomenon along the progress of severe accident is provided.
PAR for H2 control
RDS and a convoluted path for DCH
PECS for ex-vessel corium cooling
SACSS for containment pressure control
CFVS for containment integrity without uncontrolled release of fission products
More detailed description will be provided next slides in a gradual manner

17. III. Severe Accident Mitigation System
3.1 SAM Strategy Concept (2/2)
Per EUR requirements, the dedicated system for a severe accident mitigation is designed as like that you can see, safety class3, dedicated and N+1 failure criteria. Such a general design criteria would be helpful to improve their functionality and reliability.
Basically, EU-APR1400 plant has the redundant and diverse engineered safety features to safely control a accident and the consequence. And also for control of a severe accident, a dedicated system to each phenomenon along the progress of severe accident is provided.
PAR for H2 control
RDS and a convoluted path for DCH
PECS for ex-vessel corium cooling
SACSS for containment pressure control
CFVS for containment integrity without uncontrolled release of fission products
More detailed description will be provided next slides in a gradual manner

18. III. Severe Accident Mitigation System
3.2 Emergency Reactor Depressurization System
RCS depressurization performed as the 1st step in SAM strategy
Purpose
To prevent high pressure melt ejection
To prevent steam generator tube rupture
ERDS is manually opened by operator 30 minutes after the CET is greater than 648.9C
Performance Analysis
Tool : MAAP5.01
Sensitivity Analysis with respect to
the actuating timing( e.g., 0.min,
30min, 60min, etc)
Considering the effects of late re-flooding
and enhanced heat transfer
After entrance of a severe accident condition and diagnosis of accident conditions using several containment monitoring equipment, Operator should take an action to depressurize the pressure of RCS in the event of a high pressure accident sequence. The purpose is to prevent high pressure melt ejection and steam generator tube rupture
The performance target of ERDS reduces the pressure of RCS below 20 bars before breach of reactor vessel
As a considering time for the diagnosis, 30 minutes are assumed to be delayed to take an operator actions for RCS depressurization. The ERDS consists of 2 trains, each train has a 100% capacity of depressurization.
The discharge path of ERDS is from Pressurizer through Reactor Drain Tank to finally the lower part of Steam Generator compartments. Such a path would be helpful to reduce the effects of jet impingement and to improve overall mixing of containment atmosphere

19. III. Severe Accident Mitigation System
3.3 Hydrogen Mitigation System (1/2)
HMS consists of Passive Autocatalytic Recombiners (PAR)
Purpose
To prevent the occurrence of Flame Acceleration
To prevent the occurrence of Deflagration-to Detonation Transition
To prevent hydrogen explosion by keep the indexes of FA & DDT less than one over the containment
Design Criteria
Do not require external power supply for actuation
Reliable equipment in single failure
100% MWR as well as long tern hydrogen sources such as radiolysis and corrosion
46 PARs composed of a full size, medium size and small size ones are distributed over the containment
Early and late containment failure could be cased from a hydrogen explosion. So, in order to prevent hydrogen explosion, the hydrogen concentrations over the containment should be controlled to prevent the atmospheric composition that is vulnerable to hydrogen explosion. The total capacities of Passive Autocatalytic Recombiners are determined to keep the sufficiently low level of hydrogen concentration. The possibility of hydrogen explosion is confirmed to be extremely low by evaluating the possibilities of Flame Acceleration and Deflagration-to-Detonation Transition.
The hydrogen mitigation system is composed of 46 PARs which consists of 3 types, Large, Medium, and small sizes. Considering the space of the installation place, there is possible to make a combination of PARs.

20. III. Severe Accident Mitigation System
3.3 Hydrogen Mitigation System (2/2)
Performance Analysis
Tool : MAAP5.01, CFD codes
Base Case : 100% MWR Equivalent
Hydrogen Mass
Sensitivity Case : Considering the
effects of late re-flooding and
enhanced hydrogen generation
PARs PAR disposition
Early and late containment failure could be cased from a hydrogen explosion. So, in order to prevent hydrogen explosion, the hydrogen concentrations over the containment should be controlled to prevent the atmospheric composition that is vulnerable to hydrogen explosion. The total capacities of Passive Autocatalytic Recombiners are determined to keep the sufficiently low level of hydrogen concentration. The possibility of hydrogen explosion is confirmed to be extremely low by evaluating the possibilities of Flame Acceleration and Deflagration-to-Detonation Transition.
The hydrogen mitigation system is composed of 46 PARs which consists of 3 types, Large, Medium, and small sizes. Considering the space of the installation place, there is possible to make a combination of PARs.

21. III. Severe Accident Mitigation System
3.4 DCH Mitigation Design
DCH phenomenon is minimized by the ERDS composed of reliable 2 train ERDVs
Geometries of the Reactor cavity and the Reactor Annulus
To reduce the entrained corium from the reactor cavity
To design Convoluted flow path
To provide the corium chamber room
To minimize the direct flow path area into the upper containment region
To minimize the fraction of entrained corium
The possibility of DCH phenomena might be extremely low in EU-APR1400 plant and, even though it is possible, the consequences is regarded to have minor impacts since reliable and redundant the ERDS and a convoluted discharge path of entrained corium.
The area of direct path to the upper containment is minimized and the major flow path is designed to be convoluted. Such a geometric characteristics play an important role of reducing the amount of entrained corium.

22. III. Severe Accident Mitigation System
3.5 Severe Accident Containment Spray System (1/3)
The SACSS is a dedicated spray system for control of containment pressure in a severe accident
Function
Pressure and temperature control by spraying cooling water
Reduction of concentrations of radioactive aerosols in containment atmosphere
To provide the function of long term cooling of the PECS
Design Criteria
Fulfillment of single failure
Sufficient cooling as a heat removal system
System Configuration
Consists of two 100% trains
Each train consists of SA CS Pump (2), SA CS HX (2), SA CS Mini-flow HX (2), Containment spray header, piping, valves, controls, and instrumentation.
Powered by dedicated AAC power supply systems (2 AAC GTs)
This slide shows the major features of the severe accident containment spray system.

23. III. Severe Accident Mitigation System
3.5 Severe Accident Containment Spray System (2/3)
Performance Analysis
Tool : MAAP5.01
Automatically actuated when the accident conditions meet the criteria of a severe accident
Design Target : reduce the containment atmosphere pressure less than 50% containment design pressure within 24 hours following actuation of the SACSS
This slide shows the major features of the severe accident containment spray system.

24. III. Severe Accident Mitigation System
3.5 Severe Accident Containment Spray System (3/3)
This slide shows the major features of the severe accident containment spray system.

25. III. Severe Accident Mitigation System
3.6 Severe Accident Dedicated Component Cooling Water System (1/2)
System Function
Removes heat generated from the severe accident containment spray HX during SA events.
Removes heat generated from the essential components connected CCWS when the CCWS/ESWS are available during DEC events.
System Configuration
Consists of two 100% trains
Each train consists of dedicated surge tank (1), dedicated CCW pump (1), dedicated chemical addition tank (1), dedicated CCW HX (1), piping, valves, controls, and instrumentation.
Powered by dedicated AAC power supply systems (2 AAC GTs)
This slide shows the major features of the severe accident dedicated CCWS.

26. III. Severe Accident Mitigation System
3.6 Severe Accident Dedicated Component Cooling Water System (2/2)
This slide shows the major features of the severe accident dedicated CCWS.

27. III. Severe Accident Mitigation System
3.7 Severe Accident Dedicated Essential Service Water System (1/2)
System Function
Remove heat generated from the dedicated CCWS during DEC or SA events.
System Configuration
Consists of two 100% trains
Each train consists of dedicated ESW pump (1), dedicated ESW debris filter (1), piping, valves, controls, and instrumentation.
Powered by dedicated AAC power supply systems (2 AAC GTs)
This slide shows the major features of the severe accident dedicated ESWS.

28. III. Severe Accident Mitigation System
3.7 Severe Accident Dedicated Essential Service Water System (2/2)
This slide shows the major features of the severe accident dedicated ESWS.

29. III. Severe Accident Mitigation System
3.8 Passive Ex-vessel corium retaining and Cooling System (1/3)
The PECS is a Passive Ex-vessel corium retaining and Cooling System
System Function
To provide corium retention in core catcher
To provide coolability using water stored in IRWST
To achieve a long term safe state
Design Criteria
Provision of sufficient corium coolability
Provision of corium retaining to prevent the interaction between hot corium and supporting structures of containment
Installation of instruments for minitoring
Passive actuation
Independence of flooding system
The PECS consists of a refractory layer, sacrificial concrete layer, body, cooling gap, supporting studs, downcomers, several monitors, and 2 train gravity cooling water supply sub-system.
This slide shows the major features of the passive ex-vessel corium retaining and cooling system.

30. III. Severe Accident Mitigation System
3.8 Passive Ex-vessel corium retaining and Cooling System (2/3)
Concept diagram of PECS in APR1400
Performance Analysis
Tool : MAAP5.01, CFD codes
Automatically operated by the actuation signal generated from the reactor cavity temperature
void fraction short term mode long term mode
Still we are considering the need of a refractory material layer. The refractory material layer is located around the region below the reactor vessel. Some experts say a little bit thicker sacrificial material layer can withstand the corium jet erosion during discharge of corium from the breach point of reactor vessel without complete erosion.
In a injection mode, the cooling water is provided by a gravity-driven flow from the IRWST. The cooling water fills up the cooling channel of PECS and the downcomers. The cooling water will reach the top of downcomer before the SM is gone by ablation mechanism.
And then, there is a transient mode to have both injection flow and recirculation flow. After the level balancing of cooling water between the reactor cavity and the IRWST, there is only a recirculation mode.
In a recirculation mode, two-phase flow going out of the cooling channel flows up and is mixed with the upper cooling water. Relatively cold water around the inlets of downcomers comes into the downcomers and goes back to the cooling channel.

31. III. Severe Accident Mitigation System
3.8 Passive Ex-vessel corium retaining and Cooling System (3/3)
Experiments
In the CHF experiment, we will measure the Critical heat flux of exterior surface of ex-vessel core catcher. During the experiment, we can also indirectly get some data for evaluation of heat transfer coefficient, flow rates, and steam quality. Such measured data can be used for the validation and verification of CFD analysis. We will perform the CFD analysis to figure out the flow pattern and other thermo hydraulic characteristics.
In the SM ablation experiment, we are measuring the ablation speed of EU-APR1400 SM. The results will be used for V/V of analysis tool and adjusting the thickness of SM layer in the ex-vessel core catcher.
In the jet erosion experiment, we are looking for the possibility of replacement of a refractory material layer with a SM layer. After measuring the jet erosion speed, the results will be applied to the determination of thickness of SM layer below the reactor vessel.

32. III. Severe Accident Mitigation System
3.9 Containment Filtered Vent System (1/2)
The CFVS consists of two vent trains with a common venturi scrubber as a passive component which will be located on Auxiliary Building.
System Function
Protection of containment over-pressurization by filtered venting system as an alternative method when SACSS is unavailable
Maintaining the containment integrity approximately 24 hours following the onset of core damage
Design Criteria
Exhausting sufficiently filtered air
Sampling and measuring arrangements
Each train shall vent air from the containment atmosphere by the operator’s remotely opening of the containment isolation valve in the main control room
This slide shows the major features of the containment filtered vent system.

33. III. Severe Accident Mitigation System
3.9 Containment Filtered Vent System (2/2)
This slide shows the major features of the containment filtered vent system.

34. Thank You !