PHWR Group of Countries - Nucleus
Transcript of PHWR Group of Countries - Nucleus
PHWR Group of Countries Implementation of Lessons Learned from Fukushima Accident in CANDU Technology
Greg Rzentkowski Director General Directorate of Power Reactor Regulation Canadian Nuclear Safety Commission on behalf of CANDU Senior Regulators Vienna, August 2013
e-Doc 4186422
Countries Operating CANDU Pressurized Heavy Water Reactor (PHWR)
CANDU reactors have operated for over 700 reactor years across 34 reactors in 7 countries Source: CANDU Energy Inc.
No major events in CANDU reactor history
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Canada Bruce 1-8 Darlington 1-4 Gentilly 2 Pickering 1-8 Point Lepreau
Pakistan Kanupp
India Rajasthan 1-2
Romania Cernavoda 1-2
Argentina Atucha 1 Embalse
Under Construction
Atucha 2
China Qinshan 3 (1-2)
South Korea Wolsong 1-4
Outline/Objectives
• CANDU Design Overview
• Inherent Robustness of CANDU/PHWR Design to Severe Accident
• Safety Reviews by CANDU Countries in light of Fukushima Event
– Response to Fukushima Accident
– CANDU Design Enhancements
– Severe Accident Management
• Summary
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CANDU Design Overview Based on the CANDU-6 design
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CANDU Nuclear Power Plant
PHWR Group of Countries
Primary Circuit
Secondary Circuit Circuit
Cooling Water
Primary and secondary circuits are separated (unlike Fukushima Dai-ichi)
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CANDU Design Overview Based on the CANDU-6 design
PHWR Group of Countries
Source: CANDU Energy Inc.
Opens automatically Boiler makeup water isolation valves fail open on loss of power or instrument air
Emergency water supply (simplified)
Main Steam Safety Valves (MSSVs) Opens automatically to depressurize primary and secondary sides
Feedwater supply
Dousing Tank
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PHWR Group of Countries
~ 500 Mg H2O in calandria vault
~ 200 Mg D2O calandria vessel
~ 100 Mg D2O in heat transport system
CANDU Design Overview Passive Heat Removal
Heavy water coolant
Fuel
Pressure tube
Annulus Spacer Annulus Gas
Calandria Tube
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Inherent Robustness of CANDU Design to Severe Accident Scenario: Unmitigated Total Loss of Heat Sinks
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Core Heat-up Leading to In-core Fuel Channel Ruptures
PHWR Group of Countries
Channel rupture at high
pressure can be delayed by
~ 1-7 days by gravity feed
using existing dousing tank
water inventory
~ 4 hours for unmitigated
total loss of heat sinks
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Suspended Debris Bed
PHWR Group of Countries
Core debris from
disassembled channels
held up by intact channels
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Core Collapse
PHWR Group of Countries
Core collapses into residual water pool in calandria vessel
Porous terminal debris bed quenches
Residual water pool boil-off continues
Reactor vault water subcooled at this point
Calandria vessel
Calandria Vessel Rupture Disc
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Corium In-vessel Retention
PHWR Group of Countries
~ 40 hours to calandria vessel failure for unmitigated total loss of heat sinks Calandria vessel failure not expected until calandria vault water level is below corium level inside calandria vessel A design change to permit water makeup to calandria vault will maintain calandria vessel integrity
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Safety Reviews by CANDU National Regulatory authorities in light of Fukushima Event
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Response to Fukushima Accident
Assessments equivalent to stress tests
• Canada, India and Korea completed independent comprehensive safety assessment, and started implementation
• Argentina – performed stress test similar to EU
• Romania – performed EU Stress Test and started implementation
• China – comprehensive safety assessments nearing completion
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Response to Fukushima Accident
1. External events
2. Design issues
3. Severe accident management / recovery
4. National organizations
5. Emergency preparedness
6. International cooperation
PHWR Group of Countries
CANDU “Stress Test”
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External Events
Seismic • NPPs were designed or assessed for at least 0.1g and • Re-assessed for 0.2g or higher by Seismic Margin Assessment (SMA), or PSA
based SMA • Margins enhanced for more severe events
− some seismic upgrades performed or planned − Certain countries are installing or considering seismic trip
• Higher seismic requirements for new plants − typically 1 in 10,000 years for design basis
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Sufficient seismic margins.
External Events (con’t)
Flooding • Adequate safety margins against tsunami for affected NPPs
− upgrades have been identified for one of India’s reactors
• All NPPs are improving defences
− e.g. raised barriers, additional pump out capability, strengthened sea defences
Other enhancements • Additional fire defences needed have been identified and are being installed.
PHWR Group of Countries
Increased protection against wide range of hazards.
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Design Enhancements - Water Supply
Existing Passive Heat Sinks • CANDU has large inventory of water
– many hours of passive cooling – no reactivity or structural concerns with rapid cool-down and depressurization
Added Makeup Water Sources • Installing accessible connection points to:
– steam generators – calandria vessel – calandria vault
• Deploying portable pumps – on and off-site
PHWR Group of Countries
Making CANDUs safer.
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Design Enhancements – Power Supply
Existing Electrical Systems • CANDU’s have good diversity and redundancy
– standby and emergency power has many days supply of fuel
Added Power Sources • Providing mobile Diesel Generators
– stationed on- or off-site – accessible connection points – adequate fuel
• Considering additional battery capacity or charging capability – may be limited to specific purposes, e.g. accident management related I&C – may add portable charging generators
PHWR Group of Countries
Strenghtening defence-in-depth.
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Design Enhancements – Spent Fuel
Existing Spent Fuel Cooling • Slow heat-up
– low heat load – large water inventory
• Pools are robust – in-ground and seismically qualified – pools are not in reactor building
• accessible • steam can be vented
• No criticality issue
Added Safety Features • Accessible pool makeup connections • Better instrumentation – for monitoring water level
PHWR Group of Countries
Spent fuel pools are safe.
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Design Enhancements – Containment
Hydrogen Control • All CANDUs installing or evaluating passive recombiners
– complementary to igniters • Hydrogen concentration monitoring under consideration or already installed
Venting • All CANDUs have filtered containment venting
– in place and adequate for DBA – Design Basis Accident – Additional filtered system are being considered or already have been installed
PHWR Group of Countries
Enhancements to ensure containment integrity.
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Severe Accident Management
SAMGs (Severe Accident Management Guidelines) • Enhancements are being developed:
– to address credible worst case scenario including multi-unit sites
– In some countries implementation underway
Instrumentation • Survivability of equipment and instrumentation in severe accident conditions:
– hardening of key I&C
– additional instrumentation to support severe accident management
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Even well designed and well built systems may fail.
Summary
• CANDU has inherent safety features • Enhancements to strengthen Defence-in-Depth:
– Preventing Fuel Failure • Back-up Mitigation Equipment
– Preventing Severe Core Damage • Makeup capability to calandria vessel
– Arresting Severe Core Damage • Makeup capability to calandria vault
– Preventing Containment Failure • Containment venting/Passive Recombiners
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Thank You
PHWR Group of Countries Argentina, Canada, China, India, Korea, Pakistan, Romania