application of selected safety requirements from iaea ssr ...€¦ · ec6 heat transport system...
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Application of Selected Safety Requirements from IAEA SSR-2/1 in the EC6 Reactor Design ›Technical Meeting on Safety Challenges for New NPPs
22-25 June 2015, Vienna, Austria
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Safety first
Remember that all
SNC-Lavalin meetings
begin with a Health
& Safety moment.
Safety doesn’t happen by accident.
SAFETY FIRST
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Outline
› EC6 Design Overview
› Requirement 7 for Level 2 defence-in-depth: Distributed Control System approach
› Requirement 53: Emergency Heat Removal System (Level 3 defence-in-depth)
› Requirements 54 and 20: Containment envelope (Levels 3 and 4 defence-in-depth)
› Requirement 20: Emergency Containment Filtered Venting System (Levels 4 and 5 defence-in-depth)
› Requirement 20: Severe Accident Heat Removal System (Level 4 defence-in-depth)
› Requirement 67: Emergency Support Centre (Levels 4 and 5 defence-in-depth)
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EC6 Design
EC6 Reactor
• Natural uranium fuel
• Heavy water moderator and coolant
• Pressure tubes; calandria not a
pressure vessel
• Coolant physically separated from
moderator
• Small/simple fuel bundle
• On-power refuelling
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EC6 Heat Transport System › The HTS comprises the heat transport
pumps, steam generators, feeders, fuel
channels, and associated piping,
arranged in two “figure eight” loops.
› Heat generated by nuclear fuel in the
fuel channels passes through the outlet
feeders to the outlet headers, then
delivered to the SGs.
› Heat is transferred to the feedwater on
the secondary side of the SGs to
generate steam.
› The primary coolant is pumped from the
SGs back to the inlet headers, then back
to the core via the inlet feeders.
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Examples of Design Approaches to Address SSR-2/1
• Safety classification
• Requirement 7: Digital Control Systems
• Requirements 53 and 25: Emergency Heat Removal System
• Requirements 54 and 20: Containment envelope
• Requirement 20: Emergency Containment Filtered Venting System
• Requirement 20: Severe Accident Heat Removal System
• Requirement 67: Emergency Support Centre
Safety Classification for EC6 Design
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Safety Class A
A system is assigned to Class A if it meets any of the following criteria:
a) Maintains pressure boundary integrity of heat transport system where a failure would lead to a non-isolatable
loss of coolant accident.
b) Performs an immediate fast reactor shutdown function to prevent an initiating event from leading to
unacceptable consequences that exceed the design bases of Heat Transport System or safety systems.
Safety Class B
A system not included in Safety Class A is assigned to Safety Class B if it meets any of the following criteria:
a) Performs the function of core cooling to prevent an initiating event from leading to unacceptable
consequences that exceed the design bases.
b) Performs the function of containment to prevent an initiating event from leading to unacceptable radioactive
releases that exceed the design bases.
Safety Class C
A system not included in Safety Class A or B is assigned to Class C if it meets any of the following criteria:
a) Supports the operation of Class A or B systems.
b) Performs safety functions in longer term as a backup.
c) Failure of the system during operation (i.e. running failure) initiates a design basis accident.
d) Provides monitoring of safety functions during a design basis accident.
Safety Class D
A system not included in Safety Class A, B or C is assigned to Safety Class D if it meets any of the following
criteria:
a) Maintains adequate operating conditions for safety systems or systems important to safety during normal
plant operation, AOO or DBAs.
b) Prevents minor releases of radioactive materials.
c) Prevents/minimizes radiation exposure of plant staff.
d) Provides monitoring that the plant remains within normal conditions.
e) Provided for mitigation or monitoring of DEC.
Not important to safety A system not included in Safety Class A, B, C or D is assigned to not important to safety.
Safety Classification for EC6 Design
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System Safety Class Code/Standard
Fuel Handling Control and Display Systems C
CSA N290.4
IEC standards or
equivalent
Essential Control Sub-System C
CSA N290.4
IEC standards or
equivalent
Emergency Heat Removal System C CSA N285 series
Containment envelope B CSA N287 series
Emergency Filtered Containment Venting
System D CSA N285 series
Severe Accident Recovery Heat Removal
System D CSA N285 series
Emergency Support Centre D CSA N290.6
Digital Control Systems
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› Control systems are designed to assure that failures and
deviations from normal operation are minimized
› EC6 improvements using distributed control systems:
› Fuel handling control and display systems: fully
separate system with independent and diverse digital
sub-system to implement protective safety interlocks
› Fully digital device control sub-systems to interface via
digital communications with the digital group controls
› Read-backs provided from the digital device control
sub-systems to the PDS, advanced alarm
annunciation system, and computer based procedures
› All important to safety mitigating functions in a
separate IEC Class 2 mitigating controller, called the
“essential control sub-system”
› Additional parameters for setback and stepback, e.g.,
end shield outlet temperature high and Shield Cooling
System pump differential pressure low
SSR-2/1
Requirement 7: Defence in depth
Clause 4.11 (a)
Clause 4.11 (b)
Clause 4.11 (b)
Clause 4.11 (c)
Clause 4.11 (c)
Emergency Heat Removal System
› With feedwater available to the steam generators,
thermosyphoning in the intact Heat Transport System
(HTS) will prevent fuel failures
› EHRS addresses design basis accidents that cause
› Loss of heat removal from the steam generators, or
› Loss of heat removal from the Emergency Core
Cooling System (ECCS) heat exchangers
› EHRS performs the following functions:
› Gravity-fed make-up to the steam generators from
the Reserve Water Tank
› Pumped make-up to the steam generators from an
external water supply
› Seismically qualified back-up cooling water for the
ECCS heat exchangers
› Water supply to HTS to make up for small leaks or
losses (as a back-up to ECCS)
› All system components required to perform on demand
for a flow permissive function are duplicated in parallel
› All system components required to perform on demand
for a flow isolation function are duplicated in series
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SSR-2/1
Requirement 53: Heat transfer to an
ultimate heat sink
Systems shall be provided to transfer
residual heat from items important to
safety at the nuclear power plant to an
ultimate heat sink. This function shall
be carried out with very high levels of
reliability for all plant states..
SSR-2/1
Requirement 25: Single failure criterion
Containment
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› Containment design pressure of 400 kPa (g)
› Design leakage rate of 0.2%/day
› Seismically qualified to design basis
earthquake with PGA of 0.3 g
› Steel liner plate on the entire inside surface
of the containment structure
› Ensures leak-tightness is within
acceptable limits and
› Prevents spalled concrete from being
generated as a result of an external impact
› Increased thickness of the containment
structure from the CANDU 6 plants
› Meets current safety requirements for
radiation shielding, missile protection,
aircraft crash, and fire protection
›SSR-2/1
›Requirement 54: Containment system for the
reactor
›A containment system shall be provided to
ensure, or to contribute to, the fulfilment of the
following safety functions at the nuclear power
plant: (i) confinement of radioactive substances
in operational states and in accident conditions,
(ii) protection of the reactor against natural
external events and human induced events and
(iii) radiation shielding in operational states and
in accident conditions.
Severe Accident Recovery and Heat Removal System
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› With removal of decay heat from the core
available, progression of core damage can be
arrested
› With removal of heat from containment
available, containment integrity can be
maintained
› SARHRS addresses design extension
conditions that cause
› Loss of heat removal from the steam
generators with intact fuel channels,
› Loss of heat removal from the calandria
vessel with failed fuel channels, and/or
› Loss of heat removal from containment
Low-flow Spray
Header
RESERVE WATER TANK
ECCS
Strainer
CALANDRIA
MODERATORHEAD TANK
Inspection Port
On-site FreshWater Source
External Water
Supply
Severe Accident Recovery and Heat Removal System
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› SARHRS performs the following functions:
› Pumped make-up to directly to the steam
generators from an external water supply
› Gravity-fed make-up from the RWT to the
calandria vessel
› Gravity-fed make-up from the RWT to the
calandria vault
› Containment low flow spray from the RWT
› Pumped make-up to the RWT from external
source to continue gravity-fed make-up and
low flow spray
› Recover water from the reactor building
basement, cool it using a heat exchanger,
and deliver it to the RWT for make up
› Drain contaminated water from RB post-
severe accident
›Requirement 20: Design extension conditions
›5.27 … to prevent accident conditions not
considered design basis accident conditions, or
to mitigate their consequences, as far as is
reasonably practicable. This might require
additional safety features for design extension
conditions, or extension of the capability of safety
systems to maintain the integrity of the
containment. …
Emergency Containment Filtered Venting System
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› With containment integrity
maintained, uncontrolled,
unfiltered releases of radioactivity
can be prevented
› ECFVS addresses design
extension conditions that cause
› Overpressurization of
containment
› ECFVS performs the following
functions:
› Controlled, filtered venting of
containment
› Monitoring of radioactive
releases
Summary
Some examples of the design approach that has been taken in the EC6
design to implement safety requirements from IAEA SSR-2/1 have been
presented:
› Level 2 defence-in-depth: improvements through the use of the Distributed Control System, addition of setback and stepback parameters and a separate system for fuel handling
› Requirement 53: EHRS for level 3 defence-in-depth
› Requirements 54 and 20: Containment envelope for levels 3 and 4 defence-in-depth
› Requirement 20: ECFVS for levels 4 and 5 defence-in-depth
› Requirement 20: SARHRS for level 4 defence-in-depth
› Requirement 67: Emergency Support Centre for levels 4 and 5 defence-in-depth
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Enhanced CANDU 6 Technical Summary
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