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Reproduction and distribution are prohibited. Thank you IAEA - NS-G-1.9 – EPR model

Etienne COURTIN. – 10/11/2014 - p.1

Engineering & Projects Organization


Reproduction and distribution are prohibited. Thank you

Etienne COURTIN

October 3rd 2016

Novel design and safety principles of NPP – EPR model


Reproduction and distribution are prohibited. Thank you IAEA – Novel Design – EPR model

Etienne COURTIN. – 03/10/2016- p.3

Summary

Plant overview

Main plant data

Main safety functions and associated systems

Defense in depth in safety analysis

Design Basis Accidents (DBC)

DEC without core melt (DEC-A): list, features and rules

DEC with core melt (DEC-B): list, features and rules

Practical elimination

Types of situation practically eliminated

Associated features

Feedback from Fukushima: extreme hazards




Summary

Plant overview

Main plant data








Associated features





Main plant data

EPR is a 4 loops Pressurized Water Reactor

Electrical power: 1650 MWe

Fuel assemblies: 241

Primary pressure: 155 bar abs

Primary average temperature: 312°C

SG saturation pressure: 78 bar abs




Plant overview

Turbine building

Fuel

building

Reactor

building

Safeguards buildings

Diesel Generator

building

Pumping Station

Waste building

Nuclear

auxiliary

building

Diesel Generator

building




PWR concept




EPR Main systems

Cours d'introduction à l'EPR J.P. Py – Février 2004 »CCT/EPR/001 Rev. C




EPR Main systems

PRZ Connected

to Loop 3

PRZ

PRZ SV’sRD

PRZ RT

Loop 1 Loop 2

IRWST IRWST

RCPACCU

SIS/RHRS

MHSI

LHSI

EBT

EBS

CHRS

Spreading Area

Surge

Tank

ESWSCCWS

Surge

Tank

CHRS Dedicated

Cooling Chain

Ultimate Heat Sink Ultimate Heat Sink

M

MSRV MSSV MSIV

NI TI

Main

Feedwater

EFWS

EFWT

Co

ola

nt

Sto

rag

e

De

ga

sifie

r

Co

ola

nt

Pu

rifica

tio

n

N2

VCTHydrogen

Generation

CVCS

HP Charg.

Pumps

HP -Red.

Station

HP

Cooler

HP

Cooler

Regen.-Hx

Auxiliary Spray (CVCS)




Main safety functions Major systems for the performance of the main safety

functions are the following:

Function Normal operation Safety system Diversification

Reactor shutdown Control rods Shutdown rods Boron injection

(EBS)

RCS water

inventory control

Chemical and

Volume Control

System (CVCS)

Safety injection

(SIS)

N/A

Heat removal –

Hot shutdown

Steam generators:

Main Feedwater

and Condenser

Steam generators:

Emergency

Feedwater and

Main steam relief

trains

Feed & Bleed

Heat removal –

Cold shutdown

Residual Heat

Removal System

(RHR)

Residual Heat

Removal System

(RHR)

Containment Heat

removal system

(CHRS)




Summary

Plant overview

Main plant data








Associated features






All relevant single initiating events are considered to build the list of DBC

All plant modes are considered, in particular in shutdown mode (RCS open, refuelling,…)

Accidents in fuel pool are postulated

Analysis rules:

Credit safety systems

Normal operation systems not credited

Conservative assumptions and methodologies

Single failure criteria and preventive maintenance, if applicable




Design Extension without core melt (DEC-A)

Common cause failures are postulated on safety systems

=> complex sequences with multiple failures

Two main types of DEC-A:

Frequent DBC combined with CCF on a safety system

Example: ATWS = DBC2 combined with CCF on reactor shutdown

CCF on a safety system during normal operation (support system)

Example: Total loss of heat sink

All plausible combination of DBC and CCF on safety system is analyzed




Overall DEC-A approach

1 1E-1 1E-2 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9

DBC2

DBC3

DBC4

1E-2

1E-4

1E-6

1

Initiatin

g ev

ent freq

uen

cy

Main line, SF + PM

Diverse line, Overall CDF target

Objective for each elementary

sequence (to reach the overall

objective)

High frequency Low frequency




DEC-A How to build the list of DEC-A?

Build a list of plausible complex sequences

List all safety systems credited in DBC

For each safety system:

Postulated a CCF on the safety system

Identify the DBC where this system is credited

Keep the plausible combinations

Group the various sequences in a representative sequence that covers the others

Identify a diversified system that can be used as a back-up to ensure the same function:

Either an existing safety system not affected by the CCF

Or a dedicated feature for DEC

Normal operation systems are not credited

Identify the representative sequences that need a safety demonstration: list of DEC-A

If fulfillment of safety criteria cannot be proved without calculation




DEC-A Some examples

DEC-A sequence Diversified strategy Dedicated DEC-A feature

AOO + Failure of reactor

trip

Boron injection by

EBS

Automatic start of EBS

AOO + Failure of

Protection System (PS)

Diversified

protection signals

Diversified automation

system

AOO + Failure of SG Feed & Bleed Opening of pressurizer

discharge system

LOOP + failure of EDGs Ultimate DG Manual start of Ultimate DG

SB LOCA + CCF on

MHSI

Injection into RCS

by LHSI

Fast cooling

Total loss of heat sink –

Hot shutdown

Heat removal by

Steam Generators

Refill EFW tanks

Total loss of heat sink –

Cold shutdown

RCS Boiling and

containment cooling

Heat removal by CHRS

(diversified cooling chain)




DEC-A Analysis rules

DEC-A analysis are less conservative than DBC

However DEC-A analysis are still conservative in order to provide high confidence in the results

Usually:

Use of DBC models

Penalization of the most significant plant parameters

No SFC nor maintenance

PSA support studies are NOT accepted as DEC-A analysis:

Realistic assumptions and modelling

=> insufficient confidence in the safety margins




DEC-B Core melt sequences

Credible core melt sequences are postulated to design the DEC-B features

Regardless of the efficiency core melt prevention implemented

The reference sequences are the following:

SB LOCA

LB LOCA

Loss of AC power

Total loss of feedwater

Additional extreme sequences are analyzed in order to assess the DEC-B features robustness and the available grace time

Realistic analysis

Modelization of phenomena is based on experiments: uncertainties to be considered according to the knowledge sometimes limited




DEC-B Core melt phenomena

Aim of DEC-B analysis is to prove that radiological consequences only require protective measures limited in area and time => Main concern is the efficiency of containment

Containment failure mode DEC-B features

Direct Containment Heating

(DCH)

Fast depressurization by Pressurizer

Discharge System (PDS)

Hydrogen explosion H2 recombiners

Steam explosion Dry vessel pit

Corium spreading before flooding

Basemat failure Spreading area

Flooding of spreading area

High pressure Containment spray

Containment heat removal with dedicated

and diversified cooling chain

Dedicated ultimate diesel generator




Summary

Plant overview

Main plant data








Associated features





Practical elimination Situations

Situations where large or early releases cannot be reasonably prevented have to be practically eliminated

Specific stress on early releases (Tchernobyl feedback)

Situations practically eliminated are core melt situations resulting from different kinds of failures

=> defense in depth concept is not fully applied for those sequences

Different kinds of demonstration according to the type of SPE

According to WENRA, 3 types of SPE




Practical elimination Demonstration

Accident with core melt and containment failure

Ex: Core melt with DCH

Practical SPE provisions efficiency are demonstrated within the DEC-B analysis, according to DEC-B rules

Defense in depth is sufficient as all the core melt prevention lines are applied

Initiating event that is excluded because the consequences cannot be reasonably controlled

Ex: RPV failure

Defense in depth is limited to 1st and possibly 2nd level of DID

Design of components with a very detailed knowledge of:

The failures modes,

The structural characteristics regarding these failure modes

The anticipated loads

Manufacturing of components with very high quality process

Extensive in service inspection program guaranteeing the absence of damage to the structure




Practical elimination Demonstration

Accident complex sequences leading to unmanageable fuel degradation situation

Examples:

Fuel melt in spent fuel pool

VLOCA

Dilution

Initiating event is not excluded

Consequences of the initiating event are limited with a very high confidence (eg. size of a clear water slug)

Several successive barriers are implemented to prevent core melt even when a CCF is combined with the initial failure (DID level 2, 3, 4a)

DID principles are applied but not up to the stage of core melt

Deterministic demonstration is supported by a probabilistic assessment confirming the low core melt frequency




Summary

Plant overview

Main plant data








Associated features






All safety systems and major normal operation systems are protected against design basis external hazards (DBH)

=> no DBA or DEC-A expected after a DBH

In case of extreme hazard:

Objective to prevent Large or Early releases

No dedicated feature but robustness of existing features

Major accidents are prevented (LOCA)

Core melt prevention is demonstrated for a representative sequence (Loss of ultimate heat sink + Loss of unprotected diesel generators)

Demonstration is based on diversified cooling chain and ultimate diesel generator

Robustness proved against extreme hazard loads

All DEC-B means are robust against extreme hazards => the demonstration of the limitation of releases remains valid

In addition: plugs implemented to allow connection of mobile means (Make-up pump and diesel generator)

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